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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]

The aim of the present paper is to establish the detailed numerical results for bioconvection boundary-layer flow of a two-phase dusty nanofluid. The dusty fluid contains gyrotactic microorganisms along an isothermally heated vertical wall. The physical mechanisms responsible for the slip velocity between the dusty fluid and nanoparticles, such as thermophoresis and Brownian motion, are included in this study. The influence of the dusty nanofluid on heat transfer and flow characteristics are investigated in this paper. The governing equations for two-phase model are non-dimensionalized and then solved numerically via two-point finite difference method together with the tri-diagonal solver. Results are presented graphically for wall skin friction coefficient, rate of heat transfer, velocity and temperature profiles and streamlines and isotherms. To ensure the accuracy, the computational results are compared with available data and are found in good agreement. The key observation from the present analysis is that the mass concentration parameter, D ρ , extensively promotes the rate of heat transfer, Q w , whereas, the wall skin friction coefficient, τ w , is reduced by loading the dust parameters in water based dusty nanofluid. [ABSTRACT FROM AUTHOR]

The fluid flow and heat transfer behavior of liquid–liquid two phase flows have led to significantly improve the heat transfer rates in microchannels. Both numerical and experimental studies are reviewed in this paper to gain useful insights into the effect of a number of parameters such as film thickness, Peclet number, working fluid and flow geometry on hydrodynamic and thermal behavior of microchannels using liquid-liquid two phase flow. In addition, the paper summarises information about common correlations proposed to predict the pressure drop and heat transfer coefficient in the form of Nusselt number (Nu). The present study shows that there is little agreement across the literature between measured pressure drop and Nusselt number and predictions based on these correlations. Finally, the conclusions and important summaries, and some possible future development of this field are presented. [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]

Ice slurry heat transfer characteristics in a straight horizontal tube were numerically studied in this paper. By using of CFD software, the VOF model in FLUENT 6.3, the simulation of ice slurry was performed to cover laminar and turbulent flow regimes with C is varying from 0% to 20%. The obtained results were firstly validated by the published experimental values. It is found that the discrepancy between the measured and calculated Nu is number is mainly in the range of ±15%. Then the model was applied to analyze the impact of initial conditions on ice slurry heat transfer. The present work shows that, ice slurry heat transfer performance are strongly dependent on the flow velocity and ice mass fraction, and heat flux density only has a small influence on the value of Nu is when ice slurry flows in turbulence. This paper aimed to use this model as a design tool to analyze ice slurry flow in different operating conditions. [ABSTRACT FROM AUTHOR]

• Heat transfer advancement using a coated tube and pulse flows in a cryogenic quenching process is reported. • Pulse flow effects with various duty cycles, periods and coolant mass flow rates were investigated. • In general, the quenching efficiencies cover a range between 41.1% and 81.8%. • The maximum percentage increase in the quenching efficiency is at 65.77%. • The pulse flows could save up to 38.22% in the amount of cryogen consumption. This paper reports heat transfer enhancement techniques for the cryogenic quenching process. An experiment was performed to evaluate the enhancement of quenching heat transfer by two techniques: 1. Using intermittent pulse flows and 2. Coating the inner surface of a test tube with layers of low thermal conductivity films. Pulsed liquid nitrogen flows with various duty cycles and periods were applied in the quenching of a room-temperature metal tube coated with four layers of Teflon thin films on the inner surface. In general, the results obtained indicate that the quenching thermal efficiency that measures the effectiveness of heat transfer enhancement increases with decreasing duty cycle, however it is relatively independent of the period. Comparing with non-coated bare surface test tube, the low-conductivity coating substantially improved the thermal efficiency and reduced the total quenching time. Additionally, the thermal efficiency was found to increase with decreasing source inlet pressure or coolant mass flow rate. The savings on the amount of cryogen consumed follow the same trends as those for the thermal efficiency with pulse flows and tube coating. [ABSTRACT FROM AUTHOR]

• Model form uncertainty is quantified in the modular Bayesian approach. • The "curse of high-dimensionality" is avoided through sensitivity analysis. • The computational cost is significantly reduced using surrogate modeling. • The quantified simulation uncertainties are in good agreement with data. In this paper, we present an approach to inversely quantify the uncertainty of MCFD simulations through a data-driven modular Bayesian inference. Both the model parameter uncertainty and the model form uncertainty are evaluated in the proposed approach. Considering the high-dimensionality of parameter space related to the solver, we performed a sensitivity analysis to reduce the input parameter dimension for the Bayesian inference. Based on the reduced parameter dimension, surrogate models based on Gaussian Process (GP) and Principal Component Analysis (PCA) are constructed to reduce the computational cost in the Bayesian inference. Two case studies based on the proposed approach are performed, focusing on two-phase flow dynamics and on wall boiling heat transfer, respectively. In case study I, we are able to construct a GP-based surrogate model based on 8 principal components to represent the total 208 MCFD solver outputs. Moreover, both cases show that the proposed approach is able to quantify and reduce the parameter uncertainties with the support of experimental measurements. The posterior uncertainties of investigated parameters have 50%-90% narrowed uncertainty ranges compared to their prior uncertainties. Furthermore, a forward uncertainty propagation of the MCFD solver with the obtained uncertainties shows that the agreement between the solver predictions and experimental measurements are significantly improved. [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]

Abstract: Nanofluids attract researchers in many ways for their enhanced heat transfer properties. Nanorefrigerant is one kind of nanofluids. It has better heat transfer performance than traditional refrigerants. Thermal conductivity, viscosity and density are the basic thermophysical properties that must be analyzed before performance analysis. This paper presents a comprehensive review of nucleate pool boiling, flow boiling, condensation and two-phase flow of refrigerant-based nanofluids. The effects of nanolubricants on boiling and two-phase flow phenomena are presented as well. Furthermore, studies of applications and preparation of refrigerant-based nanofluids are presented. For the limited studies done so far, there are some controversies from one study to another. Based on results available in the literatures, it has been found that nanorefrigerants have a much higher and strongly temperature-dependent thermal conductivity at very low particle concentrations than conventional refrigerant. This can be considered as one of the key parameters for enhanced performance for refrigeration and air conditioning systems. Because of its superior thermal performances, latest up to date literatures on this property has been summarized and presented in this paper as well. [Copyright &y& Elsevier]

• 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]

Abstract: The Non-Inertive-Feedback Thermofluidic Engine (NIFTE) is a two-phase thermofluidic oscillator which, by means of persistent periodic thermal-fluid oscillations when placed across a steady temperature difference, is capable of utilising low-grade (i.e., low temperature) heat to induce a fluid motion. Two linearised models of the NIFTE are presented in this paper, both containing a description of the phase-change convective heat transfer that takes place between the working fluid and the heat exchangers. The first model (LTP) imposes a steady linear temperature profile along the surface of the heat exchangers; whereas the second model (DHX) allows the solid heat exchanger blocks to store and release heat dynamically as they interact thermally with the working fluid. In earlier work [Solanki R, Galindo A, Markides CN. Appl Therm Eng; [24]] it was found that these models predict the oscillation (i.e., operation) frequency of an existing NIFTE prototype pump well, but significantly overestimate its reported efficiency. Specifically, the LTP and DHX models predicted exergetic efficiencies 11 and 30 times higher than those observed experimentally, respectively. In the present paper, a dissipative thermal loss parameter that can account for the exergetic losses due to the parasitic, cyclic phase change and heat exchange within the device is included in both models in an effort to make realistic predictions of the exergetic efficiencies. The LTP and DHX models, including and excluding the thermal loss parameter, are compared to experimental data. It is found that the inclusion of the thermal loss parameter increases the predicted oscillation frequencies in the DHX model, but has a negligible effect on the frequencies predicted by the LTP model. A more significant effect is observed with respect to the efficiencies, whereby the inclusion of the thermal loss parameter leads to a greatly improved prediction of the exergetic efficiencies of the prototype NIFTE pump by both the LTP and DHX models, both in trend and approximate magnitude. From the results it is concluded that, on accounting for thermal losses, the DHX model achieves the best predictions of the key performance indicators of the NIFTE, that is, of the oscillation frequency and exergetic efficiency of the device. [Copyright &y& Elsevier]

Abstract: Thermal transport in metal foams has received growing attention in both academic research and industrial applications. In this paper the recent research progress of thermal transport in metal foams has been reviewed. This paper aims to provide the comprehensive state-of-the-art knowledge and research results of thermal transport in open celled cellular metal foams, which covers the effective thermal conductivity, forced convection, natural convection, thermal radiation, pool boiling and flow boiling heat transfer, solid/liquid phase change heat transfer and catalytic reactor. The forced convection and thermal conductivity have been extensively investigated, while less research were performed on two-phase (boiling and solid/liquid phase change heat transfer) and thermal radiation in metal foams. Also most research still treats the metal foam as one type of effective continuous porous media, very few researchers investigated the detailed thermal behaviours at the pore level either by numerical or experimental approaches. [Copyright &y& Elsevier]

• Two-phase heat transfer coefficient and dryout data for refrigerant R452A. • Dryout vapor quality decreasing with heat flux. No effects of mass flux and bubble temperature. • Assessment of existing two-phase heat transfer and dryout correlations. • Similar convective contributions to R404A and penalized nucleate boiling due to high glide. • Glide-based modification of two-phase heat transfer coefficient prediction methods. This paper presents an experimental investigation on two-phase heat transfer and dry-out occurrence for refrigerant R452A in a single horizontal circular stainless-steel tube having an internal diameter of 6.0 mm. The effects of mass flux (from 150 to 600 kg/m2s), saturation (bubble) temperature (from 23 to 55 °C) and heat flux (from 10 to 65 kW/m2) are investigated and discussed. Heat transfer coefficient and dry-out vapor quality data are then compared to R404A results in the same operating conditions, observing that the nucleate boiling contribution of the new blend is penalized by its very high temperature glide during evaporation. The assessment of some dry-out and flow boiling heat transfer coefficient prediction methods is finally carried-out and a correction factor on the nucleate boiling term is proposed to take into account the negative effect of the temperature glide difference on the mass diffusion in the liquid. By implementing this modification on two chosen asymptotic models, the statistical error analysis is considerably improved. [ABSTRACT FROM AUTHOR]

The flow and heat transfer characteristics of mist/steam coolant in U-shaped gas turbine cooling passage are studied in this paper. The mist/steam mass ratio and mist diameter are carefully analyzed with the numerical method. The results show that the heat transfer of the ribbed-wall gradually increases with the increasing of mist/steam mass ratio. Under the condition of water mist diameter = 10 μm, as mist mass ratio increases from 2% to 10%, the averaged Nusselt number of the First Flow Passage increased by 60.13% and the Second Flow Passage increased by 112.5%. The evaporation rate and surviving time of mist can be controlled by controlling the mists diameter. Larger mist diameter is not always good for enhancing the heat transfer of the ribbed walls. When mist/steam = 8%, as for the First Flow Passage and the Second Flow Passage, water mists of 10 μm diameter has higher average Nu number of ribbed wall than mists of 5 μm, 15 μm and 20 μm. The improvement of the heat transfer performance obtained from the evaporation of the water mist is bound to the increasing of the heat transfer non-uniformity of the heated walls. [ABSTRACT FROM AUTHOR]

Highlights • Mechanistic model for swirling annular flow pattern transition was established. • Two physical mechanisms for swirling annular flow pattern transition were revealed and modeled. • Effects of parameters (e.g., hydraulic diameter, working pressure and swirl angle) on the transition boundary was presented. • Generalized correlation of swirling annular flow pattern transition was proposed. Abstract The accurate prediction of flow patterns and their transition is extremely important for proper design, operation and optimization of two-phase flow systems, since the parameters such as pressure loss and heat and mass transfer are strongly dependent on the flow pattern. So far, the non-swirling gas-liquid flow in straight pipes have been widely studied and various mechanisms that lead to flow pattern transition have been clarified and modeled. However, the dynamics of gas-liquid flow under swirling condition are not well understood, and no detailed models are available for the prediction of swirling flow pattern transition. To address this, in our previous work (Liu and Bai, 2018), a visualization experiment aimed at classifying flow regimes in swirling gas-liquid flow was presented and three typical swirling flow regimes, i.e., swirling gas column flow, swirling intermittent flow and swirling annular flow were classified and defined, respectively. As the swirling annular flow can be regarded as a special case of conventional annular flow (i.e., when tangential velocity does not equal zero), in present paper, a mechanistic model for the prediction of the swirling annular flow pattern transition was developed considering its physical interest and great practical significance. Two physical mechanisms that lead to the transition from swirling annular flow to other flow patterns were revealed and modeled, respectively. The model was evaluated against a wide range of swirling and non-swirling experimental data and based on this model, the effects of different parameters (e.g., hydraulic diameter, working pressure and swirl angle) on the boundary of flow pattern transition were presented. Results revealed that the range of swirling annular flow enlarges with the increase of the working pressure and swirl angle but narrows with the hydraulic diameter. Taking these influencing factors into account, a generalized formula for the prediction of the swirling annular flow pattern transition was proposed. Compared with existing empirical correlations for annular flow, the newly developed correlation provided more accurate and reasonable prediction of flow pattern transition for both swirling annular flow and annular flow. [ABSTRACT FROM AUTHOR]

Highlights • Conditions for onset of dryout in a heat-releasing porous bed are considered. • Functional form of the dryout heat flux (DHF) is derived for three drag models. • A surrogate model for DHF derived in the form of three functions of single-argument. • Fast DHF calculation possible with accuracy acceptable for engineering applications. • Model application to multidimensional porous beds is discussed. Abstract This work is motivated by the problem of restraining temperature escalation inside a porous heat-releasing media submerged in a pool of liquid coolant. When coolant temperature reaches saturation, boiling begins in the bulk of the porous bed, with void generation rate determined by the heating power. Amount of void determines hydrostatic pressure difference that drives natural circulation of two-phase flow through the porous material. At a certain critical value of the heat release rate, the driving head cannot overcome drag of the two phase porous media flow, which results in complete evaporation of coolant in some zone. Temperature of material in the dry zone increases significantly due to deterioration of heat exchange with single phase vapor flow in comparison with boiling flow heat transfer. The paper considers the problem of determining the critical conditions for onset of dryout in a heat-releasing porous bed of an arbitrary shape. The well-known one-dimensional problem for a flat top-flooded bed is revisited, and the functional form of the dryout boundary (expressed as the dryout heat flux, DHF) is derived using non-dimensional parameters. Asymptotic behavior of the solution is analyzed, and, by the method of asymptotic interpolation, a surrogate model is proposed consisting of three single-argument, non-dimensional functions. It is shown that such a model provides acceptable accuracy even in the cases where complete similarity of solutions is not achieved. The results obtained provide important insights into the physics of the problem, reduce the number of free parameters, and enable fast evaluation of dryout conditions without the need of numerical solution of algebraic equations involved in the exact formulation. The ultimate goal of the surrogate model development, i.e. its application to multidimensional configurations, is discussed. [ABSTRACT FROM AUTHOR]

Highlights • A two-phase flow visualisation and heat transfer experiment bench was established. • Flow patterns at various crank angles in an open cooling gallery were captured. • The effects of motor speed and heating on flow patterns were discussed. • The temperature distribution was obtained and the effect of speed was discussed. • Transition from laminar flow to turbulent flow has been identified. Abstract The demand of increasing power density of the Internal Combustion Engine promotes the technology development using internal cooling gallery of the piston. The oil flow and heat transfer performance of the piston internal cooling gallery have great influence on the cooling performance, which affects the thermal conditions of the piston and of the entire engine. The experimental study using visualising method is therefore critical and important to observe the flow patterns inside the piston gallery. In this paper, we presented the visualised study of two-phase flow patterns within an open cooling gallery by using a high-speed camera at various crank angles. The effects of motor speed both on the flow patterns and temperature distribution were investigated and the heat transfer mechanisms in gas-liquid two-phase flow during reciprocating motion were explored. Based on the study, the period-doubling phenomenon, which represented the transition from laminar flow to turbulent flow, was observed when the speed was around 400–500 rpm. Results indicated the increase of motor speed can effectively improve the cooling performance. Results also showed when the speed was higher than 600 rpm, the reduction of temperatures was quite close, which proven the existence of optimal or minimum requirement of motor speed to achieve good quality cooling performance. The results obtained in this study could be used as a critical reference for numerical studies and important experimental reference to further investigate the design and optimisation of engine cooling gallery for vehicle application. [ABSTRACT FROM AUTHOR]

Highlights • Boiling flow phases are identified to obtain stable two-phase flow. • The flow rate decreases when the two-phase frictional resistance is large enough. • Existing models are roughly unsuitable for the natural circulation in rod bundle. • A new correlation for two-phase frictional pressure drop in rod bundle is fitted. Abstract Natural circulation boiling flow resistance characteristics in a 3 × 3 rod bundle channel are experimentally studied in this paper. In order to achieve a stable two-phase natural circulation flow, the boiling flow phases are identified and measures such as decreasing the inlet fluid subcooling degree and increasing the inlet local resistance are adopted. The ONB during the experiment is determined by the wall temperature drop. The variation of pressure drop components, including the gravity pressure drop, the frictional pressure drop and the acceleration pressure drop, is given. For high mass quality conditions, the mass flow rate is observed declining with the increasing heat flux when the frictional pressure drop and the acceleration pressure drop grow faster than the gravity pressure drop reduction. A few homogeneous flow models and separated flow models for the two-phase frictional pressure drop are compared against experimental results. The homogeneous flow models can well predict the frictional pressure drops for low mass quality conditions and the Duckler model predictions are the most accurate with a mean relative error of 15.7%. In terms of the separated flow models, the Mishima and Sun models predictions are in good agreement with experimental results respectively for low and high mass quality conditions. Based on the Chisholm correlation, a new correlation is fitted by experimental results. The friction factors obtained in single-phase experiments are used to calculate the liquid phase frictional pressure drops. The comparison against experimental results and data from other literatures shows the new correlation can well predict the two-phase frictional pressure drops under given flow conditions. [ABSTRACT FROM AUTHOR]

Abstract: Tilting influences the flow patterns and thus the heat transfer and pressure drop during condensation in smooth tubes. However, few studies are available on diabatic two-phase flows in inclined tubes. The purpose of the present paper is to review two-phase flow in inclined tubes, with specific reference to condensation. Firstly, the paper reviews convective condensation in horizontal tubes. Secondly, an overview is given of two-phase flow in inclined tubes. Thirdly, a review is conducted on condensation in inclined tubes. It is shown for convective condensation in inclined tubes that the inclination angle influences the heat transfer coefficient. The heat transfer coefficient can be increased or decreased depending on the experimental conditions, and especially the flow pattern. Under certain conditions, an inclination angle may exist, which leads to an optimum heat transfer coefficient. Furthermore, this paper highlights the lack of experimental studies for the prediction of the inclination angle effect on the flow pattern, the heat transfer coefficient and the pressure drop in two-phase flows during phase change. [Copyright &y& Elsevier]

Abstract: This paper deals with heat transfer analysis for boiling flow in staggered tube bundle. A local analysis is performed to determine the heat transfer coefficient linked to local flow regimes by optical fibre. The first part of the paper is devoted to the literature survey of the main existing studies on the topic. We show that published heat transfer correlations deviate largely from each others and also from the experimental results that have been carried out. On these features, a new approach has been developed. It is based on the relationship between flow regimes and thermal characteristics. An experimental setup has been developed for the determination of the local heat transfer and the two-phase flow void fraction. A detailed analysis of the two-phase flow has been performed in a previous paper in which two regimes were identified. In the present paper, focus is done on the heat transfer analysis in relation with the flow regime map. This new approach allows a better prediction of the heat transfer coefficient. For the bubbly flow, the heat transfer coefficient is well predicted by a classical correlation corresponding to nucleate boiling regime. For the dispersed flow, classical correlations for convective boiling are not adapted anymore for tube bundle. We evidenced that heat coefficient is mainly controlled by the vapour flow and a heat transfer law is derived using the vapour Reynolds number and vapour Prandlt number. These two heat transfer laws are used to evaluate heat transfer coefficient in the intermediate regime. [Copyright &y& Elsevier]

Abstract: This paper explores the through-/in-plane characteristics of water transport in the cathode gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC). Theoretical analysis is performed on the non-isothermal two-phase flow under flow channels. A dimensionless group Da (Damkohler number for PEFC operation), defined as the ratio of water generation rate to water vapor-phase removal rate, is formulated to characterize the flow regimes in a PEFC. This group, lumping geometrical parameters and physical properties, compares the water vapor-phase removal capability (via water diffusion and holding capacity) with the rate of water production by the oxygen reduction reaction. We find that this dimensionless group can be used to characterize the non-isothermal, two-phase phenomena: when Da→0, the fuel cell is subjected to single-phase operation; while as Da→∞ we have full two-phase operation. A more precise expression is explored for the dimensionless group at the channel central line, i.e. Da 0: when Da 0>1 the entire cathode GDL–CL (catalyst layer) interface is in two-phase region, whereas part of the interface is free of liquid water for Da 0<1. The latter scenario is the concept that this paper proposes for improving fuel cell water management: the consequent co-occurrence of single- and two-phase flows in the in-plane direction at Da 0<1 is beneficial to avoid severe dryout and flooding. A two-phase transport model, describing the water and heat transport on the PEFC cathode side, is employed to perform a two-dimensional numerical study. Detailed liquid and temperature distributions are displayed. Simulation predictions are in reasonably good agreement with the dimensionless-group analysis. [Copyright &y& Elsevier]

Abstract: Cryogenic loop heat pipes (CLHPs) are efficient heat transfer devices based on two-phase flow. Loop heat pipes for room temperature applications have achieved satisfactory thermal control functions with the benefits of no mechanical moving part, vibration isolation, thermal insulation, long heat transport distance and so on. While there exist many problems for low temperature applications of loop heat pipes, such as limited heat transport capacity, which could not meet the increasing requirement of instrument heat dissipation. This paper presents an advanced CLHP operating at liquid-nitrogen temperature range. An improved condenser structure is introduced to the CLHP, which greatly reduces the flow resistance and increases the cooling capability of the condenser. Many experiments have been carried out on the CLHP prototype for performance test, and one set of the experimental results with a 3.2MPa fill pressure at room temperature is presented in this paper. It is shown that the advanced CLHP prototype can be operated reliably with a high heat transfer capacity up to 41W and a limited temperature difference of 6K across a 0.48m transport distance. [Copyright &y& Elsevier]

Highlights • Two-phase flow thermal–hydraulic non-equilibrium is investigated in detail. • An advanced image processing technique is used for liquid droplet study. • A complete data reduction methodology is formulated and employed. • It is found that significant thermal–hydraulic non-equilibrium exists between the liquid and vapor phases. Abstract The current research aims at investigating the thermal–hydraulic non-equilibrium encountered in the dispersed flow film boiling (DFFB) regime under reflood conditions. A series of constant reflood tests have been performed at the 7 × 7 Rod Bundle Heat Transfer (RBHT) test facility and the experimental results have been analyzed in detail. The data not only is useful for the development of theoretical models for two-phase interfacial mass and heat transfer, void fraction, pressure drop, and rod bundle cooling prediction, but also can be used as a benchmark data base for evaluating the performance of various thermal–hydraulic analysis codes. In this paper, a complete data reduction methodology is employed to reveal and interpret important two-phase flow phenomena during reflood transients. The results show that for the region far away from the quench front location, significant thermal non-equilibrium may exist during reflood and thus poor heat removal capability of the flow mixture is expected. On the other hand, for the region immediately downstream of the quench front, the two-phase flow is found to be close to the thermal equilibrium state, indicating that both the interfacial heat transfer and the heat transfer between the rod surface and fluid mixture are very efficient. In addition, based on the liquid droplet measurement and force balance calculations, it is found that the slip ratio between the two separate phases is a strong function of the droplet size and reflood time. With a larger droplet size and a shorter distance from the quench location, the slip ratio is larger. The liquid carryover fraction at the exit of the test section is also determined as a function of the reflood time. Results indicate that only a small portion of the injected coolant mass is stored within the bundle. A significant amount of liquid mass into the test section is either evaporated or entrained in the vapor flow and eventually left the bundle. [ABSTRACT FROM AUTHOR]

Highlights • Flow condensation of refrigerants HFO-1234yf and HFC-134a in a circular tube. • Heat transfer performance depends on the fluid properties and flow patterns. • Major controlling properties strongly depend on the two-phase flow pattern. • Inconsistency between different refrigerants can be explained by Flow pattern analysis. Abstract HFO-1234yf has similar thermodynamic properties to HFC-134a but much lower GWP value. It is expected as a good candidate to replace the refrigerant HFC-134a in the near future. However, only very few papers have been published in the past years regarding to the flow condensation heat transfer performance of this new refrigerant. This study provides an experimental analysis of flow condensation heat transfer and pressure drops of refrigerants HFO-1234yf and HFC-134a in a small circular tube. The test results show that both pressure drop and condensation heat transfer performance depend on the fluid properties, flow conditions and flow patterns. The major controlling properties on pressure drops and heat transfer coefficients is strongly depending on their two-phase flow pattern at various flow conditions. At the lowest mass velocity, gravity is the major force that dominates the heat transfer mechanism and the flow pattern is slug. Higher liquid viscosity retarded the condensate flow but higher liquid conductivity provided better heat transfer through the liquid film. Both liquid viscosity and conductivity are the important controlling properties. While mass velocity and vapor quality increased, the effect of shear force increased and the flow pattern transferred to annular. Liquid thermal conductivity became the major controlling property at high vapor qualities. But at low vapor qualities, gravity is still important and therefore liquid viscosity is one the major controlling parameter. At the highest mass velocity conditions, gravity effect is negligible even though at very low vapor quality condition. Shear force dominated the condensation heat transfer mechanism and liquid conductivity is the most important controlling property. It can be concluded that the flow condensation heat transfer performance is strongly depending on the two-phase flow patterns at various flow conditions. [ABSTRACT FROM AUTHOR]

Highlights • Heat transfer at boiling of water and emulsions in a horizontal minichannel has been investigated. • Flow conditions of two-phase flows have been visualized with the use of high-speed video filming. • The boundaries of flow conditions of two-phase flows at boiling of water and emulsions. • Fluctuations of the liquid temperature and the excess pressure in the system have been examined. Abstract The paper presents the results of an experimental investigation of heat transfer at flow boiling of water and n-pentane/water and freon-11/water emulsions in a horizontal minichannel with a hydraulic diameter of 1.1 mm. The flow conditions of two-phase flows have been visualized at the exit from an evaporator with the use of high-speed video filming. At bubble and slug flow conditions of a two-phase flow the mean coefficient of heat transfer from the channel wall to emulsions has proved to be higher than to water. The results of a synchronous measurement of variations in the liquid temperature at the channel inlet and outlet and the excess pressure in the system at the instability of a two-phase flow have been examined. [ABSTRACT FROM AUTHOR]

This paper presents a direct numerical simulation (DNS) technique, the Finite Element Fictitious Boundary Method (FEM-FBM) for the simulation of fluid-solid two-phase flows with heat transfer. The heat transfer equation is introduced to study thermal convection in fluid-solid two-phase flows. The Boussinesq approximation is considered for the coupling of momentum and temperature flow fields. Multi-grid finite element solver is used to compute flow equations of mass, momentum, and energy on a fixed Eulerian mesh which is independent of time and the solid particles are allowed to move freely in the whole computational domain. Fictitious boundary method (FBM) is used to treat the particles inside the fluid, FBM takes account of the thermal and momentum interaction between the fluid and the particles. The accuracy and stability of presented method are validated by comparing our test cases results with the results reported in the available literature. Numerical tests are performed to show that this method is potentially powerful and provides an efficient approach to simulate complex thermal convective particulate flows with a large number of particles. [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]

This paper presents experimental results obtained during tests on the evaporation heat transfer inside tube of a coaxial heat exchanger for residential and industrial geothermal heat pumps. The experimental device and instrumentation of the test section using evaporation inside a water heated double tube, have been described. The results obtained for evaporation local heat transfer of refrigerants R134a and R407C have been compared to the existing correlations. The aim of this comparison is to evaluate the validation of the well-known correlations to predict evaporation heat transfer in the geothermal operating conditions. A complete experimental analysis of the local evaporation of the R134a and R407C is carried out as a function of vapor quality inside a smooth horizontal tube. Various refrigerant mass fluxes are tested. Moreover, new correlations for local heat transfer and pressure drop predictions have been developed and validated basing on the measurements. [ABSTRACT FROM AUTHOR]

In annular two-phase gas-liquid flow, the liquid film on the wall consists of relatively quiescent substrate regions which are traversed by large amplitude, high velocity waves known as disturbance waves. The turbulent disturbance wave regions have relatively high average heat transfer coefficients (low average wall temperatures) compared to the (probably laminar) substrate regions. Nevertheless, there is evidence that nucleate boiling (necessitating a higher wall temperature) occurs first in the wave regions. This paper explores the hypothesis that wall temperature fluctuations due to turbulence in the disturbance waves are of sufficient magnitude to give localized triggering of nucleation sites and hence nucleate boiling. This hypothesis was explored using Computational Fluid Dynamics (CFD). The turbulence was modelled using wall-resolved LES. The results lend weight to the hypothesis that the nucleate boiling observed in disturbance waves is due to transient local high temperatures induced by the turbulence. [ABSTRACT FROM AUTHOR]

In the present paper, convective boiling heat transfer results of R134a for circular, square and triangular tubes are presented. The evaluated channels present the same external perimeter and equivalent diameters of 1.100, 0.977 and 0.835 mm, respectively. Experiments were performed for mass velocities ranging from 200 to 800 kg/m 2 s, heat fluxes from 15 to 85 kW/m 2 , saturation temperatures of 31 and 41 °C, and vapor qualities from 0.05 to 0.95. In order to perform reasonable comparisons among the test sections, the tests were run under similar mass velocities for the three geometries. The experimental data were carefully analyzed and discussed focusing on the effect of the cross-sectional geometry. It was found that for low heat fluxes, the heat transfer coefficient for the circular channel is higher. While for high heat fluxes, the heat transfer coefficient for the triangular channel is higher than for circular and square channels. Subsequently, the experimental data were compared with predictive methods from literature which are usually developed based only on data for single circular channels. Statistically, Kanizawa et al. [24] and Kim and Mudawar [34] provided reasonable predictions of the overall database. However, none of the methods captured adequately the experimental trends for the triangular channel. [ABSTRACT FROM AUTHOR]

In an attempt to analyze the effect of red blood cells on heat transfer, a two-phase flow heat transfer model of flowing blood was proposed in this paper. In this model, blood is treated as a two-phase fluid composed of red blood cells as the particle phase and plasma as the carrier fluid. The micro-particle dynamics, including aggregation, deformation, mutual collisions of red blood cells and relative movement of red blood cells to plasma, are considered by some equations or simplified methods. The velocity field, temperature field, fluid viscosity and volume fraction of red blood cells in a two-phase blood flow can be calculated using this model. To verify the accuracy of the simulation results of the model, the velocity distribution and heat transfer efficiency calculated by this model were compared with available experimental data from the literature. Good agreement was obtained between these data; thus, this model can accurately simulate the blood flow and heat transfer in a vessel. Then, the model was used to analyze how the micro-particle red blood cells affect the heat transfer of blood, and red blood cells were found to enhance heat transfer efficiency of blood. The viscosity properties of blood, the solid pressure and the lateral movement of red blood cells are the important determinants of this enhancement. The influence of red blood cells on the heat transfer efficiency of pulsatile blood flow was also calculated using this model. [ABSTRACT FROM AUTHOR]

In this paper, numerical results for conjugate natural convection flow and heat transfer in a heat exchanger divided by a partition with finite thickness and thermal conductivity are presented using Buongiorno's two phase model. A series of numerical simulation is carried out using the finite volume method over a wide range of the Rayleigh number (10 4 ≤ Ra ≤ 10 7 ), volume fraction (0 ≤ φ ≤ 0.05), diameter (25 nm ≤ d p ≤ 145 nm) and type of the nanoparticles (Cu, Al 2 O 3 and TiO 2 ). In addition, the effects of three types of influential factors such as: thermal conductivity ratio (0.1 ≤ K r ≤ 25), orientation of conductive partition and segmentation of the conductive obstacle on fluid flow and heat transfer characteristics are investigated. Results show that at low Ra , by dividing the conductive obstacle into the nine small segments, the heat transfer rate and absolute values of stream function decrease significantly. It is also observed that by increasing the Ra and thermal conductivity ratio ( K r ) the heat transfer rate increase. Moreover, it is found that by changing the orientation of the conductive partition from vertical to horizontal mode, the heat transfer rate alters significantly. Finally, the results demonstrate that the effect of the thermophoresis force for solid particles with high thermal conductivity (like Cu) is negligible. [ABSTRACT FROM AUTHOR]

A horizontal two-phase loop thermosiphon (HLTS) which incorporates a U-turn section to prevent bidirectional flow was investigated in this study via flow visualization and three-dimensional computational fluid dynamics (CFD). The experimental device consisted of a glass-water HLTS which was tested under a range of filling ratios and operating conditions to visualize the two-phase flow in the loop. At the same time, a three-dimensional, volume of fluid (VOF) CFD model was developed to further elucidate the underlying heat and mass transfer regimes for this HLTS. It was found that by increases the filling ratio, the flow regime changes from smooth unidirectional flow to geyser oscillation flow. At a filling ratio of <70%, nucleate boiling and drop-wise condensation occur in the presence of clear unidirectional flow. At >83% filling ratios, geyser oscillation occurs, in which ascending/descending flow of the working fluid was repeatedly observed. During ascending flow, a plug flow pattern was observed in the riser and a bubble flow pattern was observed in the downcomer section. During descending flow, an annular flow pattern was observed in the riser, while plug-to-annular flow was observed in the downcomer section. The simulation results revealed these same two-phase flow regimes and predicted and measured temperatures and thermal resistances deviated by less than 9%. Overall, this valid CFD model can easily be extended to evaluate the heat and mass transfer mechanisms and performance of any geometrically similar HLTS systems across the range of filling ratios and flow regimes studied in this paper. [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]