SwePub
Sök i SwePub databas

  Utökad sökning

Träfflista för sökning "WFRF:(Ma Weimin) "

Sökning: WFRF:(Ma Weimin)

  • Resultat 1-50 av 209
Sortera/gruppera träfflistan
   
NumreringReferensOmslagsbildHitta
1.
  • Li, X., et al. (författare)
  • Flow Pattern Identification of Porous Media Based on Signal Feature Extraction and SVM
  • 2022
  • Ingår i: Kung Cheng Je Wu Li Hsueh Pao/Journal of Engineering Thermophysics. - : Science Press. - 0253-231X. ; 43:11, s. 2957-2965
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper, the visualization experiment of gas-liquid two-phase flow in porous media is carried out. The typical flow patterns of bubbly flow, slug flow and annular flow are photographed by high-speed camera, and the corresponding differential pressure fluctuation signals are measured and recorded, Using probability density function (PDF) and power spectral density (PSD) curves, the time-domain and frequency-domain characteristics of differential pressure signals corresponding to each flow pattern are analyzed, and the quantitative characteristic parameters are introduced to construct the characteristic vector reflecting the time-frequency characteristics of differential pressure signals. A two-phase flow pattern identification method in porous media based on support vector machine (SVM) is proposed. The results show that the overall recognition rate of the three flow patterns measured by the method is 98.18%, which can provide a new technical support for the on-line recognition of gas-liquid two-phase flow patterns in porous media. 
  •  
2.
  • Li, Xiangyu, et al. (författare)
  • Identification of two-phase flow pattern in porous media based on signal feature extraction
  • 2022
  • Ingår i: Flow Measurement and Instrumentation. - : Elsevier BV. - 0955-5986 .- 1873-6998. ; 83
  • Tidskriftsartikel (refereegranskat)abstract
    • The statistical analysis methods based on differential pressure signals of two-phase flow are employed in the present study to identify the flow patterns in packed porous bed. The typical flow pattern images of two-phase flow in the packed porous beds are recognized and the corresponding differential pressure signals are recorded based on the visualization experiments. Then the statistical analysis methods, including probability density function (PDF), power spectral density (PSD), and wavelet energy spectrum (WES), are employed to extract the features of differential pressure signals in the time domain, frequency domain, and time-frequency domain respectively. The dimensionless parameters are proposed as the evaluation index to quantify the differences among flow patterns. The results show that the PDF, PSD, and WES methods can effectively characterize different flow patterns in the time, frequency, and time-frequency domain, respectively. The comprehensive recognition efficiency is about 88.5% using the introduced dimensionless parameters.
  •  
3.
  • Liu, Jiebin, et al. (författare)
  • Influence of an External Perpendicular Oscillation on Stability of a Vertical Falling Liquid Film
  • 2020
  • Ingår i: Microgravity, science and technology. - : Springer Science+Business Media B.V.. - 0938-0108 .- 1875-0494. ; 32:5, s. 787-805
  • Tidskriftsartikel (refereegranskat)abstract
    • A vertical falling Newtonian liquid film flow is inherently unstable to surficial long-wave disturbances. Imposing external oscillation can stabilize the long-wave instability, but also triggers additional parametric instabilities. The effect of oscillation frequency on the stability is subtle. By using the “viscosity-gravity” scaling, the effect of oscillation frequency on the stability can be investigated exhaustively by separating it from other control parameters. In this paper, the effects of external perpendicular oscillation on the stability of a vertical falling liquid film are then investigated by a combination of linear stability analyses based on Floquet theory and numerical simulations with an unsteady weighted residual model (WRM). The linear analyses show that, increasing oscillation amplitude always has a stabilizing effect on the long-wave instability. On the other hand, increasing or decreasing oscillation frequency can suppress the long-wave instability, depending on whether the oscillation amplitude or the acceleration is fixed. The effect of varying oscillation frequency on the long-wave instability is opposite to that on the parametric instabilities. The long-wave and parametric instabilities compete with each other as the oscillation amplitude and frequency are varied with the Reynolds number fixed. A weakness of the long-wave instability always accompanies enhancements of the parametric instabilities, and vice versa. As a contrast, an increase of Reynolds number always results in more unstable long-wave and parametric instabilities. The numerical simulations with the WRM show that the wave amplitudes and the minimal local thickness of film are proportional to the unstable wavenumbers range rather than the growth rate of the instability. For a given oscillation frequency and Reynolds number, there exist a critical oscillation amplitude above which externally imposed oscillations perpendicular to the transversal direction of the film can also trigger a chaotic behavior in the film, just like what happens in the case where the oscillation is parallel to the stream-wise direction of the film.
  •  
4.
  • Liu, J., et al. (författare)
  • Linear stability of a fluid mud–water interface under surface linear long travelling wave based on the Floquet theory
  • 2021
  • Ingår i: European journal of mechanics. B, Fluids. - : Elsevier BV. - 0997-7546 .- 1873-7390. ; 86, s. 37-48
  • Tidskriftsartikel (refereegranskat)abstract
    • The Floquet theory is combined with the unsteady Orr–Sommerfeld equations for the first time to model the linear stability of a fluid mud–water interface under the influence of a linear long travelling wave (or linear shallow water wave). The modelling results reveal three instability modes that could appear on the fluid mud–water surface: the Kelvin–Helmholtz (K–H) and finite-wavelength (F-W) instabilities, which are also present in steady two-layer systems, and parametric instability, which is only seen in periodic problems. The growth rate of the parametric instability is generally small, but it affects the growth rate curves of the other two instabilities. The K–H and F-W instabilities are found to be dominant, and each plays an important role in determining the evolution of the fluid mud–water interface. Both the K–H and F-W instabilities grow with increasing water depth and decreasing wave period as well as with decreasing thickness and density of the mud layer. However, they exhibit distinct dependencies on the fluid mud-to-water viscosity ratio and compete near the critical conditions. For unstable flow near the critical conditions, the K–H instability dominates over the F-W instability at a low viscosity ratio and vice versa at a high viscosity ratio, while for unstable flow far beyond the critical conditions, the K–H instability is dominant regardless of the viscosity ratio. These results are practically instructive for waterway and harbour construction and protection since they provide valuable insights into the early dynamics of the instability mechanisms of the fluid mud–water interface.
  •  
5.
  • Lu, Junjing, et al. (författare)
  • An improved sectional model to simulate multi-component aerosol dynamics in a containment of pressurized water reactor
  • 2021
  • Ingår i: Journal of Aerosol Science. - : Elsevier BV. - 0021-8502 .- 1879-1964. ; 157
  • Tidskriftsartikel (refereegranskat)abstract
    • Simulating an evolving aerosol population in a reactor containment is essential for estimating the radioactivity that is possible to leak to the environment. In this study, a sectional model is developed to simulate multi-component aerosol dynamics in the containment during severe ac-cidents of a pressurized water reactor by improving the widely used MAEROS (Multicomponent AEROSol) model. An important advantage of the improved model is its simplified calculation method by introducing a series of correction factors to the equation coefficients when the thermal boundary conditions and the aerosol particle density in the containment change continuously. In addition, the restriction of the maximum section number in the MAEROS model is removed. The reliability of the model is validated against four analytical solutions and three sets of test data. Moreover, the improvements in the model are also proven to be necessary to effectively capture the influences of thermal boundary conditions and aerosol particle density on aerosol dynamics.
  •  
6.
  • Yu, Peng, et al. (författare)
  • A numerical study of heat transfer in bottom-heated and side/top-cooled liquid metal layers with different aspect ratios
  • 2022
  • Ingår i: Annals of Nuclear Energy. - : Elsevier BV. - 0306-4549 .- 1873-2100. ; 177, s. 109328-
  • Tidskriftsartikel (refereegranskat)abstract
    • A liquid metal layer heated from bottom and cooled from both side and top can be encountered in indus-trial applications. A special interest is from safety design of advanced pressurized water reactors that adopt the so-called in-vessel melt retention (IVR) to mitigate severe accident risk. Quantification of heat transfer in a stratified melt pool in the lower head of a reactor pressure vessel (RPV) is of great impor-tance to the qualification of the IVR strategy. The upper liquid metal layer of the stratified melt pool is heated by the lower molten oxide layer (with decay heat) underneath, and cooled by water outside the reactor vessel and by radiation or flooded water at the top. This is essentially a problem of natural convection and heat transfer in a liquid metal layer heated from bottom and cooled from both side and top. The present study is conducted to numerically investigate the heat transfer characteristics of such layer with an emphasis on the influence of the aspect ratio (ratio of radius to height; R/H) of the liq-uid metal layer. Based on the numerical outcomes, three correlations of heat transfer coefficients (for downward, upward and sideward flows) are also developed to account for the impact the aspect ratio. The numerical simulation results show that, under the same Rayleigh number, the bulk temperature and the upward and sideward heat fluxes all increase with R/H, but the downward heat flux decreases with R/H. The Nusselt numbers in all directions decrease with increasing R/H, as a reduced cooling -heating area ratio due to increasing R/H shall suppress the cooling efficiency and the convection. When R/H is larger than a threshold (-8), the heat transfer characteristics are no longer sensitive to R/H. Each correlation of heat transfer coefficient is developed as the product of two terms: a base correlation of heat transfer coefficient that is Ra dependent only, and an aspect ratio factor that considers the effect of aspect ratio R/H. The developed correlations are compared with the numerical simulation results of cases with different aspect ratios and Rayleigh numbers, and good agreements achieved.
  •  
7.
  • Yu, Peng, et al. (författare)
  • An assessment of the lumped parameter model for the two-layer melt pool heat transfer
  • 2023
  • Ingår i: Annals of Nuclear Energy. - : Elsevier BV. - 0306-4549 .- 1873-2100. ; 180
  • Tidskriftsartikel (refereegranskat)abstract
    • Lumped parameter codes for the two-layer melt pool heat transfer in IVR analysis are usually only verified against the UCSB case and lack of validation. We assessed the performance of the lumped parameter model against the recently conducted two-layer LIVE2D experiment. Both test series with/without top water cooling and with different pool heights were simulated. Influences of the heat transfer correlations in both layers were also investigated. Results showed that the lumped parameter model with existing correlations tends to over -predict sideward heat flux, regardless of the selection of heat transfer correlations in the bottom and top layers. The deficiency could be related to the simplified treatment by modelling the top layer heat transfer as two independent mechanisms: correlations obtained independently from Rayleigh-Be ' nard convection and sideward cooled convection are directly applied to calculate the corresponding upward/downward and sideward heat transfer coefficients, respectively. Evidences from calculations of top layer experiments also support this observation. To solve this issue, additional consideration of the interaction between the two convection mech-anisms may be needed and then perhaps proper model corrections be introduced, or new correlations be developed for this specific convection.
  •  
8.
  • Zhang, Zhengzheng, et al. (författare)
  • Experimental and numerical studies on the two-dimensional flow characteristics in the radially stratified porous bed
  • 2022
  • Ingår i: International Communications in Heat and Mass Transfer. - : Elsevier BV. - 0735-1933 .- 1879-0178. ; 133, s. 105940-
  • Tidskriftsartikel (refereegranskat)abstract
    • The experiment and numerical simulations are both conducted in the present study to better understand the twodimensional flow characteristics in the radially stratified porous bed. Spherical particles of two different sizes are packed in the left part and right part of a cylindrical test section separately to form a two-layer bed with the configuration of radial stratification. The variations of pressure drops in each part of the stratified bed are measured when water flows up through the packed bed. Meanwhile, the numerical simulation is also carried out to investigate the flow field in the stratified bed, especially the flow characteristics around the interface of two parts. The results indicated that the pressure drops in the two layers of radial stratified bed are almost equal. When the fluids flow up through the radially stratified porous layers, the lateral flowing from the low permeability layer to the high permeability layer leads to a decrease of pressure drop in the low permeability layer and an increase of pressure drop in the high permeability layer. Most of the lateral flow occurs in the initial part of the test section. Besides, the lateral and vertical pressure gradient can be well predicted by Ergun equation respectively.
  •  
9.
  • Zhang, Zhengzheng, et al. (författare)
  • Investigation on Flow Characteristics in Radial Stratified Debris Bed
  • 2022
  • Ingår i: Yuanzineng Kexue Jishu/Atomic Energy Science and Technology. - : Atomic Energy Press. - 1000-6931. ; 56:10, s. 2032-2040
  • Tidskriftsartikel (refereegranskat)abstract
    • During the severe accident of light water reactors (LWRs), the particulate debris bed with porous structure may be formed at different places in the reactor after molten corium fuel coolant interaction (FCI). The coolability of the debris bed therefore plays an important role in corium risk quantification, which is crucial to the stabilization and termination of a severe accident in LWRs. Many experimental and analytical studies have been conducted towards quantitative understanding of debris bed coolability. However, most of previous studies were conducted based on the homogeneous debris beds packed with single size particles, and only a few investigations were performed with the heterogeneous debris beds like stratified debris bed. In fact, scoping studies on debris bed formation and configuration based on FCI experiments indicate that the stratified debris bed would be most expected. In order to study the flow characteristics in heterogeneous debris beds, the packed porous beds with radial stratification were constructed in the present study using two different sizes glass spheres with the diameter of 2 mm and 8 mm respectively. Besides, the homogeneous packed beds packed with single size particles and uniform mixture by the above two size particles were also constructed for comparison. The particles were packed in a cylindrical test section with the inner diameter of 120 mm and the height of 600 mm. Single-phase flow tests were performed on the homogeneous beds and heterogeneous bed firstly to investigate the flow resistance characteristics in the packed beds with different configurations. Then numerical simulation was also conducted to reveal the flow redistribution of stratified bed, especially on the flow field at the stratified interface. The experimental results show that the pressure drops of single-phase flow in the homogeneous beds can be well predicted by Ergun equation. For the radial stratified packed bed with different permeability layers, the pressure drops in each layer of the stratified bed are almost equal and increase with the liquid inlet flowrate. Comparing with those in the homogenous beds packed with the same size particles as those in different layer of stratified bed, the pressure drops in the stratified bed are much lower than those of homogenous bed packed with smaller size particles, while slightly higher than those with larger size particles. The numerical simulation results state that there is a two-dimensional flow phenomenon in the radial stratified bed. In addition to dominate upward flow in the stratified bed, a lateral flow flows from low permeability layer to high permeability layer. The two-dimensional flow in stratified bed decreases the flowrate and pressure drops in low permeability layer and increases the pressure drops in high permeability layer. With the increase of liquid flowrate, the average lateral flowrate at the stratified interface increases, but the ratio of lateral volume flowrate to total fluid volume flowrate decreases.
  •  
10.
  • Bandini, G., et al. (författare)
  • Assessment of systems codes and their coupling with CFD codes in thermal-hydraulic applications to innovative reactors
  • 2015
  • Ingår i: Nuclear Engineering and Design. - : Elsevier BV. - 0029-5493 .- 1872-759X. ; 281, s. 22-38
  • Tidskriftsartikel (refereegranskat)abstract
    • The THINS project of the 7th Framework EU Program on nuclear fission safety is devoted to the investigation of crosscutting thermal hydraulic issues for innovative nuclear systems. A significant effort in the project has been dedicated to the qualification and validation of system codes currently employed in thermal hydraulic transient analysis for nuclear reactors. This assessment is based either on already available experimental data, or on the data provided by test campaigns carried out in the frame of THINS project activities. Data provided by TALL and CIRCE facilities were used in the assessment of system codes for HLM reactors, while the PHENIX ultimate natural circulation test was used as reference for a benchmark exercise among system codes for sodium-cooled reactor applications. In addition, a promising grid-free pool model based on proper orthogonal decomposition is proposed to overcome the limits shown by the thermal hydraulic system codes in the simulation of pool-type systems. Furthermore, multi-scale system-CFD solutions have been developed and validated for innovative nuclear system applications. For this purpose, data from the PHENIX experiments have been used, and data are provided by the tests conducted with new configuration of the TALL-3D facility, which accommodates a 3D test section within the primary circuit. The TALL-3D measurements are currently used for the validation of the coupling between system and CFD codes.
  •  
11.
  • Bechta, Sevostian, et al. (författare)
  • On the EU-Japan roadmap for experimental research on corium behavior
  • 2019
  • Ingår i: Annals of Nuclear Energy. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0306-4549 .- 1873-2100. ; 124, s. 541-547
  • Tidskriftsartikel (refereegranskat)abstract
    • A joint research roadmap between Europe and Japan has been developed in severe accident field of light water reactors, focusing particularly on reactor core melt (corium) behavior. The development of this roadmap is one of the main targets of the ongoing EU project SAFEST. This paper presents information about ongoing severe accident studies in the area of corium behavior, rationales and comparison of research priorities identified in different projects and documents, expert ranking of safety issues, and finally the research areas and topics and their priorities suggested for the EU Japan roadmap and future bilateral collaborations. These results provide useful guidelines for (i) assessment of long-term goals and proposals for experimental support needed for proper understanding, interpretation and learning lessons of the Fukushima accident; (ii) analysis of severe accident phenomena; (iii) development of accident prevention and mitigation strategies, and corresponding technical measures; (iv) study of corium samples in European and Japanese laboratories; and (v) preparation of Fukushima site decommissioning.
  •  
12.
  • Bian, Boshen (författare)
  • CFD Study of Molten Pool Convection in a Reactor Vessel during a Severe Accident
  • 2023
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • During severe accidents in nuclear reactors, the core and internal structures can melt down and relocate into the reactor pressure vessel (RPV) lower head (LH) forming there a stratified molten corium pool. The pool generally consists of superheated oxidic and metallic liquid layers imposing thermo-mechanical loads on the RPV wall. The in-vessel retention (IVR) strategy employs external cooling with water to maintain RPV integrity. Investigating the thermo-fluid behaviour of corium and predicting heat flux distribution on the vessel wall are crucial. The molten pool exhibits natural convection, which can typically consist of two stratified layers. There exists internally heated (IH) natural convection in the oxidic layer and Rayleigh-Bénard (RB) convection in the surface metallic layer.This study starts by illustrating the mathematical models that involve the numerical study of natural convection flow in molten corium. A verification work of the model has been done using a previous direct numerical simulation (DNS) study, and the results show good agreement. In addition, a scaling theory of the natural convection flow is demonstrated to facilitate the pre-estimation based on the Rayleigh number (Ra) and Prandtl number (Pr). After that, the numerical approaches involved in the numerical simulation of the corium are illustrated, especially focusing on the DNS method. A DNS mesh strategy is proposed in the form of a pipeline from the pre-estimation to the post-check. A scalability study of Nek5000 is performed on four different HPC clusters based on a DNS case of the IH molten convection in a hemispherical geometry with Ra=1.6×1011. The results show a super-liner speedup property of Nek5000 on each cluster within a certain range.Then, three numerical studies focusing on turbulent natural convection flow within both the oxidic and metallic layers of corium are demonstrated and discussed. Through these simulations, the thermos-fluid behaviour of the system is examined in detail, including flow configuration, temperature distribution, heat flux profiles on cooling boundaries, and turbulent quantities.1. A DNS investigation is performed on the IH molten pool convection within a hemispherical domain, employing a Rayleigh number of 1.6×1011 and a Prandtl number of 0.5. The results show a turbulent flow characterized by three distinct regions, consistent with the observation from the BALI experiments. Detailed information regarding turbulence, including turbulent kinetic energy (TKE), turbulent heat flux (THF), and temperature variance, is presented. Furthermore, the study offers comprehensive 3D heat flux distributions along the boundaries, showing heat flux fluctuations along the top boundary due to nearby turbulent eddies and a nonlinear increase in heat flux along the curved boundary from bottom to top.2. A numerical study investigates the effect of Prandtl number on the natural convection of an IH molten pool in a 3D semi-circular test section. Prandtl numbers of 3.11, 1.0, and 0.5 are considered, with a Ra= 6.54×1011. Smaller Prandtl numbers result in more vigorous turbulent motion and a thicker layer of intense turbulent mixing in the upper region. The descending flow extends further down the bottom, creating a stronger circulation at the bottom with smaller Pr. Additionally, smaller Pr leads to more thermal stripping structures and less stable stratification layers. Comparing heat fluxes on the top and curved walls reveals higher fluctuation frequency with smaller Pr for heat fluxes to the top boundary. However, the maximum heat fluxes to the side walls are lower with smaller Pr.3. A numerical study investigates the turbulent natural convection in a 3D fluid layer based on the BALI-Metal 8U experiment. Different methods, including DNS and three Reynolds-averaged Navier-Stokes (RANS) models, are employed. The results are compared with experimental data, and the performance of the RANS models is evaluated using DNS as a reference. DNS reproduces a two-distinct region flow structure observed in experiments, while the k-ω SST model exhibits similar flow patterns and TKE profiles. However, all simulations overpredict temperature compared to experimental data, with DNS providing the closest results. The DNS results also achieve better agreement with experimental data in terms of heat flux distribution and energy balance, specifically capturing the transient maximum heat flux on the lateral cooling wall. This transient behaviour plays a crucial role in accurately estimating the ‘focusing effect’.
  •  
13.
  • Bian, Qingzhen, 1988-, et al. (författare)
  • Vibronic coherence contributes to photocurrent generation in organic semiconductor heterojunction diodes
  • 2020
  • Ingår i: Nature Communications. - : NATURE PUBLISHING GROUP. - 2041-1723. ; 11:1
  • Tidskriftsartikel (refereegranskat)abstract
    • Charge separation dynamics after the absorption of a photon is a fundamental process relevant both for photosynthetic reaction centers and artificial solar conversion devices. It has been proposed that quantum coherence plays a role in the formation of charge carriers in organic photovoltaics, but experimental proofs have been lacking. Here we report experimental evidence of coherence in the charge separation process in organic donor/acceptor heterojunctions, in the form of low frequency oscillatory signature in the kinetics of the transient absorption and nonlinear two-dimensional photocurrent spectroscopy. The coherence plays a decisive role in the initial 200 femtoseconds as we observe distinct experimental signatures of coherent photocurrent generation. This coherent process breaks the energy barrier limitation for charge formation, thus competing with excitation energy transfer. The physics may inspire the design of new photovoltaic materials with high device performance, which explore the quantum effects in the next-generation optoelectronic applications.
  •  
14.
  • Buck, M., et al. (författare)
  • The LIVE program : Results of test L1 and joint analyses on transient molten pool thermal hydraulics
  • 2010
  • Ingår i: Progress in nuclear energy (New series). - : Elsevier BV. - 0149-1970 .- 1878-4224. ; 52:1, s. 46-60
  • Tidskriftsartikel (refereegranskat)abstract
    • The development of a corium pool in the lower head and its behaviour is still a critical issue. This concerns, in general, the understanding of a severe accident with core melting, its course, major critical phases and timing, and the influence of these processes on the accident progression as well as, in particular, the evaluation of in-vessel melt retention by external vessel flooding as an accident mitigation strategy. Previous studies were especially related to the in-vessel retention question and often just concentrated on the quasi-steady state behaviour of a large molten pool in the lower head, considered as a bounding configuration. However, non-feasibility of the in-vessel retention concept for high power density reactors and uncertainties e.g. due to layering effects even for low or medium power reactors, turns this to be insufficient. Rather, it is essential to consider the whole evolution of the accident, including e.g. formation and growth of the in-core melt pool, characteristics of corium arrival in the lower head, and molten pool behaviour after the debris re-melting. These phenomena have a strong impact on a potential termination of a severe accident. The general objective of the LIVE program at FZK is to study these phenomena resulting from core melting experimentally in large-scale 3D geometry and in supporting separate-effects tests, with emphasis on the transient behaviour. Up to now, several tests on molten pool behaviour have been performed within the LIVE experimental program with water and with non-eutectic melts (KNO3-NaNO3) as simulant fluids. The results of these experiments, performed in nearly adiabatic and in isothermal conditions, allow a direct comparison with findings obtained earlier in other experimental programs (SIMECO, ACOPO, BALI, etc.) and will be used for the assessment of the correlations derived for the molten pool behaviour. Complementary to other international programs with real corium melts, the results of the LIVE activities also provide data for a better understanding of in-core corium pool behaviour. The experimental results are being used for the development and validation of mechanistic models for the description of molten pool behaviour, In the present paper, a range of different models is used for post-test calculations and comparative analyses. This includes simplified, but fast running models implemented in the severe accident codes ASTEC and ATHLET-CD. Further, a computational tool developed at KTH (PECM model implemented in Fluent) is applied. These calculations are complemented by analyses with the CFD code CONV (thermal hydraulics of heterogeneous, viscous and heat-generating melts) which was developed at IBRAE (Nuclear Safety Institute of Russian Academy) within the RASPLAV project and was further improved within the ISTC 2936 Project.
  •  
15.
  • Buerger, M., et al. (författare)
  • Coolability of particulate beds in severe accidents : Status and remaining uncertainties
  • 2010
  • Ingår i: Progress in nuclear energy (New series). - : Elsevier BV. - 0149-1970 .- 1878-4224. ; 52:1, s. 61-75
  • Tidskriftsartikel (refereegranskat)abstract
    • Particulate debris beds may form during different stages of a severe core melt accident; e.g. in the degrading hot core, due to thermal stresses during reflooding, in the lower plenum, by melt flow from the core into water in the lower head, and in the cavity by melt flow out of a failing RPV into a wet cavity. Deep water pools in the cavity are used in Nordic BWRs as an accident management measure aiming at particulate debris formation and coolability. It has been elaborated in the joint work of the European Severe Accident Research Network (SARNET) in Work Package (WP) 11.1 that coolability of particulate debris, reflooding of hot debris as well as boil-off under decay heat (long-term coolability), is strongly favoured by 2D/3D effects in beds with non-homogeneous structure and shape. Especially, water inflow from the sides and via bottom regions strongly improves coolability as compared to 1D situations with top flooding, the latter being in the past the basis of analyses on coolability. Data from experiments included in the SARNET network (DEBRIS at IKE and STYX at VTT) and earlier ones (e.g. POMECO at KTH) have been used to validate key constitutive laws in 2D codes as WABE (IKE) and ICARE/CATHARE (IRSN), especially concerning flow friction and heat transfer. Major questions concern the need of the explicit use of interfacial friction to adequately treat the various flow situations in a unified approach, as well as the adequate characterization of realistic debris composed of irregularly shaped particles of different sizes. joint work has been supported by transfer of the WABE code to KTH and VTT. Concerning realistic debris, the formation from breakup of melt jets in water is investigated in the DEFOR experiments at KTH. Present results indicate that porosities in the debris might be much higher than previously assumed, which would strongly support attainment of coolability. Calculations have been performed with IKEJET/IKEMIX describing jet breakup, mixing and settling of resulting particles. Models about debris bed formation and porosity are developed at KTH. The codes have been applied to reactor conditions for analysing the potential for coolability in the different phases of a severe accident. Calculations have been performed with WABE (MEWA) implemented in ATHLET-CD and with ICARE/ICATHARE for degraded cores and debris beds in the lower plenum, under reflooding and boil-off. Ex-vessel situations have also been analysed. Strong effects of lateral water inflow and cooling by steam in hot areas have been demonstrated. In support, some typical basic configurations have been analysed, e.g. configurations with downcomers considered as possible AM measures. Melt pool formation or coolability of particulate debris is a major issue concerning melt retention in the core and the lower head. Present conclusions from those analyses for adequate modelling in ASTEC are outlined as well as remaining uncertainties. Experimental and analysis efforts and respective continued joint actions are discussed, which are needed to reach resolution of the coolability issue.
  •  
16.
  • Chen, Yangli, et al. (författare)
  • A sensitivity study of MELCOR nodalization for simulation of in-vessel severe accident progression in a boiling water reactor
  • 2019
  • Ingår i: Nuclear Engineering and Design. - : ELSEVIER SCIENCE SA. - 0029-5493 .- 1872-759X. ; 343, s. 22-37
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper presents a sensitivity study of MELCOR nodalization for simulation of postulated severe accidents in a Nordic boiling water reactor, with the objective to address the nodal effect on in-vessel accident progression, including thermal-hydraulic response, core degradation and relocation, hydrogen generation, source term release, melt behavior and heat transfer in the lower head, etc. For this purpose, three meshing schemes (coarse, medium and fine) of the COR package of MELCOR are chosen to analyze two severe accident scenarios: station blackout (SBO) accident and large break loss-of-coolant accident (LOCA) combined with station blackout. The comparative results of the MELCOR simulations show that the meshing schemes mainly affect the core degradation and relocation to the lower head of the reactor pressure vessel: the fine mesh leads to a delayed leveling process of a heap-like debris bed in the lower head, and a later breach of the vessel. The simulations with fine mesh also provide more detailed distributions of corium mass and temperature, as well as heat flux which is an important parameter in qualification assessment of the In-Vessel Melt Retention (IVR) strategy.
  •  
17.
  • Chen, Yangli, et al. (författare)
  • Coupled MELCOR/COCOMO analysis on quench of ex-vessel debris beds
  • 2022
  • Ingår i: Annals of Nuclear Energy. - : Elsevier BV. - 0306-4549 .- 1873-2100. ; 165
  • Tidskriftsartikel (refereegranskat)abstract
    • The cornerstone of severe accident strategy of Nordic BWRs is to flood the reactor cavity for the long-termcoolability of an ex-vessel debris bed. As a prerequisite of the long-term coolability, the hot debris bedformed from fuel coolant interactions (FCI) should be quenched. In the present study, coupling of theMELCOR and COCOMO codes is realized with the aim to analyze the quench process of an ex-vessel debrisbed under prototypical condition of a Nordic BWR. In this coupled simulation, MELCOR performs an integralanalysis of accident progression, and COCOMO performs the thermal–hydraulic analysis of the debrisbed in the flooded cavity. The effective diameter of the particles is investigated. The discussion on thebed’s shape shows a significant effect on the propagation of the quench front, due to different flow patterns.Compared with MELCOR standalone simulation, the coupled simulation predicts earlier cavity poolsaturation and containment venting.
  •  
18.
  • Chen, Yangli, et al. (författare)
  • Development and application of a surrogate model for quick estimation of ex-vessel debris bed coolability
  • 2020
  • Ingår i: Nuclear Engineering and Design. - : Elsevier BV. - 0029-5493 .- 1872-759X. ; 370
  • Tidskriftsartikel (refereegranskat)abstract
    • During a hypothetical severe accident of a Nordic boiling water reactor (BWR), an ex-vessel particulate debris bed is expected to form in the flooded lower drywell due to melt-coolant interactions after vessel failure. The key parameter to evaluate debris bed coolability is the dryout heat flux (DHF) or dryout power density, representing the limit of heat removal capacity by the coolant. Several numerical codes such as COCOMO have been developed to simulate thermal hydraulics in multi-dimensional debris beds and predict the cooling limit, but they are computationally expensive and not suitable for probabilistic risk analysis. This paper aims to develop a surrogate model which can serve as a quick-estimate tool for the dryout power density of a heap-like debris bed in a saturated water pool. The dryout power density predicted from the COCOMO code is treated as the full model. A characteristic factor is introduced as the dryout power density ratio between the multi-dimensional debris bed (predicted by COCOMO code) and the corresponding one-dimensional debris bed (predicted by Lipinski 0-D model). The characteristic factor is correlated by the Kriging method with six parameters: bed porosity, particle diameter, debris mass, bed slope, cavity radius and containment pressure. After the surrogate model is trained and validated, it is employed to analyze the coolability of prototypical debris beds of a reference Nordic BWR, given the bed mass and containment pressure from MELCOR simulation. Coolability maps are produced as quick look-up diagrams for identification of coolable domain with the variation of porosity, particle diameter and slope angle. A preliminary uncertainty analysis is performed to demonstrate the effect of uncertain input parameters on non-coolable domain.
  •  
19.
  • Chen, Yangli, et al. (författare)
  • Development of surrogate model for debris bed coolability analysis
  • 2019
  • Ingår i: 18th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, NURETH 2019. - : American Nuclear Society. ; , s. 6770-6779
  • Konferensbidrag (refereegranskat)abstract
    • The cornerstone of severe accident management (SAM) strategy of a Nordic boiling water reactor (BWR) is to flood the reactor cavity with water from the pressure suppression pool before failure of the reactor pressure vessel (RPV). The idea is to form a deep water pool which can accommodate the corium ejected from the RPV breach and cool the debris bed in the reactor cavity. Hence, assessment of debris bed coolant in the deep water pool is of paramount importance to the qualification of this SAM strategy. For the coolability analysis of a debris bed, one needs to estimate the dryout heat flux/power density of the particle bed, which is considered as the limit for heat removal capacity of coolant. For a multi-dimensional debris bed, the dryout power density can be assessed only by numerical simulation of two-phase flow and heat transfer in porous media. Since the numerical simulation is computationally expensive, it is neither suitable for massive calculations, nor feasible to be implemented into a system code (e.g. MELCOR). There is a clear need to develop a fast-running tool to estimate the dryout power density of a prototypical debris bed. The present study is concerned with development of a surrogate model which is sufficient for PSA study or capable of coupling with the MELCOR code without significant sacrifice of computational efficiency. The surrogate model is conceived from the coolability database predicted by COCOMO which is a mechanistic code for simulating thermal-hydraulic response of debris bed and has been extensively validated and applied in our previous studies [1][2]. The comparative results show that the surrogate model is not only able to predict the coolability limit of a debris bed, but also employed in the sensitivity study of bed’s characteristics (e.g., particle diameter, bed geometry and porosity) and the uncertainty and risk analysis.
  •  
20.
  • Chen, Yangli (författare)
  • MELCOR Capability Development for Simulation of Debris Bed Coolability
  • 2021
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • The severe accident management (SAM) strategy for a Nordic boiling water reactor (BWR) employs cavity flooding prior to vessel failure, so that the core melt (corium) discharged from the vessel could fragment and form a particulate debris bed. The key to the success of this SAM strategy is the coolability of ex-vessel debris beds.The safety analysis involves knowledge about the reactor response to severe accidents under this SAM strategy, which requires the integral simulation of a system code such as MELCOR. Since currently the MELCOR code lacks the modeling of ex-vessel particulate debris beds, the present study aims to develop the capability of MELCOR for the simulation of debris bed coolability through the coupling of MELCOR with other codes, which are dedicated to this phenomenon.The study is started from the qualification of a MELCOR model for severe accident analysis of a reference Nordic BWR, with the aim to help identify a proper core nodalization. For this purpose, three different core meshes (coarse, medium, and fine) are employed to obtain their impacts on corium release conditions. It is found the coarse mesh is sufficient in the present study, since it is not only computationally efficient, but also predicting earlier vessel failure and faster corium release, providing a more conservative condition for debris bed coolability analysis.Two couplings are then adopted: (i) coupling of MELCOR with the COCOMO code, which is a mechanistic code for simulation of thermal hydraulics in debris beds; and (ii) coupling of MELCOR with a surrogate model developed in the present study. The first method can simulate the transient behavior of a debris bed during quench process. The second method can efficiently predict the coolability limit (dryout power) required in safety analysis. The surrogate model is developed based on the COCOMO prediction of two-dimensional debris beds.The developed simulation tools, including the coupled codes and the surrogate model, are applied to the safety analysis of the reference Nordic BWR. The coupled MELCOR/COCOMO simulation is used to investigate the debris bed properties. The effective particle diameter is found as approximately 10% larger than the surface mean diameter of a debris bed with distributed sizes, quantified by the quench rate. For the effect of debris bed shape, it shows a faster quench process with a lower bed slope angle. The quench front propagation as well as the responses of local temperature and containment pressure are obtained.The coupled MELCOR/surrogate model simulation is performed to estimate the coolability of ex-vessel vessel debris beds. The results show that debris beds are coolable under prototypical conditions with probable bed properties. The surrogate model is used to generate coolability maps, which show the debris bed coolability with the variation of bed properties. The sensitivity analysis indicates that the porosity and the geometry are the most influential to coolability limit. An uncertainty analysis methodology is proposed to obtain the probability of non-coolable debris beds.
  •  
21.
  • Chen, Yaodong, et al. (författare)
  • Numerical investigation of Fukushima Daiichi-2 SBO scenario
  • 2014
  • Ingår i: International Congress on Advances in Nuclear Power Plants, ICAPP 2014. - 9781632668264 ; , s. 995-1003
  • Konferensbidrag (refereegranskat)abstract
    • Simulations of the severe accident progression for Fukushima Daiichi NPP Unit 2 (1F2) are performed using the MELCOR code. Detailed modeling of the plant is developed to represent the whole reactor system and its safety systems. The predicted results are compared with the plant data measured during the accident. By applying the main actions taken during the accident and the assumptions into the full plant MELCOR modeling, the major physical phenomena from core uncovery and degradation till reflooding of reactor core by fire pump injection are reproduced in the simulations. The trend of simulation results agree in general with the limited data (e.g., pressures) measured by the plant. The closed RCIC cycle, which involved steam flow and working process, and its interacting with reactor cooling status was modeled by user defined control function in the simulation. The simulations reveal that: The operations of RCIC kept the reactor core flooded to the top for more than 70 hours after the earthquake until the suppression pool water got saturated. Sea water might have flooded into the TORUS room to more extent than as assumed, which kept cooling of suppression pool, and delayed the failure of RCIC. Around 2 hours before the cooling water by fire pump was able to inject water into the reactor, the core damage started at around 76.5hr and got oxidized severely within 2 hours. While no further degradation occurred, the core geometry was maintained, and capable of being cooled by sea water injection.. A leakage has possibly occurred somewhere in RCS steam phase region, to account for pressurization of containment dry well before suppression pool got saturated.
  •  
22.
  • Chen, Yangli, et al. (författare)
  • Sensitivity and uncertainty analysis of trace simulation against FIx-II experiments
  • 2016
  • Ingår i: 17th International Topical Meeting on Nuclear Reactor Thermal Hydraulics, NURETH 2017. - : Association for Computing Machinery (ACM).
  • Konferensbidrag (refereegranskat)abstract
    • In a previous study [1], the US NRC code TRACE was employed to simulate the FIX-II tests which were carried out to investigate the loss of coolant accident (LOCA) of a boiling water reactor (BWR). Results exhibited that the TRACE simulation was sensitive to modelling parameters. In order to further qualify the TRACE code for BWR safety analysis, and to increase our confidence in the simulation results, sensitivity and uncertainty analysis is performed in this paper for the possible uncertain parameters, so as to identify the most influential ones. 12 parameters related to the simulated physical phenomena are selected by resorting to phenomena identification and ranking tables (PIRTs) in relative references. The sensitivity analysis method chosen is based on Finite Mixture Models (FMM) together with Hellinger distance and Kullback-Leibler divergence. Kolmogorov-Smirnov test is first introduced to combine FMM, and it has better performance in screening. Sensitivity analysis results of FMM method show that decay power, choked flow multipliers and break area have the most important influence on calculating peak cladding temperature (PCT). Although previous study failed to predict PCT, uncertainty analysis provides a certain range that successfully covers experiment result.
  •  
23.
  • Chen, Yangli, et al. (författare)
  • Uncertainty quantification for TRACE simulation of FIX-II No. 5052 test
  • 2020
  • Ingår i: Annals of Nuclear Energy. - : Elsevier. - 0306-4549 .- 1873-2100. ; 143
  • Tidskriftsartikel (refereegranskat)abstract
    • The Best Estimate Plus Uncertainty approach requires the knowledge of input uncertainties for the uncertainty propagation with best-estimate codes. Inaccurate judgement of some model parameter uncertainties related to the dominant physical phenomena may result in misestimation of the safety margin. This paper presents a framework of inverse uncertainty quantification (UQ) to quantify model parameter uncertainties in order to address this issue. It is applied to TRACE simulation of a large break loss of coolant accident conducted on the FIX-II facility, and peak cladding temperature (PCT) is the simulation output. Sensitivity analysis identifies the parameters of the critical flow model as the most influential to the PCT. The inverse UQ is performed based on Bayesian framework, which adopts Markov Chain Monte Carlo sampling and surrogate modelling algorithms. The quantified uncertainties of the model parameters are the desired results from the inverse UQ process, which are useful in BEPU studies.
  •  
24.
  • Cheng, X., et al. (författare)
  • European activities on crosscutting thermal-hydraulic phenomena for innovative nuclear systems
  • 2015
  • Ingår i: Nuclear Engineering and Design. - : Elsevier BV. - 0029-5493 .- 1872-759X. ; 290, s. 2-12
  • Tidskriftsartikel (refereegranskat)abstract
    • Thermal-hydraulics is recognized as a key scientific subject in the development of innovative reactor systems. In Europe, a consortium is established consisting of 24 institutions of universities, research centers and nuclear industries with the main objectives to identify and to perform research activities on important crosscutting thermal-hydraulic issues encountered in various innovative nuclear systems. For this purpose the large-scale integrated research project THINS (Thermal-Hydraulics of Innovative Nuclear Systems) is launched in the 7th Framework Programme FP7 of European Union. The main topics considered in the THINS project are (a) advanced reactor core thermal-hydraulics, (b) single phase mixed convection, (c) single phase turbulence, (d) multiphase flow, and (e) numerical code coupling and qualification. The main objectives of the project are: Generation of a data base for the development and validation of new models and codes describing the selected crosscutting thermal-hydraulic phenomena. Development of new physical models and modeling approaches for more accurate description of the crosscutting thermal-hydraulic phenomena. Improvement of the numerical engineering tools for the design analysis of the innovative nuclear systems. This paper describes the technical tasks and methodologies applied to achieve the objectives. Main results achieved so far are summarized. This paper serves also as a guidance of this special issue.
  •  
25.
  • Chikhi, N., et al. (författare)
  • Evaluation of an effective diameter to study quenching and dry-out of complex debris bed
  • 2014
  • Ingår i: Annals of Nuclear Energy. - : Elsevier BV. - 0306-4549 .- 1873-2100. ; 74, s. 24-41
  • Tidskriftsartikel (refereegranskat)abstract
    • Many of the current research works performed in the SARNET-2 WP5 deal with the study of the coolability of debris beds in case of severe nuclear power plant accidents. One of the difficulties for modeling and transposition of experimental results to the real scale and geometry of a debris bed in a reactor is the difficulty to perform experiments with debris beds that are representative for reactor situations. Therefore, many experimental programs have been performed using beds made of multi-diameter spheres or non-spherical particles to study the physical phenomena involved in debris bed coolability and to evaluate an effective diameter. This paper first establishes the ranges of porosity and particle size distribution that might be expected for in-core debris beds and ex-vessel debris beds. Then, the results of pressure drop and dry-out heat flux (DHF) measurements obtained in various experimental setups, POMECO, DEBRIS, COOLOCE/STYX and CALIDE/PRELUDE, are presented. The issues of particle size distribution and non-sphericity are also investigated. It is shown that the experimental data obtained in "simple" debris beds are relevant to describe the behavior of more complex beds. Indeed, for several configurations, it is possible to define an "effective" diameter suitable for evaluating (with the porosity) some model parameters as well as correlations for the pressure drop across the bed, the steam flow rate during quenching and the DHF.
  •  
26.
  • Concilio Hansson, Roberta, 1972- (författare)
  • An Experimental Study on the Dynamics of a Single Droplet Vapor Explosion
  • 2010
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • The present study aims to develop a mechanistic understanding of the thermal-hydraulic processes in a vapor explosion, which may occur in nuclear power plants during a hypothetical severe accident involving interactions of high-temperature corium melt and volatile coolant. Over the past several decades, a large body of literature has been accumulated on vapor explosion phenomenology and methods for assessment of the related risk. Vapor explosion is driven by a rapid fragmentation of high temperaturemelt droplets, leading to a substantial increase of heattransfer areas and subsequent explosive evaporation of the volatile coolant. Constrained by the liquid-phase coolant, the rapid vapor production in the interaction zone causes pressurization and dynamic loading on surrounding structures. While such a general understanding has been established, the triggering mechanism and subsequent dynamic fine fragmentation have yet not been clearly understood. A few mechanistic fragmentation models have been proposed, however, computational efforts to simulate the phenomena generated a large scatter of results. Dynamics of the hot liquid (melt) droplet and the volatile liquid (coolant) are investigated in the MISTEE (Micro-Interactions in Steam Explosion Experiments) facility by performing well-controlled, externally triggered, single-droplet experiments, using a high-speed visualization system with synchronized digital cinematography and continuous X-ray radiography, called SHARP (Simultaneous High-speed Acquisition of X-ray Radiography and Photography). After an elaborate image processing, the SHARP images depict the evolution of both melt material (dispersal) and coolant (bubble dynamics), and their microscale interactions, i.e. the triggering phenomenology. The images point to coolant entrainment into the droplet surface as the mechanism for direct contact/mixing ultimately responsible for energetic interactions. Most importantly, the MISTEE data reveals an inverse correlation between the coolant temperature and the molten droplet deformation/prefragmentation during the first bubble dynamics cycle. The SHARP observations followed by further analysis leads to a hypothesis about a novel phenomenon called pre-conditioning, according to which dynamics of the first bubble-dynamics cycle and the ability of the melt drop to deform/pre-fragment dictate the subsequent explosivity of the so-triggered droplet. The effect of non-condensable gases on the perceived mechanisms was investigated on the MISTEE-NCG test campaign, in which a considerable amount of non-condensable gases (NCG) are present in the film that enfolds the molten droplet. The SHARP images for the MISTEE-NCG tests were analyzed and special attention was given to the morphology (aspect ratio) and dynamics of the air/ vapor bubble, as well as the melt drop preconditioning and interaction energetics. Analysis showed twomain aspects when compared to the MISTEE test series (withoutentrapped air). First, the investigation showed that the meltpreconditioning still strongly depends on the coolant subcooling. Second,in respect to the energetics, the tests consistently showed a reducedconversion ratio compared to that of the MISTEE test series. The effect of the melt material in the steam explosion triggerability was also summoned, since it would in principle directly implicate the melt preconditioning. Since a number of the thermo-physical properties of the material would influence the triggering process, we focused on the material properties by using the same dioxide material with difference concentrations, i.e. eutectic and non-eutectic. Unfortunately, due to the high melt superheat the possible differences were not perceived. Thus, inaddition to other materials, lower melt superheat tests were schedule inthe future.
  •  
27.
  • Deng, Yucheng, et al. (författare)
  • A numerical study on the levitation system for droplet preparation in a fuel-coolant interaction experiment
  • 2023
  • Ingår i: Progress in nuclear energy (New series). - : Elsevier BV. - 0149-1970 .- 1878-4224. ; 159
  • Tidskriftsartikel (refereegranskat)abstract
    • The MISTEE facility at KTH was designed to investigate the process and phenomena of a molten droplet falling into a water pool that may be encountered in fuel-coolant interactions (FCI) during a severe accident of light water reactors. An aerodynamic levitation mechanism is proposed to hold the molten droplet during its preparation (melting and heating up to a prescribed temperature) in an induction furnace. The crucible is flushed with argon through an injection nozzle at the bottom to prevent the droplet from falling out of the crucible. A numerical simulation of the aerodynamic levitation system is performed in the present study with the objective of determining and optimizing the design. The problem was simplified as an isothermal two-phase flow in an axisymmetric geometry. The simulation is realized through ANSYS Fluent v17 platform, which employs the VOF method to track interfaces between two phases and the SST k-omega model to describe turbulence flow of argon gas. The numerical model is validated against tests performed in the MISTEE facility after mesh sensitivity study. It is then applied to investigate the impacts of various parameters on the facility levitation capability and the droplet stability. According to the simulation results, stable molten droplets can be obtained in the designed experimental setup. The simulation also provides the appropriate values of argon inlet velocity and sample mass at which a stable droplet can be obtained inside the crucible before its discharge. Either higher or lower inlet velocity will destabilize the formation of the droplet. Considering the temperature-dependent melt properties, both surface tension and viscosity affect the movement and deformation of the molten droplet. The wettability of melt on the crucible wall is critical to droplet formation, and it is found that a poor wettability can ensure the levitation of droplet.
  •  
28.
  • Deng, Yucheng, et al. (författare)
  • An experimental study on droplet quench and steam explosion in boric acid solutions
  • 2023
  • Ingår i: Progress in nuclear energy (New series). - : Elsevier BV. - 0149-1970 .- 1878-4224. ; 166
  • Tidskriftsartikel (refereegranskat)abstract
    • Boric acid (H3BO3) is widely adopted as an additive in the coolant of light water reactors for reactivity control, but its effect on fuel coolant interactions (FCI) during severe accidents (especially on steam explosion) was rarely investigated. To examine the effect of the boric acid additive in coolant on steam explosion, a series of molten droplet-coolant interaction tests using H3BO3 solutions (with concentration ranging from 0-3.2% by weight) is carried out in the present study. The characteristics of melt-coolant interactions are the occurrence probability of typical phenomena (no fragmentation, minor fragmentation, or spontaneous steam explosion), lateral deformation ratio, quench depth, pressure impulse and debris particle size distribution. The statistical data of such characteristics are obtained through repeating 20 runs of the same test category. The experimental results show that the H3BO3 addition in coolant has various impacts on the above-mentioned characteristics of melt-coolant interactions, depending on the H3BO3 concentration. In particular, the probability of steam explosion sightly decreases as the H3BO3 concentration increases from zero to 1.2 wt.%, but significantly increases as the H3BO3 concentration further increases to 3.2 wt.% trough 2.2 wt.%. Namely, the inhibiting effect of boric acid on steam explosion is diminishing with increasing H3BO3 concentration beyond 1.2 wt.%. It is also found that both melt and coolant temperatures are crucial parameters impacting the likelihood and energetics of steam explosion.
  •  
29.
  • Deng, Yucheng, et al. (författare)
  • An Experimental study on steam explosion of multiple droplets in different chemical solutions
  • 2024
  • Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 226
  • Tidskriftsartikel (refereegranskat)abstract
    • Motivated by the interest in steam explosion in chemical solutions and seawater, a series of tests were carried out in the MISTEE facility at KTH to investigate steam explosion characteristics as multiple molten droplets of tin were falling through a coolant pool containing deionized water, boric acid solution, neutral solution of boric acid and sodium phosphate, and seawater, separately. The experimental results revealed distinct and complex characteristics of steam explosion of multiple droplets, which were not observed in previous single-droplet steam explosion experiments. The tin melt samples of 5 g and 20 g were employed to formulate different numbers of multiple droplets. In the test with 5 g melt, steam explosion was more energetic at a deeper explosion location − a similar trend found in the single-droplet steam explosion test with 1 g melt. However, the test of 20 g melt did not show a clear trend in a wide range of explosion depth. The peak pressure and impulse increased with increasing mass of melt sample. The steam explosion occurred more closely to the coolant pool surfaces in the seawater and chemical solutions than in deionized water. Steam explosion intensity was significantly reduced in a neutral solution containing 1.2 wt.% boric acid and sodium phosphate. The influence of the chemical solutions on steam explosion was diminishing in the tests with multiple droplets.
  •  
30.
  • Deng, Yucheng, et al. (författare)
  • An experimental study on the effect of chemical additives in coolant on steam explosion
  • 2024
  • Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier Ltd. - 0017-9310 .- 1879-2189. ; 218
  • Tidskriftsartikel (refereegranskat)abstract
    • In assessment of severe accident risk in light water reactors (LWRs), steam explosion is a nonnegligible phenomenon following a relocation of core melt (corium) into coolant, and thus various research efforts have been paid to steam explosion. There had been numerous studies showing that the occurrence of steam explosions is influenced by several factors such as melt and coolant temperatures, melt materials, non-condensable gasses, etc. However, most of the existing experiments used deionized (DI) water or tap water as coolant, with little consideration of the effect of chemicals (e.g. boric acid, sodium hydroxide, sodium phosphate) commonly applied in reactor coolant. To examine the effect of the chemical additives in coolant on steam explosion, the present study performs a series of molten Tin droplet-coolant interaction tests using DI water and different chemical solutions, including H3BO3 solutions, NaOH + H3BO3 neutral solutions, and Na3PO4 + H3BO3 neutral solutions. The experimental results show that adding NaOH and Na3PO4 in boric acid solution significantly affects the occurrence probability of spontaneous steam explosion, because of the presence of PO43− and H+ ions. When different solutions have equivalent concentrations of H3BO3, the peak pressure values of the spontaneous steam explosion of Sn droplets are similar among various solutions. Compared with those in DI water, steam explosion in the chemical solutions occurs predominantly within a narrow range of depth from 28 mm to 40 mm and produces a much higher peak pressure. This implies that more energetic steam explosions may occur in the chemical solutions.
  •  
31.
  • Deng, Yucheng, et al. (författare)
  • An experimental study on the effect of coolant salinity on steam explosion
  • 2024
  • Ingår i: Annals of Nuclear Energy. - : Elsevier BV. - 0306-4549 .- 1873-2100. ; 201
  • Tidskriftsartikel (refereegranskat)abstract
    • The steam explosion plays an essential role in the safety analysis of light water reactors (LWRs). Some studies have demonstrated that the occurrence of steam explosions is dependent on many factors such as melt and coolant temperatures, melt and coolant properties, non -condensable gases, etc. After the Fukushima accident, seawater as an emergency coolant and its impact on fuel coolant interactions are receiving attention. However, there is still little knowledge on the impact of seawater on steam explosion. The present study is intended to examine the effect of coolant salinity on steam explosion through a series of tests with single molten droplet falling in different coolant pools (DI water, and seawater at different salinities from 7.7 g/kg to 35 g/kg). The experimental results reveal that the salinity of coolant significantly influences the probability of spontaneous steam explosion of molten tin droplets. The probability of steam explosion generally increases with increasing salinity from 0 to 17.5 g/kg. The molten droplet in seawater experiences more pronounced deformation at same depth before the vapor film of the droplet collapses. What's more, the peak pressure generated by steam explosion in seawater is notably higher than that in DI water. The fragmentation of molten tin droplet after the explosion is enhanced accordingly.
  •  
32.
  • Deng, Yucheng, 1993- (författare)
  • Experimental Study on Steam Explosions in Chemical Solutions and Seawater
  • 2024
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Steam explosions may be encountered in severe accidents of light water reactors (LWRs), which are thermal detonations caused by rapid and intense vaporization of the coolant upon its direct contact with the core melt (corium). Motivated by the interest in understanding and mitigation of severe accident progression, many studies have been conducted to investigate the steam explosion phenomena during severe accidents. However, most of the previous studies did not consider the effect of chemical additives in the coolant of nuclear power plants, such as additions of H3BO3, NaOH and Na3PO4 for water chemistry control, and direct utilization of seawater (NaCl additive) under an extreme condition like the Fukushima accident. The present thesis work is motivated to fill the knowledge gap concerning the impacts of chemical additives (H3BO3, NaOH, Na3PO4, and NaCl) on steam explosions.  The primary objective of the present research is to obtain characteristics of steam explosions in seawater and chemical solutions of H3BO3/NaOH/Na3PO4 with prototypical concentrations. To achieve this goal, a series of experiments have been carried out in the MISTEE experimental platform at KTH, involving single droplet and multiple droplets falling into a variety of coolant pool filled with seawater or chemical solutions of H3BO3/NaOH/Na3PO4 additives. The thesis work consists of four parts as follows.The first part is a description of the experimental methodology developed in the present study. Two experimental facilities, dubbed MISTEE-CE and MISTEE-SEA of respective mechanical plug and aerodynamic levitation for melt delivery, were designed on the MISTEE platform. Both setups were equipped with high-speed cameras for visualization, a pressure sensor for dynamic pressure measurement, and a fragment catcher for debris collection. A double-crucible design was employed to enable induction heating while avoiding melt contamination. The aerodynamic levitation system was implemented in MISTEE-SEA to reduce the disturbance of the mechanical plug. All chemical solutions were prepared in the laboratory with degassed deionized water. Tin (Sn) was chosen as the melt material due to benign properties suitable for safe handling in the laboratory.The second part is the presentation of visual observations and parameters selected to characterize steam explosions. The visualization includes the phenomena of droplet-coolant interactions and steam explosion occurrences. A molten single droplet falling into the coolant pool with deionized water or chemical solution might experience one of the three typical phenomena: deformation without fragmentation, minor fragmentation, or spontaneous steam explosion. In contrast, a multi-droplet test might involve merging and multiple explosions of droplets, resulting in a more complex set of phenomena. The quench depth and the lateral deformation ratio were defined and used to analyze the dynamic process of a single droplet in the coolant, while the peak pressure was employed to compare steam explosion energetics. In addition, the size distribution of debris particles was scrutinized.The third part is a summary and highlights of the experimental study on single-droplet steam explosion in different chemical solutions, using 1g of melt sample. The results revealed that the H3BO3 additive had little impact on steam explosion when the H3BO3 concentration was lower than 1.2 wt.%, but the risk of steam explosion in 3.2 wt.% H3BO3 solution was higher. The addition of NaOH and Na3PO4 to an H3BO3 solution significantly offset the influence on steam explosion. This suggests that the presence of PO43- and H+ ions play a significant role in spontaneous steam explosions. Additionally, seawater enhanced the occurrence of spontaneous steam explosions, with a clear correlation between increasing salinity and a higher likelihood of steam explosion. Compared to deionized water, chemical solutions (including seawater) caused more pronounced deformation in molten droplets at equivalent depths prior to direct contact of melt with coolant. Furthermore, the peak pressures of steam explosions in chemical solutions had the potential to reach notably higher values than those in deionized water. The chemical solutions except for the one of 1.2 wt.% H3BO3 tended to produce higher fractions of finer debris particles. The fourth part is about the experimental results of an investigation on steam explosion involving multiple droplets falling into deionized water and chemical solutions, using 5 g and 20 g of melt samples, respectively. It was found that under identical test conditions, the peak pressure of steam explosion increased with melt sample mass, resulting in a noticeably higher fraction of fine debris particles in the case of 20 g melt sample. The steam explosion location was concentrated within a shallower range when using chemical solutions instead of deionized water. In contrast to single-droplet experiments, the influence of the chemical solutions on the steam explosion was diminishing in the tests with multiple droplets.
  •  
33.
  • Dong, Shichang, et al. (författare)
  • Mechanistic critical heat flux model development for subcooled flow boiling based on superheated liquid sublayer depletion
  • 2022
  • Ingår i: Progress in nuclear energy (New series). - : Elsevier BV. - 0149-1970 .- 1878-4224. ; 153, s. 104445-
  • Tidskriftsartikel (refereegranskat)abstract
    • Critical heat flux (CHF) refers to the limit of boiling transfer systems, and crossing this limit may jeopardize system safety. However, a clear understanding of the physical mechanisms of CHF is still lacking. In this study, a new CHF prediction model based on superheated sublayer depletion was established for subcooled flow boiling in an upward vertical tube at low pressure. The model is characterized by its developed determination of the superheated liquid sublayer thickness, net vapor generation location, forced convection heat transfer and liquid supplement caused by bubble turbulent fluctuations. The proposed CHF model was validated by a database covering the low pressure subcooled operational ranging over P = 0.1-2.15 MPa, G = 0.7-35 Mg/m2s, Delta Tin = 11-183.16 K, D = 0.7-12 mm, L/D = 4.2-115.55, and xeqout = -0.2673-0.0843. The model can accurately predict the trend of thermal-hydraulic and geometric factors' effects on the CHF. The prediction results have good prediction accuracy with an root-mean-square error (RMSE) of 15.21%, and overall error of +/- 25%. The proposed model also shows good adaptation to a non-water (refrigerant 113 and liquid nitrogen) system.
  •  
34.
  • Dong, Shichang, et al. (författare)
  • Theoretical model for subcooled upward flow boiling heat transfer and critical heat flux for an inclined downward heated surface
  • 2023
  • Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 213
  • Tidskriftsartikel (refereegranskat)abstract
    • The in-vessel retention system and ex-vessel retention system are very important to the safety of nu-clear power plants under severe accidents. While the success of such safety systems relies on well un-derstanding the corresponding physical mechanisms of boiling heat transfer and critical heat flux (CHF). Challenges till remain in accurately predicting the subcooled flow boiling curve especially in the low-pressure and low-flow conditions due to its complex boiling phenomenon. The present study introduces a theoretical model to predict the boiling curve and critical heat flux for subcooled flow boiling in in-clined downward heated rectangular channel. The proposed model well estimates the transition from forced convection, isolated bubble nucleate boiling to fully developed boiling regime by considering the growth and interaction of bubbles. Through probability analysis of bubbles' interaction, the proportion of heat flux in different boiling regimes is determined. In addition, the flow boiling CHF is predicted based on the probability analysis of dry spots. The new model is validated by the subcooled flow boil-ing experiments with vertical single-side heated channel under low-pressure and low-flow conditions. The predicted boiling curves are consistent with experimental results corresponding to different thermal-hydraulic parameters, such as pressure, mass flux, inlet subcooling and wall wettability (hydrophilic and hydrophobic), and the prediction error of CHF is within & PLUSMN;15%. Furthermore, the inclination effect on CHF is validated by the subcooled flow boiling experiments in inclined channel with the inclination angle varying from 0 & DEG; to 90 & DEG;, which shows the good applicability of the developed model. 
  •  
35.
  • Fang, Di, et al. (författare)
  • A numerical study of liquid film dynamics in multi-nozzle spray cooling of downward-facing surface
  • 2023
  • Ingår i: International Journal of Multiphase Flow. - : Elsevier BV. - 0301-9322 .- 1879-3533. ; 161
  • Tidskriftsartikel (refereegranskat)abstract
    • In a consideration of spray cooling as the potential cooling mechanism for the in-vessel melt retention (IVR) strategy of nuclear reactors because of its superior heat removal efficiency, the SPAYCOR experiment has been conducted at KTH to investigate the spray cooling capacity of multiple nozzles applied to a downward-facing heated surface. In the present study, the dynamics of liquid film on the downward-facing surface resulting from the multi-nozzle spray are numerically simulated by using a coupled Eulerian-Lagrangian method implemented in the OpenFOAM platform. Prior to simulation of the SPAYCOR experiment, the numerical approach is used to calculate two theoretical setups which have known analytical solutions, with the objective to validate the models in predicting liquid film dynamics either in spray or on an inclined surface. In the simulation of the SPAYCOR experiment, the predicted film morphology shows a good agreement with the experimental observation. What's more, the influential factors, including the inclination of the downward-facing heater surface, the nozzle-to-surface distance as well as the nozzle-array layout, are also investigated numerically in the present study. The simulation results show that a decreasing nozzle-to-surface distance does not only lead to a thicker liquid film and a lower velocity in the vicinity of each spray coverage, but also increases non-uniformity of the liquid film. The nozzles-array layout has little influence on the average liquid film thickness and velocity, but significantly affects the film morphology.
  •  
36.
  • Fang, Di, et al. (författare)
  • A numerical study on multi-nozzle spray cooling of downward-facing heater surface
  • 2024
  • Ingår i: Progress in nuclear energy (New series). - : Elsevier BV. - 0149-1970 .- 1878-4224. ; 173
  • Tidskriftsartikel (refereegranskat)abstract
    • An experimental study on multi-nozzle spray cooling of a downward-facing heater surface has been carried out in the SPAYCOR facility at KTH, to provide data assessing the feasibility of spray cooling for in-vessel melt retention (IVR) in light water reactors. To help understand the characteristics and influential factors of the liquid film formed on the heater surface in spray, a numerical study on the dynamics of an isothermal liquid film on the heater surface has also been performed by adopting the OpenFOAM platform, and Eulerian and Lagrangian methods for liquid film and droplets, respectively. The present study is an extension of the previous modeling from hydrodynamics to thermal-hydraulics of the spray cooling problem, via adding heat flux of the heater and two convective heat transfer models between the heater wall and the liquid film. Moreover, droplets-film interaction model is modified. The SPAYCOR experiment is simulated by the numerical models, and the simulation results show a good agreement between the numerical and experimental data, in particular when the modified droplets-film interaction model is applied. After the validation of the numerical models against the SPAYCOR experiment, the numerical models are employed to investigate influential factors on heat transfer, such as mass flux, nozzle-to-surface distance, and nozzle matrix layout. The results indicate that heat transfer is enhanced by increasing mass flux and decreasing nozzle-to-surface distance, and the change of nozzle matrix from inline to staggered layout has little impact on heat removal capacity or temperature distribution of the multi-nozzle spray cooling.
  •  
37.
  • Gajev, Ivan, et al. (författare)
  • Sensitivity analysis of input uncertain parameters on BWR stability using TRACE/PARCS
  • 2014
  • Ingår i: Annals of Nuclear Energy. - : Elsevier BV. - 0306-4549 .- 1873-2100. ; 67, s. 49-58
  • Tidskriftsartikel (refereegranskat)abstract
    • The unstable behavior of Boiling Water Reactors (BWR), which is known to occur at certain power and flow conditions, could cause SCRAM and decrease the economic performance of the plant. For better prediction of BWR stability and understanding of influential parameters, two TRACE/PARCS models of Ringh-als-1 and Oskarshamn-2 BWRs were employed to perform a sensitivity study. Using the propagation of input errors uncertainty method's results, an attempt has been made to identify the most influential parameters affecting the stability. Furthermore, a methodology using the spearman rank correlation coefficient has been used to identify the most influential parameters on the stability parameters (decay ratio and frequency).
  •  
38.
  • Gajev, Ivan, 1985- (författare)
  • Sensitivity and Uncertainty Analysis of Boiling Water Reactor Stability Simulations
  • 2012
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • The best estimate codes are used for licensing of Nuclear Power Plants (NPP), but with conservative assumptions. It is claimed that the uncertainties are covered by the conservatism of the calculation. Nowadays, it is possible to estimate certain parameters using non-conservative data with the complement of uncertainty evaluation, and these calculations can also be used for licensing. As NPPs are applying for power up-rates and life extension, new licensing calculations need to be performed. In this case, evaluation of the uncertainties could help improve the performance, while staying below the limit of the safety margins.Given the problem of unstable behavior of Boiling Water Reactors (BWR), which is known to occur at certain power and flow conditions, it could cause SCRAM and decrease the economic performance of the plant. Performing an uncertainty analysis for BWR stability would give better understating of the phenomenon and it would help to verify and validate (V&V) the codes used to predict the NPP behavior.This thesis, reports a sensitivity/uncertainty study of numerical, neutronics, and thermal-hydraulics parameters on the prediction of the BWR stability within the framework of OECD Ringhals-1 (R1 stable reactor) and OECD Oskarshamn-2 (O2 unstable reactor) stability benchmarks. The time domain code TRACE/PARCS was used in the analyses. This thesis is divided in three parts: space-time convergence; uncertainty; sensitivity.A space-time convergence study was done for the numerical parameters (nodalization and time step). This was done by refining nodalization of all components and time step until obtaining space-time converged solution, i.e. further refinement doesn’t change the solution. When the space-time converged solutions were compared to the initial models, much better solution accuracy has been obtained for the stability measures (decay ratio and frequency), for both stable (R1) and unstable (O2) reactors with the space-time converged models.Further on, important neutronics and thermal-hydraulics parameters were identified and an uncertainty calculation was performed using the Propagation of Input Errors (PIE) methodology. This methodology, also known as the GRS method, has been used because it has been extensively tested and verified by the industry, and because it allows identifying the most influential parameters using the spearman rank correlation method.Using the uncertainty method’s results, an attempt has been done to identify the most influential parameters affecting the stability. A methodology using the spearman rank correlation coefficient has been implemented, which helps to identify the most influential parameters on the stability (decay ratio and frequency). Additional sensitivity calculations have been performed for better understanding of BWR stability and parameters that affect it.
  •  
39.
  •  
40.
  • Gajev, Ivan, et al. (författare)
  • Space–time convergence analysis on BWR stability using TRACE/PARCS
  • 2013
  • Ingår i: Annals of Nuclear Energy. - : Elsevier BV. - 0306-4549 .- 1873-2100. ; 51, s. 295-306
  • Tidskriftsartikel (refereegranskat)abstract
    • Unstable behavior of Boiling Water Reactors (BWRs) is known to occur during operation at certain power and flow conditions. Even though BWR instability is not a severe safety concern, it could cause reactor scram and significantly decrease the economic performance of the plant. This paper aims to (a) quantify TRACE/PARCS space–time discretization error for simulation of BWR stability, (b) establish space (nodalization) and time discretization necessary for space–time converged model and (c) show that the space–time converged model gives more reliable results for both stable and unstable reactor. The space–time converged model is obtained when further refinement of numerical discretization parameters (nodalization and time step) has negligible effect on the solution. The study is significant because performing a space–time convergence analysis is a necessary step of qualification of the TRACE/PARCS model, and use of the space–time converged model increases confidence in the prediction of BWR stability.
  •  
41.
  • Gajev, Ivan, et al. (författare)
  • Uncertainty analysis of the OECD/NRC Oskarshamn-2 BWR stability Benchmark
  • 2014
  • Ingår i: Proceedings of the International Conference on Physics of Reactors, PHYSOR 2014. - : Japan Atomic Energy Agency, JAEA.
  • Konferensbidrag (refereegranskat)abstract
    • On February 25, 1999, the Oskarshamn-2 NPP experienced a stability event which culminated in diverging power oscillations with a decay ratio of about 1.4. The event was successfully modeled by the TRACE/PARCS coupled system code, and further uncertainty analysis of the event is described in this paper. The results show very good agreement with the plant data, capturing the entire behavior of the transient including the onset of instability, growth of the oscillations (decay ratio) and oscillation frequency. This provides confidence in the prediction of other parameters which are not available from the plant records. This paper shows also how an uncertainty method was implemented for the event. Comparing the calculated uncertainty with the measured uncertainty gives confidence in the BWR stability prediction.
  •  
42.
  • Gong, Shengjie, et al. (författare)
  • An experimental investigation on bubble dynamics and boiling crisis in liquid films
  • 2014
  • Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 79, s. 694-703
  • Tidskriftsartikel (refereegranskat)abstract
    • This paper presents an experimental study of boiling and boiling crisis in a liquid film on a heater surface. The critical heat flux (CHF) values obtained in the present experiment mirror that of pool boiling, irrespective of initial liquid film thickness and liquid supply rate in the liquid film boiling case. This observation reinforces to the "scale separation" concept that high-heat-flux boiling and burnout are governed by micro-hydrodynamics in the liquid film on the heater surface. In addition to the CHF data, evolutions of bubbles and dry spots in the boiling liquid film are captured by means of high-speed high-resolution video camera. The dry spots were observed over surface heat flux ranging from 0.3 MW/m(2) to CHF, typically covering an area less than 10% of the heater surface. Three types of dry spot evolution are observed: (1) under the low heat flux, dry spots are rewetted by receding water dam upon rupture of corresponding bubbles; (2) as the heat flux reaches 1.25 MW/m(2), dry spots rewetting is additionally aided by liquid flow driven by growth of bubbles nucleated in the vicinity; (3) upon approaching the CHF, dry spot(s) cannot be rewetted anymore and expand laterally, leading to boiling crisis (burnout of the heater surface). The richness of observations and characterization of micro-hydrodynamics in the present study further demonstrates that observations and measurements on boiling liquid films provide a paramount window for investigation and understanding of physical mechanisms of boiling and boiling crisis.
  •  
43.
  • Gong, Shengjie, et al. (författare)
  • An experimental study of rupture dynamics of evaporating liquid films on different heater surfaces
  • 2011
  • Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 54:7-8, s. 1538-1547
  • Tidskriftsartikel (refereegranskat)abstract
    • Experimental data were obtained to reveal the complex dynamics of thin liquid films evaporating on heated horizontal surfaces, including formation and expansion of dry spots that occur after the liquid films decreased below critical thicknesses. The critical thickness of water film evaporating on various material surfaces is measured in the range of 60-150 mu m, increasing with contact angle and heat flux while decreasing with thermal conductivity of the heater material. In the case of hexane evaporating on a titanium surface, the liquid film is found resilient to rupture, but starts oscillating as the averaged film thickness decreases below 15 mu m.
  •  
44.
  •  
45.
  • Gong, Shengjie, et al. (författare)
  • An Experimental Study on Boiling Phenomenon in a Liquid Layer
  • 2010
  • Ingår i: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings. - : International Conference on Multiphase Flow (ICMF).
  • Konferensbidrag (refereegranskat)abstract
    • This work investigates boiling phenomena by means of imaging and characterization of bubble dynamics in the vicinity of the bubble’s nucleation site. A silicon wafer is used as heat transfer surface so that MEMS fabrication can be applied to create artificial cavity for prescribed nucleation site. The well-controlled bubbles growing on such nucleation site can facilitate measurement and observation. High-speed video camera is employed in visualization, and the instantaneous thickness of the liquid layer is recorded by a confocal optical sensor. Tests are first performed on a water layer with the thickness of 7.5mm±0.5mm, and the bubble departure diameter and frequency as well as the transient evolution of bubble diameter and foot size are obtained in isolated bubble regime. Bubble departure diameter enlarges with increasing heat flux, and the measured maximum diameter is around 3.2 mm. With the decrease of the liquid layer thickness to 2 mm, the bubbles are found to remain on the heater surface for a relative long period, with a dry spot initiation under the bubble that becomes rewetted after the bubble bursting. As the water layer thickness decreases further, irreversible dry spot appears, suggesting a minimum “safe” film thickness in the range from 1.2 to 1.9 mm under the tested heat flux range from 26 kW/m2 to 52 kW/m2.
  •  
46.
  •  
47.
  • Gong, Shengjie (författare)
  • An Experimental Study on Micro-Hydrodynamics of Evaporating/Boiling Liquid Film
  • 2011
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Study of liquid film dynamics is of significant importance to the understanding and control of various industrial processes that involve spray cooling (condensation), heating (boiling), coating, cleaning and lubrication. For instance, the critical heat flux (CHF) of boiling heat transfer is one of the key parameters ensuring the efficiency and safety of nuclear power plants under both operational and accident conditions, which occurs as the liquid layers (microlayer and macrolayer) near the heater wall lose their integrity. However, an experimental quantification of thin liquid film dynamics is not straightforward, since the measurement at micro-scale is a challenge, and further complicated by the chaotic nature of boiling process. The object of present study is to develop experimental methods for the diagnosis of liquid film dynamics, and to obtain data for the film instability under various conditions. A dedicated test facility was designed and constructed where micro conductive probes and confocal optical sensors were used to measure the thickness and dynamic characteristics of a thin liquid film on various heater surfaces, while a high speed camera was used to get visual observation. Extensive tests were performed to calibrate and verify the two thickness measuring systems. The micro conductive measuring system was proven to have a high reliability and repeatability with maximum system error less than 5µm, while the optical measuring system is capable of recording the film dynamics with spatial resolution of less than 1 mm. The simultaneous measurement on the same liquid film shows that the two techniques are in a good agreement with respect to accuracy, but the optical sensors have a much higher acquisition rate up to 30 kHz, which are more suitable for rapid process. The confocal optical sensors were therefore employed to measure the dynamic thickness of liquid films (ethanol, hexane and water) evaporating on various horizontal heater surfaces (aluminum, copper, silicon, stainless steel and titanium) to investigate the influences of heat flux, the surface and liquid properties on the film instability and the critical thickness. The critical thickness of water film evaporating on various surfaces was measured in the range of 60-150 mm, increasing with the increased contact angle or increased heat flux (evaporating rate) and also with the decreased thermal conductivity of the heater material. The data suggest the conjugate heat transfer nature of the evaporating liquid film dynamics at higher heat fluxes of interest to boiling and burnout. In the case of hexane on the aged titanium surface with contact angle of ~3o, the liquid film is found resilient to rupture, with film oscillations at relatively large amplitude ensuing as the averaged film thickness decreases below 15 µm. To interpret our experimental findings on liquid film evolution and its critical thickness at rupture, a theoretical analysis is also performed to analyze the dynamics of liquid films evaporating on heater surfaces. While the influences of liquid properties, heat flux, and thermal conductivity of heater surface are captured by the simulation of the lubrication theory, influence of the wettability is considered via a minimum free energy criterion. The thinning processes of the liquid films are generally captured by the simulation of the lubrication theory. For the case with ideally uniform heat flux over the heater surface, the instability of the liquid film occurs at the thickness level of tens micro meters, while for the case of non-uniform heating, the critical thicknesses for the film rupture are closer to  the experimental data but still underestimated by the lubrication theory simulation. By introducing the minimum free energy criterion to considering the influence of surface wettability, the obtained critical thicknesses have a good agreement with the experimental ones for both titanium and copper surfaces, with a maximum deviation less than ±10%. The simulations also explain why the critical thickness on a copper surface is thinner than that on a titanium surface. It is because the good thermal conductivity of copper surface leads to uniform temperature distribution on the heat surface, which is responsible for the resilience of the liquid film to rupture. A silicon wafer with an artificial cavity fabricated by Micro Electronic Mechanical System (MEMS) technology was used as a heater to investigate the dynamics of a single bubble in both a thick and thin liquid layer under low heat flux (<60 kW/m2). The maximum departure diameter of an isolated bubble in a thick liquid film was measured to be 3.2 mm which is well predicted by the Fritz equation. However, in a thin liquid layer with its thickness less than the bubble departure diameter, the bubble was stuck on the heater surface with a dry spot beneath. A threshold thickness of the liquid film which enables the dry spot rewettable was obtained, and its value linearly increases with increasing heat flux. In addition, another test section was designed to achieve a constant liquid film flow on a titanium nano-heater surface which helps to successfully carry boiling in the liquid film from low heat flux until CHF. Again, the confocal optical sensor was employed to measure the dynamics of the liquid film on the heater surface under varied heat flux conditions.  A statistical analysis of the measured thickness signals that emerge in a certain period indicates three distinct liquid film thickness ranges: 0~50 µm as microlayer, 50~500 µm as macrolayer, 500~2500 µm as bulk layer. With increasing heat flux, the bulk layer disappears, and then the macrolayer gradually decreases to ~105 µm, beyond which instability of the liquid film may lose its integrity and CHF occurs. In addition, the high-speed camera was applied to directly visualize and record the bubbles dynamics and liquid film evolution. Dry spots were observed under some bubbles occasionally from 313 kW/m2 until CHF with the maximum occupation fraction within 5%.  A dry spot was rewetted either by liquid receding after the rupture of a bubble or by the liquid spreading from bubbles’ growth in the vicinity. This implies that the bubbles’ behavior (growth and rupture) and their interactions in particular are of paramount importance to the integrity of liquid film under nucleate boiling regime.   
  •  
48.
  • Gong, Shengjie, et al. (författare)
  • An experimental study on the effect of liquid film thickness on bubble dynamics
  • 2013
  • Ingår i: Applied Thermal Engineering. - : Elsevier BV. - 1359-4311 .- 1873-5606. ; 51:1-2, s. 459-467
  • Tidskriftsartikel (refereegranskat)abstract
    • Experiments were conducted to investigate the boiling phenomenon in various liquid layers on a silicon heater surface with an artificial cavity. Deionized water is employed as working liquid. The emphasis is placed on how the liquid layer thickness affects bubble behaviour and liquid layer integrity for nucleate boiling under the isolated bubble regime. The experimental results show that for boiling in a liquid layer of ∼7.5 mm, the bubble dynamics reproduce the typical pool boiling characteristics with the averaged maximum diameter of 3.2 mm for the isolated bubbles growing on the cavity. As the water layer thickness decreases to the level comparable with the bubble departure diameter, the bubble is found to remain on the heater surface for an extended period, with a dry spot forming under the bubble but rewetted after the bubble rupture occurs. Further reducing the liquid layer thickness, an irreversible dry spot appears, suggesting a minimum rewettable thickness ranging from 1.2 mm to 1.9 mm corresponding to heat flux of 26 kW/m2 to 52 kW/m2. The void measured in the cavity confirms that it is dry inside the artificial cavity at high heat flux.
  •  
49.
  • Gong, S., et al. (författare)
  • An investigation on dynamic thickness of a boiling liquid film
  • 2015
  • Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 90, s. 636-644
  • Tidskriftsartikel (refereegranskat)abstract
    • Motivated by understanding the micro-hydrodynamics of boiling heat transfer and the mechanism of critical heat flux (CHF) occurrence, the present study is to investigate the boiling phenomenon in a liquid film whose dynamic thickness is recorded by a confocal optical sensor with the measurement accuracy of micrometres, while the bubble dynamics of the boiling in the film is visualized by a high-speed photography. This paper is focused on a statistical analysis of the measured thickness signals for the boiling condition ranging from low heat flux to high heat flux (near or at CHF). The dynamic thickness of liquid film appears oscillating with peak values, resulting from the liquid film movements due to nucleation of bubble(s) and its growth and rupture. The statistical analysis in a certain period indicates there emerge three distinct liquid film thickness ranges: 0-50 μm, 50-500 μm and 500-2500 μm, seemingly corresponding to the microlayer, macrolayer and bulk layer. With increasing heat flux to a specific extent, the bulk layer disappears, and then the macrolayer gradually decreases to ∼105 μm, beyond which the liquid film may lose its integrity and CHF occurs at 1.563 MW/m2.
  •  
50.
  • Gong, Shengjie, et al. (författare)
  • Diagnostic techniques for the dynamics of a thin liquid film under forced flow and evaporating conditions
  • 2010
  • Ingår i: MICROFLUID NANOFLUID. - : Springer Science and Business Media LLC. - 1613-4982. ; 9:6, s. 1077-1089
  • Tidskriftsartikel (refereegranskat)abstract
    • Motivated by quantification of micro-hydrodynamics of a thin liquid film which is present in industrial processes, such as spray cooling, heating (e.g., boiling with the macrolayer and the microlayer), coating, cleaning, and lubrication, we use micro-conductive probes and confocal optical sensors to measure the thickness and dynamic characteristics of a liquid film on a silicon wafer surface with or without heating. The simultaneous measurement on the same liquid film shows that the two techniques are in a good agreement with respect to accuracy, but the optical sensors have a much higher acquisition rate up to 30 kHz which is more suitable for rapid process. The optical sensors are therefore used to measure the instantaneous film thickness in an isothermal flow over a silicon wafer, obtaining the film thickness profile and the interfacial wave. The dynamic thickness of an evaporating film on a horizontal silicon wafer surface is also recorded by the optical sensor for the first time. The results indicate that the critical thickness initiating film instability on the silicon wafer is around 84 mu m at heat flux of similar to 56 kW/m(2). In general, the tests performed show that the confocal optical sensor is capable of measuring liquid film dynamics at various conditions, while the micro-conductive probe can be used to calibrate the optical sensor by simultaneous measurement of a film under quasi-steady state. The micro-experimental methods provide the solid platform for further investigation of the liquid film dynamics affected by physicochemical properties of the liquid and surfaces as well as thermal-hydraulic conditions.
  •  
Skapa referenser, mejla, bekava och länka
  • Resultat 1-50 av 209
Typ av publikation
tidskriftsartikel (124)
konferensbidrag (60)
doktorsavhandling (15)
annan publikation (4)
forskningsöversikt (4)
rapport (1)
visa fler...
bokkapitel (1)
visa färre...
Typ av innehåll
refereegranskat (187)
övrigt vetenskapligt/konstnärligt (22)
Författare/redaktör
Ma, Weimin (191)
Bechta, Sevostian (34)
Li, Liangxing (23)
Dinh, Truc-Nam (19)
Guo, Qiang (18)
Gong, Shengjie (18)
visa fler...
Yuan, Yidan (18)
Xiang, Yan (17)
Karbojian, Aram (17)
Kudinov, Pavel (13)
Chen, Yangli (13)
Deng, Yucheng (13)
Yu, Peng (12)
Fang, Di (10)
Komlev, Andrei A. (9)
Zhao, Lu (9)
Villanueva, Walter (8)
Thakre, Sachin, 1984 ... (8)
Manickam, Louis (8)
Kozlowski, Tomasz (7)
Yuan, Y. (6)
Sehgal, Bal Raj (5)
Wang, Ke (5)
Zhang, Li (5)
Villanueva, Walter, ... (5)
Konovalenko, Alexand ... (5)
Buck, M. (5)
Gajev, Ivan (5)
Mei, Y. (5)
Li, W. (4)
Miassoedov, A. (4)
Sehgal, Balraj (4)
Zhang, Huimin (4)
Gong, S (4)
Buck, Michael (4)
Qi, Z. (4)
Gong, Yaopeng (4)
Wang, C. (3)
Wang, K. (3)
Zhang, Y. (3)
Li, Wei (3)
Fichot, F. (3)
Bandini, G (3)
Li, Xiangyu (3)
Bai, B (3)
Pohlner, G. (3)
Gu, H. (3)
Ma, Weimin, ASSOCIAT ... (3)
Zhang, Botao (3)
Huang, Shanfang (3)
visa färre...
Lärosäte
Kungliga Tekniska Högskolan (202)
Linköpings universitet (4)
Lunds universitet (3)
Luleå tekniska universitet (1)
Chalmers tekniska högskola (1)
Linnéuniversitetet (1)
visa fler...
Karolinska Institutet (1)
visa färre...
Språk
Engelska (209)
Forskningsämne (UKÄ/SCB)
Teknik (135)
Naturvetenskap (65)
Medicin och hälsovetenskap (1)

År

Kungliga biblioteket hanterar dina personuppgifter i enlighet med EU:s dataskyddsförordning (2018), GDPR. Läs mer om hur det funkar här.
Så här hanterar KB dina uppgifter vid användning av denna tjänst.

 
pil uppåt Stäng

Kopiera och spara länken för att återkomma till aktuell vy