| 1. |
- Dong, Shichang, et al.
(författare)
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Mechanistic critical heat flux model development for subcooled flow boiling based on superheated liquid sublayer depletion
- 2022
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Ingår i: Progress in nuclear energy (New series). - : Elsevier BV. - 0149-1970 .- 1878-4224. ; 153, s. 104445-
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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.
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| 2. |
- Dong, Shichang, et al.
(författare)
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Theoretical model for subcooled upward flow boiling heat transfer and critical heat flux for an inclined downward heated surface
- 2023
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 213
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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.
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| 3. |
- Gong, Shengjie, et al.
(författare)
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An experimental investigation on bubble dynamics and boiling crisis in liquid films
- 2014
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 79, s. 694-703
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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.
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| 4. |
- Gong, Shengjie, et al.
(författare)
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An experimental study of rupture dynamics of evaporating liquid films on different heater surfaces
- 2011
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Ingår i: International Journal of Heat and Mass Transfer. - : Elsevier BV. - 0017-9310 .- 1879-2189. ; 54:7-8, s. 1538-1547
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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.
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| 5. |
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| 6. |
- Gong, Shengjie, et al.
(författare)
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An Experimental Study on Boiling Phenomenon in a Liquid Layer
- 2010
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Ingår i: 7th International Conference on Multiphase Flow - ICMF 2010 Proceedings. - : International Conference on Multiphase Flow (ICMF).
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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.
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| 7. |
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| 8. |
- Gong, Shengjie
(författare)
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An Experimental Study on Micro-Hydrodynamics of Evaporating/Boiling Liquid Film
- 2011
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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.
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| 9. |
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| 10. |
- Gong, Shengjie, et al.
(författare)
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An experimental study on the effect of liquid film thickness on bubble dynamics
- 2013
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Ingår i: Applied Thermal Engineering. - : Elsevier BV. - 1359-4311 .- 1873-5606. ; 51:1-2, s. 459-467
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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.
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| 11. |
- Gong, Shengjie, et al.
(författare)
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Diagnostic techniques for the dynamics of a thin liquid film under forced flow and evaporating conditions
- 2010
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Ingår i: MICROFLUID NANOFLUID. - : Springer Science and Business Media LLC. - 1613-4982. ; 9:6, s. 1077-1089
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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.
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| 12. |
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| 13. |
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| 14. |
- Gong, Shengjie, et al.
(författare)
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Simulation and validation of the dynamics of liquid films evaporating on horizontal heater surfaces
- 2012
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Ingår i: Applied Thermal Engineering. - : Elsevier BV. - 1359-4311 .- 1873-5606. ; 48, s. 486-494
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Tidskriftsartikel (refereegranskat)abstract
- In this study a non-linear governing equation based on lubrication theory is employed to model the thinning process of an evaporating liquid film and ultimately predict the critical thickness of the film rupture under impacts of various forces resulting from mass loss, surface tension, gravity, vapor recoil and thermo-capillary. It is found that the thinning process in the experiment is well reproduced by the simulation. The film rupture is caught by the simulation as well, but it underestimates the measured critical thickness at the film rupture. The reason may be that the water wettability of the heater surfaces is not taken into account in the model. Thus, the minimum free energy criterion is used to obtain a correlation which combines the contact angle (reflection of wettability) with the critical thickness from the simulation. The critical thicknesses predicted by the correlation have a good agreement with the experimental data (the maximum deviation is less than 10%).
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| 15. |
- Li, Liangxing, et al.
(författare)
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Experimental Study of Two-Phase Flow Regime and Pressure Drop in a Particulate Bed Packed with Multidiameter Particles
- 2012
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Ingår i: Nuclear Technology. - : American Nuclear Society. - 0029-5450 .- 1943-7471. ; 177:1, s. 107-118
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Tidskriftsartikel (refereegranskat)abstract
- This paper documents an experimental study on two-phase flow regimes and frictional pressure drop characteristics in a particulate (porous) bed packed with multidiameter (1.5-, 3-, and 6-mm) glass spheres. The experimental results provide new data to validate/develop hydrodynamic models for coolability analysis of debris beds formed in fuel-coolant interactions during a postulated severe accident. The POMECO-FL test facility is employed to perform the experiment, with the spheres packed in a test section of 90 mm diameter and 635 mm height. The pressure drops are measured for air/water two-phase flow through the packed bed, and flow patterns are obtained by means of visual observations. Meanwhile, local void fraction in the center of the bed is measured by a microconductive probe.The experimental results show that the frictional pressure drop of single-phase flow through the bed can be predicted by the Ergun equation, if the area mean diameter of the particles is chosen in the calculation. Given the so-determined effective particle diameter, the estimation of the Reed model for two-phase flow pressure gradient in the bed has a good agreement with the experimental data. The characteristics of the local void fraction can be used to predict flow pattern and mean void fraction. It is observed that slug flow prevails when the mean void fraction is <0.5, whereas annular flow dominates after the mean void fraction is >0.7. If the effective particle diameter is further used as an influential parameter in flow pattern identification, the observed flow regimes of two-phase flow in porous media are well predicted by the existing flow pattern map.
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| 16. |
- Mei, Y., et al.
(författare)
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A study on steam-water two phase flow distribution in a rectangular channel with different channel orientations
- 2018
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Ingår i: Experimental Thermal and Fluid Science. - : Elsevier Inc.. - 0894-1777 .- 1879-2286. ; 99, s. 219-232
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Tidskriftsartikel (refereegranskat)abstract
- Experimental study on steam-water two phase vertical and inclined upward flow (15–90°) was performed in a rectangular channel with cross section of 17 mm × 10 mm under atmospheric pressure to investigate the phase distribution and the average void fraction in the cross section which were obtained from the local void fraction measurement by a conductivity probe. The inlet superficial velocities of the steam and water varied from 0.72 to 3.85 m/s and from 0.11 to 0.3 m/s respectively. A high speed camera was used to identify the flow patterns. Experimental results show that the phase distribution curves are significantly affected by channel orientation and the average void fraction first decreases and then increases with the increase of orientation. Based on the drift-flux model, two parameters, namely, the distribution parameter (C0) and the drift velocity (Ugm) have been studied in detail. Both the distribution parameter and the drift velocity are found to be functions of orientation. The distribution parameter decreases with the increase of orientation while the drift velocity first increases and then decreases with the increase of orientation., Based on the experimental data, an improved drift-flux model is proposed especially for the slug and churn flow, which predicts the void fraction in an inclined channel with good accuracy.
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| 17. |
- Wang, Ke, et al.
(författare)
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Experimental Study on the Dynamics of a Thin Liquid Film under Shearing Force
- 2017
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Ingår i: Jixie Gongcheng Xuebao/Journal of Mechanical Engineering. - : Chinese Journal of Mechanical Engineering. - 0577-6686. ; 53:24, s. 70-76
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Tidskriftsartikel (refereegranskat)abstract
- The "scales-separation" phenomenon indicates that high heat-flux boiling and boiling crisis is dominated by micro-hydrodynamics of liquid microlayer on the heater surface. The techniques for liquid film measurement such as acoustic methods, nucleonic techniques, electrical methods, and optical methods are discussed in detail. Accordingly, a confocal optical sensor system is used to detect the dynamics of liquid film sheared by the co-flowing air from above in a horizontal aluminum channel. The impact of the gas shearing on film behaviors is analyzed and the integrity of liquid film is discussed in detail. The results indicate that the liquid film thickness decreases due to the entrainment and shows a linear or nonlinear variation under different flow conditions. Additionally, for a specific surface, the critical film thickness for an integral film is found to have no relation with the gas and liquid flow rates but the fluctuation of the liquid film increases with the increasing gas velocity.
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| 18. |
- Wang, Ke, et al.
(författare)
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On the Relation between Nucleation Site Density and Critical Heat Flux of Pool Boiling
- 2018
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Ingår i: Heat Transfer Engineering. - : TAYLOR & FRANCIS INC. - 0145-7632 .- 1521-0537. ; 39:17-18, s. 1498-1506
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Tidskriftsartikel (refereegranskat)abstract
- It is traditionally accepted that the critical heat flux (CHF) decreases with increasing nucleation site density (NSD). However, such a CHF-NSD relation was no longer observed in the BETA-B experiment performed on nano-film heaters; instead the increase of NSD resulted in a gain in CHF. To address this seeming contradiction in the relation between critical heat flux and nucleation site density, the present work employed probabilistic analysis to reveal the different tendencies. A concept of effective NSD was proposed, which concerns the active nucleation sites appear within a bubble lifetime, and the resulting bubbles have the chance of direct interaction. We assumed that the boiling crisis on a heater surface is mainly induced by two mechanisms: dry spot expanding in isolated bubble regime for low-NSD surface, coalescence of dry spots under multiple bubbles in fully developed nucleate boiling regime for high-NSD surface, or a combination of the two in the transition regime for medium-NSD surface. Accordingly, we estimated the critical heat flux of each boiling regime at which the boiling crisis occurs. The result indicated that there is a threshold of nucleation site density below which the increase of NSD is contributing to CHF enhancement, while the trend is inverted beyond the threshold.
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