| 1. |
- Li, X., et al.
(författare)
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Flow Pattern Identification of Porous Media Based on Signal Feature Extraction and SVM
- 2022
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Ingår i: Kung Cheng Je Wu Li Hsueh Pao/Journal of Engineering Thermophysics. - : Science Press. - 0253-231X. ; 43:11, s. 2957-2965
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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.
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| 2. |
- Li, Xiangyu, et al.
(författare)
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Identification of two-phase flow pattern in porous media based on signal feature extraction
- 2022
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Ingår i: Flow Measurement and Instrumentation. - : Elsevier BV. - 0955-5986 .- 1873-6998. ; 83
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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.
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| 3. |
- Liu, Jiebin, et al.
(författare)
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Influence of an External Perpendicular Oscillation on Stability of a Vertical Falling Liquid Film
- 2020
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Ingår i: Microgravity, science and technology. - : Springer Science+Business Media B.V.. - 0938-0108 .- 1875-0494. ; 32:5, s. 787-805
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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.
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| 4. |
- Liu, J., et al.
(författare)
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Linear stability of a fluid mud–water interface under surface linear long travelling wave based on the Floquet theory
- 2021
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Ingår i: European journal of mechanics. B, Fluids. - : Elsevier BV. - 0997-7546 .- 1873-7390. ; 86, s. 37-48
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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.
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| 5. |
- Lu, Junjing, et al.
(författare)
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An improved sectional model to simulate multi-component aerosol dynamics in a containment of pressurized water reactor
- 2021
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Ingår i: Journal of Aerosol Science. - : Elsevier BV. - 0021-8502 .- 1879-1964. ; 157
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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.
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| 6. |
- Yu, Peng, et al.
(författare)
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A numerical study of heat transfer in bottom-heated and side/top-cooled liquid metal layers with different aspect ratios
- 2022
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Ingår i: Annals of Nuclear Energy. - : Elsevier BV. - 0306-4549 .- 1873-2100. ; 177, s. 109328-
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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.
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| 7. |
- Yu, Peng, et al.
(författare)
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An assessment of the lumped parameter model for the two-layer melt pool heat transfer
- 2023
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Ingår i: Annals of Nuclear Energy. - : Elsevier BV. - 0306-4549 .- 1873-2100. ; 180
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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.
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| 8. |
- Zhang, Zhengzheng, et al.
(författare)
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Experimental and numerical studies on the two-dimensional flow characteristics in the radially stratified porous bed
- 2022
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Ingår i: International Communications in Heat and Mass Transfer. - : Elsevier BV. - 0735-1933 .- 1879-0178. ; 133, s. 105940-
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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.
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| 9. |
- Zhang, Zhengzheng, et al.
(författare)
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Investigation on Flow Characteristics in Radial Stratified Debris Bed
- 2022
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Ingår i: Yuanzineng Kexue Jishu/Atomic Energy Science and Technology. - : Atomic Energy Press. - 1000-6931. ; 56:10, s. 2032-2040
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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.
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| 10. |
- Bandini, G., et al.
(författare)
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Assessment of systems codes and their coupling with CFD codes in thermal-hydraulic applications to innovative reactors
- 2015
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Ingår i: Nuclear Engineering and Design. - : Elsevier BV. - 0029-5493 .- 1872-759X. ; 281, s. 22-38
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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.
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