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This paper is the second of a two-part series that presents experimental data and analysis on the liquid nitrogen quenching heat transfer process of a stainless-steel tube with an inner surface low-thermal conductivity thin-film coating. This paper focuses on the effects of different flow directions and two tube wall thicknesses. Additionally, this paper also provides an analysis on the chilldown thermal efficiency of the quenching process. Three flow directions with four different inner surface coating modifications, and three tube wall thicknesses were examined. The experimental data covers the Reynolds numbers ranging from 3500 to 140,000. The chilldown efficiency, along with chilldown time and LN 2 (liquid nitrogen) mass consumption were analyzed to assess the overall performance of the LN 2 line chilldown process. For thin tube wall cases, the chilldown efficiencies cover a range between 3% and 41%, and the maximum chilldown efficiency value is found for the tube with 3 L coating at Re = 5278 in the vertical up flow direction. For thick wall tube cases, the efficiencies cover a range between 4% and 52%, and the maximum chilldown efficiency value was found for the tube with 4 L coating at Re = 5308 in the vertical up flow direction. [ABSTRACT FROM AUTHOR]

• Optimize the thermosyphon loop thermal performance by controlling loop wettability. • Long-term effective hydrophilic and hydrophobic modification method was determined. • The optimum wall wettability under different filling ratios was clarified. • The optimized R loop is reduced by 6.9% to 53.5% under different filling ratios. The behavior of vapor-liquid two-phase transport within the riser of the two-phase thermosyphon loop significantly impacts the heat transfer process from the evaporation to the condensation, as well as the circulation velocity, thus playing a crucial role in the heat transfer performance. However, existing research has predominantly focused on optimizing the performance of the evaporator, overlooking the optimization of vapor-liquid two-phase flow patterns and transport characteristics within the loop. This paper aims to optimize the heat transfer performance of the two-phase thermosyphon loop by controlling the wall wettability of the loop. The strategy employed involves modifying the wettability of the riser to regulate the two-phase flow within the loop, while visualization techniques are utilized to investigate the influence of riser wettability on the vapor-liquid two-phase distribution and heat transfer performance of the two-phase thermosyphon loop. The findings reveal that the impact of wall wettability on flow behavior and heat transfer performance within the loop varies significantly with changes in the filling ratio. This paper sheds some light on the riser wettability of two-phase thermosyphon loops to optimize the thermal performance. [ABSTRACT FROM AUTHOR]

• 1 Flow and boiling heat transfer in a 5 mm tube were studied experimentally. • 2 Heat transfer enhancement is obviously founded in the region where x e < 0.05. • 3 New empirical correlations are developed based on the experimental data. • 4 New correlations have higher prediction accuracy than the existing correlations. Experimental investigation on flow boiling heat transfer in a small tube under high system pressure has been conducted in this study. The test-section is constructed with a 316L stainless steel tube. The inner diameter and heated length of the test-section are 5 mm and 930 mm, respectively. The working fluid is deionized water. The operating conditions of the experiments are as follows, the mass flux from 548 to 978 kg/(m2·s), the wall heat flux from 136 to 238 kW/m2, and the system pressure from 1.0 to 2.9 MPa. The experimental results showed that the inlet subcooling mainly affects the onset of nucleate boiling, but its impact on the heat transfer coefficient at the same vapor quality is almost negligible. The heat transfer coefficient increases with the increase of mass flux and wall heat flux, especially in the region where x e < 0.02. The average heat transfer coefficient is higher under high system pressure owing to the variation of the thermophysical properties. Additionally, the existing flow and heat transfer correlations cannot predict the flow boiling heat transfer coefficient and frictional pressure drop in the experiments accurately enough. Therefore, the new flow and heat transfer correlations are developed based on the experimental data in this paper, and the mean absolute deviation of the flow boiling heat transfer correlation and frictional pressure drop correlation are 13.3% and 12.1%, respectively. [ABSTRACT FROM AUTHOR]

• A simplified two-fluid model is proposed for critical two-phase flow in microchannels. • The model preserves precision while incorporating fewer constitutive relations. • Proposed an enhanced metastable liquid nucleation model, achieving precise predictions of nucleation pressure. • Advance the understanding of critical flow in microchannels. Critical flow phenomena are present in many chemical engineering processes, such as microchannel heat exchangers, modeling of jet pumps, nuclear safety analysis, and various industrial facilities that involve subcooled or two-phase pressurized fluids. This paper presents a simplified, reliable two-fluid model for accurately simulating critical flow in microchannels. This study develops a five-equation two-fluid model by constructing a mixed energy equation based on the assumption of metastable liquid phase and saturated gas phase. The model utilizes an improved metastable liquid nucleation model as the gas–liquid phase transition boundary to obtain a more accurate flashing inception. It also uses fewer interfacial constitutive relation equations to reduce the occurrence of unstable solutions. The improved nucleation model is validated using experimental data from previous researchers, with prediction errors controlled within 10%. Furthermore, the five-equation model demonstrates substantial improvements in numerical convergence compared to its six-equation counterpart. By comparing critical mass flux predictions, along-channel pressure variations, and interfacial transfer terms, the five-equation model effectively mitigates instability issues observed in the six-equation model while maintaining prediction accuracy. As a result, this model boasts a broader range of applicability and excels in accurately predicting critical flow, even under more complex conditions. [ABSTRACT FROM AUTHOR]

To address the issues with the multi-phase flow and heat transfer characteristics of the gas inside solid rocket motor combustor, this paper first establishes a mathematical model form that can express the heat transfer characteristics between gas and wall, and obtains the mathematical model of heat transfer by means of experiment and numerical simulation. The results show that heat transfer in the wall surface has obvious hysteresis during the actual working process of solid rocket motor, and that temperature in the wall surface will gradually reach the peak value after the motor works. The heat transfer trend between the numerical simulation results and the experimental data is relatively consistent, and the errors of heat transfer calculation in the stable working section of the motor can be controlled by 15%. It shows that the calculation model with the Sommerfeld number as the criterion selected in this paper can better reflect the heat transfer characteristics between the alumina droplet and the wall in combustor. The maximum error between the fitted value and calculated value is 9.43%, which indicates that the numerical model of heat transfer established under the state of two-phase gas and wall scouring has certain reliability. [ABSTRACT FROM AUTHOR]

[Display omitted] • A one-dimensional whole-loop CFB dynamic model was established with the flow properties. • The model explicitly depicts the solids throughput behavior at the cyclone and standpipe. • Factors affecting the dynamic characteristics of CFB boiler under load regulation were discussed. • An optimization on a 135 MW CFB boiler demonstrated a maximum load ramp rate of about 4 %/min. CFB boilers face an increasing significance for load change rate in China. However, there is currently a lack of mechanistic understanding on limitation on fast peak shaving. Based on the mass and energy balance in the full loop, this paper conducts a dynamic model to describe the coupling effect of gas–solid two-phase flow, heat transfer, and combustion processes under load regulation conditions. In particular, the material throughput pattern of cyclone and standpipe are systemically described. Finally, the model was validated by field test data in a 135 MW commercial CFB boiler. On this basis, the impact of factors such as the draining ash strategy and adding/discharging circulating ash on the load regulation characteristics of the boiler were deeply explored, and a comprehensive optimization solution was proposed. The results showed that the boiler's ramp could be maximized to about 4 %/min after adopting this solution. [ABSTRACT FROM AUTHOR]

This paper is the first of a two-part series that presents experimental data and analysis on the liquid nitrogen quenching heat transfer process of a stainless-steel tube with an inner surface low-thermal conductivity thin-film coating. The experimental data covers a wide range of flow mass fluxes (43 to 1077 kg/m2s). The experiments were conducted using test tubes with four different coating thicknesses to investigate the effects of various Teflon coating thicknesses. An analysis of the chilldown time and LN 2 mass consumption data are presented to evaluate the effects of the flow mass flux, axial distance from the tube inlet, and coating layer thickness. The chilldown time decreases with increasing flow Re. However, the chilldown mass consumption increases with increasing flow Re. In general, the larger the flow Reynolds number, the lower the percentage reduction in chilldown time is observed. For the percentage reduction in the chilldown time by the coating, the minimum is 27.8% for the tube with 7-layer coating at flow of Re = 100,000 and the maximum is 61.3% for the tube with 14-layer coating at flow of Re = 10,000. The percentage reduction in mass consumption due to coating falls in the range between 35.3% and 54.1%. [ABSTRACT FROM AUTHOR]

• Physics-Informed neural networks (PINNs) are used for single and two-phase flows. • The study investigates the rising bubble problem with and without heat transfer. • The pertinent physics of wake interaction and thermal boundary layer are discussed. • Physics-Informed neural networks are agnostic to geometry and fluid properties. • The maximum error in the position of the centre of mass by PINN is less than 2.8 %. This paper presents data-driven simulations of two-phase fluid processes with heat transfer. A Physics-Informed Neural Network (PINN) was applied to capture the behaviour of phase interfaces in two-phase flows and model the hydrodynamics and heat transfer of flow configurations representative of established numerical test cases. The developed PINN approach was trained on simulation data derived from physically based Computational Fluid Dynamics (CFD) simulations with interface capturing. The present study considers fundamental problems, including tracking the rise of a single gas bubble in a denser fluid and exploring the heat transfer in the wake of a bubble rising close to a heated wall. Tracking of a rising bubble phase interface of fluids with disparate properties was performed, revealing a maximum error of only 5.2% at the interface edge and a maximum error of 2.8% at the position of the centre of mass. Inferred (hidden variable) flows are studied in addition to a purely extrapolative inverse isothermal bubble case. When no velocity data was supplied, velocity field predictions remained accurate. Rise of an inferred isothermal bubble with unseen fluid properties was found to produce a maximum mean-squared error of 0.28 and centre of mass error of 1.25%. For the case of the rising bubble with a hot wall, the maximum error in the temperature domain using specified boundary conditions was 6.8%, while the bubble position analysis reveals a maximum positional error of 3.6%. These results demonstrate that PINN is agnostic to geometry and fluid properties when studying the combined effects of convection and buoyancy on two-phase flows for the first time. This work serves as a starting point for PINN in multiphase cases involving heat transfer over a range of geometries. Eventually, PINN will be used in such cases to provide solutions for forward, inverse, and extrapolative cases. Each of which represent a dramatic saving in computational cost compared to traditional CFD. Contours of volume fraction (α) and temperature (T*) predicted by the CFD (Top) and PINN (Bottom) for the evolution of flow showing how the rising bubble influences the thermal boundary layer on the wall. [Display omitted] [ABSTRACT FROM AUTHOR]

• A review of experimental and CFD methods used to analyze the thermal-hydraulic characteristics of sub-channels in rod bundle geometries is presented. • Both adiabatic and boiling two-phase flow studies are examined. • The effects of the design and flow parameters on the two-phase flow hydrodynamics and heat transfer performance are summarized. • Based on previous numerical studies, a basic CFD model for two-phase flows in rod bundle geometries is proposed. Thermal-hydraulic analysis of two-phase flow plays a crucial role in the optimal design of nuclear fuel assemblies and in nuclear safety. This paper presents the state of the art in two-phase flow hydrodynamics and heat transfer in nuclear fuel assemblies. A comprehensive review of experimental and CFD methods used in the last 30 years to analyze the thermal-hydraulic characteristics of sub-channels in rod bundle geometries under different flow conditions (adiabatic and boiling two-phase flow) is presented. The effects of design parameters, e.g. rod-pitch length, mixing vane angle, and flow parameters, e.g. pressure, mass flux and heat flux, on flow hydrodynamics and heat transfer performance are summarized. In addition, the capabilities of CFD modelling of two-phase in rod bundles are discussed. Moreover, based on the existing studies, some recommendations for future studies are given. [ABSTRACT FROM AUTHOR]

• Obtained local surface temperature (T w), boiling curves and liquid loss rate (m loss). • Heat transfer rate (Q w) and m loss increase markedly at onset of nucleate boiling. • Jet outlet velocity (v) affects Q w and m loss more obviously than impact height (H). • Effects of v and H on T w are more significant at non-phase transition stage. • Growing of Q w and M loss slows down at anaphase of nucleate boiling. Combining the benefits of jet impact and phase-transition heat transfer, jet impact boiling has been widely adopted in industrial scenarios. Notably, owing to limitations of high-velocity jets, such as high impact stress and pumping pressure in liquid circuits, low-velocity jet impacting deserves more attention and explorations. In this paper, under an atmospheric environment with wall heating temperatures ranging from 36 °C to 128 °C, the two-phase flow and heat transfer characteristics (local surface temperature, boiling curves and liquid loss rate) of low-velocity jet impacting on a cylindrical surface are investigated by experiment, followed by the analysis on the effects of jet outlet velocity and impact height. The findings reveal that, with the increase of heat transfer rate, the heat transfer regimes in sequence are non-phase transition (single-phase convection and air-liquid two-phase convection) and nucleate boiling. The influences of jet impact height and outlet velocity on local surface temperatures are pronounced at non-phase transition stage. In impact region, along with average superheat, the growth rates of heat transfer rate and liquid loss rate increase significantly from non-phase transition to nucleate boiling stage, and slow down at anaphase of nucleate boiling stage. Interestingly, the increase in outlet velocity delays the onset of slowdown in growth of liquid loss rate to some extent. Overall, the heat transfer rate and liquid loss rate are significantly enhanced at higher jet outlet velocity of 0.60 m/s, but not affected significantly by impact height. [ABSTRACT FROM AUTHOR]

• A revised self-induced jet impingement device is introduced in pool boiling using R1336mzz(Z). • Liquid-vapor separation promoted by the revised device is confirmed from the visualization results. • The revised device does nearly no effort on the nucleate boiling heat transfer coefficient until a relatively high heat flux. • Parametric investigations on the characteristics of the revised device are conducted. • The revised device gains up to 69.5 % and 36.7 % in the critical heat flux and the maximum nucleate boiling heat transfer coefficient, respectively. Pool boiling performance enhancement has attracted considerable interest on high power electronic cooling. In this paper, we introduce a self-induced jet impingement device for practical boiling enhancement, achieved by integrating the guidance tube with an orifice plate. With R1336mzz(Z) as the working fluid, we conduct visualization and parametric investigations on the pool boiling performance, considering the varying characteristics of this device. Visualization snapshots confirm the liquid-vapor separation and indicate that the two-phase flow pattern in the guidance tube eventually stabilizes as a churn-annular flow. Our findings suggest that self-induced liquid jet impingement has a minimal effect on the nucleate boiling heat transfer coefficient (h NB) under a certain heat flux, around 75 % CHF of the standard pool boiling, as the single-phase heat transfer is weakened due to the low thermal conductivity of R1336mzz(Z). As the heat flux intensifies, we observe substantial enhancements in both the Critical Heat Flux (CHF) and the Maximum Nucleate Boiling Heat transfer coefficient (h MNB) resulted from the delayed onset of boiling crisis, owing to the additional liquid supply and the promotion on vapor exhausting. The parametric studies reveal that the flow rate of the liquid jet, amplified by either extending the length or the inner diameter of the guidance tube, or by increasing the number of jet holes, positively impacts the boiling performance. However, these improvements show diminishing returns. Our study shows enhancements in CHF and h MNB of up to 69.5 % and 36.7 %, respectively, compared to the standard pool boiling. [ABSTRACT FROM AUTHOR]

• Different strategies are analyzed to enhanced microchannels based heat sinks. • Nanofluids to enhance heat performance of microchannel based heat sinks is analyzed. • Experiments are performed for Al 2 O 3 , Au and Ag nanoparticles at various concentrations. • The flow-boiling regimes are identified and discussed for the working conditions considered. • Two-phase flow conditions may not always be beneficial, low concentrations of nanoparticles (ϕ ≪ 1%) can have a notable impact on heat transfer mechanisms. This paper investigates the advantages and challenges associated with two-phase flows, specifically flow boiling of pure liquids and nanofluids, for cooling applications in microchannel heat sinks. The study explores various two-phase flow patterns, their related issues, and examines the potential of nanoparticles to enhance heat transfer. Alumina (Al 2 O 3), gold (Au), and silver (Ag) nanoparticles at different concentrations were tested. Experimental tests were conducted under different working conditions using various working fluids, including water, Al 2 O 3 1 wt%, Ag 1 wt%, Au 1 wt%, Au 0.75 wt%, Au 0.5 wt%. The heat fluxes used were 1.026 kW/m2, 1.696 W/m2 and 2.403 kW/m2, while the volumetric flows ranged between 0.5 mL/min and 1.5 mL/min. The observed results indicate that even for the lowest particle concentration tested, the water-Au nanofluid exhibits superior cooling performance compared to the other examined fluids. The findings suggest that although two-phase flow conditions may not yield significant benefits, even small concentrations of nanoparticles (ϕ ≪ 1%) can significantly impact heat transfer mechanisms. This approach provides a cost-effective and efficient alternative for cooling microchannel heat sinks without necessitating the use of two-phase flow conditions. [ABSTRACT FROM AUTHOR]

• Comprehensive and critical review of the available research on flow patterns and flow pattern maps in microchannels. • Summary of advanced experimental techniques for flow visualization and sensing in micro-channels. • Experiments on adiabatic and diabatic flow patterns and flow pattern maps in microchannels. • Flow pattern based heat transfer prediction methods in microscale channels. • Analysis of flow patterns and flow pattern transition mechanisms. • Unstable and transient two phase flow patterns and heat transfer in microscale channels. • Research needs for flow patterns and flow pattern maps are identified and recommended. This paper mainly presents comprehensive review on the research regarding adiabatic and diabatic gas–liquid two-phase flow patterns, bubble growth, flow pattern transitions and flow pattern maps in microchannels over the past 15 years. First, criteria for distinction of macro- and micro-channels are discussed. Then, fundamentals of gas liquid two-phase flow patterns, flow pattern maps and techniques for two phase flow visualization and sensing are presented. Next, experimental studied of adiabatic and diabatic two phase flow patterns, bubble behaviour, flow pattern transitions and flow pattern maps in microchannels with plain and enhanced structures are reviewed. Finally, applications of flow patterns and flow pattern maps are discussed. Flow pattern based mechanistic heat transfer prediction methods are focused on and studies on unstable and transient two phase flow patterns and heat transfer in microscale channels are addressed. According to the review and analysis, recommendations on the future research needs have been given. Systematic and accurate experimental data on flow patterns, bubble growth, flow pattern transitions are still needed. In particular, there are lacks general flow pattern transition criteria. Therefore, effort should be made to develop generalized flow pattern transition criteria based on well documented experimental observation and data. Furthermore, studies of mechanistic and theoretical models for flow patterns, flow pattern transitions bubble growth in microchannels should be further conducted. As an important topic, unstable and transient gas liquid two phase flow patterns and heat transfer in microchannels should be systematically investigated as well in order to understand the flow pattern transition mechanisms in microchannels with plain and enhanced structures. [ABSTRACT FROM AUTHOR]

Coolant two-phase flow in microchannel heat sinks can provide efficient and attractive solutions for disposing of high heat fluxes generated in electronic components and other similar engineering devices. Moreover, the coolant flow in double-layer microchannels has been proven as an also more effective way than a single layer. This paper introduces some useful results of an experimental study with coolant two-phase flow conditions in double-layer microchannels. The double layers microchannels were fabricated by end milling using a CNC machine. The effects of the applied heat flux up to 82 W/cm2, the mass flux in the range of 200 to 426 kg/m2.s, and the maximum microchannel depth of 550 μm, on the thermal behaviour and associated coolant pressure drop, were investigated. The study handles 'in integrated approach' the important topics of the boiling heat transfer characteristics and hydraulic performance. Among these topics, the flow boiling curve and average boiling heat transfer coefficients have received much attention in the present work. Numerical values of this coefficient were predicted using an accurate mathematical model combined with in-house solution code which was developed and reported in this paper. The coolant two-phase flow good square surface temperature uniformity for the double layer microchannels. • The double layers microchannels were precisely manufactured by end milling CNC machines. • The effects of the applied heat flux, the coolant mass flux, and the microchannel depth on the boiling heat transfer characteristics and associated coolant pressure drop were investigated for parallel flow configuration. •. • The study mainly involves visualization of flow boiling pattern in the upper layer microchannel. • The boiling heat transfer coefficients values were predicted using in- house code developed in the present study. • The coolant two-phase flow good square surface temperature uniformity for the double layer microchannels. • The results for the boiling heat transfer coefficients indicated a satisfactory validation of the present in-house code. [ABSTRACT FROM AUTHOR]

• Heat transfer coefficient and pressure gradient of R1233zd(E) and R1336mzz(Z) measured in a 0.643 mm microchannel tube. • Flow pattern captured at exit of each data point and related to the heat transfer and flowing behavior of the two fluids. • Result compared to existing correlations and the best predictive correlations are recommended. This paper presents the heat transfer coefficient, pressure gradient, and flow patterns of R1233zd(E) and R1336mzz(Z) in a microchannel tube. Both heat transfer coefficient and pressure gradient are measured simultaneously and presented in the same plot in this paper. The experiment was conducted on a 24-port microchannel tube with an average hydraulic diameter of 0.643 mm. Experiment covers mass flux from 100 to 200 kg- m −2s−1, heat flux from 0 to 6 kW- m −2, vapor quality from 0 to 1. The saturation temperature of R1233zd(E) is from 30 to 50 °C, and R1336mzz(Z) is from 40 to 60 °C. The pressure gradient is a strong function of mass flux and saturation temperature (properties). Heat flux has an insignificant effect on the pressure gradient because of low accelerating pressure drop. The heat transfer coefficient is strongly affected by mass flux, heat flux, and saturation temperature. When mass flux is higher, the flow has more waves and turbulence, and the HTC and dP/dz are higher. R1233zd(E) and R1336mzz(Z) have much smaller HTC and higher dP/dz than R1234yf and R1234ze(E) under the same condition. R1234yf has almost the same HTC as R1234ze(E) at high quality (x >0.5), but higher at low quality (x <0.5). Hwang and Kim (2006) is recommended for predicting pressure gradient of the two low pressure fluids (R1233zd(E): MAE=14.1%, ME=-3.7%, Dev=16.8%; R1336mzz(Z): MAE=16.7%, ME=-2.0%, Dev=22.0%). Liu and Winterton (1991) is recommended for predicting HTC of R1336mzz(Z) (MAE=15.2%, ME=-4.6%, Dev=17.9%). None model has good agreement with the measured HTC of R1233zd(E). [ABSTRACT FROM AUTHOR]

The numerical simulation of phase change heat transfer is important for predicting the gas-liquid two-phase flow and heat transfer characteristics and optimizing the phase change heat dissipation system. For the isovolumetric phase change heat transfer process, the compressibility of the gas phase has a significant impact on the simulation accuracy. A numerical simulation method of flow and phase change heat transfer considering gas-liquid two-phase compressibility is established based on VOF method using OpenFOAM in this paper. This method introduces a generalized phase change source term and a compressible source term to improve the isoAdvector geometric reconstruction algorithm and establishes a method for capturing the phase interface of a compressible two-phase flow. The phase change flux based on the phase interface is converted into a phase change source term based on the unit volume by calculating the phase interface density. The compressible term of the governing equation is constructed and its implicit discrete method is proposed. The coupling between velocity and pressure is realized based on the PISO algorithm. A numerical calculation method for phase change heat transfer of compressible two-phase flow is established. The effectiveness and accuracy of the proposed numerical method for predicting the heat transfer characteristics of phase change flows are verified by the simulation of vertical flat film condensation and film boiling. The numerical simulation results are in good agreement with the experiment. Compared with the other numerical simulation methods, the proposed method considering the influence of the compressibility of the gas phase on the flow and heat transfer characteristics in the heat pipe has high accuracy in predicting the gas-liquid two-phase flow, capturing the evolution characteristics of the phase interface and predicting the phase change heat transfer. The numerical simulation method can provide algorithm support for the numerical simulation of compressible two-phase flow phase change heat transfer and the design of a phase change heat dissipation system. [ABSTRACT FROM AUTHOR]

• Flow boiling heat transfer of water in a coil was experimentally investigated. • Severe inhomogeneity of circumferential wall temperature existed. • The flow pattern of two-phase boiling flow in the coil was divided into four regions. • Influencing factors on CWTI and heat transfer coefficient were explored and analysed. • A new FBHT correlation in the HCT with better accuracy was proposed. Helically coiled tubes (HCTs) have been widely used in chemical industry and nuclear engineering because of its compact structure and excellent heat transfer performance. This paper designed and built an experimental platform to study the flow boiling heat transfer (FBHT) of subcritical water with high temperature and high pressure in an HCT. The results showed that the wall temperature on the inner side is higher and it gradually decreases when moving to the outer side along the circumferential direction. The variation law of circumferential wall temperature inhomogeneity (CWTI) under different pressures, mass velocities, heat fluxes and vapour qualities was obtained. The distribution of circumferential wall temperature varies greatly in the single-phase region while it varies slightly and is relatively uniform in the two-phase boiling region. As the vapour quality increases, the CWTI decreases first, and maintains at a low value when the vapour quality is about 0.0 ∼ 0.9. When the vapour quality reaches about 0.9, the CWTI increases rapidly. When the vapour quality is less than 0.5, the heat flux and mass velocity have little effect on the CWTI. When the vapour quality is larger than 0.5, reducing the heat flux or increasing the mass velocity can effectively reduce the CWTI. In addition, the flow pattern of two-phase boiling flow in the HCT is divided into four regions, namely bubble flow, slug flow, annular flow and mist flow, and the transition vapour quality x t1 = 0.2, x t2 = 0.5 and x t3 = 0.93 are respectively selected according to the sudden abrupt change of the wall temperature or the differential pressure. Finally, the existing FBHT correlations are collected and evaluated and the calculation accuracy needs to be improved for more accurate calculation. Therefore, a new FBHT correlation with better accuracy is proposed for calculating the heat transfer coefficient of two-phase flow boiling in the HCT. [ABSTRACT FROM AUTHOR]

• New mechanism for supercritical water HTD is proposed. • Coupling effects of nanoparticles and corrugated tubes on HTD are investigated. • Mitigation effect of nanoparticles on "HTD cycles" is investigated. • Mechanism for suppression of HTD by nanoparticles and corrugations is investigated. When faced with high thermal loads, the dramatic variations in supercritical water (SCW) thermophysical properties can lead to heat transfer deterioration (HTD). Therefore, the present paper first proposes a new mechanism of HTD based on the pseudo boiling theory, and then numerically investigates the mitigation effects of nanoparticles and corrugations on the HTD. By the investigation of SCW in vertically upward smooth tube, the mechanism study illustrates that the pseudo film with the characteristics of low turbulence intensity and thermal conductivity induces the HTD, and the pseudo two-phase flow instability dominates the growth and dissipation of pseudo film. With the help of nanoparticles and corrugations, the wall temperature has a maximum decrease of 348 K, the wall temperature peak mitigation ratio was 33.5%, and the maximum heat transfer enhancement is 164%. Since the corrugations within the corrugated tube (CT) does not change the growth rate of pseudo film, "HTD cycles" occurs in the CT with large corrugation pitch and further deteriorates the overall heat transfer performance. By the inhibition of pseudo film growth and enhancement of gas-like SCW thermal conductivity, the coupling effect of nanoparticles and corrugations can effectively suppress the HTD as well as "HTD cycles". [ABSTRACT FROM AUTHOR]

The main aim of this study is to experimentally investigate two-phase flow patterns during flow boiling of R245fa in a micro channel array. The array consists of 10 rectangular micro channels connected in parallel, each having a hydraulic diameter of 1 mm. Base area of the channels is 14.5 x 180 mm. Micro channels without and with 0.5 mm wide inlet restrictions are examined to determine their influence on the flow. Four two-phase flow patterns are observed using a high-speed camera: bubbly, slug, churn-annular and annular flows. Two former are grouped together as intermittent flow, and two latter as annular flow. In the paper a special focus is put on the determination of the transition vapor quality corresponding to the change from intermittent to annular flow as well as how it is influenced by heat flux, mass flux and saturation temperature. Tested heat flux ranges from 30 to 50 kW/m 2 , mass flux from 400 to 1000 kg/m 2 s, examined saturation temperatures are 49, 63 and 82 ° C and inlet subcooling varies between 13 and 19.4 K. It was found that transition vapor quality increases at lower mass flux, higher heat flux or saturation temperature. Coupling data on two-phase flow patterns with the data from our previous study on heat transfer (Halon et al., 2022) shows that flow patterns do not determine the dominant heat transfer mechanism. Intermittent flow occurs both in forced convection and nucleate boiling dominated zones, and annular flow, typically associated with the domination of forced convection, accompanies nucleate boiling dominated heat exchange. Micro channels with restrictions exhibit similar behavior, but transition vapor qualities shift to higher values. Comparison with four recognized models for transition vapor quality shows that it is best predicted by the model of Costa-Patry and Thome (2013) with M A P E of 47.9% calculated for both studied geometries. • Two-phase flow patterns during flow boiling in micro channels are investigated. • Two micro channel geometries are tested: without and with inlet restrictions. • The focus is on transition from intermittent to annular flows. • Transition shows dependence on heat flux, mass flux and saturation temperature. • The link between flow patterns and the dominant heat transfer mechanism is analyzed. [ABSTRACT FROM AUTHOR]

• Flow boiling system with two novel microchannels for electronic cooling was experimentally studied. • The effects of mass flow rate, inlet temperature and heat flux on the thermal performances were explored. • The relationship among flow pattern, boiling heat transfer coefficient and pressure drop were investigated. • More outstanding microchannel structure for flow boiling cooling system was found. • Proper correlation is selected and revised for the heat transfer prediction for novel microchannels. This paper reports experimental research of the microchannel flow boiling system for thermal management in electronic devices. Under different operational conditions, the flow boiling characteristics of HFE7100 in two novel microchannels: microchannel with Tesla valve (MCTV), microchannel with sector bump (MCSB) were studied. The effects of mass flow rate, inlet temperature and heat flux to the flow boiling characteristics regarding the two structures were analyzed and more outstanding structure for flow boiling cooling system was found. The results report that the boiling state in both two novel microchannels are stable, the pressure drop fluctuation is about 0.8% to 3.9%, but the heat transfer coefficient of MCSB is approximately 45 % higher and the pressure drop of MCSB is 25% lower than that of MCTV in average. One reason is the ribs of MCSB has stronger ability to segment bubble and vapor slug which contributes to smaller average volume of bubble/vapor slug and proportion of vapor phase. Another is that the geometry of MCSB is smoother and the local flowing velocity near the rib is larger which is beneficial for bubble nucleation and separation. In addition, the effective heat flux for ONB of MCSB is larger than that of MCTV. It is concluded that MCSB has superior thermal-hydraulic performance in flow boiling process than MCTV. The proper correlation is selected and revised for the prediction of heat transfer coefficient of two microchannels. Generally, the prediction error of the revised correlation is no more than 30%. [ABSTRACT FROM AUTHOR]

• Flow patterns in helically coiled tubes (HCTs) are classified by visual observation. • Flow-regime map is proposed and mechanisms of flow transitions are presented. • Void fraction for the entire tube cross-section is measured by WMS technique. • New correlations of distribution parameter and drift velocity are proposed. Helically coiled tubes (HCTs) are widely used in industries owing to their compact structure and excellent heat and mass transfer efficiency. Due to the complexity of three-dimensional flow in HCTs, profound knowledge on the flow pattern characteristics is not well documented in existing literature. In this paper, we employ the wire-mesh sensor (WMS) and image tomography technique to measure a series of essential parameters (e.g. flow structures, phase fraction) to explore the flow pattern transition mechanism. According to our observation, bubbly, plug, slug, wavy and annular flow are recognized, and the mechanisms of flow pattern transitions are carefully discussed based on the analysis of the forces. In addition, the variation of void fraction in both temporal and spatial dimensions is discussed in detail. By introducing the drift flux analysis, new correlations of distribution parameter and drift velocity are proposed taking both HCTs structure and flow condition into account. [ABSTRACT FROM AUTHOR]

• Visualizations of flow patterns of two-phase flow in smooth and hierarchical tube. • Measurements of heat transfer coefficients of two-phase flow in smooth and hierarchical tube. • Flow boiling correlation in three-dimensional hierarchical tube. • Flow condensation correlation in three-dimensional hierarchical tube. Development of novel enhanced surface tubes is an important way to advance two-phase flow heat transfer. To make correlations of two-phase flow heat transfer in enhanced tubes with acceptable prediction has been a challenge for many years. The paper presents a method: two-phase heat transfer correlations in enhanced tubes are directly developed through existing two-phase heat transfer smooth-tube models by introducing several modifying factors and dimensionless numbers. An enhanced heat transfer tube containing staggered arrays of three-dimensional dimples and secondary petal-shaped protrusions (EHT) is one of the most complicated internal surface-tube configurations on the market. Visualizations of flow patterns and measurements of heat transfer coefficients of two-phase flow using refrigerant R410A in smooth and EHT tube are conducted. Detailed visualization experiments during two-phase flows show good agreements between flow regimes and flow-pattern maps in smooth and EHT tube. The equation of continuity is adopted to provide understanding of the mechanism of the agreements. Both theoretical analysis and experimental studies support the method. As a result of using it, new two-phase heat transfer correlations are developed in enhanced tubes as the EHT tube is an example. Flow boiling correlation can predict data points within +10% to -20% and flow condensation correlation can predict data points within ±10% in the EHT tube. [ABSTRACT FROM AUTHOR]

This paper presents the flow boiling heat transfer coefficient and pressure gradient measurements of R32 and R1234yf mixtures in a microchannel tube at three concentrations: 15/85, 50/50, and 85/15 by mass. R32/R1234yf mixture has a temperature glide curve that the 15/85 R32/R1234yf mixture has low-temperature glide (0.5 K) while the opposite concentration (85/15) has the highest glide (7 K). The experiment was conducted on a 24-port microchannel tube with an average hydraulic diameter of 0.643 mm. The experiment covers mass flux from 100 to 200 kg-m−2 s−1, heat flux from 0 to 6 kW-m−2, and vapor quality from 0 to 1. Mass flux has a strong effect on both HTC and dP/dz. Heat flux affects HTC but not dP/dz. In addition, the results are compared to pure fluid to discuss the effect of temperature glide. When the glide is higher, the HTC is lower and flatter. At fixed quality, as R1234yf concentration increases, HTC of the mixture decreases slightly first and then increases dramatically. Furthermore, two experiment methods are compared: evaporation in one pass and in one test section. The non-equilibrium in two-phase flow affects concentration gradient in the two phases and further affects the HTC of mixtures with high-temperature glide. • Heat transfer coefficient and pressure gradient of R32/R1234yf are measured. • Three mixtures with different compositions are studied. • Non-equilibrium in two-phase flow affects heat transfer coefficient measurements. • Heat transfer coefficient is lower when the mixture has a larger temperature glide. [ABSTRACT FROM AUTHOR]

This work is an extension of our recent article, which was inspired by discussions with the reviewers. The paper presents an experimental study of the gas-liquid segmented flow as a method of intensifying microfluidic heat exchangers (MFHE) with a focus on studying the influence of the gas volume fraction. It was found that the highest heat transfer coefficient is achieved when the bubble size coincides with the channel diameter, which corresponds to a gas hold-up of 12%. Experimental studies have also shown that segmented flow makes it possible to increase the Nu up to 1.54 and reduce thermal resistance up to 1.67 times compared to single-phase flow. • The heat transfer coefficient of MFHE peaks at gas hold-up of 12%. • The highest heat transfer when the bubble length coincides with the channel diameter. • The dependence between dimensionless bubble length and gas hold-up in MFHE is established. • Nu is 37% higher for two-phase flow than for single-phase flow at the same pressure drop. [ABSTRACT FROM AUTHOR]

• A high-order implicit scheme for multi-phase flows with heat and mass transfer is proposed. • The scheme is developed for a generalized non-orthogonal grid and it is relatively simple to implement. • Accuracy of the proposed scheme is demonstrated for a series of direct contact condensation of steam bubbles in upward liquid subcooled flows. • The effects of subcooling degree and steam mass flow rate on heat and mass transfer characteristics are investigated. Reliable prediction of unsteady multiphase flows with mass changes, large density ratios, and complex flow features is very challenging due to the need to accurately resolve both complex interface deformations and steep gradients associated with material interfaces. This paper describes an application of our recently developed high-order monotonicity preserving (MP) scheme – a reliable and robust scheme for long-term computations of flows contain complex smooth structures interspersed with discontinuities (Ha and Lee, 2020) − to the solution of unsteady incompressible two-phase flows in the presence of heat and mass changes. The flow is governed by the multiphase Navier–Stokes equations, which comprises of the mixture continuity equation, the mixture momentum equations, the mixture energy equation, and the continuity equation for the individual phase. The numerical discretization is implemented on generalized curvilinear grids for facilitating flow simulations of the complex geometries. Our high-order MP scheme is applied to the calculations of the convective and surface tension terms. To obtain an effectively simultaneous solution of all equations, a novel dual-time stepping approach in conjunction with an implicit Runge–Kutta three-stage scheme is proposed. The reliability of the proposed numerical algorithm is then examined for a 1D Stefan condensation problem and two condensing flows including single bubble and steam-jet condensation problems. Particularly, for the single bubble condensation test, the bubble characteristics at different time instants are evaluated both qualitatively and quantitatively. For the steam-jet condensation test, the capability for capturing the evolution of interface deformations under different thermal and hydrodynamic flow conditions is highlighted. Finally, the effects of subcooling degree and steam mass flow rate on heat and mass transfer characteristics are investigated and discussed. [ABSTRACT FROM AUTHOR]

• Stability of an unsteady, two-phase flow with heat and mass transfer considered. • Work motivated by loss of coolant accidents in nuclear power plants. • Flow is mathematically unstable, regions of ǣengineering stabilityǥ identified. • Engineering stability quantified using solution growth behavior and doubling time. • Framework provided for application to other types of problems. This paper considers the stability of an unsteady, two-phase flow with heat and mass transfer. The model problem is motivated by loss of coolant accidents in nuclear power plants. For the example problem, two flow geometries are considered: inverted annular flow boiling and an annular mist flow. The model is comprised of coupled Mathieu equations so that stability can be determined using a Floquet analysis. The flow is found to be mathematically unstable to all perturbative wavenumbers, but for practical purposes there are regions of stability. Using the solution's growth behavior and doubling-time, the notion of practical stability, which is termed herein as "engineering stability," is quantified and a method is provided for application to other engineering stability problems. [ABSTRACT FROM AUTHOR]