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Flow boiling heat transfer is a promising technology for thermal management. In this paper, flow boiling heat transfer characteristics in a rectangular vertical flow channel are experimentally and numerically investigated at low flow rate. Visualization results provide guidance for the prediction of flow patterns. The flow boiling correlation at low flow rates is improved with an error of less than 30%. Numerical results reveal vortices enhanced local heat transfer. Absolute vorticity is introduced to quantify the secondary flow. The results indicate that the secondary flow intensity increases with heat flux, verifying that boiling flow produces the local convection enhancement. [ABSTRACT FROM AUTHOR]

Boiling flow presents a significant concern, especially when a liquid surpasses its boiling point, potentially leading to catastrophic consequences. This research utilizes a two-phase code in the OpenFOAM software to investigate bubble formation during flow boiling. The well-established empirical models for calculating wall heat components were selected based on the operating conditions. The study incorporates experimental data from high-pressure boiling flow (10–30 bars) with variable properties of refrigerant R-12. The predictions reveal underpredictions in void fraction and liquid temperature compared to experimental observations. Significantly, the impact of the subcooling degree on void fraction behaviour is emphasized, and a potential underprediction of the evaporation portion is highlighted, particularly near the wall. Challenges in modelling bubble size distribution are evident through discrepancies in bubble diameter and velocity data, indicating the necessity for further advancements in the code. In summary, this numerical study provides valuable insights into the intricate dynamics of high-pressure subcooled boiling flow, especially when considering variable working fluid properties. Future efforts will focus on refining models for nucleation site density, bubble departure size, and lift-off frequency to enhance prediction accuracy.

Two-phase flow loop cooling system is required for increasing heat transfer rate, heat transport distance, and cooling heat flux in thermal control of space structures. The understandings on thermo-fluid dynamics of gas-liquid two-phase flows with phase change under microgravity is necessary for the system design. To clarify the phenomena and verify the operation of two-phase flow loops under microgravity condition, two-phase flow loop experiments were conducted as a JAXA project named TPF experiment onboard the International Space Station (ISS). The loop was a pump-driven loop and the pressure was maintained by an accumulator. Perfluorohexane was used as the working fluid because of low latent heat of vaporization and moderate boiling point under the restriction of power input and for the safe operation. The loop had two kinds of heating section, such as a copper heating tube and a transparent glass heating tube, and a condenser. Flow observation sections made of transparent polycarbonate resin were installed just downstream of each heating tube in order to observe and measure the interface structure of boiling two-phase flow in detail. In this report, the design concept and system characteristics of the experimental loop and the results of the gas-liquid two-phase flow measurements in the flow observation section are presented. [ABSTRACT FROM AUTHOR]

• Experimental data on R290 flow boiling inside a micro-fin tube were obtained. • Heat transfer coefficient and pressure drop distributions were analyzed. • Effects of conditions on the heat transfer characteristics were illustrated. • The experimental data were compared with several correlations. The present study focuses on the analysis of heat transfer and pressure drop of R290 in a horizontal micro-fin tube. In this regard, experiments were conducted, and flow patterns, heat transfer coefficient, and friction pressure drop were analyzed under various conditions. A wide range of conditions was studied, including saturation pressures ranging from 0.215 to 0.415 MPa, mass fluxes ranging from 70 to 190 kg m−2 s−1, heat fluxes in the range of 10.6–73.0 kW m−2, and vapor qualities from 0 to 0.96. The performed analyses revealed four flow patterns, including bubbly flow, plug flow, slug flow, and annular flow. It is found that the onset of dry-out occurs when the liquid film thickness is equal to the fin height and occurs earlier at high heat fluxes and low saturation pressures. The maximum heat transfer coefficient and friction pressure drop occur at the onset of dry-out, and the heat transfer coefficient exhibits a significant correlation with the heat flux. The obtained results were verified using the heat transfer correlation of Cavallini and the friction pressure drop correlation of Rollman, where the average relative deviation (ARD) was −24.01% and −8.65%, respectively. [ABSTRACT FROM AUTHOR]

In this work, argon flow within different microchannels with square cross-section is simulated by the molecular dynamics simulation (MDS) method. The first simulation is done within ideal microchannel and then, internal surfaces of this microchannel are roughened by cone, cubic and spherical barriers respectively to report influences of roughness with different geometries on the distribution of fluid flows. It is reported that cone and spherical geometry of barriers do not strongly affect flow characteristics of argon fluid within the microchannel, while the cubic geometry of barriers delays density distribution of argon flow in the middle region of the microchannel due to their destructive role against translocation of fluid particles between lateral and central layers of the microchannel. For the density results, it is reported that two physical phenomenon of thermal and external energies play important roles on the distribution of fluid particles in different sections of microchannel due to their different applying directions. Moreover, it is observed that the temperature profile of fluid flow within microchannel with cubic barriers touched a maximum temperature of 420 K at the central layers which are in the highest temperature level among all cases studied. Results of this study are applicable for practical application in small scale designs such as medical probes to inject destructive fluid for destroying tumors in cold surgeries.

An experimental investigation has been carried out to evaluate the characteristics of the evaporation heat transfer within a rifled tube for R22 refrigerant. This study is mainly aimed at evaluating the contribution of the rifle tubes to the heat transfer coefficient in horizontal boiling flow and comparison of the results with the flow within the smooth tubes. The test section is made of a six head rifled tube with a length of 2 m, which is uniformly heated by the joule heating effect. The range of test parameters included the evaporation temperatures from 4 to 15 , the heat flux range of 5–17 kW/m2, the refrigerant mass flux of 70–350 kg/m2s, and the quality range of 0.1 to 0.8. The results showed that increased vapor quality and temperature led to increased heat transfer coefficient within the rifled tubes. Also, increased heat flux and mass flux resulted in an increased heat transfer coefficient. The enhancement of heat transfer coefficients of approximately 28 to 56% (i.e., enhancement factors of 1.28 to 1.56) have been recorded compared to the smooth tube for qualities of 0.3 and 0.7, respectively. In lower mass fluxes, the enhancement factor increases significantly; however, as mass flux is further increased, the enhancement factor is reduced and approaches a constant value. As a result, the use of rifled tubes in high-quality flows at lower mass fluxes enhances heat transfer significantly. [ABSTRACT FROM AUTHOR]

• Advanced heat transfer tubes with dimple and sandblast surfaces. • Two phase heat transfer characteristics of refrigerant R32 on the annulus side of the tubes. • Influence of refrigerant mass flux and vapor quality on heat transfer characteristics. Enhancing the heat transfer performance of two-phase flow in heat transfer tubes is of paramount importance. In pursuit of this goal, the research group fabricated heat transfer tubes featuring both microscopic and macroscopic surfaces containing staggered arrays of three-dimensional dimples (depth of 2 mm, diameter of 3 mm, and spaced at intervals of 10 mm), petal-shaped protrusions (depth of 850 μm, diameter of 980 μm, and spaced at intervals of 50 μm), and sandblast structure (diameter ranging from 5 to 10 μm and depth of approximately 1 μm). Experiments were carried out to examine boiling and condensation two-phase flow heat transfer in annulus outside the tubes, including smooth tube, sandblast tube (E1), dimple tube (E2), and sandblast/dimple composite surface tube (E3), all constructed from stainless steel with a tube diameter of 19.05 mm. Both heat transfer coefficient (HTC) and frictional pressure drop exhibited a positive correlation with mass fluxes. In condensation, HTC increased as vapor quality increased. Sandblasting of the dimple surface had a detrimental effect on HTC, with the HTC of the E3 tube falling between that of the E2 and E1 tubes. In flow boiling, sandblasting the dimple structure's surface further enhanced the vaporization core, resulting in the E3 tube displaying the highest HTC, benefiting from advantages of both sandblast and dimple structures. The effect of vapor quality on HTC was found to be minor at low mass fluxes, while at high mass fluxes, HTC increased initially and then decreased as vapor quality increased. [ABSTRACT FROM AUTHOR]

The rectangular channel is of particular interest for nuclear reactors due to its compact structure and high heat transfer efficiency. Two-phase flow characteristics in rectangular channels are different from those in conventional circular channels, which is important for the design and safety analysis of reactors. A subcooled boiling flow experiment was conducted using a single-side heated vertical rectangle channel at various system pressures of 0.80 MPa–1.80 MPa. Local two-phase parameters, namely void fraction, interfacial area concentration, bubble interfacial velocity, and bubble chord length, were measured by a double-sensor conductivity probe, and flow visualization images were captured by a high-speed camera. The bubble distribution feature of subcooled boiling flow in rectangular channels is revealed in this study. A "W" type distribution of void fraction was observed in cap-bubbly flow and the interfacial area concentration has a similar distribution as the void fraction, which illustrates that bubbles gather at the center and sides of the channel. Different from the adiabatic flow, the system pressure is a positive factor for the void fraction which is observed in this experiment. Furthermore, the accuracy of void fraction prediction models is assessed based on the experimental data. A large deviation is observed between the predictive and the experimental void fraction, which is related to the single-side heating feature of the flow channel. [ABSTRACT FROM AUTHOR]

• Coupled continuity-IATE model predicts the growth in void and IAC in boiling flow. • Cap/slug bubbles are formed through the source terms, not flow-regime transition criteria. • The sensitivity to the boundary conditions and nucleation uncertainty is evaluated. • Improved model predictions of total void fraction compared to one-group models. The Two-Fluid Model is the backbone of thermal-hydraulics and system-analysis codes for nuclear design. The Two-Fluid Model tracks the transfer of conserved quantities—mass, momentum, and energy—without the need for bubble interface tracking. However, two-phase flows are characterized by their different flow regimes which change as more vapor is present in the flow, from bubbly flow to cap/slug flow to annular flow. The two-group Two-Fluid Model can track the progression of boiling flows beyond the bubbly region without the need for flow-regime maps. A two-group wall-boiling model is implemented and coupled with the two-group Interfacial Area Transport Equation. The resulting model is compared against experimental data and predicts the growth in cap/slug bubbles through interaction models rather than through flow-regime maps. The coupled model can predict the growth in void fraction and interfacial area concentration beyond the capability of the one-group model, demonstrating the applicability of a coupled two-group approach to modeling boiling flow. [ABSTRACT FROM AUTHOR]

A novel approach for the solution of Stefan problem within the framework of the multi fluid model supplemented with Volume of Fluid (VOF) method, i.e. two-fluid VOF, is presented in this paper. The governing equation set is comprised of mass, momentum and energy conservation equations, written on a per phase basis and supplemented with closure models via the source terms. In our method, the heat and mass transfer is calculated from the heat transfer coefficient, which has a fictitious function and depends on the local cell size and the thermal conductivity, and the implementation is straightforward because of the usage of the local value instead of a global parameter. The interface sharpness is ensured by the application of the geometrical reconstruction scheme implemented in VOF. The model is verified for three types of computational meshes including triangular cells, and good agreement was obtained for the interface position and the temperature field. Although the developed method was validated only for Stefan problem, the application of the method to engineering problems is considered to be straightforward since it is implemented to a commercial CFD code only using a local value; especially in the field of naval hydrodynamics wherein the reduction of ship resistance using boiling flow can be computed efficiently since the method handles phase change processes using low resolution meshes.

The interfacial area concentration (IAC) is of vital importance in determining the interfacial transfer terms of mass, momentum, and energy between phases in the two-fluid model. The two-group (2-G) interfacial area transport equation (IATE) dynamically models the IAC evolution of large and small bubbles, and the intergroup transfer bookkeeps the mass transferring between two groups. Different intergroup mass transfer coefficient correlations are employed in the 2-G IATE model, which include one old correlation and three new correlations previously developed by the authors. The two-group IATE is benchmarked with subcooled boiling experimental dataset. The group-1 interfacial area concentration results show that the three modified correlations improve the physically incorrect prediction by the old correlation. With the modified correlations, the 2-G IATE is foundationally capable of predicting the group-1 interfacial area concentration in subcooled boiling flow.

A subchannel void sensor (SCVS) acquires the two-phase flow in a rod bundle as the time-series data of cross-sectional distributions. Herein, the temperature and pressure ranges of an SCVS were extended to include the rated conditions of boiling water reactors. The improved SCVSs were installed in a 5 × 5 heated rod bundle at eight height levels. In a boiling experiment using the rod bundle, the three-dimensional distributions of the boiling two-phase flow were measured over a wide pressure range (up to 7.2 MPa). The new experimental data were compared with existing experimental data and the results of a subchannel analysis. Experimental results were consistent with those of a high-energy X-ray computed tomography study of a heated rod bundle with the same geometry and under the same heat and flow conditions as those used in our study. The subchannel analysis code reproduced the experimental results fairly well, and the obtained database is applicable for validating and improving thermal-hydraulic analysis codes. [ABSTRACT FROM AUTHOR]

To solve the problem of thermal protection posed by the long working time of Mg/CO 2 powder rocket engines, an experimental system is designed to study the heat-transfer characteristics of liquid CO 2 in the regenerative cooling channel. How the heat flux, coolant temperature, mass flux, and back pressure influence the heat-transfer coefficient, the thermodynamic quality, and the gas-phase formation point of nearly saturated CO 2 is studied. The heat-transfer coefficient is found to increase with increasing mass flux or decreasing inlet temperature. Higher heat flux increases the rate of bubble formation and promotes nucleate-boiling heat transfer, but it also intensifies the film boiling, thereby degrading the heat-transfer performance. Lower back pressure does not affect the heat-transfer performance in the nucleate-boiling region, but it worsens it in the film-boiling region. It is also found that the classical boiling-heat-transfer models do not predict the experimental heat-transfer coefficient well. Combined with experimental data, how reduced pressure affects heat transfer is considered, and a new empirical correlation formula is proposed for the heat-transfer coefficient of CO 2 near saturation. The optimized model can successfully predict 90.23% of the experimental data, and the prediction accuracy is improved greatly. The present research provides a theoretical basis for designing a regenerative cooling scheme for an Mg/CO 2 powder rocket engine. • Liquid CO 2 is used as coolant for the first time to conduct heat transfer experiments in a regenerative cooling channel. • A method for calculating the heat transfer coefficient along the microchannel is established. • The effects of different heat fluxes, coolant temperatures, mass fluxes and back pressures on the heat transfer performance of CO 2 are obtained. • An empirical correlation for heat transfer of CO 2 boiling flow in a regenerative cooling channel at near saturation state is obtained. [ABSTRACT FROM AUTHOR]

The pipeline is a convenient and safe way to transport natural gas liquids (NGLs). However, the NGL is easy to boil due to the variations of pressures and temperatures along the pipeline. The bubble lift-off diameter is an essential parameter to calculate the mass and heat transfer rates between vapor and liquid phases for the NGL two-phase saturated boiling flow. This paper proposed a novel bubble lift-off diameter model based on the force-balance principle of bubbles, which considers the effects of the pressure, shear lift force, unstable drag force, surface tension, gravity force, buoyancy force, gas-phase density, bubble volume, bubble flow velocity, and bubble growth time on the bubble’s lift-off diameters at various pipe inclination angles. A total of 136 experimental data points are applied to validate the new model. Results demonstrate that the average relative deviation (ARD) between the experimental bubble’s lift-off diameters and calculated values based on the new model is in the range from 5.75% to 29.95%. In contrast, for horizontal and vertical pipes, the minimum ARDs of seven existing models (Fritz, Kocamustaf, Zeng, Lee, Situ, Hamzekhani, Chen models) are in the range from 19.42% to 42.58%, respectively. Moreover, the in-depth force analysis results reveal that the shear lift force, buoyancy force, drag force and surface tension force are dominant factors affecting the bubble lift-off diameters in inclined pipes. The new model provides an effective method to calculate the bubble lift-off diameter in the pipe at various inclination angles, overcoming the deficiencies of most existing models that only can be applied to either horizontal or vertical pipes.

In this research, a novel method to investigate the transient heat transfer coefficient in a channel is proposed, in which the water flow, itself, is considered both single-phase and two-phase. The experiments were designed to predict the temporal and spatial resolution of the Nusselt number. The inverse technique method is non-intrusive, in which time history of temperature is measured, using some thermocouples within the wall to provide input data for the inverse algorithm. The Tikhonov method is used as an inverse method. Error for estimation heat flux is around 0.06qmean. The temporal and spatial changes of heat flux, Nusselt number, vapor quality, convection number, and boiling number have all been estimated, showing that the estimated local Nusselt numbers of flow for without and with phase change are close to those predicted previously. This study suggests that the extended inverse technique can be successfully utilized to calculate the local time-dependent heat transfer coefficient of boiling flow.

A novel approach for the solution of Stefan problem within the framework of the multi-fluid model supplemented with Volume of Fluid (VOF) method, i.e. two-fluid VOF, is presented in this paper. The governing equation set is comprised of mass, momentum and energy conservation equations, written on a per-phase basis and supplemented with closure models via the source terms. In our method, the heat and mass transfer is calculated from the heat transfer coefficient, which has a fictitious function and depends on the local cell-size and the thermal conductivity, and the implementation is straightforward because of the usage of the local value instead of a global parameter. The interface sharpness is ensured by the application of the geometrical reconstruction scheme implemented in VOF. The model is verified for three types of computational meshes including triangular cells, and good agreement was obtained for the interface position and the temperature field. Although the developed method was validated only for Stefan problem, the application of the method to engineering problems is considered to be straightforward since it is implemented to a commercial CFD code only using a local value; especially in the field of naval hydrodynamics wherein the reduction of ship resistance using boiling flow can be computed efficiently since the method handles phase change processes using low-resolution meshes. [ABSTRACT FROM AUTHOR]

This study tries to employ the computational fluid dynamics (CFD) approach to investigate the influence of the simultaneous use of the metal foam (porous media) and the water-based nanofluid on the boiling flow regime. For this purpose, boiling flow heat transfer of water-based copper nanofluid inside of both simple pipe and aluminum metal foam pipe is simulated using the Eulerian two-phase CFD model. The nanofluid is supposed to flow inside the pipe with the mass flux of 1927 kg/m2s and the subcooled temperature of 10oc at atmospheric working pressure. The pipe is also under a constant wall heat flux (50–110 KW/m2). Different copper nanoparticle volume fractions (i.e., 0.5, 1, and 1.5%) are considered for this investigation. The porosity and pore density of the metal foam are 0.8 and 10 pore per inch (PPI) respectively. The heat transfer parameters of boiling flow including the wall temperature, the vapor volume fraction on the wall, the onset of the nucleate boiling (ONB), the local heat transfer coefficient, and the mean diameter of the vapor bubble are considered in this study. All heat transfer coefficients predicted by the CFD are compared with the Chen correlation. As far as the authors know, for the first time, the boiling flow of nanofluid-based water inside a metal foam pipe was simulated by the two-phase model of Eulerian. The results reveal that for the metal foam pipe, after happening of the ONB, the wall temperatures suddenly jump over the saturated temperature and the critical heat flux (CHF) happens. But no such sudden increase of the wall temperature is seen along the simple pipe before and after the ONB. Besides, the convective heat transfer coefficient in the metal foam pipe before the beginning of the boiling is more than double the boiling flow in the simple pipe. Although in metal foam pipe, once the boiling happened, the heat transfer coefficient was degraded by 50%, this value is still 25% more than the heat transfer coefficient of the boiling flow in the simple pipe. The results of this study could be practically used for boiling heat transfer analysis of heat exchangers filled by porous media and nanofluid.

• Experimental studies of R1234yf/R32 mixtures were conducted. • Flow boiling heat transfer and pressure drop inside multiport tubes were examined. • Heat transfer and pressure drops of mixtures and pure components were compared. • Effects of mass flux, heat flux, and quality on flow boiling were clarified. • A predictive heat transfer correlation for refrigerant mixtures was proposed. The flow boiling heat transfer and pressure drop of zeotropic binary mixture R1234yf/R32 were experimentally investigated inside a horizontal multiport tube with rectangular minichannels. Local heat transfer coefficients were quantified under mass fluxes in the range 50–400 kgm−2s−1, heat fluxes in the range 5–20 kWm−2, and circulation compositions of 80/20 and 50/50mass%. The obtained heat transfer coefficient of the mixtures were compared with those of pure components under the same experimental conditions. The heat transfer of the mixtures was strongly influenced by mass flux, vapor quality, and mass fraction, whereas the influence of heat flux on heat transfer was small. The heat transfer coefficients of the mixtures were lower than those of the pure components under most conditions owing to mass diffusion resistance and temperature glide; however, the heat transfer coefficients of the mixtures were same or higher than those of R1234yf at high mass flux and high vapor quality regions. The frictional pressure drops of the mixtures increased with increasing mass flux, vapor quality, and mass fraction of R1234yf. The database encompassing more than 900 and 190 for boiling heat transfer coefficient and frictional pressure drop were compared with available previous correlations. Previous correlations underestimated the heat transfer coefficients of the mixtures, especially for data with higher temperature glide and dominant forced convective heat transfer. The proposed correlation shows good agreement with the heat transfer coefficients of the R1234yf/R32 mixtures with mean and mean absolute deviations of −5.0% and 10.3%, respectively. [ABSTRACT FROM AUTHOR]

In this research, a novel method to investigate the transient heat transfer coefficient in a channel is proposed, in which the water flow, itself, is considered both single-phase and two-phase. The experiments were designed to predict the temporal and spatial resolution of the Nusselt number. The inverse technique method is non-intrusive, in which time history of temperature is measured, using some thermocouples within the wall to provide input data for the inverse algorithm. The Tikhonov method is used as an inverse method. Error for estimation heat flux is around 0.06qmean. The temporal and spatial changes of heat flux, Nusselt number, vapor quality, convection number, and boiling number have all been estimated, showing that the estimated local Nusselt numbers of flow for without and with phase change are close to those predicted previously. This study suggests that the extended inverse technique can be successfully utilized to calculate the local time-dependent heat transfer coefficient of boiling flow. [ABSTRACT FROM AUTHOR]

It is key in several industrial applications to accurately describe rapid depressurization of liquid and dense phase states. Examples include refrigeration systems, nuclear reactor cooling and CO 2 capture and storage (CCS). It is expected that large-scale CO 2 pipeline transportation must be deployed as a vital part of reaching net zero emissions by 2050. During rapid depressurization of liquid-like CO 2 , boiling will in many cases occur out of equilibrium, at a lower pressure than the local saturation pressure. Capturing the non-equilibrium effects is necessary to predict outflow rates and the resulting pressure and temperature inside the pipe. In the present work, we quantify the non-equilibrium effects by studying a series of CO 2 pipe depressurization experiments from liquid-like states at initial temperatures from 10 °C to 40 °C. We compare the experimental results to predictions of the homogeneous equilibrium model (HEM) and a homogeneous relaxation-type non-equilibrium model (HRM*) where the mass-transfer rate from liquid to gas is tuned by a relaxation time. The relaxation time was found to decrease for increasing temperatures, and it was observed to be approximately 60 times longer for the coldest experiment than for the warmest one. [Display omitted] • We present pipe depressurization experiments of CO 2 at various initial temperatures. • The CO 2 was depressurized from supercritical pressures at a dense, liquid-like state. • We compare the results to the homogeneous equilibrium model and a relaxation model. • Shorter relaxation times were found for the higher-temperature experiments. • The relaxation time correlated with the scaled, relative initial entropy. [ABSTRACT FROM AUTHOR]

Microchannel two‐phase flow is an effective cooling method used in microelectronics, in which the heat flux density is unevenly distributed usually. The paper is focused on numerical study the effect of aspect ratio on the flow boiling of microchannels with nonuniform heat flux. The heat source is a three‐dimensional (3D) integrated circuit. 3D microchannel model and volume of fluid method are coupled in numerical simulation. The results show that the aspect ratio has no relationship with the two‐phase pressure drop of the microchannel. It has a certain influence on the distribution of bubble shape. In terms of the heat transfer coefficient, the aspect ratio has a certain influence on a section of the inlet. Due to the nucleate boiling, the convective heat transfer in the remaining areas is the dominant factor and the average heat transfer coefficient is mainly determined by the heat flux at the bottom of the channel. [ABSTRACT FROM AUTHOR]

Determination of local heat transfer coefficient at the interface of channel wall and fluid was the main goal of this experimental study in microchannel flow boiling domain. Flow boiling heat transfer to DI-water in a single microchannel with a rectangular cross section was experimentally investigated. The rectangular cross section dimensions of the experimented microchannel were 1050 μm × 500 μm and 1500 μm × 500 μm. Experiments under conditions of boiling were performed in a test setup, which allows the optical and local impedance measurements of the fluids by mass fluxes of 22.1 kg·m−2·s−1 to 118.8 kg·m−2·s−1 and heat fluxes in the range of 14.7 kW·m−2 to 116.54 kW·m−2. The effect of the mass flux, heat flux, and flow pattern on flow boiling local heat transfer coefficient and pressure drop were investigated. Experimental data compared to existing correlations indicated no single correlation of good predictive value. This was concluded to be the case due to the instability of flow conditions on one hand and the variation of the flow regimes over the experimental conditions on the other hand. The results from the local impedance measurements in correlation to the optical measurements shows the flow regime variation at the experimental conditions. From these measurements, useful parameters for use in models on boiling like the 3-zone model were shown. It was shown that the sensing method can shed a precise light on unknown features locally in slug flow such as residence time of each phases, bubble frequency, and duty cycle.

In this work, argon flow within different microchannels with square cross-section is simulated by the molecular dynamics simulation (MDS) method. The first simulation is done within ideal microchannel and then, internal surfaces of this microchannel are roughened by cone, cubic and spherical barriers respectively to report influences of roughness with different geometries on the distribution of fluid flows. It is reported that cone and spherical geometry of barriers do not strongly affect flow characteristics of argon fluid within the microchannel, while the cubic geometry of barriers delays density distribution of argon flow in the middle region of the microchannel due to their destructive role against translocation of fluid particles between lateral and central layers of the microchannel. For the density results, it is reported that two physical phenomenon of thermal and external energies play important roles on the distribution of fluid particles in different sections of microchannel due to their different applying directions. Moreover, it is observed that the temperature profile of fluid flow within microchannel with cubic barriers touched a maximum temperature of 420 K at the central layers which are in the highest temperature level among all cases studied. Results of this study are applicable for practical application in small scale designs such as medical probes to inject destructive fluid for destroying tumors in cold surgeries. [ABSTRACT FROM AUTHOR]

• Interfacial drag forces for interior, edge, and corner subchannels were formulated in a rod bundle. • Subchannel-average void fraction covariance was empirically modeled for saturated boiling flow. • Subchannel-average void fraction covariance was analytically modeled for subcooled boiling flow. • The covariance models were validated by steam-water data up to 8.6 MPa. • The relative velocity covariances were calculated from the void fraction covariances. Accurate simulation of boiling two-phase flows in a rod bundle is indispensable for the robust, economical, and safe design of various heat transfer systems using the rod bundle configuration. Subchannel analysis codes are used for this purpose. Interfacial drag force modeling significantly affects the prediction accuracy of the void fraction. The void fraction and relative velocity covariances constitute the interfacial drag force. However, the covariances are currently not considered in existing subchannel codes due to a lack of reliable constitutive equations to calculate the void fraction and relative velocity covariances. This study aims to model the subchannel-average void fraction and relative velocity covariances for subcooled and saturated boiling flows in three types of subchannels in a rod bundle. The considered subchannels are interior, edge, and corner subchannels. The subchannel-average void fraction and relative velocity covariances for saturated boiling flow are modeled by the data obtained from local void fraction data collected for saturated boiling flow in an 8 × 8 rod bundle under pressures from 1.0 to 8.6 MPa. The subchannel-average void fraction and relative velocity covariances for subcooled boiling flow are modeled based on the bubble-layer thickness model. The modeled subchannel-average void fraction and relative velocity covariances are well validated with the experimental data. The modeled subchannel-average void fraction and relative velocity covariances are expected to be implemented in subchannel analysis codes to improve the void fraction prediction accuracy in each subchannel type. [ABSTRACT FROM AUTHOR]

Highlights • The boiling heat transfer and pressure drop of R32 inside a horizontal small-diameter microfin tube were investigated. • The adiabatic and boiling flow patterns were observed. • The effects of mass velocity, quality, heat flux, and flow pattern on boiling flow characteristics were investigated. Abstract This study experimentally investigated the flow boiling heat transfer and pressure drop characteristics of R32 inside a horizontal small-diameter microfin tube with an equivalent diameter of 3.48 mm. The boiling heat transfer coefficient and pressure drop were measured in a mass velocity range of 50–400 kgm−2 s−1, heat flux range of 5–20 kWm−2, and saturation temperature of 15 °C. The flow pattern at the outlet of the test microfin tube was observed and classified as wavy-slug flow and annular flow regime. The effects of quality, mass velocity, heat flux, and flow pattern on the flow boiling characteristics were clarified. The heat transfer mechanism was characterized by forced convection, nucleate boiling, and evaporation heat transfer through the thin liquid film inside the grooves. No difference in the frictional pressure drop on adiabatic and boiling flows was observed. The measured heat transfer and frictional pressure drop were compared with those in previous correlations. [ABSTRACT FROM AUTHOR]

In CFD simulations of boiling flow and CHF phenomenon, the solid domains are usually neglected to reduce the complexity and computational time. However, based on the simulation of three test sections, the azimuthal heat conduction in the solid domains is found to have a significant influence on the numerical results, especially on the wall temperature distribution. This technical note tries to give the researchers some suggestions about the azimuthal heat conduction effects on boiling flow by CFD simulation of steady state flows in vertical pipes and rod bundle geometry. The CFD modeling methodology was developed using FLUENT. Eulerian two-phase flow is used to model the flow and heat transfer phenomena of boiling flow. Three boundary conditions are tested to investigate the effect of azimuthal heat conduction. The results show that the three boundary conditions are nearly equivalent to each other for subcooled boiling flow in centrosymmetric and non-centrosymmetric geometry, like vertical pipe and rod bundle. But for the CHF phenomenon in rod bundle, the simplified BC overestimates the azimuthal inhomogeneity and the peak temperature of the rod surface. During this process, azimuthal heat conduction plays a key role in this result. The conclusions require researchers should model the entire domain in the CFD simulation and set the heating boundary according to the experiment as accurately as possible to better predict the flow and heat transfer characteristics in the boiling flow. [ABSTRACT FROM AUTHOR]

Tube vibration inevitably occurs on transfer lines of liquid hydrogen (LH2) and affects the heat transfer characteristic of LH2. In this study, a three-dimensional numerical method based on RPI boiling model and vibration model has been built to investigate the influence of tube vibration on boiling flow with LH2. The model has been partly verified by the experimental data from the literature and considered effective for liquid hydrogen boiling flow. The changes in the partition of heat flux were analyzed under certain conditions and the relative heat transfer coefficients under different amplitudes, frequencies and inlet velocities were compared. The numerical results indicate that the vibration can significantly enhance the convective heat flux while weaken the quenching heat flux and the evaporative heat flux. It illustrates that the changes of relative heat transfer coefficients are corresponding to the vibration velocity. In addition, the enhancement of heat transfer is more obvious when the Reynolds number of LH2 is relatively low. [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]

• X-ray CT experiments are performed for boiling flow in a 3 × 3 rod bundle with a spacer. • High-resolution data on the void fraction distribution are obtained. • The influence of mixing vanes, mass and heat fluxes is investigated. • The effect of heat flux is more remarkable at high mass flux than at low mass flux. • The mixing vanes cause bubble accumulation even at high mass flux or low heat flux. We conducted boiling flow experiments in a 3 × 3 rod bundle containing a spacer grid with split type mixing vanes where the central rod was heated. Void fraction around the spacer grid was measured with high-resolution X-ray computed tomography. The study focusses on the effects of mass and heat flux on the void fraction downstream of the spacer for mass flux and heat flux ranging from 535 to 1950 kg/m2 s and 42.8 to 107.2 kW/m2, respectively. As the working fluid, refrigerant Octafluorocyclobutane (RC 318) was used and fluid scaling was applied. We found that the void fraction increases as the flow passes through the vanes and then decreases downstream until Z ≈ 4 D h , and then increases again. In addition, we found that the mixing vanes cause a local increase in void fraction even at low heat flux or high mass flux, and that the arrangement of the vanes influences the size and location of the high and low void content regions. We also found that the effect of heat flux on the relative void fraction is more noticeable at high mass flux than at low mass flux. In the second part of the study, which will be presented in a separate paper, we investigated the effect of different vane angles on the void fraction. [ABSTRACT FROM AUTHOR]

We study the evolution of flow and temperature of a fluid moving upstream in a long, thin vertical pipe when a boiler element is involved. The main goal of this work is to understand how the size and position of the boiler will affect the flow and temperature in the pipe over time, as current literature considers cases where the heater or boiler covers the whole length of the pipe, or when already boiling fluid enters a pipe without a boiler. Therefore, we shall allow for a boiling element which covers only a fraction of the pipe when devising out mathematical model. The boiling process results in a transition to different multiphase flow regimes, and we therefore consider a two-phase flow model. From this model, we obtained a simplified one-dimensional model, since we are concerned with a long, thin pipe, under reasonable assumptions and reductions which still preserve the desired physics. We performed a stability analysis for the boiling boundary denoting the phase change in this model. We then obtained numerical simulations for the steady and transient solutions. The numerical results suggest that both the size and position of the boiler strongly affect the flow regime. In particular, depending on the size of the boiler, transition to other phases might not always occur, and depending on its position along the pipe, the fluid coming out at the top of the pipe might not have the desired thermal profile. As such, one may tailor the position and size of the boiler element in order to obtain a useful thermal profile for particular applications. Such results are of possible relevance in industrial applications where heating or boiling of fluid is required. [ABSTRACT FROM AUTHOR]

The boiling flow or condensation is widely encountered in many industrial applications for both cooling as well as heating processes. Compact heat transfer devices, such as micro-heat exchangers and evaporators, are extensively used for both cooling as well as heating processes over conventional heat exchangers, such as microelectronic circuits, automobile and aerospace industries, due to high surface area to volume ratio and heat transfer rates, compactness and easy thermal control. For better design of micro- or mini-heat exchangers, a detailed specific knowledge of the multiphase flow and its properties such as the flow pattern during flow boiling, critical heat flux (CHF) and stable operation are very important. This paper provides a state of art review on boiling flow in microchannels since year 2000 till date. Flow patterns formed and the parameters influencing flow pattern transitions, during multiphase heat transfer in micro- or mini-channels, have been reviewed in detail. The flow regimes and flow pattern maps, and modeling approaches considered for boiling flow in micro-channels/devices with various challenges have been discussed. A lot of contradiction between the experimental data has been observed for the analysis of flow regimes and flow pattern maps. Further, the effect of hydrodynamics during flow boiling and CHF on heat transfer coefficient has been discussed in detail. Recently, with the advancement in measurement techniques, the heat transfer measurement technologies have been synchronized with the visualization techniques, which helped in understanding the boiling flow physics in micro- and mini-channels. Therefore, an in-depth understanding of flow patterns and regimes under boiling flow conditions in mini- and micro-channels can be used to predict the boiling heat transfer mechanism, which can be further used for developing better heat transfer models for boiling flow. Further, enhancement in heat transfer coefficient for boiling flow in microchannels, either by using complex microchannel configurations or nanocoating on the microchannel surface, have received attention recently, which have been discussed and analyzed in the present review. Both micro- and mini-channels have number of applications in aerospace, refrigeration and computational systems; therefore further attention is needed for more robust and precise design. [ABSTRACT FROM AUTHOR]

This study investigated the effect of tube diameter on flow boiling characteristics of refrigerant R32 in horizontal small-diameter tubes with 1.0, 2.2, and 3.5 mm inner diameters. The boiling heat transfer coefficient and pressure drop were measured at 15 °C saturation temperature. The effects of mass velocity, heat flux, quality, and tube diameter were clarified. The flow pattern of R32 for adiabatic two-phase flow in a horizontal glass tube with an inner diameter of 3.5 mm at saturation temperature of 15 °C was investigated. Flow patterns such as plug, wavy, churn, and annular flows were observed. The heat transfer mechanisms of forced convection and nucleate boiling were similar to those in conventional-diameter tubes. In addition, evaporation heat transfer through a thin liquid film in the plug flow region for low quality, mass velocity, and heat flux was observed. The heat transfer coefficient increased with decreasing tube diameter under the same experimental condition. The fictional pressure drop increased with increasing mass velocity and quality and decreasing tube diameter. The experimental values of the heat transfer coefficient and frictional pressure drop were compared with the values calculated by the empirical correlations in the open literature. [ABSTRACT FROM AUTHOR]

Boiling heat transfer technology is widely used in the barge-mounted nuclear power plant and received increasing attention recently, because that the boiling flow behaviors under ship motion conditions are very complicated and important. In order to study the boiling heat transfer characteristics of pulsating flow in a rectangular channel under rolling motion, a series of experiments were performed. The results demonstrate that the boiling heat transfer coefficient fluctuations have the same period with the rolling motion. The fluctuation intensity of the coefficient increases with the increase of rolling amplitude and period. The time average boiling heat transfer coefficient of pulsating flow under rolling motion condition is equal to that in the steady state. Based on this phenomenon a new correlation for predicting the time average boiling heat transfer coefficient of rectangular channel under rolling motion and steady state was proposed, with a MAE of 15.1%, as well as 95.4% and 81.3% of the data points within ±30% and ±20% error bands, respectively. Furthermore, the predicted correlations that used to calculate the relative pulsation amplitude and instantaneous boiling heat transfer coefficients of pulsating flow were developed based on the heat transfer characteristics of pulsating flow under rolling motion conditions. [ABSTRACT FROM AUTHOR]

In this study, two-phase flow pressure drop was experimentally measured in condensation and evaporation fluid flow. Refrigerants R134a and R32 were tested in a multiport extruded aluminium tube (MPEs) with hydraulic diameters of 0.715 and 1.16 mm. Experimental conditions were in a range: mass velocity 200–1229 kg/m 2 s, heat flux 2.55–70 kW/m 2 , saturated temperatures (5,7.5,12.5,30,35,40,45,50,55) °C. To do so, two installations have been used. They were constructed at the Technical University of Cartagena Spain, for the study of boiling and condensation in tubes. They are briefly described in this paper jointly with the experiments performed. The experimental data are compared with some well-known correlations which were developed for macro/mini-channel tubes. The homogeneous mixture model slightly underestimates the experimental boiling data. Classic macro-channel correlations: Friedel and Müller-Steinhagen and Heck predict satisfactorily well our experimental pressure drop data. Some of the new correlations specially developed for mini/micro-channels have been tested, finding that Cavallini et al. and Zhang and Webb predict with reasonable accuracy the experimental two-phase flow pressure drop data. [ABSTRACT FROM AUTHOR]