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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]

• Air-water annular flow experiments are conducted in 3 × 3 simulating PWR fuel rod-bundle. • A conductivity-based technique is developed to measure local dynamical liquid film thickness. • The inception of disturbance wave is attributed to down-burst mechanism caused by highly turbulent gas flow. • Film thickness of central region is thicker than that of gap region for disturbance wave dominated film flow. • Cross-correlation analysis indicates film flow having a circumferential coherent wave structure. Local liquid film behavior of annular flow on rod bundle geometry is vital for accurate prediction of dryout. This paper is the first part of a two-part study concerning measurement and analysis of local liquid film behavior of annular flow on rod bundle geometry. In this paper, air-water annular flow experiments have been performed at different gas and liquid superficial velocity conditions in a 3 × 3 simulating PWR fuel rod-bundle test-section, and a conductivity-based technique is developed to measure the local dynamical liquid film thickness. Large amplitude disturbance waves appear at all low liquid phase superficial velocity conditions through down-burst mechanism caused by the highly turbulent gas flow and strong inter-subchannel interaction. Wave shapes do not change drastically by the variation of superficial liquid velocity since liquid flow tends to fill out the rod bundle corner due to the nature of annular flow. For disturbance wave dominating the film flow case, secondary flow in subchannel produces a circumferential force to shear off the liquid film, and the liquid droplets are redistributed, resulting in the liquid film thickness for the central region being thicker than that of gap region. Cross-correlation analysis indicates that gap and central regions are in possession of circumferential coherent wave structure with the same interfacial velocity, and the wave position for the central region is a little ahead of that for gap region since a higher wave has a longer leeward and upwind faces. The current study on local liquid film behaviors for rod bundle and the possible mechanisms that causing the discrepancies in local liquid film structures can be further used for the mechanism model development for the liquid film flow on the rod bundle. Part II of this study will introduce a mechanism model as an alternative approach to local liquid film thickness prediction for annular flow in rod bundle geometry. [ABSTRACT FROM AUTHOR]

• The channel geometry affects the velocity profile, average film thickness, wave height, base film thickness, and wave frequency, while it has little effect on wave velocity. • The system pressure affects the magnitude of average film thickness, base film thickness, and wave characteristics. • For boiling flow, the heating state should be considered as an influencing factor. • Wave velocities in the wispy annular flow regime and other annular flow regimes behave differently. • An accurate description of the structure of the gas-liquid interface is necessary for a better understanding of annular flow and modeling of liquid film characteristics and wave characteristics. A comprehensive understanding of annular flow characteristics is needed to improve the transport efficiency of petroleum and natural gas pipelines, and the efficiency and safety of heat transfer equipment, such as condensers, evaporators, steam generators, and nuclear reactors. For the annular flow characteristics, liquid film characteristics and wave characteristics are of essential importance for heat and mass transfer in annular flows. This paper reviews previous studies on these parameters, including prediction models and correlations for liquid film characteristics and wave characteristics of annular flows. Then, the paper examines the predictive capability of the models and correlations using existing databases. The data comparison considers the following aspects: the channel geometry effect, pressure effect, and heating state. The channel geometry affects the velocity profile, average film thickness, wave height, base film thickness, and wave frequency, while it has little effect on wave velocity. The pressure effect on these parameters is evident as it changes the gas inertia. For wave velocity in boiling flows, the predictions of current adiabatic flow models do not agree well with the current database values indicating the importance of heating state. [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]

The study of bubble dynamics is important in various engineering applications as it can provide a fundamental understanding of the bubbly flow behavior. This paper presents a novel method for tracking bubble trajectories and classifying bubble behaviors, including formation, extinction, continuous movement, fragmentation, and coalescence. The method is based on the theory of perfect matching in bipartite graphs. By establishing the perfect matching between bubbles identified at adjacent time instants, judging the continuity of bubble motion, and simply calculating the number of bubbles in the neighbor of matching bubbles at adjacent time instants, the method can deal with the challenging situation where the bubble position and volume significantly change between two adjacent time instants. The method is validated in a simulated vertical pipe boiling flow, showing that the tracking accuracy is 100% and classification accuracies for continuous movement, fragmentation, and coalescence events are estimated to be larger than 90% in a wide range of time steps between two adjacent time instants. • A Kuhn-Munkres-based algorithm for precise multi-bubble tracking is proposed. • Multi-bubble behavior through changes in bubble position and quantity is classified accurately. • A quantitative analysis of tracking accuracy in a 2D pipe bubble flow by LBM is conducted. • 100% tracking accuracy and >90% classification accuracy over wide time steps are achieved. [ABSTRACT FROM AUTHOR]

• Flow boiling of refrigerants in small to micro tubes and channels. • Two-phase pressure was obtained for different refrigerants, geometries and operating conditions. • Past pressure drop correlations did not compare well with current data bank on two-phase pressure drop. • A new correlation for predicting two-phase pressure drop in flow boiling in small to micro passages is proposed. An accurate and acceptable correlation for the prediction of two-phase pressure drop is considered a crucial step in heat exchanger design. Although many existing models and correlations were developed and proposed in the past, their ability to predict within acceptable error bands when applied in general to flow boiling flows is limited, even within their originally recorded ranges. The discrepancies are worse when predicting two-phase pressure drop in small to micro-scale passages. Therefore, the aim of the work described in this paper is to assess the most well-known models and correlations using a large experimental data-bank. The data-bank includes four refrigerants, namely R134a, R245fa, HFE-7100 and HFE-7200, small to micro heat exchangers made of different metals and different channel configurations. In addition, the data cover a large range of operating conditions, which can allow generally applicability of new correlations developed. This range covers channel hydraulic diameter of 0.46−4.26 mm, heated length of 20−500 mm, system pressure of 1 − 14 bar, mass flux of 50−700 kg/m2 s, wall heat flux ranging from 2 to 234 kW/m2 and exit vapour quality up to one. Twenty six existing models and correlations that were developed and proposed for vertical/horizontal flow, single/multi-channels and circular/non-circular channels were evaluated. Moreover, the effect of using different equations for calculating the two-phase mixture viscosity, void fraction, Fanning friction factor and Lockhart–Martinelli parameter was assessed and discussed. The mean absolute error of the existing correlations when compared with our data-bank was more than 30 %. Therefore, a new correlation for calculating the two-phase multiplier, which was strongly dependent on the Boiling number and the Lockhart–Martinelli parameter, was developed and then the frictional component and total two-phase pressure drop relationships were completed. [ABSTRACT FROM AUTHOR]

Closure models for transient cryogenic quench flow boiling heat transfer prediction are finding significant attention to support the design of energy-efficient, cost-effective, and reliable cryogenic fluid management systems. In this regard, the characterization of the film boiling crisis, which indicates the beginning of transient wetting of the wall by liquid, is essential due to the significant heat transfer change past this stage. In the current work, a new rewetting database is developed from 112 chill-down experiments conducted using LCH 4 , LOX, LAr, and LN 2 in feed lines having 0.11 m to 11 m length, 0.4 cm to 2.2 cm inner diameter, 0.5 mm to 5 mm wall thickness, and low-thermal conductive coating thickness up to 157 μm, under several pipe orientations. Additionally, the dataset comprises a mass flux from 10.5 kg / m 2 s to 1805 kg / m 2 s , pressure from 109.8 kPa to 1153.8 kPa and gravitational acceleration from terrestrial condition to ±0.01 g. Subsequently, 30 existing seminal rewetting correlations are analyzed for their applicability to transient cryogenic rewetting prediction. Moreover, the importance of feed line and flow parameters in this phenomenon is also studied. Accordingly, a new correlation for rewetting temperature is proposed with 4.15% mean absolute percentage error (MAPE). Further, applying a low-thermal conductive coating to the feed line inner wall surface has already been proven to enhance the chill-down efficiency up to 80%. For modeling such feed systems, determining bulk wall rewetting point is necessary. Hence, a new apparent rewetting temperature correlation to characterize the film boiling crisis in cryogenic coated tubes is proposed for the first time with a MAPE of 4%. Finally, the paper presents the predictions from a non-homogeneous two-phase thermal-hydraulic code, employing the previously reported film boiling correlations and the rewetting correlations from the present work. The obtained model performance indicates considerable improvement in prediction while using the proposed closure models. This shows their potential to support optimization tasks to achieve higher chill-down efficiency. • LCH 4 , LOX, LAr, and LN 2 quench rewetting database is developed from 112 experiments. • 30 existing rewetting temperature correlations are comprehensively reviewed. • The effect of the feed line and flow parameters on the rewetting phenomenon is studied. • Proposed rewetting temperature correlations for bare and coated tubes with 4.15% and 4% MAPE, respectively, employing statistical methods and genetic algorithm. • Thermal-hydraulic code prediction performance with proposed closure models is presented. [ABSTRACT FROM AUTHOR]

Highlights • 41 cryogenic heated tube critical heat flux data points are gathered. • 3122 cryogenic heated tube heat transfer coefficient data points gathered. • 20 CHF, film, and nucleate boiling correlations were assessed. • Heated-tube correlations generally deviate from cryogenic heated tube data. • Heated tube disparity between data and correlations is less than disparity for quenching case. Abstract The design and development of future cryogenic transfer systems depends on accurate correlations for modeling two-phase boiling and heat transfer at reduced temperatures in both quenching and heating configurations. The penalty for poor models translates into higher margins, increased safety factors, and an overall increase in cost. Existing correlations used in fluid/thermal design codes are based on room temperature fluids and have been shown to not accurately predict heat transfer for cryogenic fluids in the quenching configuration. This paper presents the results of a deep literature review of 40 studies conducted to gather and consolidate a database of cryogenic flow boiling data in the heating configuration. The purpose of this paper is to assess the existing correlations against the gathered experimental data. Twelve sets of cryogenic two-phase heated tube data were collected which include 872 nucleate boiling, 2250 film boiling, and 41 critical heat flux data points. The data was compared against a total of 20 heat transfer coefficient and critical heat flux correlations. As in the case of the quenching configuration, results show that existing correlations do not accurately predict cryogenic heated tube data, but the disparity between heated tube data and models is not nearly as high as in the quenching configuration. [ABSTRACT FROM AUTHOR]

Highlights • Flow regimes experimentally identified for R32, R134a, R1234e(E), R245fa. R1233zd(E). • Flow regime transitions described. • Flow visualizations are compared with the existing flow regime maps. • The comparison suggests future directions. Abstract Two-phase flow during condensation in smooth horizontal round tubes of R245fa, R1233zd(E), R1234ze(E), R134a, R32 from superheated vapor is visualized and presented in this paper. Flow regimes under different mass fluxes, heat fluxes, saturation pressures, specific enthalpies and tube sizes (1, 4, 6 mm) are identified. The paper describes to the flow regime transitions according to the visualizations. The driving force behind the annular-stratified flow transition is identified to be the force balance between shear, gravity and surface tension. The mechanism that dictates the annular-intermittent flow transition is the comparison between wave-height and the tube size. The slip ratio, which generates the Kelvin-Helmholtz instability, is considered to be the reason of transition from stratified-wavy to the fully-stratified flow. The more complicated scenarios where characteristics of different flow regimes coexist are detailed and methods for simplification are provided. The results are also compared to two different flow regime maps. The flow regime map that does not consider the non-equilibrium effects does not provide information beyond bulk quality 1 and 0. Additionally, it does not capture the annular entrance during condensation either. The flow regime map with non-equilibrium taken into account addresses issues above while having its own defects. For instance, it is highly empirical and some transition lines do not properly reflect experimental observations. A more mechanistic flow regime map is recommended. [ABSTRACT FROM AUTHOR]

The remarkable progress made in power electronics has been coupled with the miniaturization of systems, requiring high-performance cooling. A solution that has garnered significant interest involves the use of two-phase loops, in which heat transfer is associated with the phase change of the working-fluid. This paper aims at modeling two-phase flow with separated phases in a gravity-driven two-phase loop. The Volume-of-Fluid method is employed to capture the moving interface. Amid the variety of the phase change models available in the literature, we have chosen to employ a hybrid model: a subgrid-scale model to generate phase change at the wall in the absence of an interface, and a interface-resolved model for phase change at pre-existing liquid-vapor interfaces. These models have been implemented in the OpenFOAM computational code. The developed solver makes possible the simulation of compressible laminar two-phase flow with diabatic liquid-vapor phase change. Conjugate heat transfer between the wall and the fluid is also taken into account. Two-dimensional numerical simulations demonstrate that the developed solver is capable of reproducing flow regimes obtained from experiments, including the formation of Taylor bubbles and the propulsion of liquid from the evaporator to the condenser during geyser boiling. Furthermore, this study examines how the filling ratio affects flow regimes and performance. • Simulations of a Two-phase loop thermosyphon are performed with a 4-equation model. • A compressible Volume-of-Fluid method with isoAdvector advection formulation is used. • A hybrid Lee-Tanasawa phase change model is implemented in the OpenFOAM framework. • Thermal resistance of the loop and liquid oscillation frequency are computed. • The effect of filling ratio of methanol on the loop global performances is analyzed. [ABSTRACT FROM AUTHOR]

• Artificial Intelligence to predict two-phase frictional pressure gradient. • Collection of a pressure drop database for a wide range of operating conditions. • Effect of the output and input type and numerosity on prediction accuracy. • Good accuracy with non-dimensional output and mixed input values. • Conventional methods always preserve physical trend with respect to neural networks. This paper presents a discussion on several possibilities to predict the frictional pressure gradient during two-phase flow, with both the application of artificial intelligence and the implementation of conventional correlations and predictive methods. To this purpose, a huge database of approximately 8000 data points has been collected from 49 sources available in scientific literature, including 23 working fluids and the following ranges of parameters: mass fluxes from 32.7 to 2000 kg/m2s, saturation temperatures from -190°C to +120°C (reduced pressures from 0.021 to 0.780), tube diameters from 0.5 to 14.0 mm. This consolidated database has been used to train several artificial neural networks (ANNs), by using only two hidden layers (shallow neural networks) and evaluating the effect of: training and testing datasets choice (either test data included or outside the training domain), the number of neurons for each hidden layer (from 1 to 50), the type of output (either dimensional or non-dimensional), the type and number (from 1 to 22) of input parameters. The best results (MAPE of 16.8% and 88% of data within ±30%) have been obtained by using the liquid-only two-phase multiplier Φ L O 2 as non-dimensional output and 12 mixed input parameters. Compared to the statistics of well-established literature correlations for frictional pressure drop (best MAPE of 22% and 73% of data points predicted within a ±30% error range, provided by Mauro et al. mechanistic method), the ANN demonstrates therefore a higher general accuracy. However, the use of Artificial Neural Networks does not guarantee a physical trend, which is instead preserved with conventional prediction methods. [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]

High fidelity 3D computational simulation for gas–liquid two-phase flow is getting important to simulate various thermal–hydraulic phenomena in nuclear related components. Single-phase computational simulation techniques have reached to a certain level of reasonable prediction accuracy for the purposes of designs and performance analyses of industrial equipment. However, two-phase computational simulation techniques have not reached to the level of reliable prediction due mainly to the difficulty in the modeling of interfacial transfer terms. Accurate modeling of the interfacial forces including the interfacial area modeling is one of the keys to predict the distribution of gas phase in various two-phase flow systems successfully. This paper is aiming at reviewing the interfacial force modeling including recent advance of the interfacial area transport equation. This paper discusses the frame-work of bubble-wall collision force which is potentially used in place of the wall lubrication force applicable for laminar flow. This paper also discusses the frame-work of the bubble collision force considering the effect of the bubble coalescence on the bubble collision frequency. The review of existing interfacial force models including newly developed bubble-wall collision force and recent advance of the interfacial area transport equation offers the most advanced knowledge of constitutive equations necessary for improving the predictive capability of the two-phase flow computational simulation codes. [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]

Abstract: Pore-scale simulations of two phase flows in a packed-sphere bed and in a carbon paper gas diffusion layer (GDL) are carried out using the free energy multiphase lattice Boltzmann method (LBM). The simulations are performed based on the detailed microstructure of the porous media under periodic boundary conditions such that the average phase saturations in the porous medium remain constant. A comparison of the simulated and measured relative permeabilities for the packed sphere bed as a function of non-wetting phase saturation is performed, and effects of the wettability and the anisotropic characteristics of relative permeabilities of the GDL are investigated. [Copyright &y& Elsevier]

The paper presents flow characterization of condensation based on experiments with R134a in 6.1 mm inner diameter horizontal round tube. The paper presents flow visualizations and liquid film thickness measurements with mass fluxes from 50 to 200 kg m −2 s −1 and heat fluxes from 5 to 15 kW m −2 , showing the effect of mass flux and heat flux on the onset of condensation, flow regime, film distribution and void fraction. All the measurements are taken at constant pressure of 1.319 MPa, which corresponds to a saturation temperature of 50 °C. The result of flow visualization reveals that the condensation always starts in the bulk superheated region, which is generally defined as the beginning of condensing superheated region, as annular flow and the flow regime is strongly affected by the mass flux, instead of the heat flux. Based on the flow visualization results and underlying physics in the condensing superheated region, a new diabatic flow regime map is proposed to better represent the physics and predict flow regimes in both the two-phase and the condensing superheated region. The range of prediction of the new flow regime map extends over quality one up to the onset of condensation and the early stages of condensation are strictly in annular flow regime. A film thickness measurement technique for round tube is described as well as results of calibration. The film thickness measurement demonstrates that void fraction drops below one in superheated region, which is not taken into account for most conventional void fraction models. In addition, both the mass flux and heat flux affect the void fraction by altering the onset of condensation, a new void fraction model is proposed to include this mechanism. Moreover, having information of film distribution provides an opportunity to more realistically model the film geometry inside of the tube. [ABSTRACT FROM AUTHOR]

This is the second of a two-part series of papers which presents the experimental results of a parametric series of liquid nitrogen chilldown tests of a stainless steel tube. Whereas the first paper in this series focused on the effect of mass flux, inlet subcooling, equilibrium quality, pressure, and axial distance from the inlet on two-phase convection heat flux for vertical upward flow only, this paper focuses on the effect of flow direction with respect to gravity. Nine different flow directions were examined, including horizontal, 30° inclined and declined, 45° inclined and declined, 60° inclined and declined, and vertical upward and downward. The experimental data covers liquid Reynolds numbers ranging from 800 to 230,000 and local pressures ranging from 150 to 725 kPa. Analysis is presented on the effect of flow direction on the measured film, transition, and nucleate boiling heat transfer coefficients as well as the critical heat flux. Finally, new heat transfer correlations to predict the data are presented for film boiling, nucleate boiling, and transition boiling. These correlations can be used for numerical simulations of cryogenic chilldown to help predict the chilldown time and propellant consumption for different system variables. [ABSTRACT FROM AUTHOR]

• A fully visualized experimental facility combined with PIV system was designed to study the reverse flow phenomenon in the EHDPHRS. • Single-phase reverse flow velocity is proportional to heating power and inversely proportional to subcooled-water temperature. • Two-phase reverse flow velocity increases first and then decreases with heating power. • Buoyancy lift induced by density difference and pressure difference induced by direct contact condensation are the driving force for reverse flow. • Flow inertia force caused by forward flow is the restraining force for reverse flow. The equal-height-difference passive heat removal system (EHDPHRS) has a large potential in floating nuclear power plants (FNPPs) due to its ability to fully utilize the marine environment to achieve residual heat removal. However, the horizontal connection between the heat sink and the heat source could easily lead to the subcooled-water reverses flow and thus trigger serious flow instabilities. In the present paper, a fully visualized experimental set was built, combined with a particle image velocity (PIV) system to investigate the subcooled-water reverses flow in the EHDPHRS. First, PIV observations clearly highlighted five flow stages during a complete natural circulation, including the reverse flow occurred in the single-phase and two-phase flow. Then, it was shown that the single-phase reverse velocity is proportional to heating power and inversely proportional to subcooled-water temperature. While, the two-phase reverse velocity increases first and then decreases with the increase in heating power. Moreover, the two-phase reverse flow phenomenon shows obvious periodicity. Finally, the buoyancy force (F b) caused by the density contrast, and the pressure difference (F Δ P) caused by the direct contact condensation (DCC) are the triggering forces for the reverse flow, while the flow inertia force (F i) caused by the forward flow represents the restraining force. [ABSTRACT FROM AUTHOR]

• A new method for measuring the void fraction of a two-phase flow is presented. • A new data on pressure, ball size and flow effects in packed beds were collected. • A robust drift-flux correlation for the packed beds was developed. • Existing slip ratio formulas reviewed and adapted for the packed beds. • The liquid velocity is introduced into the one-parameter void fraction equation. This paper presents the results of a void fraction study for a steam-water flow in a channel that contains a close-packed bed of solid spherical particles. Investigations of a two-phase flow motion through random layers of solid particles are crucial for the energy industry to design future nuclear reactors with spherical coated fuel particles and analyze post-emergency cooling of material fragments in the core of light water reactors. An experimental setup and a new measurement method combining a flow cutoff method and the use of additional water are described. Experiments were carried out with a packed bed of particles with diameters of 2 and 4 mm at a pressure of 0.12–0.9 MPa and a mass velocity of 28–300 kg m − 2 s − 1 . Some measurements were made in a particle-free pipe 39 mm in diameter. The findings have established that the void fraction in packed beds is higher than in a particle-free pipe, is insensitive to the particle size, and, in the considered range of values, increases with a rise in the flow rate or a decline in pressure. The analysis of the known correlations for heated and unheated channels of different geometries and packed beds is conducted. The obtained experimental data is generalized using the drift-flux model and adapted slip ratio models. The paper proposes a modified version of the one-parameter equation for the void fraction in a packed bed with constraints imposed on the difference in the vapor and water filtration velocities. [ABSTRACT FROM AUTHOR]

This paper presents analysis of a novel multiphase scroll pump to transport gas–liquid two-phase mixtures, in which each compression chamber has a flank clearance which forms a pressure unloading gap connected to the discharge port. Therefore, this multiphase scroll pump has the function of unloading gas–liquid mixture pressure automatically during the compression process, so the frequently occurring problem of liquid slugging in displacement multiphase pumps is solved. Hence, safety and reliability are improved. In this paper the working process model of the presented multiphase scroll pump is discussed; and equations of fluid motion and geometry of scroll wrap profiles are given. Grid generation and numerical methods are discussed, and numerical simulation of the working process for a multiphase scroll pump is conducted using the commercial CFD software package Fluent, with results suggesting key parameter values for design of multiphase scroll pumps. [ABSTRACT FROM AUTHOR]

• The experimental system and model of condensation in spiral tube are established. • The effects of structural parameters on condensation characteristics are studied. • The enhancement factor of spiral tubes with different parameters are compared. • Compared with the helix angle and spiral diameter, the effect of inner diameter is momentous. • To provide guidance for the design of large-scale LNG spiral wound heat exchanger. In this paper, a two-phase flow experimental system is proposed based on the condensation flow of the hydrocarbon mixture in the spirally wound tube to study the characteristics of the mixture (methane/ ethane/propane) in the spiral tube. An investigation is conducted on the effects of spiral diameter (D), inner diameter (d) and helix angle (β) on heat transfer coefficient (h mix) and friction pressure drop (Δ P fric). Moreover, the performance evaluation criterion (PEC) is adopted to measure the effect of structural parameters. As indicated by this paper, h mix decreases with the increase in β, d and D , whereas Δ P fric changes more significantly with d. If d increases or β decrease properly, the PEC will increase, which is of high significance to the design of tube-wound heat exchangers. The results of this paper can provide a reference for investigating the mechanism of condensation heat transfer in spiral tubes and the design of spiral wound heat exchangers. [ABSTRACT FROM AUTHOR]

• Experiment study of dryout characteristic in helically coiled tubes covers a wide range of system and geometric parameters. • The dryout patterns are divided into three groups according to the distribution of the liquid film thickness. • The effects of system and geometric parameters on the dryout quality and pattern are analyzed. • A "dryout map" and the correlations of dryout quality are proposed and can accurately predict the dryout characteristics in helical coiled tubes. In helically coiled tubes, the geometry-induced centrifugal force exerts upon the fluid to make the dryout phenomenon complicated. Hence, it is essential to understand the dryout mechanism and develop a model to accurately predict the dryout quality and dryout pattern in helically coiled tubes that can cover a wide range of parameters. In this paper, 17 helically coiled tubes with different coil diameters (160 ∼1350 mm), inner diameters (9 ∼14 mm) and inclined angles (2.5 ∼ 10°) are used to conduct the experimental study of the dryout characteristic under different conditions, including pressure between 2 and 7 MPa, mass flow rate between 100 and 1000 kg/(m2·s), heat flux between 50 and 700 kW/m2. Three types of dryout patterns in helically coiled tubes were observed by analyzing the circumferential distribution of the wall temperature, including the gravity-dominated pattern, the secondary-flow-dominated pattern and the centrifugal-force-dominated pattern. Then the effects of the operational and geometric parameters of helically coiled tubes on the dryout characteristic have been studied, such as system pressure, mass flow rate, heat flux, tube diameter and coil diameter. Finally, a "dryout map" was developed based on the experimental data (297 data points), and an analytical model of dryout quality was proposed in terms of dimensionless groups for each zone. The correlations were verified by comparing with the experimental results in the open literature. The proposed correlations in this paper can give a better prediction for different experimental results. [ABSTRACT FROM AUTHOR]

• Transition criterion for channel flow was developed in terms of void fraction and particle diameter. • Mechanistic interfacial friction models were suggested based on two-phase flow regimes of a packed bed. • A zero-dimensional dryout heat flux model was suggested using the suggested interfacial friction models. • Predictions of the pressure drop and dryout heat flux were improved using the suggested models. In this paper, two-phase drag models for a packed bed of uniform-size particles were suggested, and they were applied to the calculation of pressure drop and dryout heat flux. We provided physical basis for the two-phase flow regime model through the analysis of the interfacial friction (F i). The suggested model provides flow patterns representing bubbly, slug, and channel flow and considering three criteria including d2 F i /d α 2 = 0, F i = maximum, and F i = 0. The results obtained from the three criteria were drawn with several observation-based experimental ones to generate the flow regime map (void fraction vs. particle diameter). Through the current flow regime map, we clearly saw the existence of channel flow in a packed bed with particles smaller than around 3.5 mm. Then, mechanistic interfacial friction models were developed on basis of the current two-phase flow map of bubbly flow, slug flow, channel flow and annular flow. The suggested interfacial friction models were validated with top- and bottom-flooding air-water experiments and boiling experiments. We found out that the capability of pressure drop estimation by the current model were significantly improved for a bed with small particles. Finally, a zero-dimensional dryout heat flux (DHF) model was derived using the suggested interfacial friction models, and validated against DHF experimental data for beds with 1-D configuration. The root-mean-square error (RMSE) of the suggested DHF model was 35%, which was the smallest among the RMSEs of the previous DHF models. [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]

• Mass transfer due to droplets entrainment in churn and annular flows is studied. • Relation between droplet entrainment mechanism and features of droplets is discussed. • Influence of initial droplets on gas-droplets momentum transfer is figured out. In this paper, the air-water vertical two-phase pipe flow experiment was carried out at room temperature and pressure to study the characteristics of entrained droplets in churn and annular flows with the high-speed camera and phase Doppler anemometry. The results show that there are five kinds of droplet entrainment modes in churn and annular flows, namely shearing-off of waves, bag break-up of waves, ligament break-up of waves, droplet impacting liquid film, and burst of bubbles. With the flow pattern transition from churn flow to annular flow, the dominant droplet entrainment changes from shearing-off of waves to ligament break-up of waves. The gas superficial velocity plays a decisive role in determining the characteristics of entrained droplets, while the effect of the liquid superficial velocity can be neglected. In churn flow, the shearing-off of waves produces a small amount of entrained droplets with low sphericity and initial velocity. Large droplets tend to deposit, and small droplets undergo a certain degree of fragmentation and coalescence during the accelerated migrating in the gas core. In annular flow, the ligament break-up of waves produces a large amount of entrained droplets with high sphericity and initial velocity. Droplet size presents continuous distribution. The size of droplets increases slightly due to the coalescence of small droplets during the accelerated migrating in the gas core. Compared with churn flow, the momentum exchange between the gas phase and droplets is greater in annular flow, which leads to a significant increase in droplet momentum. The size distribution of the entrained droplets determines the momentum exchange between the gas core and the entrained droplets to a certain extent. [ABSTRACT FROM AUTHOR]

• The proposed model is able to maintain the order parameter within a physically meaningful range and conserve the volume. • The proposed mode can simulate two-phase fluid flows with the density ratio up to 1000. • A phase-field-based multiple-relaxation-time lattice Boltzmann model is proposed for incompressible multiphase flow with different densities. In this paper, a fractional step lattice Boltzmann method is proposed to model two-phase flow with large density differences by solving a modified Cahn-Hilliard phase-field equation and the incompressible Navier-Stokes equations. In order to maintain the interface profile and conserve the volume, the normal flux in the original Cahn-Hilliard equation is removed and a profile-corrected term to enforce the interface to be a hyperbolic tangent profile is added. The modified Cahn-Hilliard equation is split to two sub-equations. One is the original CH equation and the other is the correction equation. Both the original CH and the hydrodynamic equations are solved by the lattice Boltzmann method. The correction equation is solved by the finite difference method. With the multi-relaxation-time collision model and a high-order compact selective filter operation employed in the lattice Boltzmann method, numerical stability is much improved. Compared with the previous lattice Boltzmann methods, the proposed method is able to maintain the order parameter within a physically meaningful range, which is capable of tracking the interface accurately. The proposed method can also simulate two-phase fluid flows with the density ratio up to 1000. Several benchmark problems, including single vortex deform of a circle, translation of a drop, stationary droplet, capillary wave and rising bubble with large density ratios, are presented to validate the accuracy and efficiency of the present method. [ABSTRACT FROM AUTHOR]

• 3D printed clear axial micro-finned and smooth tubes are used for visualization. • Axial micro-fins do not provide additional force to pull up the liquid inside the tube. • Liquid-vapor interface in the micro-finned tube is lower and wavier than the smooth. • Part of the liquid is trapped in the micro grooves when the vapor quality is low. This paper presents a novel visualization approach to understand the effect of axial micro-fins on two-phase flow behavior inside horizontal round tubes. In order to reflect the real inner geometry in the commercial metal tubes (smooth and micro-finned tubes), clear resin tubes were made by an SLA 3D printer to reproduce the geometry. The 3D printed tube for visualization was installed right after the heat transfer test section. In the heat transfer test section, R410A flow boiling experiments were conducted at 10 °C saturation temperature with heat flux of 15 kW/m2. The results indicated the axial micro-fins do not provide additional force to lift up the liquid in the round tube and the annular flow pattern does not occur earlier (at lower vapor quality or mass flux conditions) than the smooth tube. For low mass flux and vapor quality, the liquid-phase refrigerant is easily trapped in the grooves at the top or side of the axial micro-finned tube. The main liquid level in the axial micro-finned tube is, therefore, lower than that in the smooth tube under the same conditions. This liquid level change is quantified by tracing the liquid-vapor interfaces in the captured videos with change point analysis. [ABSTRACT FROM AUTHOR]

• Stability boundaries of various instabilities are identified in a parameter plane. • Rarely reported flow excursion with compressible volume (FECV) is studied in detail. • FECV has significantly different characteristics from Ledinegg instability. • Physical explanation of each type of instability are given by phase space plots. • Various instabilities are identified by a particular type of bifurcation. The present work aims to study different types of flow instabilities in two-phase flow heated channel. These flow instabilities have an important effect on the performance of the systems with such channels. A methodology is developed to find the regions in the parametric space where different types of instabilities occur. Three types of instabilities, namely, Ledinegg instability or flow excursion, pressure drop oscillations (PDO) and flow excursion with compressible volume (FECV), are considered here. The first two instabilities, Ledinegg instability, and PDO are quite well known. However, the instability termed here as FECV is rarely studied in the literature. Although FECV is a flow excursion, it has several significant differences from the Ledinegg instability. First of all, Ledinegg instability can lead the system to two alternate equilibrium points depending on the resultant of the perturbation; however, for the system having FECV only one alternative equilibrium point exists. Moreover, the existence of FECV requires the presence of compressible volume in the system, again a feature similar to PDO and in contrast with Ledinegg instability. A detailed physical explanation of these instabilities in terms of the interaction of internal pressure drop and external curves included in this paper. Using bifurcation theory, namely, Hopf and pitchfork bifurcations are identified as the PDO and Ledinegg instability, respectively. Furthermore, FECV is identified by Hopf bifurcation (a feature similar to PDO). Moreover, numerical simulations are carried out at different locations in the parameter space to understand the dynamics of the system. These simulations are presented as time evolution of the variables as well as by trajectories in the phase space. [ABSTRACT FROM AUTHOR]

• Numerical and experimental studies of CO 2 critical flow for turbo-machinery seal. • Verification of the CO 2 critical flow model based on the isentropic flow. • Modification of the Hodkinson's and Vermes' equations for CO 2 leakage analysis. • Comparison of experimental data with choked flow and labyrinth seal flow equations. Supercritical CO 2 (S-CO 2) has the potential to be used as the working fluid in a power cycle since S-CO 2 shows a density value high as its liquid phase while the viscosity value remains closer to its gaseous phase. Thus, it requires much less work to compress due to its low compressibility as well as relatively small flow resistance. However, the S-CO 2 leakage flow from turbo-machinery via seal becomes one of the important issues since not only it influences the cycle efficiency due to parasitic loss but also it is important for evaluating the system safety under various operating conditions. In this paper, the gap effect, which is studied by changing the diameter of an orifice, cavity length effect, and the number of tooth effect in a simulated labyrinth seal geometry nozzle are presented. Experimental results under various conditions including not only single phase flow but also two phase flow conditions are presented. [ABSTRACT FROM AUTHOR]

• An experimental investigation was conducted for flow instability with two-phase interface morphology. • The flow instability rapidly happened once the annular flow appeared for D in = 2.15 mm. • A prediction model of flow instability was established for test-sections with different diameters. In this paper, an experimental investigation was carried out for the characteristics of flow instability with the evolution of two-phase interface morphology. The inner diameters of test sections included 2.15 mm, 4.95 mm, as well as 6.80 mm, and the inner diameter of bypass was 12.5 mm. The deionized water was selected as the working fluid, and the characteristics of two-phase interface morphology were recorded by a high-speed camera. It was found that the corresponding flow patterns were the typical annular flow for all the test sections as the two-phase flow instability occurred. As for the test sections (D in = 6.80 mm and D in = 4.95 mm), the flow patterns gradually transferred from the bubbly flow to the annular flow with the heating flux increasing, and the flow instability finally happened as the annular flow appeared. As for the test section (D in = 2.15 mm), the flow state remained stable before OFI (Onset of Flow Instability) with the single phase or bubbly flow. On account of the significant channel restriction and surface tension, the flow pattern rapidly evolved to the annular flow for OFI, while the flow state simultaneously transferring from the stable state to the unstable state. Finally, a prediction model of the flow instability for test sections with different inner diameters (D in = 2.15 mm, D in = 4.95 mm, D in = 6.88 mm) was established combined with the dimensionless parameters of N conf and We ; the predicted results agreed well with the experimental results. [ABSTRACT FROM AUTHOR]

• The coherent structure under two-phase flow in the tight lattice bundle was found. • For bubbly flow, the coherent structure is in the form of bubble clusters. • For cap-bubbly flow, the coherent structure is in the form of cap-shaped bubbles. • The characteristic parameters were defined to characterize the coherent structure. This paper investigated the two-phase flow characteristics in a double sub-channel tight lattice (P / D = 1.06). Compared to the prototype, the test channel was magnified by 2.7 times. A self-developed 16 × 32 wire-mesh sensor was utilized to measure the phase distribution at two fully developed positions of the test channel (Z / D h = 86.86, 115.8). The flow conditions are 0.036 m/s < j g < 1.04 m/s and 0.93 m/s < j l <1.86 m/s at Z / D h = 115.8, including the bubbly flow (B), the cap-bubbly flow (CB), and the slug flow (S). Combined with the pseudo-side view of the flow pattern and the power spectrum, the coherent structure of the gas phase under the bubbly flow and the cap-bubbly flow was confirmed. For the bubbly flow, the coherent structure is in the form of bubble clusters consisting of discrete bubbles. These bubble clusters are staggered on both sides of the gap but do not traverse across the gap, which is speculated to be caused by the large-scale vortex of the liquid flow field. For the cap-bubbly flow, the coherent structure is in the form of cap-shaped bubbles. In this study, the cap-shaped bubbles are confined in the center of the subchannels and staggered in adjacent subchannels, which is attributed to the compact arrangement of rod bundles and the inter-subchannel liquid cross-flow caused by the cap-shaped bubbles. The characteristic parameters, including the pulsation frequency f , convection velocity v , wavelength λ , and amplitude A were defined to characterize the coherent structure. Data analysis results show that the coherent structure frequency under the bubbly flow increases with the increase of the liquid-phase velocity and the coherent structure frequency under the cap-bubbly flow increases with the increase of the gas-phase velocity. In addition, the coherent structure is suppressed under three flow conditions, including the bubbly flow with low liquid velocity (j l < 1.3 m/s), the B-CB transition, and the slug flow. [ABSTRACT FROM AUTHOR]

• The channel size effect on drift-flux parameters is detailed. • Critical size such as 30 times Laplace length does not apply to non-circular channels. • The bundle distribution parameter depends on bundle casing geometry and rod array. • The channel size effect is more pronounced at low pressure and low flow conditions. • The channel size has less impact for beyond-bubbly flows. Gas-liquid two-phase flows are utilized in various heat and mass transfer processes that appear in numerous industrial apparatus. Typical examples are chemical reactors, heat exchangers, nuclear reactors, deaerators, condensers, etc. The numerical simulation of gas-liquid two-phase flows, which is crucial for efficient, safe, and optimized apparatus design, requires an accurate model developed based on the physics of the two-phase fluid dynamics. The two-fluid model is considered the most accurate two-phase conservation equations used in computer simulation codes to predict the thermal-hydraulic behavior of two-phase flows. The drift-flux parameters, such as the distribution parameter and drift velocity in the drift-flux model, are utilized in formulating the interfacial drag force in the two-fluid model-based simulation codes. The drift-flux model is an insightful model considering the difference between gas and liquid velocities. The effect of phase and velocity distributions on the void fraction is considered through the two drift-flux parameters. The distribution parameter and drift velocity are critical parameters in the two-phase flow formulation through the two-fluid model, which are the backbone of thermal-hydraulic analysis codes. The constitutive equations for the distribution parameter and drift velocity developed for medium-size channels reach a mature level but do not apply to industrial-size channels or large-size channels. The complicated two-phase flow dynamics in large-size channels affect the distribution parameter and drift velocity significantly. The distribution parameter in a large-size channel increases at low pressure and low liquid flow conditions due to induced secondary flow in the large-size channel. The drift velocity also increases due to cap bubbles formed by the surface instability of large bubbles. Multi-dimensional two-phase flow dynamics observed in large-size channels complicate the distribution parameter and drift velocity modeling. Thus, the drift-flux modeling in large-size channels is a much more subtle task than that in medium-size channels. This fact has driven the research to establish the drift-flux type correlations by modeling the distribution parameter and drift velocity in large-size channels. The current paper aims to provide state-of-the-art knowledge of the current status of the recent development of the drift-flux type correlations in large-size channels. The discussed items cover the formulation of the one-dimensional drift-flux model, typical drift-flux correlations developed for medium-size channels, distribution parameters for subcooled and saturated boiling flows, unique two-phase flow dynamics in large-size channels, critical size at the boundary between medium and large-size channels, and existing drift-flux correlations for large-size channels. The flow channel geometries discussed in the current paper are circular, annulus, rectangular, square, vertical rod bundle, and horizontal tube bundle. The future research direction is also discussed. [ABSTRACT FROM AUTHOR]

Abstract: Characteristics of air–water two-phase flow in a mini-square-channel were investigated experimentally. One sidewall of the flow channel was made of permeable porous material (carbon paper). Air was fed into the channel from its inlet, while water was injected uniformly into the channel along the permeable wall. Based on the observed results, flow patterns of this special gas–liquid two-phase flow were classified, and different kinds of surface characteristics of the permeable wall were used, flow regime maps for specific flow parameters were drawn, and the influence of different combinations of gas flow rate and liquid infiltration amount on flow patterns were also analyzed. Meanwhile, the paper also provided a detailed analysis of the channel pressure drop characteristics in this kind of two-phase flow; with special focus on the influence of different kinds of permeable wall surface characteristics on pressure drop variation. [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: 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]

Highlights • Flow patterns of R1234ze(E) (50–225 kg m−2 s−1 at 30 °C) in a 0.643 mm microchannel tube is presented. • The plug/slug flow is homogeneous. The length of liquid slug and vapor plug is affected by quality. • Liquid droplet formed from collision of liquid slug and ring. Abstract This paper presents the discussion of regimes of two-phase flow of R1234ze(E) in a 24-port microchannel tube with average hydraulic diameter of 0.643 mm, obtained by visualization. The experiment is conducted in the same facility in the previous work for R32 (Li and Hrnjak, 2019). The conditions cover mass flux from 50 to 225 kg m−2 s−1. The inlet saturation temperature is 30 °C. The two-phase flow is generated by adding heat in several steps to refrigerant initially subcooled at the entrance of test line. As vapor quality increases at a fixed mass flux, the flow is firstly plug/slug, then transitional, and finally is annular flow regime at high quality. When mass flux is 50 kg m−2 s−1, no annular flow observed in the tube. The vapor quality of boundary between two flow patterns decreases as mass flux increases. The annular flow starts at x = 0.85 (G = 100 kg m−2 s−1) and x = 0.63 (225 kg m−2 s−1). The transitional flow starts at x = 0.8 (G = 50 kg m−2 s−1) and x = 0.3 (225 kg m−2 s−1). Comparing to R32 and R134a, flow patterns of R1234ze(E) transition at lower quality due to the lower vapor density and thus higher vapor velocity. Three flow pattern maps in literature are compared to the results and they have limited agreement with our observations. The plug/slug flow behaves as homogeneous. The velocity at interface between liquid slug and vapor plug is close to the homogeneous velocity. The vapor plug length fraction also agrees to the homogeneous void fraction. The length of vapor plug increases dramatically as vapor quality increases at fixed mass flux. The length of liquid slug first increases and then unchanged as quality increases. The video supports that there are liquid droplets formed from the liquid slug and liquid ring collision. [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 • Flow patterns of R134a (50–250 kg-m−2s−1 at 30 °C) in a 0.643 mm microchannel tube is presented. • The velocity and length fraction of vapor plug in plug/slug flow indicates the flow is homogeneous. • As quality increases, the length of vapor plug grows dramatically in plug/slug flow. Abstract This paper presents visualization of the two-phase flow of R134a in a 24-port microchannel tube with the average hydraulic diameter of 0.643 mm. The experiment is conducted on the same facility in the previous work for R32 (Li and Hrnjak, 2019). Mass flux covers the range from 50 to 250 kg-m−2s−1. The two-phase flow is generated by adding heat in several steps to originally subcooled refrigerant at the very entrance. When mass flux is 50 kg-m−2s−1, no annular flow is observed in the tube. The annular flow starts at x = 0.9 when mass flux is 100 kg-m−2s−1 and x = 0.6 when 250 kg-m−2s−1. The transitional flow starts at x = 0.25 (G = 50 kg-m−2s−1) and x = 0.8 (250 kg-m−2s−1). Revellin and Thome (2007), Ong and Thome (2011), and Zhao and Hu (2000) correlations are plotted against the measurements. None of them fit our data since the experiment is in lower mass flux than the database of their correlations'. The interface velocity and vapor plug length fraction (close to void fraction) are measured from the high speed videos. The velocity and vapor fraction agree to results based on the homogeneous assumption. In plug/slug flow, both the vapor plug and liquid slug lengths are uneven. The length distribution of plug and slug seems to follow Beta distribution. When the quality is low, the quantity of short vapor plugs is larger than long plugs. At fixed mass flux, when quality increases, length of vapor plugs increases and length of liquid slugs decreases. [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: 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: 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 • The study concerns with a numerical study on natural convection of different nanofluids in a porous medium. • The Buongiorno’s model was utilized to track the nanoparticles concentration. • Thermophoresis and Brownian parameter have no significant effect on isothermal lines and the streamlines. Abstract This paper concerns with a numerical study on heat transfer by natural convection of different nanofluids inside a porous medium between two horizontal eccentric cylinders. The Buongiorno’s model was utilized to track the nanoparticles concentration. Heat transfer by natural convection of water-copper, water-diamond and water–silicon dioxide nanofluids inside a porous medium between two cylinders was studied. A set of characteristic equations were solved using finite element method. Range of parameters which are studied are Ra = 103-106, Da = 10−6-10−3, ε = 0.1–0.9 and volume fractions of 0.0–0.1, Nr = Nt = Nb = 0.1–0.5, and Le = 1–10. The porous medium was analyzed by Darcy-Brinkman model. The results showed that thermophoresis parameter and Brownian parameter have no significant effect on isothermal lines and the streamlines. The probability of collision of nanoparticles decreases with increasing the Lewis number, which consequently decreases the average Nusselt number. The maximum and minimum enhancement in natural convection was detected in water-diamond and water–silicon dioxide respectively. [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 • Attempt to combine two original vaporization models to take advantages of both. • The species solver is added to the HFM framework, the coupling strategy is assessed. • The feasibility of switching between the two frameworks is demonstrated. • The conservative level set method is utilized to reduce unphysical mass loss. • Results show good agreement with the analytical and experimental data. Abstract This paper proposes a vaporization model for detailed numerical simulations of a reactive interface. The proposed model represents a preliminary attempt to combine vaporization models based on heat flux and species mass flux to take each model’s advantages concerning numerical accuracy, robustness and applicability. We utilize a conservative level set method to reduce unphysical mass loss in capturing the deformable interface, and the ghost fluid method to accurately impose various jump conditions across the interface. Four validations are conducted to demonstrate both the strengths and limitations of the two original models. Since the vapor mass fraction is relevant in realistic applications, we add the species solver to the heat flux based model and assess the coupling strategy in different situations. The feasibility of switching between the two frameworks in the coupled vaporization model is also demonstrated. Finally, the coupled model is applied to simulations of deformable/moving droplet under high-ambient-temperature conditions. The results of the simulations are consistent with analytical and experimental data. Although additional work is required, the coupled vaporization model exhibits potential as a means for modeling spray combustion. [ABSTRACT FROM AUTHOR]