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AbstractPressurized water reactors (PWRs) are prevalent in nuclear power plants worldwide, contributing most of the electricity produced from nuclear energy. Modern PWRs allow subcooled flow boiling under their normal operating conditions to enhance the heat transfer rate. However, subcooled flow boiling might also suffer from critical heat flux (CHF) under accidental conditions, called departure from nucleate boiling, a major safety concern for the reactor’s design and operations. Thus, thermal-hydraulic analysis (THA) of subcooled flow boiling in PWRs is crucial in optimizing fuel assembly design to ensure efficient and safe operations. However, performing a THA in PWRs is a very complicated task because of the complexities involved in the operating conditions of the reactor, the physics of subcooled flow boiling, and subchannel geometry due to the presence of mixing vane grids (MVGs). In recent years, computational fluid dynamics (CFD) has emerged as a powerful tool for predicting subcooled flow boiling in various geometries, including PWR subchannels. The current study reviews CFD approaches for analyzing subcooled flow boiling in typical subchannels of a PWR under its prototypical operating condition or close to it. The impacts of various factors such as MVGs and their components (dimples, springs, and mixing vanes), cold walls, wall heat flux distributions, bowed rods, etc. on parameters such as lateral velocity, coolant pressure and temperature, void fraction, heat transfer coefficient, Nusselt number, wall heat flux partitions, CHF, etc. required to describe the behavior of coolant flow and heat transfer in subchannels, are presented. Finally, a summary of key conclusions and the scope for further research is presented. [ABSTRACT FROM AUTHOR]

This study investigated the rolling motion effect on the location of critical heat flux (CHF) for subcooled flow boiling within a mini-channel that is heated from one side. The study employed 15° and 25° as rolling amplitudes and 10 and 30 seconds as rolling periods across two different flow rate conditions. The CHF values observed under static conditions and those under rolling motion were subsequently compared. A key finding was that CHF consistently occurred near the highest positions of the rolling platform, where reduced buoyancy force, due to the absence of centrifugal acceleration, combined with only the radial buoyancy component acting in the flow direction, resulted in a slower bubble rise, facilitating easier coalescence of bubbles into a vapor film, leading to CHF. [ABSTRACT FROM AUTHOR]

Hypervapotron is a water-cooled device which relies on internal fins and boiling heat transfer to sustain ultra-high heat flux in the range of 20–30 MW/m2 from thermonuclear fusion reactor. This study investigated the design optimization of hypervapotron cooling channels using multiphase computational fluid dynamic simulation. With focus on geometric variations in fin heights and side slots installed to the cooling channel, their effects on the cooling performance and internal flow pattern of these channels were evaluated. The obtained results revealed that the incorporation of side slots significantly enhanced cooling performance by promoting bidirectional fluid inflow and optimizing heat dissipation. It was also found that adjusting fin heights significantly contributed to improved thermal management by ensuring smoother liquid ingress and effective heat exchange. This work established a link between design variations and cooling performance, providing insights and guidelines for advanced engineering of hypervapotron systems. [ABSTRACT FROM AUTHOR]

The precise design of heat exchangers in automobile air conditioning systems for more sustainable electric vehicles requires an enhanced assessment of CFD mechanistic models for the subcooled boiling flow of pure eco-friendly refrigerant. Computational Multiphase Flow Dynamics (CMFDs) relies on two-phase closure models to accurately depict the complex physical phenomena involved in flow boiling. This paper thoroughly examines two-phase CMFD flow boiling, incorporating sensitivity analyses of critical parameters such as boiling closures, momentum closures, and population balance models. Three datasets from the DEBORA experiment, involving vertical pipes with subcooled boiling flow of refrigerant at three different pressures and varying levels of inlet liquid subcooling, are used for comparison with CFD simulations. This study integrates nucleate site density and bubble departure diameter models to enhance wall boiling model accuracy. It aims to investigate various interfacial forces and examines the S-Gamma and Adaptive Multiple Size-Group (A-MuSiG) size distribution methods for their roles in bubble break up and coalescence. These proposed approaches demonstrate their efficacy, contributing to a deeper understanding of flow boiling phenomena and the development of more accurate models. This investigation offers valuable insights into selecting the most appropriate sub-closure models for both boiling closure and momentum closure in simulating boiling flows. [ABSTRACT FROM AUTHOR]

• Effect of vortex generators on flow boiling oscillation are verified. • Vortex generators suppress the large bubble's formation and reverse flow. • The max increment of limiting oscillation heat flux by vortex generators is 21.1%. • A new correlation for vortex-enhanced limiting oscillation heat flux is developed. Subcooled flow boiling in micro-channels is an effective solution for high-heat-flux electronics thermal management, and the heat flux has a limitation value (i.e., limiting oscillation heat flux) due to flow boiling oscillation. The purpose of this study is to propose a new method to suppress the flow boiling oscillation (i.e., forcing bubbles to leave the channel by installing a vortex generator), experimentally verify the effect of vortex generators on suppression of flow boiling oscillation and increasing limiting oscillation heat flux, and develop a predicting correlation for this heat flux. In the experiments, the vortex generator is a helix wire with a pitch of 1.8 mm, a length of 22.5 mm, a wire diameter of 0.2 mm and a diameter of 1.8 mm; the inlet subcooling degree and mass flux range from 10 – 50 K and 750–1750 kg m−2 s−1, respectively. The results show that, the flow boiling oscillation occurs in the flow channel without vortex generators while it doesn't occur in the flow channel containing vortex generators at the same working condition; the vortex generators can raise the limiting oscillation heat flux with a maximum increment of 21.1%, which indicates that vortex generators can suppress the flow boiling oscillation. A new correlation predicting limiting oscillation heat flux in channels with vortex generators was developed, and its predictions match 95% of the experimental data within ± 20% while the average deviation is 7.6%. [ABSTRACT FROM AUTHOR]

In an effort to reduce the charging time of electric vehicles (EVs), the thermal management of the charging system has emerged as one of the most urgent issues. The charging rate has been restricted to avoid thermal failure especially in charging cable. Recently, a direct contact cooling technique that utilizes subcooled flow boiling for charging cables has been proposed and has shown promising thermal management performance. However, there has been a lack of clear explanation regarding its thermal characteristics due to its intrinsically complicated flow features. This study investigates the thermal characteristics of subcooled flow boiling in a horizontally aligned concentric annular tube intended to simulate the direct contact cooling technique employed for charging cables. For the experimental investigation, the wall temperature measurements and high-speed flow visualization images acquired from the transparent test module annulus, with inner and outer diameters of 6.35 mm and 22.0 mm, were utilized. Three key axial thermal characteristics of subcooled flow boiling in annulus have been analyzed, and they are flow regime transition, variation of heat transfer mechanisms, and occurrence of critical heat flux (CHF). The transition of flow regimes and the variation of dominant heat transfer mechanisms in the axial direction are mutually influencing due to the coupled effects between the hydrodynamic and thermal characteristics of simultaneously developing flows. The flow regime changes from partially developed boiling (PDB) to fully developed boiling (FDB) as the intensities of nucleate boiling and bubble recondensation vary in the axial direction, while the dominant heat transfer mechanism shifts from single-phase convection to nucleate boiling, corresponding to the flow regime transition from PDB to FDB. The departure from nucleate boiling (DNB) type CHF is observed, manifested by the wavy interface propagating from the far downstream to the upstream region, and the intensities of nucleate boiling and bubble recondensation are also identified as the dominant parameters in determining the occurrence of CHF. Charging time estimation reveals that the proposed method can achieve an 80 % charge for 100-kWh EV batteries in 98 s, while maintaining the cable wire temperature below the safety limit of 80 °C. This strongly supports its potential as a promising thermal management technique for future ultra-fast charging systems.

Subcooled flow boiling contains complex bubble behaviors and interactions. Accurate prediction and analysis of the subcooled flow boiling are of great significance for the safe operation and economy of the nuclear reactors. The bubble-layer-based model is a new theoretical model in predicting the flow boiling, which divides the flow filed into two regions in the radial direction, which are the bubble layer region and the core region. Bubble behaviors in each region, mass and energy exchanges between different regions, and variations of parameters in the axial direction are analyzed and reflected through a set of two-dimensional steady-state conservation equations of mass, momentum and energy. Therefore, this model can provide more detailed information of the flow field than current one-dimensional models. However, this bubble-layer-based model for the subcooled flow boiling has only been verified in conventional tubes. When the channel type and sizes change, the accuracy of the model is still uncertain. Considering that rectangular channels are widely encountered in multiphase flow equipment, this paper aims to further extend this model to the prediction of the subcooled flow boiling in rectangular channels. Firstly, several flow boiling experiments in both conventional rectangular channels and narrow rectangular channels were selected. The working conditions are 0.1-8 MPa for pressure, 211-1 700 kW/m2 for heat flux, 240-2 200 kg/(m2·s) for mass flux, 4.42×103-3.72×105 for liquid Reynold number and 0.83-1.76 for Prandtl number. Then, variations of void fraction obtained by this model were compared with these experimental data. The results show that this bubble-layer-based model shows high accuracy when it is applied to both conventional rectangular channels and narrow rectangular channels, which reflects that the channel size and shapes have limited impact on the accuracy of this model. In addition, to further verify the accuracy of this model, one of the existing models (Cai model) in predicting the void fraction in subcooled flow boiling was also selected for comparison. The results show that the bubble-layer-based model shows higher accuracy than Cai model, with the relative error of 32.7% and 52.6% respectively. Finally, this model was applied to the analysis of the flow boiling in one narrow rectangular channel. It describes the variations of several two-phase flow parameters, such as the void fraction, pressure, velocity and temperature. These results not only provide a better understanding of the mechanism of subcooled flow boiling, but also provide basic data for the thermal physical coupling analysis of the rectangular fuel elements in nuclear reactors.

Copyright of Atomic Energy Science & Technology is the property of Editorial Board of Atomic Energy Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The current research focuses on the development of a numerical approach to forecast strongly subcooled flow boiling of FC-72 as the refrigerant in various vertical minichannel shapes for high-heat-flux cooling applications. The simulations are carried out using the Volume of Fluid method with the Lee phase change model, which revealed some inherent flaws in multiphase flows that are primarily due to an insufficient interpretation of shear-lift force on bubbles and conjugate heat transfer against the walls. A user-defined function (UDF) is used to provide specific information about this noticeable effect. The influence of shape and the inlet mass fluxes on the flow patterns, heat transfer, and pressure drop characteristics are discussed. The computational results are validated with experimental measurements, where excellent agreements are found that prove the efficiency of the present numerical model. The findings demonstrate that the heat transfer coefficient decreases as the mass flux increases and that the constriction design improves the thermal performance by 24.68% and 10.45% compared to the straight and expansion shapes, respectively. The periodic constriction sections ensure good mixing between the core and near-wall layers. In addition, a slight pressure drop penalty versus the thermal transfer benefits for the two configurations proposed is reported.

The pressure drop characteristics of subcooled water were experimentally investigated in a circular cooling channel with and without a twisted tape (TT) under high heat fluxes, which was designed for the water-cooling structure of the divertor target in a tokamak device. The working medium was deionized water, and the main parameters were mass flux G = 3000–8000 kg·m−2·s−1, inlet pressure of the test section p = 3, 4.2, 5 MPa, equivalent one-side heating flux qe = 5~10 MW·m−2. The off-center circular channel is electrically heated to simulate the unilateral radiation heating on the divertor target by high-temperature plasma. The pressure drop experiment of vertical upward circular cooling channels under high and unilateral heat flux is carried out. The influences of the TT and system parameters such as qe, G, and p on the pressure drop of the test section (Δp) were discussed in detail. In the single-phase (SP) flow region, Δp is mainly affected by the TT, G, and qe. The pressure drop with a TT is significantly higher than that without a TT, a higher G and a lower qe lead to a greater Δp. In the subcooled boiling (SB) flow region, Δp is correlated with the TT, qe, G, and p: the influence of the TT and G decreases, while the influence of p increases. The higher the qe, the higher the G, and the lower the p, the larger the Δp. The results show that almost all of the SP pressure drop correlations for heated circular channels overestimate the experimental pressure drop coefficient ratio for a given viscosity ratio. According to the test results, a new correlation of SP pressure drop under high and unilateral heat fluxes has been proposed, the average error (AE) and root mean square error (RMSE) of which are 0.26% and 3.17%, respectively. [ABSTRACT FROM AUTHOR]

It is very important to accurately analyze the heat transfer characteristics of a working fluid in the cooling system based on subcooled flow boiling. In contrast to previous computational fluid dynamics (CFD) studies for the subcooled flow boiling of water or synthetic refrigerants in a vertical tube, we study the subcooled flow boiling characteristics of CO2 in a horizontal tube at -30 ℃. Our model is built based on Eulerian multi-phase model combined with the interfacial area concentration and wall heat flux partitioning models, and it is solved by the CFD solver, ANSYS FLUENT 19.3. This paper determines a reasonable bubble waiting time coefficient (CWT), critical Weber number (Wecri), and turbulence model (TM) for modelling of flow boiling heat transfer of CO2 by combining with uniform design (UD), Taguchi method, multiple quadratic regression (MQR) model and grid search method. As the result, the reasonable values of CWT and Wecri, and reasonable TM were determined as CWT = 0.8, Wecri = 0.7, and RNG k–ε model. This work has a significant contribution to study the subcooled flow boiling heat transfer characteristics of CO2 at low temperature. The proposed approach could be actively applied to improve the solution accuracy of the heat transfer of a working fluid in various heat transfer system. [ABSTRACT FROM AUTHOR]

To study the pressure drop characteristics of hypervapotron, which was designed as a water-cooling structure in the divertor dome of the fusion reactor, the pressure drop tests of subcooled water were carried out in a vertically upward hypervapotron. To simulate the one-side radiant heating condition in the engineering application, the non-uniform heat fluxes were obtained by using the off-center electrically heating method. The system parameters were as follows: mass flux G = 2000–5000 kg·m−2·s−1, inlet pressure p = 2–4 MPa, and equivalent one-side radiating heat flux qe = 0–5 MW·m−2. The effects of the parameters on the pressure drop were discussed in detail. It was observed that in the single-phase (SP) region, the pressure drop was little influenced by the inlet fluid temperature (Tb,in). However, in the subcooled boiling region, the pressure drop increased rapidly with the increasing Tb,in. A higher G leads to a high pressure drop. In the SP region, the influence of p on the pressure drop is not obvious, and the pressure drop decreased with the increasing qe. The test data are used to evaluate the typical pressure drop correlation, and the results show that none of these correlations can predict the pressure drop well under the test conditions. Therefore, a new pressure drop correlation is proposed for subcooled water in a hypervapotron under high and non-uniform heat fluxes. The new correlation has a high prediction accuracy for the test data, and the mean relative error (MRE) and root mean square error (RMSE) are 0.72% and 4.33%, respectively. The test results have a reference value for the design of the water-cooling structure of the diverter. [ABSTRACT FROM AUTHOR]

In this paper, subcooled flow boiling heat transfer was experimentally and numerically investigated to determine the temperature distribution, outlet temperature, and heat transfer coefficient in a vertical annulus with laminar upward flow of water as a working fluid. Experiments were developed in a way that the effect of different parameters such as fluid mass flux (26, 35 and 44 kg m−2 s−1), fluid inlet temperature (333 and 343 K), and heat flux (40–90 kW m−2) to be clarified. A mathematical model was then developed to determine the temperature distribution (in r and z directions), and the heat transfer coefficient along the heat transfer surface with the help of the asymptotic model, predicting more accurate results in comparison with Chen's model. The advantage of this model over the empirical correlations is that it can predict the surface and bulk temperatures according to the operational conditions of the problem and use the obtained temperatures to calculate the thermal parameters of the system along the heat transfer surface, so, there is no need to obtain time-consuming and expensive experimental data. The maximum values of the contribution of nucleate boiling heat transfer mechanism to the total heat transfer were determined about 0.5 to 0.7 with the concept of nucleate boiling fraction. Moreover, the thermal boundary layer thickness was theoretically calculated over the heat transfer surface. The results showed that the maximum boundary layer thickness developed from 0.003 to 0.01 m when the mass flux decreases from 44 to 2.6 kg m−2 s−1. Effect of increasing mass flux and decreasing heat flux on the location of onset of nucleate boiling (ONB) demonstrated that ONB location shifted to the right from 0.017 to 0.043 m in z direction which implied the subcooled flow boiling incipience at the distance lower than 0.05 m from the beginning of the heat transfer surface. [ABSTRACT FROM AUTHOR]

The precise design of heat exchangers in automobile air conditioning systems for more sustainable electric vehicles requires an enhanced assessment of CFD mechanistic models for the subcooled boiling flow of pure eco-friendly refrigerant. Computational Multiphase Flow Dynamics (CMFDs) relies on two-phase closure models to accurately depict the complex physical phenomena involved in flow boiling. This paper thoroughly examines two-phase CMFD flow boiling, incorporating sensitivity analyses of critical parameters such as boiling closures, momentum closures, and population balance models. Three datasets from the DEBORA experiment, involving vertical pipes with subcooled boiling flow of refrigerant at three different pressures and varying levels of inlet liquid subcooling, are used for comparison with CFD simulations. This study integrates nucleate site density and bubble departure diameter models to enhance wall boiling model accuracy. It aims to investigate various interfacial forces and examines the S-Gamma and Adaptive Multiple Size-Group (A-MuSiG) size distribution methods for their roles in bubble break up and coalescence. These proposed approaches demonstrate their efficacy, contributing to a deeper understanding of flow boiling phenomena and the development of more accurate models. This investigation offers valuable insights into selecting the most appropriate sub-closure models for both boiling closure and momentum closure in simulating boiling flows.

In the current research project, two-dimensional numerical simulations are conducted to analyze the effects of geometrical configuration on flow structures and the thermal performances of subcooled flow boiling. The CFD simulations are carried out in two different configurations (straight and periodic constriction expansion) in a minichannel mounted vertically at four mass fluxes (500 kg/m2s; 836.64 kg/m2s; 1170 kg/m2s; and 2535 kg/m2s). The present predicted results exhibit excellent accordance with the previous experiments, with mean errors of 6.39% and 9.78%, demonstrating the efficiency of the present numerical study. The simulation results show that the periodic constriction expansion design provides good mixing between the layers, leading to a 43.11% mean enhancement of the thermal transfer, which is more important than the slight pressure drop penalty of 4.32 for a mass flux of 500 kg/m2s due to the combined pressure drop along the minichannel that resulted from the periodic constriction and expansion regions. Furthermore, the visualization of flow patterns shows that the bubbly flow is the dominant flow regime in the periodic constriction-expansion configuration.

In the current research project, two-dimensional numerical simulations are conducted to analyze the effects of geometrical configuration on flow structures and the thermal performances of subcooled flow boiling. The CFD simulations are carried out in two different configurations (straight and periodic constriction expansion) in a minichannel mounted vertically at four mass fluxes (500 kg/m²s; 836.64 kg/m²s; 1170 kg/m²s; and 2535 kg/m²s). The present predicted results exhibit excellent accordance with the previous experiments, with mean errors of 6.39% and 9.78%, demonstrating the efficiency of the present numerical study. The simulation results show that the periodic constriction expansion design provides good mixing between the layers, leading to a 43.11% mean enhancement of the thermal transfer, which is more important than the slight pressure drop penalty of 4.32 for a mass flux of 500 kg/m2s due to the combined pressure drop along the minichannel that resulted from the periodic constriction and expansion regions. Furthermore, the visualization of flow patterns shows that the bubbly flow is the dominant flow regime in the periodic constrictionexpansion configuration. [ABSTRACT FROM AUTHOR]

The progress reached in the high heat flux systems has required the development of appropriate thermal management approaches for dissipating the high heat fluxes, especially for small-scale devices. One of the most advantageous thermal management techniques is the utilization of subcooled flow boiling. In this work, the subcooled flow boiling of FC-72 is numerically simulated in a minichannel using ANSYS Fluent to investigate the effects of system pressure and gravitational orientation on the subcooled flow boiling thermal transfer performances. Two different orientations (vertical downflow and vertical upflow) were examined in the same conditions of heat flux (q = 191553 W/m2), mass flux (G = 836.64 kg/(m2s)) and inlet temperature (Tin = 304.54 K), and under three different system pressures (102000, 120000, and 209900 Pa). The present computational study has been validated and a good agreement with the experimental data was demonstrated. The predicted results demonstrate that the increase in system pressure improves the thermal performance of subcooled flow boiling by an average enhancement of 15.94%. In addition, the vertical upflow orientation is more advantageous than the downflow orientation due to the buoyancy force that moves the bubbles towards the flow direction and leads to less chaotic liquid-vapor interactions. An average enhancement of 1.65% in the heat transfer coefficient is reached in the upflow orientation compared to the downflow orientation for the higher system pressure of 209900 Pa.

This paper presents the effects of minichannel geometrical configuration on flow structures, hydrodynamics, and heat transfer behavior of the subcooled flow boiling process in a cooling system with high heat flux. The two-dimensional numerical simulations are quantitatively conducted for dielectric fluid FC-72 as a working fluid in two different configurations (straight and periodic constriction-expansion) of a minichannel mounted vertically at three inlet temperatures (285.35, 304.54, and 325.35 K) at constant mass flux (836.64 kg/m2s) and input heat flux (191,553 W/m2). The predicted results are in excellent agreement with the published experimental data, which proves the efficiency of the present numerical model. In both configurations, the heat transfer coefficient increases with an increase in the inlet temperature. The results show that the periodic constriction-expansion design significantly improves the heat transfer performances of subcooled flow boiling by 21.77–36.9% for Tin = 285.35–325.35 K because the periodic constriction-expansion configuration offers good mixing between layers.

The divertor is the key in-vessel component to remove the high heat flux and impurities in magnetic confinement fusion reactor. Hypervapotron (HV) has been developed as one of the candidates for cooling structure of divertor, owing to its excellent heat transfer performance in subcooled flow boiling. To address the design and development requirements of divertor, a suitable model for single-phase and two-phase flow of HV are developed. Four combination models of boiling properties expressed by different empirical correlations are evaluated. The results indicate that the LC-UN (Lemmert-Chawla, Unal) model shows a good agreement with the experimental results of Lim et al. (2022), with average deviation of 8.15%. Moreover, for the first time, the model successfully predicts the onset of fully developed nucleate boiling (OFDB) point of HV. Based on the LC-UN model, the detailed heat transfer and flow characteristics of HV are revealed. It shows that the boiling process is independent between each slot due to the fin barrier, and the bubbles are condensed by the subcooled main stream after leaving the wall. The “double vortex” in HV intensifies the interaction between vapor and liquid phase, preventing the aggregation of bubbles, allowing it has excellent heat transfer performances.

Three-dimensional subcooled flow boiling of R134a in a threaded tube was numerically simulated at the conditions of 200~400 kW/m2 heat flux, 3~20 K inlet subcooling, and 0.2~0.6 m/s inlet velocity. The bubble behavior in the horizontal threaded tube with 0.581 mm thread tooth height was observed. The effect of heat flux, inlet subcooling, and inlet velocity on bubble departure diameter and heat transfer coefficient were explored. The results presented the whole growth process of five kinds of bubbles. It was found that the bubbles either collapsed in cold liquid after leaving the heating wall or grew along the axial direction and contacted the heating wall. And there was no bubble sliding during the growth. In addition, the most important and special characteristic of bubble behavior in threaded tubes was the phenomenon of the bubble passing through the cavity. The coalescence and breakup behavior occurred after the bubble passed through the cavity. According to the discussions of the departure diameter and heat transfer coefficient, it was inferred that the bubble departure diameter increased with the increase of heat flux from 200~400 kW/m2 and subcooling from 3~20 K while decreasing with the increase of inlet velocity from 0.2~0.6 m/s. And due to the influence of the threaded tube structure, there are special points in the change of bubble departure diameter. The heat transfer coefficient of the bubbles in the threaded tube was higher than the smooth tube, which was increased by 1.5~12.5%. The heat transfer coefficient increased with the increase of heat flux and subcooling and is closely related to the bubble departure diameter. [ABSTRACT FROM AUTHOR]

Amidst the background of "double carbon", energy saving and emission reduction is a popular direction in the current refrigeration industry. Therefore, the research on the boiling heat transfer of gas–liquid two-phase flow is helpful to strengthen the heat transfer and design a more efficient heat exchanger. In this paper, a research method combining numerical simulation and experimental verification is adopted. Firstly, an experimental platform used for the subcooled flow boiling of refrigerant in casing tubes is introduced and experiments are carried out to obtain experimental data, which provides a theoretical basis for the development of numerical simulation and verifies the feasibility of numerical simulation. A numerical model of subcooled flow boiling in R22 was established and the grid independence test was carried out. Based on the simulation results, three factors affecting the boiling heat transfer of R22 are analyzed: First, the boiling heat transfer coefficient of R22 increases with the increase of the mass flow rate of R22, but the increase decreases when the mass flow rate increases from 0.018 kg/s to 0.020 kg/s. Second, the boiling heat transfer coefficient of R22 increases significantly with the increase of hot water flow rate. Third, the influence of R22 subcooling on boiling heat transfer is more complex. When the subcooling is 5 °C and 1 °C, heat transfer can be enhanced; high subcooling at 5 °C can enhance convective heat transfer and low subcooling at 1 °C can accelerate the arrival of saturated boiling. In this paper, three kinds of bubble behaviors affecting heat transfer in supercooled flow boiling, including sliding, polymerization, and bounce are also studied, which provides a basis for further research on heat transfer mechanism of supercooled flow boiling. [ABSTRACT FROM AUTHOR]

In this study, forced convection and subcooled flow boiling heat transfer coefficients of crude oil from southern oil fields of Iran were experimentally measured under different operating conditions. Despite the little data available on the heat transfer coefficient of crude oil at different operating conditions, there is an urgent need for precise and sufficient information about the status and mechanism of crude oil heat transfer for design and economic optimization of equipment such as crude oil preheaters in production units and oil refineries. The experiments were carried out in a constant heat flux heat exchanger heated by an electric heater equipped with adjustable heat flux in the range of 40–90 kW m - 2 . Firstly, in order to ensure the quality of obtained data, the heat transfer coefficients of distilled water were measured and compared with the available literature data. The experimental results indicated that the heat transfer coefficient of crude oil is relatively 31% less than that of distilled water; so, the ability of crude oil in heat transfer is less than distilled water, and the temperature of the heated surface in the crude oil experiment is higher than that of distilled water. Furthermore, enhancement of fluid flow rate and heat flux increases the heat transfer coefficient. It is very difficult to distinguish the transition from forced convective heat transfer to boiling of crude oil by enhancement of heat flux. For this purpose, (Tw–Tb) was plotted against heat flux, and the obtained results indicated that the subcooled flow boiling of crude oil starts in the range of 41.9–51.3 kW m - 2 . Finally, the convective heat transfer coefficients were correlated in dimensionless Nusselt form as a function of dimensionless numbers Re and Pr. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Harbin Institute of Technology. Social Sciences Edition / Haerbin Gongye Daxue Xuebao. Shehui Kexue Ban is the property of Harbin Institute of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Water–copper is one of the most common combinations of working fluid and heating surface in high-performance cooling systems. Copper is usually selected for its high thermal conductivity and water for its high heat transfer coefficient, especially in the two-phase regime. However, copper tends to suffer oxidation in the presence of water and thus the heat flux performance is affected. In this research, an experimental investigation was conducted using a cooper bare surface as a heating surface under a constant mass flux of 600 kg·m−2·s−1 of deionized water at a subcooled inlet temperature ΔTsub of 70 K under atmospheric pressure conditions on a closed-loop. To confirm the heat transfer deterioration, the experiment was repeated thirteen times. On the flow boiling region after thirteen experiments, the results show an increase in the wall superheat ΔTsat of approximately 26% and a reduction in the heat flux of approximately 200 kW·m−2. On the other hand, the effect of oxidation on the single phase is almost marginal.

The plate-type fuel assembly with the advantages of high specific power of active zone and compact structure has been widely adopted in many research reactors and material irradiation test reactors. The narrow rectangular channel in platetype fuel assembly has the geometric characteristics of large aspect ratio, resulting in large gradient and steep distribution of velocity in the height direction. When the subcooled boiling occurs, the bubble behavior near heated wall will be affected, and the bubble sliding phenomenon has a great influence on the boiling heat transfer. In this paper, a wall heat flux distribution model considering the sliding heat flux was built and quenching heat flux was redefined as the sensible heat released by the solid wall under the dry spot. The bubble stress analysis model and sliding model based on theoretical and semi-empirical expressions were established to calculate essential auxiliary parameters including bubble departure diameter, lift-off diameter and sliding distance. A wall boiling model suitable for upward flow boiling in the narrow rectangular channel was developed. After that, the numerical simulations under different operating conditions were performed against the Nuthel flow boiling experiments in narrow rectangular channels for model verification. The results show that the model developed in this paper can better predict the wall superheat degree of upward flow boiling in the narrow rectangular channel under medium and low pressure conditions in the range of 1-4 MPa. Compared with the RPI model, the wall temperature calculated by the developed model in this paper is much higher, and the maximum error of wall superheat in the boiling region decreases from 80% to 17% in the simulated conditions, indicating that the wall boiling model considering the sliding heat flux can describe the flow and heat transfer characteristics of upward flow boiling in the narrow rectangular channel more accurately. As for the heat flux distribution, the sliding heat flux and single-phase convective heat flux calculated by developed model take up significant fraction, and evaporation heat flux is less significant. The quenching heat flux, defined by the reconstructed expression, is almost negligible, which demonstrates that the sensible heat released by the solid wall under the dry spot has little influence on the overall heat transfer process. In RPI model, however, evaporation heat flux accounts for the majority of the total heat flux in the boiling region. Lower fraction of heat flux allocated to evaporation heat flux leads to less evaporation rate and predicted void fraction.

The subcooled flow boiling (SFB) of a water-ethanol mixture are relevant in operating heat-dissipating devices, such as smaller catalytic reactors, electronic apparatus, and hybrid electric vehicle battery components. The operative temperature should always be at a sustainable value to evade the failure or breakdown of these heat-dissipating devices. To cool these devices, a water-ethanol mixture is used as a coolant. The forced convective as well as SFB heat transfer coefficients (HTCs) for the water-ethanol mixture are estimated numerically using the volume of fluid method in a rectangular channel with dimensions of 15 mm×15 mm×150 mm. During SFB, the liquid-vapor interaction is examined by solving the bubble void fraction (BVF). For the discretization process, the Crank-Nicholson implicit method (scheme) is used, and the convective equation for the BVF is converted to an algebraic equation. The corrector predictor equation procedure is used for solving the BVF. The thermodynamic and thermophysical parameters related to subcooled boiling are estimated upon the incorporation of the bubble void fraction (α) using the mixture rule. These parameters are then incorporated into the x-momentum equation as well as into the energy equation for finding the fluid temperature, velocity, and pressure drop values. From the estimated values of temperature, subcooled flow boiling HTC is obtained. The estimated values of HTC can predict well compared with that of empirical equations. Moreover, mass flux plays a vital role in the forced convective region, while heat flux has a crucial role in the SFB region for the improvement of HTC. [ABSTRACT FROM AUTHOR]

The progress reached in the high heat flux systems has required the development of appropriate thermal management approaches for dissipating the high heat fluxes, especially for small-scale devices. One of the most advantageous thermal management techniques is the utilization of subcooled flow boiling. In this work, the subcooled flow boiling of FC-72 is numerically simulated in a minichannel using ANSYS Fluent to investigate the effects of system pressure and gravitational orientation on the subcooled flow boiling thermal transfer performances. Two different orientations (vertical downflow and vertical upflow) were examined in the same conditions of heat flux (q = 191553 W/m2), mass flux (G = 836.64 kg/(m2s)) and inlet temperature (Tin = 304.54 K), and under three different system pressures (102000, 120000, and 209900 Pa). The present computational study has been validated and a good agreement with the experimental data was demonstrated. The predicted results demonstrate that the increase in system pressure improves the thermal performance of subcooled flow boiling by an average enhancement of 15.94%. In addition, the vertical upflow orientation is more advantageous than the downflow orientation due to the buoyancy force that moves the bubbles towards the flow direction and leads to less chaotic liquid-vapor interactions. An average enhancement of 1.65% in the heat transfer coefficient is reached in the upflow orientation compared to the downflow orientation for the higher system pressure of 209900 Pa. [ABSTRACT FROM AUTHOR]

Water–copper is one of the most common combinations of working fluid and heating surface in high-performance cooling systems. Copper is usually selected for its high thermal conductivity and water for its high heat transfer coefficient, especially in the two-phase regime. However, copper tends to suffer oxidation in the presence of water and thus the heat flux performance is affected. In this research, an experimental investigation was conducted using a cooper bare surface as a heating surface under a constant mass flux of 600 kg·m − 2 ·s − 1 of deionized water at a subcooled inlet temperature Δ T s u b of 70 K under atmospheric pressure conditions on a closed-loop. To confirm the heat transfer deterioration, the experiment was repeated thirteen times. On the flow boiling region after thirteen experiments, the results show an increase in the wall superheat Δ T s a t of approximately 26% and a reduction in the heat flux of approximately 200 kW·m − 2 . On the other hand, the effect of oxidation on the single phase is almost marginal. [ABSTRACT FROM AUTHOR]

The objective of this experiment is to investigate the unsteady heat transfer of R-407C subcooled flow boiling (SCFB) due to the oscillating refrigerant flow rate within a narrow tube oriented horizontally for application in energy efficiency air conditioning system. The refrigerant flow rate is input in the cycle of triangular oscillations. Influences of oscillation amplitude (ΔG/G‾) and oscillation period (tp) of the periodic flow rate on SCFB heat transfer are studied. The present results disclose that either ΔG/G‾ or tp has minor impact on time-average SCFB heat transfer performance. However, obvious changes in the temporal variations of the wall temperature on the pipe (Tw) and convective heat transfer coefficient (hr) are found, leading to three heat transfer mechanisms of single phase convection, intermittent nucleate boiling, and persistent subcooled boiling for various input heat fluxes. The Tw of persistent subcooled boiling presents the reversed tendency compared to that of single phase convection due to the variation of the triangular R-407C oscillation. In the case of intermediate heat flux, the unusual boiling phenomenon, called intermittent nucleate boiling, can be obtained depending on the R-407C mass flux. The visualized bubble images are also provided to further describe the intermittent nucleate boiling.

To study the pressure drop characteristics of hypervapotron, which was designed as a water-cooling structure in the divertor dome of the fusion reactor, the pressure drop tests of subcooled water were carried out in a vertically upward hypervapotron. To simulate the one-side radiant heating condition in the engineering application, the non-uniform heat fluxes were obtained by using the off-center electrically heating method. The system parameters were as follows: mass flux G = 2000–5000 kg·m−2·s−1, inlet pressure p = 2–4 MPa, and equivalent one-side radiating heat flux qe = 0–5 MW·m−2. The effects of the parameters on the pressure drop were discussed in detail. It was observed that in the single-phase (SP) region, the pressure drop was little influenced by the inlet fluid temperature (Tb,in). However, in the subcooled boiling region, the pressure drop increased rapidly with the increasing Tb,in. A higher G leads to a high pressure drop. In the SP region, the influence of p on the pressure drop is not obvious, and the pressure drop decreased with the increasing qe. The test data are used to evaluate the typical pressure drop correlation, and the results show that none of these correlations can predict the pressure drop well under the test conditions. Therefore, a new pressure drop correlation is proposed for subcooled water in a hypervapotron under high and non-uniform heat fluxes. The new correlation has a high prediction accuracy for the test data, and the mean relative error (MRE) and root mean square error (RMSE) are 0.72% and 4.33%, respectively. The test results have a reference value for the design of the water-cooling structure of the diverter.

The pressure drop characteristics of subcooled water were experimentally investigated in a circular cooling channel with and without a twisted tape (TT) under high heat fluxes, which was designed for the water-cooling structure of the divertor target in a tokamak device. The working medium was deionized water, and the main parameters were mass flux G = 3000–8000 kg·m−2·s−1, inlet pressure of the test section p = 3, 4.2, 5 MPa, equivalent one-side heating flux qe = 5~10 MW·m−2. The off-center circular channel is electrically heated to simulate the unilateral radiation heating on the divertor target by high-temperature plasma. The pressure drop experiment of vertical upward circular cooling channels under high and unilateral heat flux is carried out. The influences of the TT and system parameters such as qe, G, and p on the pressure drop of the test section (Δp) were discussed in detail. In the single-phase (SP) flow region, Δp is mainly affected by the TT, G, and qe. The pressure drop with a TT is significantly higher than that without a TT, a higher G and a lower qe lead to a greater Δp. In the subcooled boiling (SB) flow region, Δp is correlated with the TT, qe, G, and p: the influence of the TT and G decreases, while the influence of p increases. The higher the qe, the higher the G, and the lower the p, the larger the Δp. The results show that almost all of the SP pressure drop correlations for heated circular channels overestimate the experimental pressure drop coefficient ratio for a given viscosity ratio. According to the test results, a new correlation of SP pressure drop under high and unilateral heat fluxes has been proposed, the average error (AE) and root mean square error (RMSE) of which are 0.26% and 3.17%, respectively.

The onset of flow instability (OFI) is a major threat to the operation of a fusion power plant. In particular, because the divertor is loaded with a high heat flux of up to 20 MW/m2, it may be vulnerable to OFI. Therefore, in this study, the OFI of a one-side heated flat heat sink was experimentally analyzed. When the effect of the system parameters on the OFI was analyzed, the faster the vapor inside the channel could be removed, the higher was the OFI that was experimentally recorded. As the flow rate and degree of subcooling increase, the OFI increases because it induces an enhancement of the forced convective heat transfer performance and a rapid coding rate, respectively. However, when the pressure is increased, the latent heat and liquid surface tension are reduced; thus, boiling occurs at a lower heat rate, which leads to a decrease in OFI. When the prediction performance of the existing OFI correlations developed under the subcooled flow boiling condition was evaluated, it showed a tendency to overpredict this experimental value significantly. This is because the evaluated correlations were developed based on relatively low heat flux conditions and narrow rectangular channels. Therefore, in this study, we developed a new OFI correlation optimized for experimental values using an artificial intelligence (AI) regression method. Because this correlation can be utilized under one-side high heat load conditions, it is expected that it will be especially useful when establishing the operation strategy of the cooling system of a fusion power plant.

The single bubble dynamics and local thermal effects in a rectangular channel are investigated in this paper under subcooled nucleate flow boiling conditions. A combined effect of Marangoni and microlayer evaporation in flow boiling regimes has rarely been reported in open literature. Therefore, a comprehensive boiling model that combines the three submodels, namely, phase change model, microlayer evaporation model, and Marangoni model, is developed that accounts for small-scale physics such as evolution of superheat layer during bubble growth, microlayer evaporation, and scavenging of the superheated liquid during the bubble departure. The verification of model has been carried out through detailed flow and temperature field validation exercises of various bubble stages with recent experimental data reported in the literature. The effects of varying subcooled conditions and Reynolds number on bubble dynamics and the associated heat transfer rates have been examined. The study reveals a decreasing trend in the bubble diameter with increasing Reynolds number and degree of subcooling. It has also been observed that the bubble shape is affected by the Marangoni phenomena. Bubble shape slightly flattens during inception, gradually becomes spherical while sliding, and later elongates after liftoff. The individual contribution of microlayer heat flow (Qmicrolayer) is estimated to be around 22-40% for flow boiling conditions and it is the second-highest heat transfer contributor after the latent heat transfer. The results obtained from the proposed model show a good match with published data and indicate the significance of microlayer in the single bubble flow boiling heat transfer. [ABSTRACT FROM AUTHOR]

Subcooled flow boiling presents an enormous ability of heat transfer rate, which is extremely important in the heat-dissipating systems of many industrial applications, such as power plants and internal combustion engines. Using an Euler-Euler-based three-dimensional numerical simulation of subcooled flow boiling in a vertical tube, we investigated different heat transfer quantities (average and local heat transfer coefficient, average and local vapor volume fraction, average and local wall temperature) and bubble dynamics quantities (bubble departure diameter, bubble detachment frequency, bubble detachment waiting time, and nucleation site density) under various boundary conditions (pressure, subcooled temperature, mass flux, heat flux). Numerical results show that an increase in heat flux leads to the increase in all of the physical quantities of interest but the bubble detachment frequency. An entirely opposite behavior is observed when we change the mass flux and inlet subcooled temperature. Furthermore, a rise in pressure reduces all of the target quantities but the wall temperature and bubble detachment frequency. Since numerical simulation of such multiphase flow requires significant computational resources, we also present a deep learning approach, based on artificial neural networks (ANN), to predicting the physical quantities of interest. Prediction results demonstrate that the ANN model is capable of accurately predicting the target quantities with mean absolute errors less than 2.5% and R-squared more than 0.93. [ABSTRACT FROM AUTHOR]

• A new dimensionless correlation for the ONB is presented. • A new dataset for the ONB with ethylene glycol-water mixture (50 % vol) is presented. • A previously published correlations error analysis with past and current experimental data is presented. • No effect in the ONB point has been observed due to the orientation of the heating plate. In this work, a new comprehensive dataset for the onset of nucleated boiling (ONB) point in flow boiling of 50% vol ethylene glycol-water mix is presented. The experimental setup basically consists of a stainless-steel plate, electrically heated and embedded in a horizontal rectangular channel flow. The new dataset ranges the following experimental conditions: bulk velocities: 0.2 ∼ 0.8 [ m s ]; mass fluxes: 243 ∼ 1000 [ k g m ² s ]; subcooling degrees: 21 ∼ 113 [ ∘ C ]; and pressures: 130 ∼ 225 [ k P a ]. The experimental matrix was repeated for four different orientations: 0, 180, 225 and 270°. In addition to the new data generated and given here as an annex, previous datasets extracted from 12 previously published works have been considered covering six different fluids in total. All the experimental data was previously evaluated using existing correlations in the literature. Finally, a new dimensionless correlation has been developed with relatively low error figures. Further work is required to accommodate different channel configurations, effects of morphology or the effect of wettability. [ABSTRACT FROM AUTHOR]

• A numerical analysis model of bubble growth was developed. • The growth characteristics of bubbles within the grid elements of petal-shaped fuel elements are investigated. • Development of a correlation formula for bubble growth prediction. Based on the VOF multiphase flow model and bubble growth model, a numerical analysis model of bubble growth of subcooled flow boiling in a petal-shaped fuel rod channel was established. The growth characteristics of a single bubble and the influence of thermal parameters were analyzed. Firstly, the growth rate, shape, and trajectory of the bubble were investigated under typical working conditions. Secondly, the influences of inlet subcooling, inlet velocity, and wall superheat on the bubble growth characteristics were clarified. Finally, the existing bubble growth rate prediction correlation was evaluated based on the results of numerical calculations. The results show that the bubble growth rate is fast at the early stage of bubble growth when the evaporation heat flow from the microlayer plays a major role. With the disappearance of microlayer evaporation, the bubble diameter increases slowly. The bubble diameter decreases continuously when the evaporation heat flow in the overheating layer is less than the condensation heat flow after the bubble lift-off. Compared to the rectangular channel, the bubble diameter in a petal-shaped fuel rod channel has a larger equivalent diameter and more obvious lateral movement. With the increase of inlet flow velocity and inlet subcooling, the total evaporation heat flow of the bubble decreases, while the condensation heat flow increases, which results in the decrease of bubble diameter. The influence of increasing wall superheat on bubble diameter is opposite to that of increasing inlet flow velocity and inlet subcooling. Based on the numerical results, a new prediction correlation for bubble growth rate in a petal-shaped fuel rod channel was developed, whose prediction error is within ±20 %. The related research results help to further reveal the subcooled boiling heat transfer mechanism in a petal-shaped fuel rod channel. [ABSTRACT FROM AUTHOR]

Experimental investigations into accident-tolerant fuel and conventional claddings are conducted across a wide range of flow boiling conditions. Two key thermal-hydraulic performances of these claddings, heat transfer coefficients, and critical heat flux, are characterized for their dependence on thermophysical properties, wall thicknesses and surface morphology. Materials that are investigated in this study include Zircaloys, FeCrAl alloys, Inconels, and stainless steels. Relevant experimental results demonstrate that the impact of thermophysical properties and wall thicknesses on heat transfer coefficients is significant and challenging to resolve using current predictive models. This difference caused by the cladding material is only appreciable under weak heat convection conditions. Increasing the mass flow rate and/or liquid subcooling can significantly reduce or even eliminate this difference in flow boiling heat transfer coefficient. To further analyze these experimental results mechanistically, material-conjugated quenching and evaporative heat fluxes are developed to understand the underlying mechanisms related to cladding properties. Similarly, the flow boiling critical heat flux exhibits a noticeable dependence on cladding material properties, including wall thickness, thermophysical properties, and surface morphology, particularly at low mass fluxes and low inlet subcoolings. However, this critical heat flux dependence on material side factors weakens with increasing mass flow rate and/or inlet subcooling. This suggests that convection-dominated heat transfer becomes increasingly dominant compared to quenching and evaporative mechanisms in determining both heat transfer coefficients and critical heat flux. [Display omitted] • Flow boiling characteristics of various claddings are obtained using subcooled water. • Material-related factors are appreciable under weak heat convections. • Gradual increasing of mass flux and/or inlet subcooling reduces the impacts from materials. • The role of cladding properties can be rationalized by heat flux partitioning model. • ATF-loaded LWR cores can be assessed by present CHF databases under nomimal operation condidtions. [ABSTRACT FROM AUTHOR]

Three-dimensional subcooled flow boiling of R134a in a horizontal tube was simulated by a VOF (volume of fluid) model combined with the level set method. Bubble characteristics were explored at heat flux of 0.3 MW/m2, inlet subcooling of 3 K, and inlet velocity of 0.4 m/s. It was observed that five representative bubbles occurred in subcooled flow boiling, including sliding bubble, coalescing bubble, non-departed bubble, bouncing bubble, and continuous-boiling bubble. The results showed that the bubble radial velocity was an important factor of bubble departure after a sliding process. Moreover, the effect of heat flux, inlet velocity, and inlet subcooling on bubble departure diameter were investigated. The departure diameter increased with increasing inlet velocity from 0.2 to 0.4 m/s and heat flux from 0.2 to 0.4 MW/m2, while diameter decreased with inlet subcooling from 3 to 10 K. Finally, based on the influence of heat flux, inlet velocity, and inlet subcooling on average departure diameter of the bubble except the coalescing bubble, a model was proposed to predict the average departure diameter. The deviation of the model was within 5%. [ABSTRACT FROM AUTHOR]

The wall boiling model in the current research is used to predict the bubble dynamics treatment in flow boiling in a vertical pipe. A random surface roughness is developed for simulating the surface roughness effects in sub-cooled flow boiling as a novel method. This novel method is called direct roughness simulation (DRS). The DRS results are compared to the smooth surface (SS) and surface roughness model (SRM). The SRM is the traditional way of simulating surface roughness. The finite volume methods and Euler-Euler are applied to investigate subcooled flow boiling. The turbulence stresses are simulated by the k-ε model. The surface roughness effect on bubble dynamics for flow boiling is investigated numerically. According to the numerical simulations, nucleation site density is only increased by augmentation of heat flux. In contrast, increasing surface roughness, pressure, mass flux, and subcooled temperature cause a drop in nucleation site density. The bubble detachment waiting time and bubble departure diameter increase with the rise in pressure; however, by increasing other boundary conditions, these two parameters decrease. Results show that the reduction in the nucleation site density at outlet was 28.05% for the DRS and 25.5% for the SRM compared to the SS. The bubble detachment frequency at oulet 2.04% decreases when using the SRM and 6.5% increases when using the DRS. Compared to the SS; the bubble departure diameter at outlet 4.3% increases when using the SRM and 11.8% decreases when using the DRS. [ABSTRACT FROM AUTHOR]

Three-dimensional subcooled flow boiling of R134a in a threaded tube was numerically simulated at the conditions of 200~400 kW/m2 heat flux, 3~20 K inlet subcooling, and 0.2~0.6 m/s inlet velocity. The bubble behavior in the horizontal threaded tube with 0.581 mm thread tooth height was observed. The effect of heat flux, inlet subcooling, and inlet velocity on bubble departure diameter and heat transfer coefficient were explored. The results presented the whole growth process of five kinds of bubbles. It was found that the bubbles either collapsed in cold liquid after leaving the heating wall or grew along the axial direction and contacted the heating wall. And there was no bubble sliding during the growth. In addition, the most important and special characteristic of bubble behavior in threaded tubes was the phenomenon of the bubble passing through the cavity. The coalescence and breakup behavior occurred after the bubble passed through the cavity. According to the discussions of the departure diameter and heat transfer coefficient, it was inferred that the bubble departure diameter increased with the increase of heat flux from 200~400 kW/m2 and subcooling from 3~20 K while decreasing with the increase of inlet velocity from 0.2~0.6 m/s. And due to the influence of the threaded tube structure, there are special points in the change of bubble departure diameter. The heat transfer coefficient of the bubbles in the threaded tube was higher than the smooth tube, which was increased by 1.5~12.5%. The heat transfer coefficient increased with the increase of heat flux and subcooling and is closely related to the bubble departure diameter.

The marine environment may change the force on the fluid and inevitably influence bubble behavior and the two-phase flow in the reactor core, which are vital to the safety margin of a nuclear reactor. To explore the effect of the marine motion on the flow and heat transfer features of subcooled flow boiling in the reactor core, the volume of fluid (VOF) method is employed to reveal the interaction between the interface structure and two-phase flow in the subchannel under rolling motion. The variations of several physical parameters are obtained, including the transverse flow, the vapor volume fraction, the vapor adhesion ratio, and the phase distribution of boiling two-phase flow with time. Sensitivity analyses of the amplitude and the period of the rolling motion were performed to demonstrate the mechanisms of the influence of the rolling motion. We found that the transverse flow in the subchannel was mainly affected by the Euler force under the rolling motion. In contrast to the two-phase flow in the static state, the vapor volume fraction and vapor adhesion ratio show different characteristics under rolling motion. Additionally, the onset of significant void (OSV) point changes periodically under rolling motion. [ABSTRACT FROM AUTHOR]

The ability to predict critical heat flux (CHF) is of considerable interest for high-heat equipment, including nuclear reactors. CHF prediction from a mechanistic model for subcooled flow boiling in rod bundles still remains unsolved. In this paper, we try to predict the CHF in an annulus, which is the most basic flow geometry simplified from a fuel bundle, using a liquid sublayer dryout model. The prediction is validated with both water and R113 data, showing an accuracy within ±30%. After the CHF in an annulus is calculated successfully, a near-wall vapor–liquid structure is proposed on the basis of the liquid sublayer dryout model. Modeling of heat transfer modes over the heating surface at CHF is performed, and predictions of the changes in liquid sublayer thickness and heater surface temperature at the CHF occurrence point are carried out by solving the heat conduction equation in cylindrical coordinates with a convective boundary condition, which changes with the change in flow pattern over the heating surface. Transient changes in the liquid sublayer thickness and surface temperature at the CHF occurrence point are reported. [ABSTRACT FROM AUTHOR]

The increasing demand of cooling in internal combustion engine (ICE) may require the shift of heat removal method from traditional single phase liquid convection to subcooled flow boiling in order to fulfill the desired functional temperature. Thus, the characteristics of subcooled flow boiling heat transfer should be studied exclusively considering the practical conditions in ICEs. Accordingly, in this article, subcooled flow boiling experiments were conducted in a rectangular channel using 50/50 volume mixture of ethylene glycol and water coolant (EG/W) as working fluid. Aluminum and cast iron surfaces were selected as the heated surfaces to simulate the material of cylinder head in gasoline and diesel engines, respectively. Experimental results showed a trend that the aluminum surface had a better performance than the cast iron surface in terms of heat transfer coefficient in the boiling region. The difference between these two surfaces was concluded as results of different surface thermophysical properties. A modified wall heat flux model was proposed based on the power-type addition method. The proposed model modified the nucleation boiling contribution by introducing a new parameter which accounts for the influence of thermophysical properties of heated surface on the boiling process. Thus, one such modified model could be useful for practical engine cooling applications.

• Analyzed effect of surface roughness on subcooled flow boiling through visualization. • Large surface roughness shifts the axial position of the ONB forward. • Modified empirical equation for subcooled boiling in a narrow rectangular channel. Due to the size limitation, the heat transfer characteristics of subcooled flow boiling in the narrow rectangular channel are different from the conventional channels. Surface roughness directly affects bubble formation and detachment in subcooled boiling. This study presents a visualization study on the effect of surface roughness on the heat transfer characteristics and the onset of nucleate boiling (ONB) in narrow rectangular channels, including roughness of 0.34 μm, 1.2 μm, and 2.5 μm. The results indicate that surface roughness can enhance the heat transfer between the fluid and the wall and reduce wall temperature. The heat transfer coefficient in the subcooled flow boiling increases nonlinearly with surface roughness. Large surface roughness can shift the axial position of the ONB forward and reduce the wall heat flux required for subcooled flow boiling. An empirical correlation for predicting the subcooled flow boiling is modified based on Hsu's nucleation criterion and the experimental data. [ABSTRACT FROM AUTHOR]

• An elaborately designed test section for narrow rectangular channel is employed. • Wall temperature distributions and subcooled boiling curves are analyzed. • Comprehensive assessments of previous heat transfer correlations are conducted. • Two new correlations are proposed to predict subcooled boiling heat transfer. This paper presents the subcooled flow boiling in vertical narrow rectangular channel with a large aspect ratio (60 × 2.0 mm, w / e = 30) at high-pressure conditions (10–16 MPa). Extensive heat transfer data are obtained from the elaborately designed experiments. Wall temperature distributions and typical subcooled boiling heat transfer curves are analyzed. Due to the larger aspect ratio of the narrow rectangular channel, a significant temperature difference in the transverse direction of the heating plate results in a headmost boiling at the center point. The applicability of three types of heat transfer correlations is systematically evaluated. The correlations of Jens-Lottes and Thom et al. exhibit the most outstanding performance among the eleven heat transfer correlations for the fully developed subcooled boiling (FDB). The prediction performances of five enhancement type heat transfer correlations are generally unsatisfactory, which mainly due to their discrepant application conditions and channel geometries. The superposition type heat transfer correlations cannot predict our experimental data with proper trend and high accuracies. Based on the boiling mechanism of bubble nucleation, two developed correlations are proposed to predict the subcooled heat transfer coefficient for narrow rectangular channels with a large aspect ratio, with emphasis on high-pressure conditions. The prediction accuracies (MAEs) have been improved to 3.25 % (foe the FDB correlation) and 5.38 % (for the enhancement type correlation). [ABSTRACT FROM AUTHOR]

This study presents a new method for predicting the Critical Heat Flux (CHF) approaching condition in subcooled flow boiling based on pressure fluctuation analysis. Through experimental investigations, water flow in an upward direction was conducted within a mini-channel with one side heated to analyze the sequential boiling phenomena, including ONB (Onset of Nucleate Boiling), OFI (Onset of Flow Instability), and CHF. While ONB was determined using the slope of the wall temperature–heat flux, OFI was determined by considering the pressure drop through the channel and outlet pressure fluctuation. By analyzing the inlet pressure fluctuations, a new condition called "the CHF approaching condition" was introduced, which serves as a CHF precursor. The CHF approaching condition was found to be approximately 90% of the CHF value across all experimental scenarios. The proposed method provides an effective tool for predicting and preventing CHF, thereby reducing the potential risk of damage to the heating surface. [ABSTRACT FROM AUTHOR]