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• 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]

• 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]

• 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]

• 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]

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]

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]

• 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]

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 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]

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]

This paper presents an experimental and theoretical investigation on two-phase flow boiling heat transfer with carbon dioxide (CO 2 or R744) as the refrigerant in horizontal small diameter tube at low temperature. The experimental data were obtained in the following condition, the heat flux is 7.5–30 kW/m 2 ; the mass flow rate is 300–600 kg/m 2 s; the saturation temperature is 233–273 K; the test section is made of stainless steel tubes whose inner diameters are 0.6 and 1.5 mm. The experimental results show that heat transfer coefficient increases with the increase of mass flow rate, but decreases with the increase of tubes’ inner diameter and saturation temperature, the boiling heat transfer of CO 2 has a greater effect on nucleate boiling and highly depends on heat flux. The comparative study indicates that: during the existed flow boiling heat transfer model, the model which has the best prediction accuracy presented with 79.8% of predicted points within ±30% and 21.8% mean deviation before the dryout phenomenon happened, while it was only 18.4% and 59.9% after the dryout phenomenon happened. [ABSTRACT FROM AUTHOR]

The present paper presents an investigation of the effects of the refrigerant type on the heat transfer coefficient during flow boiling inside micro-scale channels. Experimental results for R134a, R1234ze(E), R1234yf and R600a for flow boiling in a circular channel with internal diameter of 1.1 mm are presented. The experimental database comprises 3409 data points covering mass velocities ranging from 200 to 800 kg/m 2 s, heat fluxes from 15 to 145 kW/m 2 , saturation temperatures of 31 and 41 °C, and vapor qualities from 0.05 to 0.95. The experimental data were parametrically analysed and the effects of the experimental parameters (heat flux, mass velocity, saturation temperature and working fluid) identified. Subsequently, the experimental data were compared against the most quoted predictive methods from literature, including macro and micro-scale methods. Based on the broad database obtained in the present study, an updated version of the predictive method of Kanizawa et al. (2016) was proposed. The updated version provided accurate predictions of the present experimental database, predicting more than 97% and 86% of the results within error bands of ±30 and ±20%, respectively. [ABSTRACT FROM AUTHOR]

The purpose of this paper is to establish the detailed parametric study for compressible natural convection flow of two-phase dusty fluid along a vertical wavy cone. Typical sinusoidal surface is used to elucidate the heat transport phenomena for the compressible dusty fluid. The governing equations are transformed into parabolic coordinates and the singularity of the leading edge is removed. The resulting equations are then solved straightforwardly by implicit finite difference method. The influence of compressibility, dusty fluid and the transverse curvature of the surface are discussed in detail in terms of local heat transfer rate, skin friction coefficient, velocity and temperature distributions. In order to ensure the accuracy, comparison with some special cases are also presented and the agreement is found to be very good. The key observation from the present study is that the half-cone angle, ϕ , and mass concentration parameter, D ρ , extensively promote the rate of heat transfer into the compressible boundary layer region. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study of heat transfer enhancement with liquid-gas two-phase segmented microchannel flows by utilizing a novel technique of molecule-based temperature sensors. A serpentine microchannel was constructed and steady liquid and gas segments of ethanol and air were successfully produced at various flow rates. A molecule-based temperature sensor made of Tris(2,2′-bipyridyl) ruthenium was utilized in the liquid segments by dissolving in ethanol as temperature-sensitive fluid and mixing in a dope to produce temperature-sensitive paint in order to retrieve fluid and surface temperature data, respectively. Three cases of different liquid and gas flow rates were tested. The heat transfer enhancements of these three cases were carefully examined under constant heat flux thermal boundary conditions, and the increased heat transfer was evaluated in comparison with the single-phase flow, where only liquid was injected in the microchannel. An improvement of up to 30% in the averaged Nu number of the microchannel flow was determined for Case 3, with a high gas flow rate and short liquid segments. Velocity and vorticity profiles in the liquid segments at different gas and liquid flow rates were also studied using a micro-particle image velocimetry technique. Stronger circulation was observed in Case 3, which confirmed the results from the heat transfer analysis, namely that more efficient heat transfer occurs with strong circulation in short liquid segments when a high gas flow rate is applied. The results of the present study provide detailed temperature profiles in segmented flow as well as the evolution of the Nu number from the entrance to the exit of a serpentine microchannel. [ABSTRACT FROM AUTHOR]

Lack of understanding of flow boiling behavior in reduced gravity poses a major challenge to the development of future space vehicles utilizing two-phase thermal control systems (TCSs). A cost effective method to investigating the influence of reduced gravity on flow boiling is to perform ground experiments at different orientations relative to Earth gravity. This paper is the first part of a two-part study aimed at exploring flow boiling mechanisms of FC-72 in a rectangular channel heated along one wall or two opposite walls. Experiments are performed in vertical upflow, vertical downflow and horizontal flow, subject to large variations in mass velocity, inlet quality and wall heat flux. Detailed measurements are used to investigate the influences of orientation, and therefore gravity, on boiling curve, local and average heat transfer coefficients, and pressure drop, and their relationship with interfacial behavior is captured with high-speed video. For horizontal flow, the effects of gravity are reflected in appreciable stratification across the channel at low mass velocities, with gravity aiding vapor removal from, and liquid return to the bottom heated wall, while accumulating vapor along the top heated wall. For vertical upflow and vertical downflow, with both single-sided and double-sided heating, there is far better symmetry in vapor formation along the channel. The heat transfer coefficient shows significant variations among the different orientations and heating configurations at low mass velocities, but becomes insensitive to orientation above 800 kg/m 2 s, proving inertia around this mass velocity is effective at negating any gravity effects. For low mass velocities, pressure drops are fairly equal for vertical upflow and vertical downflow, but greater than for horizontal flows. However, fairly equal pressure drops are achieved at high mass velocities for all orientations. Overall, this study proves that gravity effects on two-phase pressure drop and two-phase heat transfer are dictated mostly by mass velocity and, to a lesser extent, by inlet quality. [ABSTRACT FROM AUTHOR]

In this paper, an experimental apparatus was set up to study the two-phase mixture flow and heat transfer over a horizontal tube bundle on the shell-side of the submerged combustion vaporizer (SCV). The influences of the static water height, superficial flue gas velocity and heat load on the heat transfer characteristics were systematically examined. The experimental results revealed some interesting observation of the SCV, such as the circulation flow of water bath accompanying a great deal of gas bubbles and the highly thermal efficiency due to the direct-contact heat transfer between the highly turbulent two-phase mixture and staggered tube bundles. Finally, based on the experimental data, a new dimensionless, semi-theoretical empirical correlation was developed for two-phase mixture convection heat transfer over the staggered tube bundle on the shell-side of SCV. The discussion and correlation provided a framework for designing the SCV. [ABSTRACT FROM AUTHOR]

The design of systems incorporating multiphase flows remains a significant challenge for applications including lab-on-chip and microreactors. Segmented two-phase flow is used for the purpose of distributing biological samples from one site to many, enhancing heat and mass transport over laminar single phase flow and rapid chemical synthesis. This paper examines hierarchical designs, regularly utilised in such applications, that transport a segmented gas–liquid flow over an area with maximal thermodynamic performance. Fundamental design rules, predicted using the constructal method, minimize flow resistance of elemental bifurcations. The predicted geometric configuration follows an alternative to the established Murray’s law (or Hess–Murray law) when the global pressure difference is dominated by short liquid slugs and dispersed gas phase. Breakup of phases in the junction region was examined numerically with a volume-of-fluid approach to develop a general criteria where junction losses are non-negligible. Although this loss is periodic, the interfacial pressure during the necking stage of the dispersed phase dominates. Using the findings for an elemental bifurcation, multiple scale hierarchical configurations for combined fluid and heat transport were assessed. The multiple scale tree-shaped design can be advantageous for low thermal resistance and pumping power requirements compared to a single scale serpentine layout. The principle-based method and results can reduce the design space in high-fidelity investigations of microscale segmented flows. [ABSTRACT FROM AUTHOR]

This paper experimentally and numerically studies the solid–liquid heat transfer mechanism of octadecane which is a phase change material (PCM) with the melting temperature of 28 °C in thermal management systems. This study finds that the convective heat transfer driven by the buoyancy force could be dominant during the melting of pure PCM: the time to melt 30 mm of PCM decreases from 453 min to 13 min when the heating direction changes from downwards to upwards. The use of porous materials such as metal foams (made from Cu, Al or C) with various porosities and pore sizes in the thermal management system significantly increases the overall heat transfer rate due to the increase of effective conductivity. The amount of heat absorption within 60 min significantly increases from 105.6 kJ to 248.9 kJ after using the Al foam with 0.97 porosity. Further decreasing the porosity to 0.925 and 0.88, however, will decrease the amount of absorbed heat to 230.7 kJ and 179.7 kJ because of the decrease of natural convection and the decrease of overall heat capacity. Considering the increase of weight and cost of the thermal management system by using porous materials, the Al foam (compared with C and Cu foams) with the porosity of 0.97 (compared with 0.88 and 0.925) and the pore size of 5.08 mm (compared with 0.64 mm, 1.27 mm, 2.54 mm) is preferred within the porous materials investigated by this study. [ABSTRACT FROM AUTHOR]

In this paper, the thermo-hydrodynamic behaviors in miniaturized two-phase thermosyphons are experimentally investigated by a visualization system. The effects of heat load and channel dimension on the thermo-hydrodynamic behaviors are examined and further quantitatively recognized by a state diagram depending on the Weber number and Bond number. In addition, the vapor–liquid two-phase flow, heat transfer regimes and thermal performances are analyzed and discussed. The results indicate that, differing from the conventional two-phase thermosyphon, the state of working fluid in a miniaturized two-phase thermosyphon is no longer only in a fashion of pool and annular-flow states, but also in a fashion of oscillation state. The oscillation state is the characteristic two-phase flow regime of a miniaturized two-phase thermosyphon and possesses the feature of random formation and oscillatory motion of liquid plug in minichannels. The pool state occurs at small Weber number, the annular-flow state is observed at large Weber number, and the oscillation state takes place at medium Weber number when Bond number is approximately smaller than 0.7. The thermal performance of oscillation state is better than that of pool state but is less powerful than that of annular-flow state. The heat transfer regimes are the film evaporation and condensation combined with thermal conduction in the pool state, the forced evaporation and condensation in the oscillation state, and the film evaporation and condensation in the annular-flow state, respectively. [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]

The present paper presents an experimental study of flow patterns and heat transfer characteristics of flow boiling of R134a in a multi-microchannel heat sink. The copper test section has 27 parallel rectangular channels with a depth of 470 μm, a width of 382 μm, a length of 40 mm, and a fin thickness of 416 μm. The experimental results are presented for saturation temperatures of 13, 18, and 23 °C, and mass fluxes of 150, 400, and 600 kg/m 2 s. The wall heat flux and inlet vapor quality values were between 3 and 127 kW/m 2 , and 0.05 and 0.92, respectively. The effects of pertinent parameters on the heat and fluid flow characteristics such as saturation temperature, mass flux, heat flux, and inlet vapor quality are studied and discussed. The heat transfer coefficient at high saturation temperatures (i.e. 23 °C) is higher than low saturation temperatures (i.e. 13 °C) in the heat flux range of 40–120 kW/m 2 . For high heat flux ranges, the heat transfer coefficient increases with increasing mass flux. The convective boiling heat transfer mechanism will play a major role in wavy and annular flow patterns. For wall heat fluxes higher than 80 kW/m 2 , the existence of a partial dry-out phenomenon in the multi-microchannel leads to a decrease in heat transfer coefficient. The results unveil the significant effect of flow patterns on heat transfer characteristics. Based on the experimental data, a correlation is proposed to calculate the heat transfer coefficient for R134a flow in the multi-microchannel heat sink that is useful in electronic cooling applications. [ABSTRACT FROM AUTHOR]

A two-phase lattice Boltzmann model considering the interaction forces of nanofluid has been developed in this paper. It is applied to investigate the flow and natural convection heat transfer of Cu–Ga nanofluid. The effects of different nanoparticle radiuses, interaction forces, aspect ratios of the enclosure, nanoparticle volume fractions, and Rayleigh numbers on the nanoparticle distributions and heat transfer characteristics are investigated. It is found that the nanofluid with small nanoparticle radius significantly improves the heat transfer compared with the nanofluid with big nanoparticle radius. The temperature difference driving force has the biggest effect on the nanoparticle distribution, followed by the Brownian force and drag force, and the other forces have the least effect. The average Nusselt number increases with the increasing aspect ratio of the enclosure, nanoparticle volume fraction and Rayleigh number. [ABSTRACT FROM AUTHOR]

This paper presents experimental results for the heat transfer coefficient during flow boiling of refrigerants R134a, R245fa and R600a inside small diameter tubes plus a new heat transfer predictive method. The experimental database comprises 2047 data points covering tube internal diameter ranging from 0.38 to 2.6 mm, mass velocities from 49 to 2200 kg/m 2 s, and heat fluxes up to 185 kW/m 2 . The data are parametrically analyzed and the effects of the experimental parameters (mass velocity, tube diameter, heat flux, refrigerant type and saturation temperature) on the heat transfer coefficient and dryout vapor quality are identified. In general, the heat transfer coefficient increases with increasing mass velocity, heat flux and saturation temperature, and decreasing the tube diameter. Moreover, the dryout vapor quality decreases with increasing mass velocity and vapor specific volume. The method proposed in the present study predicted 97% of the experimental results within an error margin of ±30% and with a mean absolute error of 11%. The new method provided better predictions of its database than seven of the most quoted predictive methods available in the literature, and was also accurate to predict independent databases. [ABSTRACT FROM AUTHOR]

High-fidelity simulation of flow boiling in microchannels remains a challenging problem, but the increasing interest in applications of microscale two-phase transport highlights its importance. In this paper, a volume of fluid (VOF)-based flow boiling model is proposed with features that enable cost-effective simulation of two-phase flow and heat transfer in realistic geometries. The vapor and liquid phases are distinguished using a color function which represents the local volume fraction of the tracked phase. Mass conservation is satisfied by solving the transport equations for both phases with a finite-volume approach. In order to predict phase change at the liquid–vapor interface, evaporative heat and mass source terms are calculated using a novel, saturated-interface-volume phase change model. This phase change model is formulated to anchor the interfacial temperature at saturation within each iteration, and thereby acts as a robust constant-temperature boundary condition. Unlike other available phase-change models, the source terms are coupled with the local temperature explicitly; therefore, numerical oscillations around the interface temperature are not observed during iterations within a time step, which reduces the numerical cost. In addition, the reference frame is set to move with the vapor slug to artificially increase the local velocity magnitude in the thin liquid film region in the relative frame. This reduces the influence of numerical errors resulting from calculation of the surface tension force, and thus suppresses the development of spurious currents. As a result, non-uniform meshes may be used which can efficiently resolve high-aspect-ratio geometries and flow features. The overall numerical expense is significantly reduced. The proposed saturated-interface-volume model is first validated against a one-dimensional Stefan problem, and then used to simulate the growth of a vapor bubble flowing in a heated, 2D axisymmetric microchannel. The bubble motion, bubble growth rate, liquid film thickness, and local heat transfer coefficient along the wall are compared against previous numerical studies. A three-dimensional flow boiling problem is studied to demonstrate the cost effectiveness of the present approach and to highlight the transport mechanisms it can reveal in more complex domains. [ABSTRACT FROM AUTHOR]

A method based on the Eulerian two-phase flow theory and the extended heat transfer model for numerically simulating three-dimensional ice accretions on an aircraft wing is presented in this paper. The governing equations for supercooled droplets in three-dimensional applications are established by considering the droplet phase as pseudo-fluid and applying the conservation laws of mass and momentum for a fixed control volume. A permeable wall boundary condition is proposed to depict the physical phenomenon of droplet impingement more properly. The droplet collection efficiency distribution is readily obtained from the solution of the three-dimensional droplet flowfield. Ice accretions can then be simulated through performing the mass and energy balances for each icing control volume. Some concepts such as critical ice thickness and inner time step as well as an iterative solution procedure for runback water motion have been proposed to facilitate the simulation for three-dimensional case. For validation purpose, multistep simulation results for the droplet impingement and ice accretion under specified icing conditions are compared with the corresponding experimental data and some previously predicted results, showing some better agreement gained for the current method. Furthermore, the effects of some meteorological parameters on ice accretion have been investigated and analyzed individually. [ABSTRACT FROM AUTHOR]

Loosening local equilibrium assumptions in two-phase flow in porous media gives rise to new, unknown variables. More specifically, when loosening the local thermal equilibrium assumption, one has to describe the heat transfer between multiple phases, present at the same mathematical point. In this paper, we calibrate a macro-scale mathematical model which is free of local equilibrium assumptions to experimental observations. We emphasize the correct determination and upscaling of necessary input parameters from the experimental data achieved by image analysis. By choosing an appropriate scaling parameter, we are able to reproduce experimental measurements satisfactorily. This is a first step towards quantifying heat transfer in two-phase flow in porous media. Ultimately, our aim is to find the limits of the applicability of local equilibrium assumptions in two-phase flow in porous media. [ABSTRACT FROM AUTHOR]

An impressive amount of investigations has been devoted to enhancing thermal performance of microchannels. The small size of microchannels and their ability to dissipate heat makes them as one of the best choices for the electronic cooling systems. In this paper, a comprehensive review of available studies regarding single and two-phase microchannels is presented and analyzed. 219 articles are reviewed to identify the heat transfer mechanisms and pressure drops in microchannels. This review looks into the different methodologies and correlations used to predict the heat transfer and pressure drop characteristics of microchannels along the channel geometries and flow regimes. The review shows that earlier studies (from 1982 to 2002) were largely conducted using experimental approaches, and discrepancies between analytical and experimental results were large, while more recent studies (from 2003 to 2013) used numerical simulations, correlations for predicting pressure drop and heat transfer coefficients were considerably more accurate. [ABSTRACT FROM AUTHOR]

Laminar and turbulent forced convection heat transfer and flow characteristics of nanofluids in small smooth tubes are numerically simulated using two kinds of multiphase-flow models. The simulated results are compared with the experimental results from the published papers and the traditional predicting correlations to investigate the applicability of these models for nanofluids. The multiphase-flow models including mixture model and Eulerian model, and both of them belong to the well-known Euler–Euler model. The effects of various parameters such as Reynolds number and nanoparticles concentration on the heat transfer and flow characteristics are investigated and discussed in each model. The study results show that little deviation exists between the simulated results and the traditional predicting correlations for low concentration nanofluid, which indicates that low concentration nanofluid has no meaningful nano-effect on forced convection heat transfer. While, non-traditional fluid characteristics occur and increase with increasing the nanoparticles concentration, and the simulated results using special models of multiphase flow are closer to the experimental data than that of the traditional correlations, which means the multiphase flow models are more accurate than traditional correlations for high concentration nanofluid. Moreover, the numerical results also indicate that the drag coefficients of simulated results have only little difference less than 0.4% with that of experimental results for nanofluids in the laminar flow region. However, the drag coefficients of simulated results have a increase by about 60–22% compared with the experimental results in the turbulent flow region. As conclusion, the present study indicates that the two-phase models, including mixture model and Eulerian model, can predict the forced convection heat transfer and flow characteristics of nanofluid well, and have important implications for the application of nanofluid. [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]

Abstract: In this paper, the heat transfer performance of the alternating discrete two phase flow in micro-tubes has been numerically investigated. Two immiscible fluids are injected into a micro-tube alternately, generating a plug flow pattern with two completely separated phases. The overall Nusselt number could be increased by sixfold over the single phase fully developed flow when the ratio of primary phase to the tube inner diameter (L 1/D) is kept at about 7. Characteristic of the average Nusselt number is found to be almost independent of the secondary phase. The friction factor is much dependent on the viscosity of the secondary phase. Secondary phase with higher viscosity would induce higher pressure loss and large pressure fluctuation is observed at regions close to the interfaces. The flow field that is far from the interfaces could be approximated as the single phase fully developed flow. Weber number of the primary phase and Capillary number of the secondary phase should be kept below certain critical values in order to maintain the discrete flow patterns. Frictional loss is a more important consideration than the heat capacity when choosing a particular secondary phase. Secondary phase with lower viscosity is preferred due to the overall heat transfer enhancement will become less beneficial if the pressure loss induced by the secondary phase becomes significant. Liquid–gas discrete flow is found to have better heat transfer performance than the liquid–liquid discrete flow. [Copyright &y& Elsevier]

• Analyzing local heat transfer data of flow boiling CO 2 in tubes with porous inserts. • Identification of the main influencing factors: wetting and convection. • Modeling based on established sub-models without fitting parameters. • Proposed model describes more than 90% of the data within an error interval of ±30%. In this paper we want to assess the mechanisms by which metal porous inserts influence flow boiling heat transfer in horizontal tubes. For this purpose, measurement data of the local and circumferentially averaged heat transfer coefficient of CO 2 flow boiling in horizontal tubes with two different porous inserts are presented. As inserts, different sponges (open-cell network structures) and a wire matrix element are chosen and compared with each other. Different operating conditions were investigated. The mass flux was varied in a range from 25 kg m−2 s−1 to 190 kg m−2 s−1 and the vapor quality from 10% to 100%. Saturation pressures of 12 bar, 19 bar and 26.5 bar and average heat fluxes up to 65 kW m−2 were realized. By comparing the local heat transfer coefficient with empty tube data, wetting and convective heat transfer can be identified as the main influencing factors. Thus, we can obtain guidelines for the development of a model to describe the heat transfer coefficient of two-phase flow in porous inserts. A model derived in this way describes the measurement data of flow boiling of CO 2 in horizontal tubes with inserts with a high degree of accuracy. The model is based on established sub-models without any additional fitting parameter. More than 90% of all measured data are predicted with less than 30% deviation [ABSTRACT FROM AUTHOR]

• The typical stripper in the PCC is replaced by an innovative solar stripper (So-St). • The conceptual design for direct solvent regeneration in the solar field is analysed. • A design protocol to calculate So-St segments and modules layout is developed. • The length and diameter of subsequent segments to the overall design is optimized. • The process control for the pressure drop and enhanced heat transfer is maintained. This paper presents an advanced thermodynamic analysis to design a solar stripper (So-St) field to replace the conventional desorption unit in a post-combustion carbon capture (PCC) plant. By eliminating the large, complex, and costly stripper, a solar collector field (SCF) consisting of a number of So-St arrays is sized to directly release the CO 2 gas from the rich solvent under transient conditions. In this study, a design protocol has been developed to address the key design challenges for the evolving two-phase flow in the solar receiver tube analysing the instantaneous flow regime, pressure drop, and vapour molar fraction (VMF) to enhance the heat transfer. This design protocol optimizes the physical properties of the So-St components and eventually leads to the whole So-St field sizing based on the nominal capture capacity. A case-study is demonstrated to capture 1.5M tonne CO2 /y from a representative 660 MW e coal-fired power plant in Sydney/Australia. It is found that 22 rich solvent loops that each consists of an array of 6 parallel So-St modules, where each module consists of 6 segments in series is needed. The length/diameter of these So-St segments/modules are optimized for this case-study, and the design procedures are clarified and simplified which can be easily applied for other case-studies. Techno-economic validation of this novel So-St technology is an essential step-forward to fully solarize and autonomies carbon capture industry. [ABSTRACT FROM AUTHOR]

• Both flow boiling and condensation in mini channels with segregated fins were investigated; • Machine learning based performance prediction models were developed; • The influential factors of different dimensionless parameters were analyzed. Two phase flow boiling and condensation heat transfer in mini channels are promising technologies in handling the challenges of modern thermal management in various fields. In this paper, experimental systems were developed to generate data for flow boiling and condensation of refrigerant R134a in two mini channels with serrated fins under various working conditions. Based on these data, machine learning based Artificial Neural Network (ANN) models were trained to predict the heat transfer performance for both flow boiling and condensation. The proposed ANN models predict the heat transfer coefficient satisfactorily with a Mean Absolute Relative Deviation (MARD) of 11.41% and 6.06% for boiling and condensation respectively compared with experimental data. The test results also indicate that using the most important parameters rather than all available parameters is able to formulate a reasonable ANN model. It also reveals that the developed ANN model can be transferred in a different dataset from the same domains but shows an unacceptable performance for different domains. The proposed ANN model can illuminate future study on two phase flow and be used in practical heat exchanger design, which will benefit the advanced thermal management system. [ABSTRACT FROM AUTHOR]

• 3D numerical simulation of flow boiling in diverging microchannel heat sink. • The effect of varying divergence angle, width, and wall contact angle has been studied. • Volume of fluid (VOF) along with phase change model is used for the numerical study. • Varying nucleation time and bubble patterns was observed for different channels. • Flow reversal was observed in the channel with divergence angle 0.38°. This paper presents a 3D conjugate numerical simulation of the diverging microchannel heat sink, which is performed using the Volume of Fluid (VOF) model coupled with the phase change model. The modeling approach and simulation results are validated with standard correlations and published experimental results. The model is used to analyze the flow boiling and transient heat transfer mechanism through simulation of bubble nucleation, bubble growth pattern, pressure, temperature, and vapor quality variations. Moreover, local and transient Heat Transfer Coefficient (HTC) is calculated and then correlated with the simulated bubble dynamics. Herein, simulations are performed on the expanding microchannel with a varying outlet to inlet width ratio for uniform heat flux 150 kW/m2 at two different mass fluxes 240 kg/m2s and 710 kg/m2s. Further, to understand the effect of wall contact angle, simulations are performed for two different values of wall contact angle of 140° and 65°. The results indicate that the channels with low divergence angle (θ) and low outlet to inlet width ratio (δ) have low nucleation time. Rapid bubble growth and elongated flow due to channel confinement were also observed. Higher values of the heat transfer coefficient are observed in the microchannels with small hydraulic diameter, and as the diameter is increased, a reduction in the HTC values is observed. The transient heat transfer coefficient fluctuation in the channel with time has been studied along with the corresponding flow pattern. Due to the smaller wall contact angle, the bubble separation rate from the wall is low, thereby creating localized hotspots. A comparative study between straight channel and diverging channel has showed that diverging microchannels have a better performance in terms of instability mitigated flow boiling. This study has considerable relevance in electronic chip cooling using expanding microchannel with instability mitigated two-phase flow. [ABSTRACT FROM AUTHOR]

• A numerical investigation to study the effect of injecting mist into the cooling airstream to cool down gas turbine vanes. • The discrete phase model (DPM) is used to simulate and track the evaporation and movement of the tiny water droplets. • A vane wall temperature reduction of 360 K–450 K is achieved when a 20% mist/air ratio with 20µm droplets size is used. This paper describes a numerical investigation to study the effect of injecting mist (tiny water droplets, micrometers in size) into the cooling airstream to cool down gas turbine vanes. In this study, the conjugate heat transfer method is employed which consists of the simulation of the air/mist fluid flow inside and outside the vanes as well as the heat conduction through the vane body. The complete 3-D vane with internal cooling passages and external film cooling holes on the surface is simulated in a rotational periodic sector. The discrete phase model (DPM) is used to simulate and track the evaporation and movement of the tiny water droplets. The effects of different parameters such as the mist/air ratio (10–20%) and the mist droplets size (20–50 µm) on mist cooling enhancement are investigated. The results show that by using a mist/air ratio of 10%, 15%, and 20% with 20 μm droplets size, on the pressure side, a maximum wall temperature reduction of 250 K, 340 K, and 450 K respectively can be achieved. On the suction side, the corresponding maximum wall temperature reductions are 160 K, 260 K, and 360 K, respectively. Using larger droplets of 50 µm did not achieve better cooling enhancement because the droplets were rushed far away from the surface by the acceleration through the film cooling holes. Using the uniform droplet size distribution provides noticeably better cooling enhancement in the first 40% of the vane's height (from the shroud) than the non-uniform droplet size distribution (Rosin-Rammler Distribution) does. [ABSTRACT FROM AUTHOR]

• This paper studies heat transfer characteristics of solar salt and vapor-liquid two phase flow. • The heat transfer performance of solar salt is affected by water pressure, water and solar salt flow fluxes, molten salt inlet temperature. • The water mass flow rate, pressure, and molten salt inlet temperature has an important effect on the subcooled boiling starting point. • The fitting correlation is first obtained to describe the heat transfer performance of solar salt and vapor-liquid two phase flow. In this study, an experiment was conducted to obtain the heat flux and heat transfer coefficient of a mixed molten salt (60% NaNO 3 , 40% KNO 3) and water at subcritical pressure in a tube in tube heat exchanger. The control variable method was used in this study. The experimental parameters included the molten salt and water mass fluxes of 38.7–54.1 kg/m2s and 212.8–259.6 kg/m2s, respectively, water pressure of 15–19 MPa, water and molten salt inlet temperatures of 200–400 °C and 440–500 °C, respectively, and heat flux of 12–160 kW/m2. The heat transfer performances of the molten salt and water in the subcooled, subcooled boiling, saturated boiling, dryout and superheated regions were analysed with respect to the molten salt and water mass flow rate, inlet salt temperature, water pressure, and inlet water temperature. The fitting correlation of the thermal performance of molten salt to the subcooled boiling or vapor-liquid two-phase flow was presented; the fitting values demonstrated good consistency with the experimental data, with a maximum deviation of –6% and +8%. [ABSTRACT FROM AUTHOR]

• Heat transfer data for flow boiling under micro, hyper and normal gravity conditions. • Two-phase flow images under micro, hyper and normal gravity conditions. • Comparison of prediction methods and experimental data for flow boiling heat transfer. • Gravitational acceleration affects two-phase distribution, drift velocity and pressure drop. • Constant pressure gradient is not reasonable for the data regression analyses. The present paper concerns an experimental investigation of the gravitational effects on subcooled and saturated flow boiling heat transfer in a vertical 3.75 mm diameter aluminum tube, using perfluorohexane as working fluid. Experimental results obtained during parabolic flights are compared with data gathered in the same test facility for normal gravity. Results were obtained for mass velocities of 133, 252, 377 and 428 kg/m2s, heat fluxes up to 106 kW/m2 and vapor qualities up to 0.28. The assessments of prediction methods from literature for flow boiling heat transfer proposed for normal gravitational conditions were evaluated through comparisons of calculated and experimental results. In general, for either a mass velocity of 428 kg/m2s or a vapor quality lower than -0.2, the gravitational effects on the heat transfer coefficient were found as negligible. For lower mass velocities and saturated flow boiling, higher heat transfer coefficients were observed under hypergravity conditions. This behavior was associated to the fact that higher gravitational forces favor bubbles detachment. The method of Kanizawa et al. [1] provided reasonable predictions of the saturated flow boiling results under microgravity and hyper gravity conditions, predicting 100% of the data within an error band of ±30%. [ABSTRACT FROM AUTHOR]

During drilling or oil recovery, it is common that the formation trap gas invades the vertical wellbore/pipeline to cause the gas-liquid two-phase flow, which brings challenges and dangers to wellbore/pipeline pressure control. In this paper, coupling the interfacial mass transfer theory of slip-rising bubbles based on bubble hydraulics to the gas-liquid two-phase flow theory, a new transient non-isothermal gas-liquid two-phase flow model in the vertical wellbore/pipeline was developed. In this model, the effects of flow regime and heat transfer on the mass transfer rate were considered. The proposed model was validated using the measured experiment data and field data. Using this model, the effect of interfacial mass transfer of slip-rising bubbles on the evolution of gas-liquid two-phase flow in a special gas kick scenario was analyzed. The simulation results indicated that the mass transfer rate between oil dispersion and invasion gas was relatively slow, which made the gas not instantaneously dissolved in the oil dispersion or not instantly saturated the oil dispersion. Under the same gas invasion rate, the fraction and mass of free gas calculated by this model were always larger than those obtained by Yin's model, resulting in a faster of the bottomhole pressure reduction rate and pit gain increase rate. Additionally, increasing the gas concentration and flow rate could promote the interphase mass transfer rate, while increasing the temperature would inhibit it. This model could characterize the wellbore/pipeline gas-liquid two-phase flow with interphase mass transfer in more detail and accurately. [ABSTRACT FROM AUTHOR]

• The effect of the confinement level and the heat flux on the boiling flow pattern and the heat transfer performance of a flat confined two-phase thermosyphon is studied in this paper. • The combination between the confinement number Co , linked to the geometry of the device, and the Weber number We , linked to the vapor phase velocity, is shown to be relevant in explaining the heat transfer and flow pattern results. • Inclining the device ± 80° from the vertical position has low impact on the heat transfer performance for confinement numbers between 0.5 and 1. Incipience of the boiling process is not affected as well. • More confinement generates more vapor velocity that prevents the liquid phase from flowing down to the evaporator. A transparent flat confined two-phase thermosyphon is experimentally investigated in order to analyze the link between the boiling flow patterns and the thermal performance of the system with several parameters such as the confinement, the saturation temperature, the filling ratio or the inclination angle. The device is constituted of a copper plate on which the heat sink and heat source are located, and of a transparent polycarbonate plate that enables the visualization of the boiling patterns. Two values of the inner thickness of the flat two-phase thermosyphon, 1 mm and 3 mm, are tested. The small values of the thickness induce confined boiling in the evaporator section, which leads to an enhancement of the heat transfer inside the thermosyphon. Two fluids are tested, water and n-pentane. The confinement difference between the two fluids enables to assess the influence of confinement on the boiling regime and heat transfer capacity, particularly its dependence on inclination. Confining the boiling in only one direction allows sufficient rewetting of the heated surface even for high confinements. It appears that the thermal performance of the two-phase thermosyphon is not affected by the inclination angle in the range 10°–170° (90° being the vertical position) with a moderate confinement of the boiling process in only one direction. [ABSTRACT FROM AUTHOR]