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• 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]

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]

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

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

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

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

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

As one of the most widely used refrigerants, R134a has been investigated extensively in the past 20years, and research into R134a two-phase flow boiling heat transfer remains a hot spot. A number of correlations for two-phase flow boiling heat transfer have been proposed. However, a correlation with high prediction accuracy for R134a two-phase flow boiling heat transfer remains needed. The purpose of this work is to propose a better correlation for R134a two-phase flow boiling heat transfer. Based on the database consisting of 2286 data points of R134a two-phase flow boiling heat transfer compiled from 19 published papers, a new correlation for two-phase flow boiling heat transfer coefficient is developed. The new correlation takes advantage of the newly defined dimensionless number Fa that is highly related to flow boiling heat transfer. It has a mean absolute deviation (MAD) of 14.2%, with 74.4% of the sample having a relative deviation within ±20%, while the best one among 20 existing models available in literature only has an MAD of 34.9%, with 31.2% of the sample having the relative deviation within ±20%. The new correlation also shows high prediction accuracy for other refrigerants. [ABSTRACT FROM AUTHOR]

Abstract: Parallel channel heat transfer system is an attractive means to enhance heat transfer by increasing the total surface area in the heat exchanger. Uniform distribution of flow in the channels is an important operation attribute in order to avoid channel dryout and the subsequent hot spots and possible device failure. For the system operating in the single-phase regime, uniform flow distribution is a stable equilibrium. However, in the two-phase boiling regime, stable equilibrium bifurcates – the uniform distribution becomes unstable, and new stable or unstable non-uniform distributions, or maldistributions, equilibria emerge. As a result, parallel channel heat exchangers typically operate in the single phase regime to avoid such “maldistribution instability”. This paper presents a stability analysis and active flow and temperature control of a parallel channel evaporator in a pumped two-phase boiling heat transfer loop. We show that for identical channels, the system is uncontrollable with the pump alone and unobservable from the overall flow rate measurement. A conventional solution involves introducing additional valve-induced pressure drop, but the drawback is a resulting higher pumping power. We present a new, somewhat surprising, result that if the channel characteristics are all distinct, the system actually becomes controllable from the pump and observable from the total loop flow rate. For non-identical parallel systems, we show a controller design approach and demonstrate its efficacy through a simulation example. [Copyright &y& Elsevier]

Abstract: The transient behavior of flow boiling in vertical porous channels is numerically studied in this paper. The velocity and temperature fields under different parameters for both aiding and opposing flows are investigated. Distinctly different flow and heat transfer features are observed in the comparison of aiding and opposing flows. An analysis of liquid saturation along the heated wall indicates that the minimum liquid saturation for aiding flow is located at the tail of the heated section, whereas for opposing flow, it is within the heated section and shifts upstream with the increase of Rayleigh number and decrease of Peclet number. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents new experimental flow boiling heat transfer results in micro-scale tubes. The experimental data were obtained in a horizontal 2.3mm I.D. stainless steel tube with heating length of 464mm, R134a and R245fa as working fluids, mass velocities ranging from 50 to 700kgm−2 s−1, heat flux from 5 to 55kWm−2, exit saturation temperatures of 22, 31 and 41°C, and vapor qualities ranging from 0.05 to 0.99. Flow pattern characterization was also performed from images obtained by high-speed filming. Heat transfer coefficient results from 1 to 14kWm−2 K−1 were measured. It was found that the heat transfer coefficient is a strong function of heat flux, mass velocity and vapor quality. The experimental data were compared against ten flow boiling predictive methods from the literature. Liu and Winterton , Zhang et al. and Saitoh et al. worked best for both fluids, capturing most of the experimental heat transfer trends. [Copyright &y& Elsevier]

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