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This study investigates the flow boiling heat transfer characteristics of refrigerants R134a and R407c in horizontal micro-fin tubes. We conducted experiments across a range of heat flux (15–35 kW·m−2), mass flux (50–250 kg·m−2 s−1), and saturation temperature (15 °C-25°C) conditions. Our findings reveal that R134a consistently exhibits a higher heat transfer coefficient (HTC) compared to R407c, making it advantageous for applications requiring efficient heat transfer. However, R134a also shows a tendency towards premature dry-out flow patterns at higher heat fluxes, which could limit its effectiveness under certain conditions. In contrast, R407c demonstrates more stable flow regimes, maintaining wavy-annular and annular flows without early dry-out, which highlights its robustness at higher heat fluxes. Flow pattern maps indicate that higher heat fluxes promote the formation of wavy and annular flow patterns in both refrigerants, with R134a transitioning to dry-out flows more readily than R407c. The influence of saturation temperature on HTC was also significant, with lower temperatures resulting in higher HTCs for both refrigerants. This can be attributed to the increased thermodynamic driving force for phase change at lower temperatures. Our study aligns with previous research, corroborating the critical role of micro-fin tubes in enhancing HTC. The findings have practical implications for the design and optimization of refrigeration systems, emphasizing the need to carefully select refrigerants and operating conditions to achieve efficient and stable heat transfer performance. [ABSTRACT FROM AUTHOR]

Flow boiling in microchannels with porous walls has received extensive attention in recent years. Compared with the emphasis on heat transfer, there is a lack of research on the effect of the porous wall structures on the pressure drop characteristics. In this study, systematic experiments are performed to measure the pressure drop of water-vapor two-phase flow in five microchannels with copper foam fins, which consist of nine or six channels and fins of copper foam. The porosities of the foam fins range from 0.78 to 0.82 and ratios of fin width to channel width range from 0.5 to 2. The channels are approximately 0.5 or 1 mm in width and 1 mm in height. Both adiabatic and flow boiling experiments are conducted with water at mass fluxes ranging from 66 to 407 kg/(m2 s). In the adiabatic experiments, the average quality in channels is between 0.017 and 0.846. In the flow boiling experiments, the outlet quality of channels is between 0.040 and 0.863. Slug flow, churn flow, annular flow, and wispy-annular flow are observed in adiabatic experiments. A two-phase frictional pressure drop correlation based on the Lockhart-Martinelli model is developed for copper foam fin microchannels by introducing the effects of the mass flux, porosity, ratio of fin width to channel width, and heating condition step by step. The mean absolute percentage errors of the new correlation are 7.53% for 325 data points under adiabatic conditions and 5.51% for 268 data points under flow boiling conditions, respectively. This work provides insight into the correlations of frictional pressure drop in microchannels with porous walls. [ABSTRACT FROM AUTHOR]

Microchannel flow boiling heat transfer has the advantages of strong heat dissipation capacity, good temperature uniformity, and compact structure. It is an excellent way to thermally manage electronic devices, but when the heat flux exceeds CHF (Critical Heat Flux), the heat transfer performance deteriorates as the working fluid dries out. Non-azeotropic mixtures have the potential to effectively delay or avoid dry-out during the boiling process due to their temperature slide characteristics which causes the mass transfer resistance. To understand the influence of non-azeotropic mixtures on microchannel flow boiling, using the phase-change microchannel heat sink as the research object, the experiments on the flow boiling heat transfer performance of R245fa/R134a mixtures under different working conditions were carried out, and the characteristics of flow boiling heat transfer were obtained under the different working conditions, and comparison was developed with those of pure substance R245fa. The results demonstrated that a small amount of low-boiling-point components in the high-boiling-point working fluid inhibited boiling heat transfer to some extent, and lowered the average heat transfer coefficient under the non-dryout condition slightly lower than that of the pure substance; however, it also effectively delayed the onset of local dry-out and prevented significant deterioration in thermal transfer performance under the lower mass flow rate and higher heat flux, which could enhance the heat sink's stability. [ABSTRACT FROM AUTHOR]

Numerous energy systems, including distillation, power production, air conditioning, cooling, and purification, use evaporation and flow boiling in minichannels. In this study, we show noticeably higher heat transfer coefficients and critical heat flux of 182% and 114% during DI water flow boiling in ZnO-nano-thin-film nanostructured (∼110–423 nm), industrial-scale-heated copper bottom surface. By using a combination of the sol–gel spin coating and annealing methods, we produce durable and highly conformal nanostructured surfaces that enable scale nano-manufacturing. Flow boiling experiments were carried out in 1.5 mm height bottom surface-heated minichannel using DI water as the working fluid. In order to measure the effectiveness of present method and clarify how the structural length scale affects it, the present study ZnO-nano-thin-film structured surfaces are compared with previously published micro/nano-scale fabricated surfaces, demonstrating the necessity and importance of the nanoscale properties of ZnO-nano-thin films for improvement. The surfaces of the nano-thin film were subjected to durability testing utilizing a seven-day continuous flow boiling experiment, which revealed minor deterioration. The higher boiling performance is achieved on ZnO-TF-423 is due to the proper bonding between polished copper bare surface (BS) and deposited ZnO thin films. Additionally, the rough surface on BS allows the copper and ZnO thin films to bind properly. It can be concluded that surfaces made using an efficient sol–gel spin coating process possesses superior boiling heat transfer capabilities at comparatively lower surface temperatures, suggesting a smaller chance of damaging the surface from rising temperatures. [ABSTRACT FROM AUTHOR]

With the advancement of micro machining technology, the high-heat-flux removal from miniature electronic devices and components has become an attractive topic. Flow boiling in micro-channels is an optimal form of heat transfer and has been widely employed in high-heat-flux cooling applications. This comprehensively-reviewed article focused on the available recent literatures of experimental investigation regarding the flow boiling heat transfer and unstable behaviors of the fluid with lower boiling point in micro-channels. The thermal-fluid characteristics and potential heat transfer mechanisms of low-boiling-point fluids flow boiling in different narrow passages were summarized and discussed. The literatures regarding the pressure drop and occurrence of the unstable phenomena existing in two-phase flow boiling process were also discussed. The emphasis was given to the heat transfer enhancement methods as well as instability elimination, and various methods such as modification of surface and channel flow geometries were considered. Some future researches in the field of micro-scale flow boiling were suggested. [ABSTRACT FROM AUTHOR]

In current study, a flow boiling experimental examination has been performed on water, water-TiO2 nanofluid, and water-Al2O3 nanofluid and Gradient Boost Regression model was established to predict the Nusselt number of TiO2 and Al2O3 nanofluids. Nusselt number was calculated by varying heat flux, Reynolds number, and volumetric concentration. Good agreement was shown by water results acquired from experiments with the correlation of Chen and CFD. Results demonstrated that the Nusselt number of TiO2 and Al2O3 nanofluid was larger than water. Increment in Nusselt number was noticed with the increment in concentration, heat flux, and Reynolds number. It was seen that Al2O3 nanofluid was superior to TiO2 nanofluid. Highest improvement in the Nusselt number was 37% shown by Al2O3 nanofluid when compared to water. Correlations of Nusselt number were developed for both nanofluids. GBR-GSO machine learning model showed a good agreement with the experimental Nusselt number. [ABSTRACT FROM AUTHOR]

In this study, the flow pattern and bubble behavior of R1233zd(E) during subcooled flow boiling in parallel channels are experimentally investigated with visualization and thermal measurement. The test section is composed of 21 rectangular mini channels with the hydraulic diameter of 1.5 mm and the length of 140 mm. Bubbly flow, slug flow, churn flow and wavy-annular flow occur in sequence with the increase of vapor quality, while transient flow pattern transition process involving multiple flow patterns are also captured. The distribution of flow pattern is non-synchronized and axial-asymmetric, with earlier flow pattern transitions observed in peripheral channels away from the center axis. The initial nucleate site in each channel also show a random and axial-asymmetric distribution, while faster bubble growth can be noted in some channel under the comprehensive effects of liquid evaporation and bubble coalescence. The variation of heat transfer coefficient is correspondence to the flow pattern transition, showing different trends along the flow direction. The increase of mass flux can lead to delayed flow pattern transition and variation of heat transfer coefficient. In addition, higher heat transfer coefficient can be noted in channels away from the center axis. [ABSTRACT FROM AUTHOR]

Heat sinks of copper foam fin microchannels are developed to deal with cooling challenges. The heat sinks consist of fins made of copper foam and channels. The channels are 0.5 mm in width and 1 mm in height, and the fins are 0.5 and 2.0 mm in width. Flow boiling experiments are conducted using R134a at subcooled and saturated inlet conditions. The heat flux is between 22 and 172 W/cm2, and the mass flux ranges from 264 to 1213 kg/(m2 s). The influence of the quality, the heat flux, and the mass flow rate on the heat transfer coefficient is obtained. It is found that wider fin raises the heat transfer coefficient. A correlation is developed based on heat transfer mechanisms, and it predicts the experimental result with a 12% mean absolute error. Compared with a solid fin microchannels heat sink, the heat transfer coefficient of the copper foam fin microchannels is higher (up to 60%) when the heat flux is lower than 100 W/cm2. The copper foam fin microchannels may enhance the heat transfer coefficient and reduce the pressure drop at the same time. [ABSTRACT FROM AUTHOR]

There are several industrial applications where boiling is used, for example boilers, refrigeration systems, nuclear reactor cooling, and microelectronic chip cooling. Experimental research has been carried out to determine the flow boiling heat transfer capabilities of copper-alumina-coated surfaces for application in heat transfer equipment. De-ionized (DI) water is used as the coolant for experimentations in a minichannel with dimensions 10 × 1.5 × 10 mm. Copper surfaces coated with thin copper-alumina nanocomposite films are created using the electrodeposition process. The coated layer created using an electrochemical technique offers strong adhesiveness with the base copper and is therefore anticipated to be suitable for real-world heat transfer appliances as part of the ongoing scientific development in subcooled flow boiling. The electrochemical technique offers easier control over its various parameters, such as current density, duration and electrolyte composition, making it possible to easily achieve a variety of surface characteristics, such as crystallinity, wettability and porosity. as required in the coated surfaces. Additionally, the copper-alumina is a hydrothermally stable oxide material that is well suited for use in boiling heat transfer devices. The boiling (subcooled flow) heat transfer tests are carried out at various mass flows. The improvement in the two-phase heat transfer coefficient (HTC) and critical heat flux (CHF) can reach up to 90 % and 93 %, respectively. The coated surfaces have improved CHF and HTC because of improved wettability, increased surface roughness, and the existence of active nucleate sites in high-density. [ABSTRACT FROM AUTHOR]

Flow boiling systems onboard some flight vehicles operate under hypergravity conditions. Many investigations in frictional pressure drop (FPD) of two-phase flow boiling in channels have been conducted extensively under normal gravity (Earth's gravity, 1 g) and microgravity, while few under hypergravity so far. This study presents the experimental investigations into the effects of hypergravity levels on two-phase flow boiling FPD, with refrigerant R1234yf flowing in an electrically heated horizontal mini-tube of 2.01-mm inner diameter. The experimental range of parameters was gravity levels of 1–3.16 g, mass fluxes of 365 and 570 kg/m2 s, saturation pressures of 0.62 and 0.70 MPa, heat fluxes of 47.6 and 97.5 kW/m2, and vapor qualities up to 0.985. The effects of gravity, vapor quality, heat flux, mass flux, and saturation pressure on FPD were analyzed. The results demonstrate that the flow boiling FPD increases with increasing vapor quality x up to around 0.7, and decreases sharply with increasing quality when x > 0.7. The FPD increases significantly with increasing mass flux and negligibly with increasing gravity level, heat flux and saturation pressure. The experimental data were compared with 13 saturated flow boiling FPD correlations for normal gravity, and the Xu and Fang (2012) correlation has the highest prediction accuracy, with the mean absolute deviation (MAD) of 8.0%. [ABSTRACT FROM AUTHOR]

This research investigates the complex phenomenon of nanofluid flow boiling and its associated heat transfer characteristics. Employing advanced numerical simulations and machine learning techniques, we explore the behavior of dimensionless heat transfer correlations in subcooled flow boiling scenarios using water and SiO2/water nanofluids. The study begins with an adaptive design of experiment, yielding a comprehensive dataset of nearly 250 simulations. In the numerical simulation phase, the extracted data are examined, and the grid independence of the modeling process is confirmed, ensuring robustness and reliability. The data, validated against experimental results, provides critical insights into the intricate interplay of dimensionless numbers influencing Nusselt number behavior. Subsequently, the extracted dataset from the numerical simulations underwent a two-stage feature selection process, incorporating Pearson correlation and iterative techniques, to identify the most influential dimensionless parameters for the calculation of the Nusselt number. Later, the data are randomly split into training and testing sets (70–30%), and predictive models are developed using Python, leveraging libraries such as Pandas, NumPy, scikit-learn, and Keras. A tenfold cross-validation approach is employed to ensure model stability and accuracy. Through response surface methodology (RSM), we establish regression equations for average and local Nusselt numbers, achieving minimal mean absolute error (MAE) and high R-squared (R2\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$${R}^{2}$$\end{document}) values, demonstrating the effectiveness of our approach. Further enhancing predictive capabilities, we explore random forest, support vector machine, and artificial neural network (ANN) models. The ANN model emerges as the top performer, offering exceptional accuracy with MAE below 2.21% and R2\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document}$${R}^{2}$$\end{document} above 0.95 for both average and local Nusselt numbers. Notably, the machine learning algorithms, from data preprocessing to the final model evaluation, required only about 10% of the time invested in the numerical simulations. [ABSTRACT FROM AUTHOR]

As per the available literature, most of the techniques used for boiling surface modification utilize high operational temperature and longer production duration, ensuing in higher energy utilization with more safety worries for large scale of manufacturing. Again, the important criterion for boiling surfaces is to control the porous layer (porosity) to achieve high rate of cooling. Considering these aspects, a nature-inspired and simple technique for surface modification is proposed in the present work, which is single-step forced convection electrochemical deposition followed by single-step sintering. In this technique, the surface properties can be easily managed. Using this technique, the nanocomposite coatings of higher thermal conductive Cu–TiO2 (~ 300 W/mK) nanoparticles are fabricated on copper heating surface. The growth in nanograins is occurred during sintering process, which enhances the connectivity between the deposited nanograins. The porosity, thickness of porous layer, roughness, and wettability of the electrodeposited structured surfaces are raised with amplified current supply up to 75%, 42 µm, 1.32 µm, and 38°, respectively. The heat transfer performances of these developed surfaces are analyzed through flow boiling experiments. The maximum enhancement in critical heat flux (~ 143%) and heat transfer coefficient (~ 153%) is attained on developed coated surface at lower mass flux. These augmentations are attributed to better surface wettability and better surface morphological characteristics of the developed surfaces, which are due to the existence of sufficient liquid microlayer on them. The proposed electrodeposition technique can be employed for practical cooling applications where effective handling of high rate of heat flux is a challenge. [ABSTRACT FROM AUTHOR]

This is the second report on the development of heat loss model for flow boiling experiments conducted onboard International Space Station (ISS). In the first paper (Inoue et al. 2021), a heat loss model was developed for subcooled flow boiling experiments, a single-phase liquid at the inlet of heated test tube was discussed. In the present report, a heat loss model is developed for saturated flow boiling experiments, where two-phase liquid and vapor mixture flows to the inlet of heated test tube. The accurate evaluation of vapor quality is essential for the exact investigation of gravity effects on flow boiling. Since long straight test tube could not be introduced due to the strict dimensional restriction of experimental setup in the ISS experiments, a high-power serpentine-type preheater is employed for adjusting vapor quality at the inlet of heated test tube. Therefore, in addition to the heat loss model for subcooled flow boiling developed in the first report, where the temperature of subcooled liquid is measured directly behind the preheater, an additional heat loss model for the sections including the preheater and the downstream piping is needed. A new heat loss model is developed based on the heat transfer data obtained in the preliminary heat loss experiments with liquid single-phase flow and in the liquid-vapor two-phase flow where power is supplied only to the preheater. For the ISS experiments, the amount of heat loss is quite large due to the existence of the avionics air flowing inside the experimental setup. Decrease of vapor quality due to the heat loss increases as the mass velocity decreases and it reaches a difference of predicting quality as much as 0.53 in an extreme example for low mass velocity of 50 kg/(m2·s). Developed heat loss model can estimate vapor quality at the inlet of heated test tube with sufficient accuracy for investigating detailed flow boiling characteristics under microgravity conditions. [ABSTRACT FROM AUTHOR]

There are plenty of literature research studies investigating two-phase heat transfer characteristics of propane under varying operational conditions. Based on the collected data retrieved from the experimental measurements, several flow boiling heat transfer correlations have been proposed up to now. However, the prediction accuracy of the proposed correlations for propane refrigerant is still in question as most of the correlation is developed for their measurements or derived for a limited range of operational conditions. To conquer this drawback, this study proposes a new flow boiling heat transfer model for smooth tubes based on a propane experimental database compiled of 2179 points obtained from different eighteen laboratories around the world. Operational conditions of the database cover mass fluxes between 50 and 600 kg/m2s, saturation temperatures between − 35.0 and 43.0 °C, heat fluxes between 2.5 and 227.0 kW/m2, hydraulic diameters between 0.3 and 7.7 mm, and thermodynamic qualities 0.01 to 0.99. Estimations performed by the new flow boiling model have been compared to those obtained by the literature correlations, and comparative results indicate that the proposed model surpasses the existing flow boiling in terms of prediction accuracy with a mean absolute error of 19.1% and mean relative error of 1.7%. [ABSTRACT FROM AUTHOR]

Flow boiling experiments in microchannels under low mass fluxes have rarely been reported in the literature. In this study, flow boiling critical heat flux (CHF) for a parallel straight copper microchannel heat sink with 41 channels and hydraulic diameter of 420 µm (width: 300 µm; height: 700 µm) is reported under low mass fluxes(< 100 kg/m2s). The flow patterns reveal that the inertial force will be too low to flush the bubble downstream. The heat transfer coefficient was observed to decrease with an increase in the heat flux or vapor quality, which pointed toward convective boiling dominancy inside the channels. When approaching the CHF, the vapor flowed back to the heat sink's inlet plenum, resulting in a flow blockage, which is the main factor for the CHF occurrence. [ABSTRACT FROM AUTHOR]

The convective flow boiling characteristics of aqueous Al2O3 and TiO2 nanofluids are presented in this paper. Stable suspensions of aqueous Al2O3 and TiO2 nanofluids at two different concentrations of 0.01 and 0.1 wt% were prepared and allowed to flow through a horizontal annulus under varying heat flux from 26,140.132 to 53,573.503 W⋅m−2 and varying flow rates from 3 to 6 L⋅min−1. Parametric effects of nanoparticle concentration, heat flux, and flow rate on the heat transfer coefficient (HTC) were studied. Variation in pressure drop and pumping power at studied concentration and flow rates was estimated. In addition to that, bubble growth and nucleation site were analyzed using flow visualization technique. Enhancement in the HTC was obtained with increasing concentration and flow rate. A maximum of 2.4 times enhancement in HTC relative to water was observed. Bubble growth and nucleation site density increased with increasing heat flux and nanofluid concentration. Pressure drop and pumping power increased with nanoparticle addition and even at such a low-flow condition. Highest pressure drop of 33.33 % was registered with nanofluids at 0.1 wt%. The change in heating surface roughness due to particle deposition affects heat transfer performance. Finally, figure of merit showed that nanofluids are better alternatives to their basefluids for superior heat transfer. [ABSTRACT FROM AUTHOR]

Two series of flow boiling experiments have been conducted onboard the International Space Station (ISS) as a part of the TPF (Two-Phase Flow) experiment promoted by JAXA during July 2017–March 2018, February 2019–July 2019. Microgravity data on two-phase flow and heat transfer in flow boiling of n-Perfluorohexane (FC-72) have been obtained by a copper heated test tube and a transparent glass heated tube in a wide experimental range of mass velocity, liquid subcooling, vapor quality and heat flux. Furthermore, detailed two-phase flow behaviors have been observed by using high frame rate camera in the unheated observation section. In order to elucidate the accurate influence of gravity on flow boiling, it is essential to compare the heat transfer data and two-phase flow behaviors obtained under normal (terrestrial) gravity and microgravity environments at the same flow and heating conditions. However, both experiments cannot be performed by using the same experimental apparatus and under the same thermal environmental conditions. In addition, the heat loss cannot be negligible due to the forced avionics air flow inside the experimental apparatus in ISS. Therefore, exact evaluation of the fluid conditions at the inlet of the heated test tube requires the heat loss model with high-accuracy. In the present paper, the heat loss models for evaluating the degree of liquid subcooling at the inlet of the heated test tube and net heat flux from the heated tube to test fluid has been developed by using the results of preliminary heat loss experiments conducted onboard ISS. The correction of the degree of liquid subcooling by the proposed heat loss models is not negligible for the accurate analysis of gravity effects. The accuracy of the heat loss models has been verified through the evaluation of heat transfer coefficients for single-phase turbulent forced convection obtained from the heating experiments onboard ISS. [ABSTRACT FROM AUTHOR]

For the present study numerical simulations of subcooled flow boiling of FC-72 in microgravity have been conducted to accompany boiling experiments performed in microgravity on the International Space Station (ISS). The numerical domain represents the geometry of the experimental test cell. For all simulations the open source framework OpenFOAM was employed, including extensions to the interFoam solver, which have been developed at the authors’ institute. A reference case has been defined applying intermediate values from the experimental parameter range as system parameters. This case has been examined thoroughly with regards to hydrodynamic phenomena and heat transfer during multiple, successive bubble cycles. Based on this reference case, the system parameters flow velocity, input heat flux, pre-heating time, and subcooling of the liquid bulk have been varied, and the impact of these quantities on bubble growth and movement as well as heat transfer have been studied. It was found, that an increased flow rate as well as increased subcooling lead to smaller bubbles and increased time between subsequent nucleations. A high input heat flux, an increased pre-heating time, and a decreased subcooling lead to a rapid cycle of bubble nucleation and coalescence. [ABSTRACT FROM AUTHOR]

The inception of the boiling, in a pool or flow boiling, is the formation of the vapor bubble at an active nucleation site that plays a crucial role in the boiling process and it becomes critical and unfolds many facets when channel size reduces to submicron. The detailed knowledge of the bubble dynamics is helpful in establishing the thermal and hydraulic flow behavior in the microchannel. In the current paper, bubble dynamics that include bubble nucleation at the nucleation site, its growth, departure, and motion along the flow in a microchannel(s) are discussed in detail. Different models developed for critical cavity radius favorable for bubble nucleation are compiled and observe that models exhibit large deviation. The bubble growth models are compiled and concluded that the development of a more generalize bubble growth model is necessary that would be capable of accounting for inertia controlled and thermal diffusion controlled regions. Bubbles at nucleation sites in a microchannel grow under the influence of various forces such as surface tension, inertia, shear, gravitational and evaporation momentum. Parametric analysis of these forces reckoned that the threshold between macro- to microchannel could be identify through critical analysis of such forces. Eventually, the possible impact of the various factors such as operating conditions, geometrical parameters, thermophysical properties of fluid on bubble dynamics in microchannel has been reported. [ABSTRACT FROM AUTHOR]

The experimental study of the frictional pressure drop characteristics of flow boiling heat transfer of CO2 in a horizontal minichannel with an inner diameter of Φ1.5 mm was carried out. Experimental conditions: heat flux (7.5 ~ 30 kW•m−2), mass flow rate (50 ~ 600 kg•m−2•s−1), saturation temperature (−40 ~ 0 °C), vapor quality (0 ~ 1). The comparison between the experimental results and theoretical flow pattern diagrams shows that the change of heat flux has little effect on the frictional pressure drop at high vapor quality area, but it has a decisive effect on the dryout and mist flow areas. The mass flow rate is the most important factor affecting the frictional pressure drop which has a decisive effect on the flow pattern experienced during the heat transfer process. The saturation temperature has an important influence on the flow pattern transition characteristics and the friction pressure drop decreases with the increase of saturation temperature. The influence of vapor quality on the frictional pressure drop is mainly caused by the flow pattern change in the minichannel. The flow visualization study shows that the theoretical flow pattern diagram can better reflect the flow pattern of CO2 in the phase-change heat transfer process and the transition trend of flow pattern under different working conditions. [ABSTRACT FROM AUTHOR]

To solve the heat dissipation problem of electronic devices with high heat flux hotspots, a diamond microchannel heat sink consisting of 37 parallel triangular microchannels with channel lengths of 45 mm and hydraulic diameters of 280 μm was designed. The flow boiling heat transfer characteristics of ammonia in the microchannels were investigated under high heat fluxes of 473.9-1000.4 W/cm2. Saturated flow boiling experiments with saturation temperatures of 25, 30, and 35°C and mass fluxes of 98–1200 kg/m2s were conducted, as well as subcooled flow boiling with inlet subcooling of 5°C as a comparison. The temperature and pressure drop measurements were analyzed, and the main conclusions below can be drawn. (1) At a given heat flux, the heat source temperature first decreased and then increased with the mass flux, and there existed an optimum mass flux to optimize the cooling performance of the heat sink. (2) The heat transfer performance under the saturated inlet condition was obviously better than that under the subcooled inlet condition. (3) A larger saturation temperature leaded to weakening of both the heat transfer capacity and the stability of the microchannel heat sink. Notably, with the high heat diffusion ability of the diamond substrate and the great heat transfer capacity of ammonia flow boiling in microchannels, the heat sink can achieve a heat removal capacity of up to 1000.4 W/cm2. [ABSTRACT FROM AUTHOR]

Numerical predictions of flow boiling in a square tube rotating in vertical direction at a uniform rotating speed are presented. There-dimensional physical model and numerical model were established for simplified problem. Realizable k−ε (RKE) turbulent model was employed to study vapor–liquid two-phase turbulent flow. Heat and mass transfer between vapor and liquid were solved based on volume of fluid multiphase flow model combined with user-defined function. The transient results show that wall superheat at onset of nucleate boiling (ONB) condition in rotating tube is obviously higher than that in stable tube. Based on Jens–Lottes formula of Bowring model, empirical formulas for wall superheat at ONB condition were modified with consideration of Rossby number. As the rotation speed increased, location of ONB shifts to the tube exit. Flow pattern in tube is determined by coupling effect of heating and rotation. Stability of flow pattern is gradually weakened during the increase in rotation speed. The numerical results are in fair agreement with two sets of experimental data for same physical model and working conditions. [ABSTRACT FROM AUTHOR]

This paper presents an investigation of the boiling heat transfer of R141b in a mini-channel with a 2.42-mm inner diameter filled with metal foam. Investigations are performed at velocity of 0.01 m s−1 and temperature of 450 K. The coupled level set and volume-of-fluid method is applied to simulate flow boiling in metal foam. It considers that the bubble number and the average bubble area have influence on the heat transfer coefficient. The simulation model is verified by experimental data with an emphasis on the effect of boiling bubbles and turbulent kinetic energy on heat transfer. The simulation results show that even though the bubble motion range and vortices area is smaller, more vortices appear in the channel filled with foam metal, which increase the mixing frequency and improve the heat transfer performance. Meanwhile, the heat transfer coefficient is inversely proportional to the average bubble area but proportional to the bubble number. The relationship between turbulent kinetic energy and heat transfer coefficient is also compared. Results indicate that the differentiation of turbulent kinetic energy to time appears consistent with the heat transfer coefficient in the mini-channel filled with metal foam. [ABSTRACT FROM AUTHOR]

To solve the heat dissipation problem of high heat flux components on spacecraft, experiments were carried out to investigate the flow boiling heat transfer characteristics of ammonia in a microchannel heat sink. Verified by gravity-independent criteria, the experimental results can be used to predict the performance of flow boiling heat transfer in microgravity environment. The heat sink consisted of 37 V-shaped microchannels with hydraulic diameters of 280 μm and channel lengths of 45 mm. Saturated flow boiling experiments were conducted with heat fluxes of 60.2~134.3 W/cm2, mass fluxes of 165~883 kg/m2s and saturation temperatures of 25 and 35 °C, as well as subcooled flow boiling experiments with inlet subcooling of 5 °C as a comparison. According to the experimental results, the following conclusions can be drawn. (1) As the mass flow rate increases, higher wall superheat is needed to trigger nucleate boiling. (2) In a lower mass flux range, the heat transfer coefficient changes drastically with mass flux, showing convective boiling characteristics, while the value varies slightly in a higher mass flux range, indicating that the nucleate boiling mechanism is dominant. (3) Under the experimental conditions in this work, the average heat transfer coefficient of ammonia reaches its peak when the outlet vapor quality is between 0.21 and 0.31. (4) A subcooled inlet condition has a suppression effect on the average heat transfer capacity of the working fluid at high mass flux, and increasing the saturation temperature reduces the average heat transfer coefficient. In addition, a comparison between the experimental results and existing correlations demonstrated that the correlation proposed by Fang predicted the experimental results well, and then a modified Fang correlation was proposed based on the experimental data obtained in this study. The results could provide references for the prediction of flow boiling heat transfer performance in aerospace environments and the design of heat dissipation systems. [ABSTRACT FROM AUTHOR]

Nanofluid flow boiling and critical heat flux (CHF) are interesting topics for many researchers and have industrial application for preventing potential damages. On this basis, numerous experimental studies have been performed on the determination of CHF; however, due to the weakness of the existing correlations for this purpose, an accurate numerical study on CHF is yet needed to be reported. In the present study, the subcooled flow boiling of nanofluid has been simulated to predict CHF. For estimating the nucleation site density (NSD), a correlation is used, into which the liquid properties, flow characteristics and surface properties contribute. The studies were performed at different concentrations of alumina nanoparticles. The simulation results indicated error percentages below 10% in CHF estimation, confirming the reliability of the results. No significant difference was observed between the CHF values calculated in the simulation studies considering constant thermal properties for the nanoparticles, as compared to those obtained with variable thermal properties for the nanoparticles. Taking the tube in a horizontal position, rather than the vertical position, increased the CHF value. However, the increase in CHF was negligible. [ABSTRACT FROM AUTHOR]

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The present study investigates chilldown characteristics of a horizontal copper transfer line with 7.94 mm outer diameter, 0.81 mm wall thickness and 500 mm length. Data presented in this paper is for the experiments conducted with different mass fluxes (66 kg (m2.s)−1 to 102 kg (m2.s)−1) in a horizontal copper transfer line under terrestrial gravity conditions. Temperature measurements were recorded at six equidistant points to a distance of 330 mm from an inlet. Inverse problem solving method is utilized to calculate corresponding heat flux and heat transfer coefficients. Considering the thermal properties of the quenched wall, an empirical relation was developed. It is found that while employing copper transfer lines instead of stainless steel, thermal mass of the section is reduced by a factor of 100, thereby encountering a reduction of 50% in critical heat flux. [ABSTRACT FROM AUTHOR]

Flow boiling heat transfer widely utilized in numerous industrial applications such as boiler tubes, evaporators and cooling of reactors in a nuclear power plant. Nanofluids are a new category of thermal fluids, made by dispersing a nanometer solid particle which is usually less than 100 nm into conventional liquids such as water, oil engine and ethylene glycol with the intent to enhance the thermal properties of the base fluids. This work reviews the recent experimental studies focusing on the flow boiling heat transfer using nanofluids. The latest results associated with this subject are presented and outlined to account the influence of several parameters, which are related to operating conditions and nanoparticles morphology on the heat transfer coefficient and the critical heat flux. Besides, the effects of nanoparticles on other related sub-phenomenon of the flow boiling by using nanofluids were discussed. Moreover, the latest review papers of the related topic were presented and briefly discussed. Finally, suggestions for future research activities related to this field were also concisely listed. [ABSTRACT FROM AUTHOR]

To predict the critical heat flux (CHF) of flow boiling under subcooled conditions, this study comes up with a new model based on the Interfacial Lift-off CHF model. The new model makes up the shortcomings of the previous one used only under saturated conditions, and it also considers the effect of condensation on the interface between liquid and vapor phases. An all-new subcooled correctional coefficient is risen up based on the CHF value for the corresponding flow velocity under near-saturated conditions. By comparison between the predicted results of this model and the experimental ones at 6 subcoolings from 3 to 30 °C, and 18 flow velocities from 0.75 to 8 m/s, with the mean absolute error of 9.4%, the predicted trends are both intuitively reasonable and numerically accurate. It reflects the rule under which CHF changes with the outlet subcooling and flow velocity under both saturated and subcooled conditions, and the vapor amount within a vapor bulk decreases with the increase of subcooling. But as the flow velocity increases, it decreases first, and then tends to be a fixed value gradually. [ABSTRACT FROM AUTHOR]

With the rapid development of modern space technologies, heat sinks with high heat flux have become a bottleneck in the development of high capacity electronic equipment with high power densities. To ensure the stability and security of heat sink devices, it's crucial to greatly enhance the heat transfer performance of the heat sink device and flow boiling in microchannels has attracted more and more attention due to its unique advantages. The objective of this investigation is to study the growth and heat transfer of single bubble and multiple bubbles during flow boiling in a microchannel using the VOSET method. In order to verify the accuracy of the numerical model adopted in this paper, the results obtained by the present method were compared with that of previous experiments and numerical calculations. After that, effects of Re and wall superheat on the bubble morphologies and heat transfer in a microchannel were studied. Increasing Reynolds number could increase the bubble growth rate and boiling flow heat transfer. However, there was a slight increase in heat transfer performance with an increase in wall superheat. In addition, when Re reached a certain value, the elongated bubble zone would appear in the microchannel. Further, the stretching length, the contact length between the heated wall and the vapour and the second kind of thin liquid layer were analyzed in detail. Finally, more importantly, the dynamics and heat transfer of multiple bubbles with different bubble waiting time were presented. With a decrease in bubble waiting time, the heat transfer performance was enhanced significantly. [ABSTRACT FROM AUTHOR]

An experimental study is conducted to investigate the effect of hydraulic diameter on the saturated flow boiling characteristics of deionized water in parallel rectangular microchannels. Experiments have been performed for the mass fluxes of 51, 65, 78 and 93 kg m−2 s−1, and the hydraulic diameters of 100, 150, 200 and 250 μm. The wall heat flux ranges from 35.9 to 105.6 kW m−2. To eliminate the effect of aspect ratio and to address clearly the effect of hydraulic diameter, all the channels are designed with square cross section. Flow visualization studies are performed for a better understanding of the underlying physical phenomena. Effects of heat flux, mass flux and vapor quality on the heat transfer and total pressure drop have been investigated, too. It is concluded that hydraulic diameter has significant influence on both of the local two phase heat transfer coefficient and the total pressure drop. [ABSTRACT FROM AUTHOR]

The flow boiling heat transfer characteristics of subcooled air-dissolved FC-72 on a smooth surface (chip S) were studied in microgravity by utilizing the drop tower facility in Beijing. The heater, with dimensions of 40 × 10 × 0.5 mm3 (length × width × thickness), was combined with two silicon chips with the dimensions of 20 × 10 × 0.5 mm3. High-speed visualization was used to supplement observation in the heat transfer and vapor-liquid two-phase flow characteristics. In the low and moderate heat fluxes region, the flow boiling of chip S at inlet velocity V = 0.5 m/s shows almost the same regulations as that in pool boiling. All the wall temperatures at different positions along the heater in microgravity are slightly lower than that in normal gravity, which indicates slight heat transfer enhancement. However, in the high heat flux region, the pool boiling of chip S shows much evident deterioration of heat transfer compared with that of flow boiling in microgravity. Moreover, the bubbles of flow boiling in microgravity become larger than that in normal gravity due to the lack of buoyancy Although the difference of the void fraction in x-y plain becomes larger with increasing heat flux under different gravity levels, it shows nearly no effect on heat transfer performance except for critical heat flux (CHF). Once the void fraction in y-z plain at the end of the heater equals 1, the vapor blanket will be formed quickly and transmit from downstream to upstream along the heater, and CHF occurs. Thus, the height of channel is an important parameter to determine CHF in microgravity at a fixed velocity. The flow boiling of chip S at inlet velocity V = 0.5 m/s shows higher CHF than that of pool boiling because of the inertia force, and the CHF under microgravity is about 78-92% of that in normal gravity. [ABSTRACT FROM AUTHOR]

The refrigerant charge reduction is one of the most challenging issues that the scientific community has to cope to reduce the anthropic global warming. Recently, mini microfin tubes have been matter of research, since they can reach better thermal performance in small domains, leading to a further refrigerant charge reduction. This paper presents experimental results about R134a flow boiling inside a microfin tube having an internal diameter at the fin tip of 2.4 mm. The mass flux was varied between 375 and 940 kg m−2 s−1, heat flux from 10 to 50 kW m−2, vapor quality from 0.10 to 0.99. The saturation temperature at the inlet of the test section was kept constant and equal to 30 °C. R134a thermal and fluid dynamic performances are presented and compared against those obtained with R1234ze(E) and R1234yf and against values obtained during R134a flow boiling inside a 3.4 mm ID microfin tube. [ABSTRACT FROM AUTHOR]

In this paper, an experimental investigation on the flow boiling heat transfer in a horizontal long mini-channel was carried out. The mini-channel was with 2 mm wide and 1 mm deep and 900 mm long. The material of the mini-channel was stainless. The working fluid was deionized water. The experiments were conducted with the conditions of inlet pressure in the range of 0.2~0.5 MPa, mass flux in the range of 196.57-548.96 kg/m2s, and the outlet vapor quality in the range of 0.2 to 1. The heat flux was in the range of 292.86 kW/m2 to 788.48 kW/m2, respectively. The influences of mass flux and heat flux were studied. At a certain mass flow rate, the local heat transfer coefficient increased with the increase of the heat flux. If dry-out occurred in the mini-channel, the heat transfer coefficient decreased. At the same heat flux, the local heat transfer coefficient would depend on the mass flux. It would increase with the mass flux in a certain range, and then decrease if the mass flux was beyond this range. Experimental data were compared with the results of previous studies. Flow visualization and measurements were conducted to identify flow regime transitions. Results showed that there were eight different kinds of flow patterns occurring during the flow boiling. It was found that flow pattern had a significant effect on heat transfer. [ABSTRACT FROM AUTHOR]

An experimental study was performed to measure FC-72(C6F14) flow boiling heat transfer and pressure drop in heat sinks for electronics cooling. The heat sink had cooling cross section area of 38.0 × 37.0 mm with rectangular fins. The height, length and thickness of a fin was 5.0, 24.0 and 1.0 mm, respectively. The width of fluid channels between the fins was 1.0 mm. The heat sink consisted of a heating and cooling section, and a cover. Two types of heat sinks were used in this study. The two heat sinks were different only in the cover, and the machined depth of the cover was 5.0 and 8.0 mm, respectively. Electric heating from 100 to 300 W was supplied by cartridge heaters and it was equivalent to the heat flux from 71.12 to 213.4 kW/m2 based on the cross section area of the cooled surface. The saturation temperatures of the FC-72 were from 59.8 °C to 71.5 °C during the experiment and the mass fluxes were from 24.2 to 230.0 kg/m2s. The trend of heat transfer and pressure drop variation with the change of vapor quality was similar to that of flow boiling in tubes such as the increase of heat transfer and pressure drop with the increase of vapor quality before dryout. Similar heat transfer coefficients and pressure drop values were measured under the same mass flow conditions for both types of heat sinks. In this study, the cooling performance with liquid water was also measured at the same heat sinks. The comparison of experimental data presented that the cooling capacity with FC-72 flow boiling was up to 330 % higher than that with liquid water. However, the FC-72 pressure drop was also significantly higher than water. [ABSTRACT FROM AUTHOR]

Flow boiling heat transfer characteristics using a new nanofluid, multi-walled carbon nanotubes (MWCNTs)/pure water were investigated in a horizontal tube under atmospheric pressure. In order to prepare nanofluid, gum Arabic (GA) as a surfactant and MWCNTs nanoparticles were used in the ratio 1:1 with concentrations of 0.001 and 0.01% by mass, and its stability was determined by measuring zeta potential. During the experiments, detailed observations of the boiling process showed physical differences between nuclear boiling of pure water and nanofluid especially after CHF phenomenon. These differences were mainly due to the deposition of carbon nanotube nanoparticles on the heat transfer surface and improvement in wettability and re-wetting characteristics. The experimental data showed that using nanofluid can improve the flow boiling characteristics of CHF. Based on these results, it was found that the nanofluids heat transfer coefficient was larger than that of pure water and it increased with increasing mass flux and also with increasing heat flux for both pure water and nanofluids. The enhancement of the maximum heat transfer coefficient of MWCNTs-GA/water nanofluid was approximately 4.3% at CHF and 0.005 volume fraction of nanoparticles. The results also showed that the pressure drop of nanofluid increases with increasing mass flux and the enhancement of maximum pressure drop was approximately 0.12 vol% at CHF and 0.005 vol% of nanoparticles. Graphical abstract: [ABSTRACT FROM AUTHOR]

This paper presents effects of heating directions on heat transfer performance of R134a flow boiling in micro- channel heat sink. The heat sink has 30 parallel rectangular channels with cross-sectional dimensions of 500μm width 500μm depth and 30mm length. The experimental operation condition ranges of the heat flux and the mass flux were 13.48 to 82.25 W/cm and 373.3 to 1244.4 kg/ms respectively. The vapor quality ranged from 0.07 to 0.93. The heat transfer coefficients of top heating and bottom heating both were up to 25 kW/m2 K. Two dominate transfer mechanisms of nucleate boiling and convection boiling were observed according to boiling curves. The experimental results indicated that the heat transfer coefficient of bottom heating was 13.9% higher than top heating in low heat flux, while in high heat flux, the heat transfer coefficient of bottom heating was 9.9%.higher than the top heating, because bubbles were harder to divorce the heating wall. And a modified correlation was provided to predict heat transfer of top heating. [ABSTRACT FROM AUTHOR]

The present study reports an experimental evaluation of heat transfer characteristic of R134a flow boiling in micro-channel heat sink. The heat sink is composed of 30 parallel rectangular micro-channels with cross-sectional dimensions of 500μm width and depth, as well as total length 30mm. Experiments were conducted with heat flux up to 80.212 W/cm, mass velocity ranging from 373.33 to 1244.44 kg/ms, vapor quality ranging from 0.06 to 0.9. The wall temperature of heat sink heated could be controlled at around 50°C. Heat transfer coefficient could be up to 180 kW/mK. Two dominating flow patterns were observed by analyzing boiling curves. The heat transfer characteristics of nucleate boiling and convective boiling were presented in the study. Revised correlations of R134a flow boiling in micro-channel heat sink were carried out with the consideration of nucleate boiling and convective boiling mechanisms. [ABSTRACT FROM AUTHOR]

Flow boiling in micro-channels and mini-channels has received significant attention due to its capability for dissipating highflux heat, especially in the thermal management of high precision electronics. A heat sink with narrow rectangular mini-channels is designed to investigate flow boiling in the mini-channels, including the effect of gravity. It contains 14 parallel channels with a cross section, of 1×4mm , of which the hydraulic diameter is 1.6mm. The cooling capability, the temperature uniformity, and the temperature stability of the flow boiling in minichannels are investigated with R22, with total mass flow flux ranges from 35 to 70kg/m s. The results show that the cooling capability of the heat- sink is up to 340W(∼ 3.0W/cm ), and the temperature difference is below 4 C(even down to 2 C) on the heat sink. The temperature uniformity isn't quite sensitive to heat flux. The instability has not been observed in the present researches. [ABSTRACT FROM AUTHOR]

An investigation is performed into the heat transfer performance of jet impingement flow boiling using AlO-water nanofluids with AlO additions of 0, 0.0001, 0.001 and 0.01 vol%, respectively. It is shown that the heat transfer performance of jet impingement flow boiling using AlO-water nanofluid is poorer than that obtained when using de-ionized (DI) water as the working fluid. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS) observations reveal that the reduction in the heat transfer performance is due to the formation of a nano-sorption layer on the heated surface, which results in an increase in the thermal resistance. However, it is shown that by applying acoustic vibration to the heated surface, the formation of the nano-sorption layer is prevented; with the result that the heat transfer performance obtained using the AlO-water nanofluids is better than that obtained using pure DI water. [ABSTRACT FROM AUTHOR]

The evaporating momentum force and the shear force acting on the meniscus of an evaporating and elongating bubble in flow boiling in microchannel have been investigated theoretically and numerically. The concept of the effective evaporation region and the theory of the liquid layer supplement between elongated bubble and microchannel are proposed, and the analytical expressions of the evaporating momentum force and shear force have been obtained. The relative importance of both forces has been determined by the method of magnitude analysis and numerical simulation. It has been found that the evaporating momentum force can always be neglected in analyzing the bubble elongation process and the motion law of meniscus of elongated bubble in microchannel flow boiling, but whether the shear force should be considered or not is determined by its relative order of magnitude and the particular conditions such as channel dimension and the operating conditions. [ABSTRACT FROM AUTHOR]

The characteristics of the confined bubble and elongated bubble in subcooled flow boiling in a single horizontal rectangular microchannel with hydraulic diameter D=1mm are studied experimentally. The channel with 1 × 1mm cross section is fabricated in a thin copper plate whose confinement number is Co=2.8 and the degassed deionized water is used as the working fluid. Visualization on the confined and elongated bubbles inside the microchannel is carried out by employing a high-speed CCD camera with a microlens. The recorded images are carefully analyzed to illustrate the behaviors of the confinement and elongation processes of the bubble. The boiling number is used as an adjustable parameter to regulate the operating conditions which is eventually found to take a vital role in the bubble elongation process. Two formation patterns of the confined and elongated bubble are identified and the interactions between the neighboring confined and elongated bubbles are elucidated. [ABSTRACT FROM AUTHOR]

The aim of the present paper is to describe the results of flow boiling heat transfer at low gravity and compare them with those obtained at earth gravity, evaluating possible differences. The experimental campaigns at low gravity have been performed with parabolic flights. The paper will show the analysis of differences between the heat transfer coefficients at normal and at zero gravity, and the study of the effects of mass flux, heat flux, and tube diameter on boiling phenomena at microgravity. Three tube diameters are tested: 6.0, 4.0, and 2.0 mm. With respect to terrestrial gravity, both heat transfer rate enhancement (up to 15-20%) and deterioration (up to 35%) have been observed. Heat transfer differences for the two gravity conditions may be related to the different bubble size in each of them. The size of a bubble in flow boiling is generally affected by the gravity level, being larger at low gravity, unless inertial forces are largely predominant over buoyancy and other forces acting on the bubble itself when detaching from a heating wall. Heat transfer enhancements at low gravity, are observed in those conditions where the flow pattern is bubbly flow at normal gravity and intermittent flow at low gravity. The results are presented in a flow boiling gravity influence map, which can be considered a useful tool for designing boiling systems for space applications. [ABSTRACT FROM AUTHOR]

Experiments were performed to study pressure drops in copper foams embedded in a rectangular copper channel. De-ionized water was used as the working fluid with mass fluxes of 30-200 kg/m s, and inlet temperature of 40-80°C. The copper foam has the porosity of 0.88 and the pore densities of 30, 60 and 90 ppi (pores per inch). Both single-phase liquid flow and boiling two-phase flow are studied. Effects of mass fluxes, vapor mass qualities, and average pore diameters of metallic foams are investigated. It is found that friction factors for the single-phase liquid flow are mainly dependent on the Reynolds number and the average pore diameter of metallic foams. The friction factors are decreased with increases in the Reynolds numbers, and will approach 0.22 at high Reynolds numbers. For the boiling two-phase flow, two-phase pressure drops are increased with increases in the outlet vapor mass qualities, mass fluxes, and ppi values. The two-phase multiplier is increased with increases in the outlet vapor mass qualities and mass fluxes, and it is decreased with increases in the Martinelli parameter and will attain a constant value depending on the mass fluxes. The larger the mass fluxes, the larger the constant value is. An experimental correlation considering the effects of vapor mass qualities, mass fluxes, and average pore diameters of metallic foams is recommended, showing good accuracy to predict the two-phase pressure drops in metallic foams. [ABSTRACT FROM AUTHOR]

This study investigates flow boiling heat transfer of aqueous alumina nanofluids in single microchannels with particular focuses on the critical heat flux (CHF) and the potential dual roles played by nanoparticles, i.e., (i) modification of the heating surface through particle deposition and (ii) modification of bubble dynamics through particles suspended in the liquid phase. Low concentrations of nanofluids (0.001-0.1 vol.%) are formulated by the two-step method and the average alumina particle size is ~25 nm. Two sets of experiments are performed: (a) flow boiling of formed nanofluids in single microchannels where the effect of heating surface modification by nanoparticle deposition is apparent and (b) bubble formation in a quiescent pool of alumina nanofluids under adiabatic conditions where the role of suspended nanoparticles in the liquid phase is revealed. The flow boiling experiments reveal a modest increase in CHF by nanofluids, being higher at higher nanoparticle concentrations and higher inlet subcoolings. The bubble formation experiments show that suspended nanoparticles in the liquid phase alone can significantly affect bubble dynamics. Further discussion reveals that both roles are likely co-existent in a typical boiling system. Properly surface-promoted nanoparticles could minimize particle deposition hence little modification of the heating surface, but could still contribute to the modification in heat transfer through the second mechanism, which is potentially promising for microchannel applications. [ABSTRACT FROM AUTHOR]

n experimental study on the saturated flow boiling heat transfer for a binary mixture of R290/R152a at various compositions is conducted at pressures ranging from 0.2 to 0.4 MPa. The heat transfer coefficients are experimentally measured over mass fluxes ranging from 74.1 to 146.5 kg/(m·s) and heat fluxes ranging from 13.1 to 65.5 kW/m. The influences of different parameters such as quality, saturation pressure, heat flux, and mass flux on the local heat transfer coefficient are discussed. Existing correlations are analyzed. The Gungor-Winterton correlation shows the best fit among experimental data for the two pure refrigerants. A modified correlation for the binary mixture is proposed based on the authors' previous work on pool boiling heat transfer and the database obtained from this study. The result shows that the total mean deviation is 10.41% for R290/R152a mixtures, with 97.6% of the predictions falling within ±30%. [ABSTRACT FROM AUTHOR]

The rapid development of micro-thermal technologies has conveyed an increasing interest on convective boiling in micro-channels. Although there is general agreement that these systems may be able to dissipate potentially very high heat fluxes per unit volume, their heat transfer characteristics are still unclear and require investigation. The present study illustrates heat transfer data for flow boiling in a single micro-channel, for two channel diameters, namely, 510 and 790 μm, three fluids, namely, R-134a, R-236fa and R-245fa, mass velocities from 300 to 2,000 kg/m2 s, and heat fluxes up to 200 kW/m2. Stable flow boiling heat transfer data are analyzed through a parametric investigation, and are also confronted with measurements in the presence of two-phase oscillatory instabilities, which were found to significantly change the trends with respect to vapor quality. [ABSTRACT FROM AUTHOR]

The flow boiling heat transfer in microchannels have become important issue because it is extremely high-performance heat exchanger for electronic devices. For a detailed study on flow boiling heat transfer in a microtube, we have used a transparent heated microtube, which is coated with a thin gold film on its inner wall. The gold film is used as a resistance thermometer to directly evaluate the inner wall temperature averaged over the entire temperature measurement length. At the same time, the transparency of the film enables the observation of fluid behavior. Flow boiling experiments have been carried out using the microtube under the following conditions; mass velocity of 105 kg/m2 s, tube diameter of 1 mm, heat flux in the range of 10 ~ 380 kW/m2 s, and the test fluid used is ionized water. Under low heat flux conditions, the fluctuations in the inner wall temperature and mass velocity are closely related; the frequency of these fluctuations is the same. However, the fluctuations in the inner wall temperature and heat transfer coefficient are found to be independent of the fluctuation in the mass velocity under high heat flux conditions. [ABSTRACT FROM AUTHOR]

The typical MEMS fabrication of micro evaporators ensures the perfect smooth wall surface that is lack of nucleation sites, significantly decreasing the heat transfer coefficients compared with miniature evaporators fabricated using copper or stainless steel. In the present paper, we performed the boiling heat transfer experiment in silicon triangular microchannel heat sink over a wide parameter range for 102 runs. Acetone was used as the working fluid. The measured boiling heat transfer coefficients versus the local vapor mass qualities are compared with the classical Chen’s correlation and other correlations for macro and miniature capillary tubes. It is found that most of these correlations significantly over-predict the measured heat transfer coefficients. New correlations are given. There are many reasons for such deviations. The major reason is coming from the perfect smooth silicon surface that lowers the heat transfer performances. New theory is recommended for the silicon microchannel heat sink that should be different from metallic capillary tubes. [ABSTRACT FROM AUTHOR]