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The hypervapotron structure was considered to be a feasible configuration to meet the high heat-dissipating requirement of divertors in nuclear fusion devices. In this work, symmetric CuCrZr-based transverse microchannels (TMHC) and longitudinal microchannels (LMHC) with an integrated hypervapotron channel were proposed and manufactured, and subcooled flow boiling experiments were conducted using deionized water at an inlet temperature of 20 °C with a traditional flat-type hypervapotron channel (FHC) for comparison. The LMHC and TMHC obtained lower wall temperatures than the FHC for all conditions, and the TMHC yielded the lowest temperatures. The heat transfer coefficients of the LMHC and TMHC outperformed the FHC due to the enlarged heat transfer area, and the TMHC had the greatest heat transfer coefficient (maximumly increased by 132% compared to the FHC) because the transverse-arranged microchannels were conductive, promoting the convection and liquid replenishment ability by introducing branch flow between fins; however, the microchannels of the LMHC were insensible to flow velocities due to the block effect of longitudinal microchannels. The LMHC obtained the largest pressure drop, and the pressure drop for the FHC and TMHC were comparable since the transverse-placed microchannels had little effect on frictional pressure loss. The TMHC attained the greatest comprehensive thermohydraulic performance which might bring significant insight to the structural design of hypervapotron devices. [ABSTRACT FROM AUTHOR]

The hypervapotron structure was considered to be a feasible configuration to meet the high heat-dissipating requirement of divertors in nuclear fusion devices. In this work, symmetric CuCrZr-based transverse microchannels (TMHC) and longitudinal microchannels (LMHC) with an integrated hypervapotron channel were proposed and manufactured, and subcooled flow boiling experiments were conducted using deionized water at an inlet temperature of 20 °C with a traditional flat-type hypervapotron channel (FHC) for comparison. The LMHC and TMHC obtained lower wall temperatures than the FHC for all conditions, and the TMHC yielded the lowest temperatures. The heat transfer coefficients of the LMHC and TMHC outperformed the FHC due to the enlarged heat transfer area, and the TMHC had the greatest heat transfer coefficient (maximumly increased by 132% compared to the FHC) because the transverse-arranged microchannels were conductive, promoting the convection and liquid replenishment ability by introducing branch flow between fins; however, the microchannels of the LMHC were insensible to flow velocities due to the block effect of longitudinal microchannels. The LMHC obtained the largest pressure drop, and the pressure drop for the FHC and TMHC were comparable since the transverse-placed microchannels had little effect on frictional pressure loss. The TMHC attained the greatest comprehensive thermohydraulic performance which might bring significant insight to the structural design of hypervapotron devices.

Continuously improving heat transfer efficiency is one of the important goals in the field of energy. Compact heat exchangers characterized by microscale flow and heat transfer have successfully provided solutions for this purpose. However, as the characteristic scale of the channels decreases, the flow and heat transfer characteristics may differ from those at the conventional scale. When considering the influence of scale effects and changes in special fluid properties, the flow and heat transfer process becomes more complex. The conclusions of the relevant studies have not been unified, and there are even disagreements on some aspects. Therefore, further research is needed to obtain a sufficient understanding of flow structure and heat transfer mechanisms in microchannels. This article systematically reviews the research about microscale flow and heat transfer, focusing on the flow and heat transfer mechanisms in microchannels, which is elaborated in the following two perspectives: one is the microscale single-phase flow and heat transfer that only considers the influence of scale effects, the other is the special heat transfer phenomena brought about by the coupling of microscale flow with special fluids (fluid with phase change (pseudophase change)). The microscale flow and heat transfer mechanisms under the influence of multiple factors, including scale effects (such as rarefaction, surface roughness, axial heat conduction, and compressibility) and special fluids, are investigated, which can meet the specific needs for the design of various microscale heat exchangers. [ABSTRACT FROM AUTHOR]

Pin fins have been widely applied in microchannels to enhance the heat transfer performance, however, they inevitably lead to a large increase in the pressure drop and the pump power consumption, which hinders their practical applications in microchannel heat sinks. In this work, we report that lowering the pin fin to channel height ratio can significantly reduce the pressure drop while largely maintaining the heat transfer enhancement for flow boiling in finned microchannels. A novel two-step deep reactive ion etching (DRIE) approach is developed to fabricate finned microchannel heat sinks with the controllable pin fin to channel height ratio. Our results show that at low to moderate heat flux, all the microchannel heat sinks with pin fins show nearly overlapping boiling curves, as pin fins elevate flow boiling heat transfer through enhanced bubble nucleation in this heat flux range, which is not affected by the height of pin fins. However, higher pin fins lead to a larger heat transfer coefficient (HTC) in the high heat flux range and a higher critical heat flux (CHF), which can be attributed to the enlarged surface area and the increased flow velocity. Importantly, compared to the microchannel heat sink with full-height pin fins, the pressure drop can be significantly reduced by 43.9 % through lowering the pin fin height ratio to 50 %, while the CHF is only slightly decreased by 10.1 %. The performance evaluation criterion (PEC) analysis shows that the calculated PEC is larger than 1 for lower pin fins at most heat flux conditions, which confirms that lowering the pin fin to channel height ratio is beneficial for flow boiling in finned microchannels. Moreover, flow reversal is effectively suppressed due to the increased flow resistance and the confinement of bubble growth by pin fins. This work sheds the light on the design of finned microchannel heat sinks with high flow boiling heat transfer performance and reduced pressure drop. • Developed a two-step DRIE process to fabricate microchannels with lower pin fins. • Pressure drop is significantly reduced by lowering the pin fin height ratio. • Flow boiling heat transfer performance is slightly affected by the pin fin height. • Flow instability is effectively suppressed by pin fins. [ABSTRACT FROM AUTHOR]

Continuously improving heat transfer efficiency is one of the important goals in the field of energy. Compact heat exchangers characterized by microscale flow and heat transfer have successfully provided solutions for this purpose. However, as the characteristic scale of the channels decreases, the flow and heat transfer characteristics may differ from those at the conventional scale. When considering the influence of scale effects and changes in special fluid properties, the flow and heat transfer process becomes more complex. The conclusions of the relevant studies have not been unified, and there are even disagreements on some aspects. Therefore, further research is needed to obtain a sufficient understanding of flow structure and heat transfer mechanisms in microchannels. This article systematically reviews the research about microscale flow and heat transfer, focusing on the flow and heat transfer mechanisms in microchannels, which is elaborated in the following two perspectives: one is the microscale single-phase flow and heat transfer that only considers the influence of scale effects, the other is the special heat transfer phenomena brought about by the coupling of microscale flow with special fluids (fluid with phase change (pseudophase change)). The microscale flow and heat transfer mechanisms under the influence of multiple factors, including scale effects (such as rarefaction, surface roughness, axial heat conduction, and compressibility) and special fluids, are investigated, which can meet the specific needs for the design of various microscale heat exchangers.

This study aims to explore the boiling heat transfer characteristics of nanofluids in the microchannels under the action of electric fields. The linear electrodes were arranged in the geometric center of microchannels to generate a 0-800V non-uniform electric field, and the SiO2-R141b nanofluids with the different mass fractions were prepared by a two-step method. A flow boiling experiment was performed on the microchannel with a cross-section of 2 mm×2 mm under the system pressure of 140 kPa, the mass flow rate of 317.71 kg/(m²·s), the heat flux density ranges from 11.53 to 29.14 kW/m², and the inlet temperature of 32℃. A systematic investigation was made on the average saturated boiling heat transfer coefficient and the range of effective heat transfer heat flux density in the microchannel with the different mass fractions of nanofluids under the action of different electric voltages. The enhanced heat transfer mechanism of the nanofluid flow boiling was determined to simulate the detachment rate of the bubbles, the velocity of the confined bubbles, and the aspect ratio of the microchannels under the action of different voltages and electric fields. The comprehensive performance evaluation of the heat transfer (PEC) method was used to enhance the heat transfer of microchannels by different strengthening techniques. Both the electric field and the nanofluid were used to enhance the flow boiling heat transfer in the microchannel. Electric fields presented a better effect on the nucleate boiling heat transfer than single-phase heat transfer. The nanofluids were stable and enhanced heat transfer performance in the single- and multi-phase flow. An optimal mass fraction of nanoparticles was achieved in the nanofluids. The combination performance was better than that of the electric field or the nanofluid alone. The average saturated boiling heat transfer coefficient of nanofluids under the action of an electric field was higher than that without an electric field, which increased with the increase of voltage. The maximum coefficients of saturation boiling heat transfer increased by 44.9%, 20.9%, and 58.0%, respectively, for the electric field enhancement alone, nanoparticle enhancement alone, as well as the electric field and nanoparticle addition enhancement, compared with pure refrigerant. There was a significantly larger effective range of heat transfer enhancement that combined electric field and nanoparticles, compared with the electric field alone and the addition of nanoparticles to the pure refrigerant. The electric field can be expected to accelerate the bubble detachment frequency and the speed of the confined bubble, thereby reducing the aspect ratio of the confined bubble. At the same time, the nanoparticles can be used to improve the thermal conductivity of the working medium for the high heat exchange between the heat exchange wall and the working medium, indicating the enhanced heat transfer under the combined action. The nanofluid presented the highest enhancement performance of heat transfer under the electric field, where the average heat transfer comprehensive performance evaluation factor was 1.34. Consequently, the electric field combined with the addition of nanoparticles can effectively enhance the heat transfer of flow boiling in the microchannels. The finding can provide new ideas to improve the heat transfer performance of microchannels under the combined electric field and nanoparticle composites. [ABSTRACT FROM AUTHOR]

• The number of publications focused on the flow boiling instabilities is increasing in the last decades. • Most of the studies are based on microchannel and pin fin structures. • The integration of combined geometries is a promising approach to further improve the efficiency and reliability of flow boiling technology. Flow boiling technology is considered one of the most promising cooling solutions for high power microelectronics. However, flow boiling instabilities cause serious reliability problems that hinder large scale commercial use of this technology. Extensive experimental research has been carried out in the last years to mitigate these issues, focusing on geometrical aspects, surface modification, wettability, working fluid, or active control strategies, among others. Nevertheless, despite the relevance of geometrical aspects, none of the studies conducted a comprehensive review on geometrical structures and their instabilities mitigation effect. This review aims to provide constructive insights into relevant studies about experimental flow boiling geometrical enhancements and their impact on instabilities mitigation. It focuses on pin-fin arrays, most of the microchannel structures, inlet restrictors, and combined geometries. Firstly, every geometrical option is analysed in detail, summarizing experimental works in terms of geometry tested, operating parameters, and instabilities improvement. Secondly, a graphical overview is performed aimed at identifying research gaps and tendencies. This study found that significant progress has been made in geometric mitigation of flow boiling instabilities, and several advanced methodologies have been proposed to improve the reliability of flow boiling technology. The integration of various geometric configurations is a promising avenue, but further research is required to reach a widely accepted solution. [ABSTRACT FROM AUTHOR]

• The influence of operating parameters on heat transfer performance and instability was studied. • The dissipated heat flux up to 117 W/cm2 for a large heating area of 30 × 40 mm2 was achieved. • Four flow patterns were observed and the flow reversal occurred when the R is higher than unity. • An improved predictive model for heat transfer coefficient was proposed. Flow boiling in parallel microchannels is an efficient method to deal with the high heat flux removal challenge for electronic modules. In this work, the visualized experiments on the R134a flow boiling inside a high-heat-flux microchannels heat sink were performed. The heat sink contains 27 parallel microchannels with a hydraulic diameter of 0.89 mm and a length of 30 mm. The experiments were conducted over a base heat flux between 2 and 117 W/cm2 (based on the footprint area), mass velocity of 282–1033 kg/m2 s, the saturation temperature ranged from 27.2 to 35.5 °C, exit vapor quality up to 0.45 and an inlet sub-cooling approximately 3 °C. The effects of mass velocity, saturation temperature, and heat flux, on the flow boiling characteristics were analyzed and discussed. Results demonstrated that the heat transfer coefficient (HTC) rise with the increase of mass flux under middle and high heat flux. At the maximum mass velocity of 1033 kg/m2 s, the flow boiling HTC reached 51456 W/m2 K. Moreover, the pressure drop increased with higher mass and heat flux, while it decreased with increasing saturation temperature. Additionally, under high heat flux conditions, the two-phase flow became more stable, and the wall surface temperatures exhibited greater uniformity when higher mass velocity or saturation temperature was applied. The visualization results illustrated that there were four flow patterns and reverse flow occurred when R exceeded unity. Finally, the experimental results were evaluated with six previous heat transfer correlations. An improved predictive model for flow boiling HTC was developed and good agreement with present data was obtained with MAE of 9.08 %. A two-phase micro-channels cold plate for high-power density electronic components could be rationally designed and optimized with reference to this work. [ABSTRACT FROM AUTHOR]

• A "Synchronous backflow" phenomenon is observed in foam fin microchannels (FFMCs) and it is the cause of heat transfer deterioration. • An HTC correlation is developed for FFMCs with different porosities and fin-channel width ratios with a MAPE of 9.05 %. • HTC increases as porosity decreases in low-quality region while the variation is opposite in high-quality range due to the balance of heat transfer area and transverse flow resistance. • Fin area utilization efficiency drops with wider fins and optimal fin-channel width ratio is 1. • FFMC with 0.78 porosity and the same fin and channel widths shows the best flow boiling performance due to higher HTCs and lower pumping powers. Flow boiling in copper foam fin microchannels (FFMC) has demonstrated better heat transfer performance and flow stability. In this work, systematic experiments are conducted to investigate the effects of structural parameters on flow boiling in five FFMCs with porosities ranging from 0.78 to 0.82 and ratios of fin width to channel width ranging from 0.5 to 2. Five mass fluxes ranging from 68 kg/(m2s) to 408 kg/(m2s) and effective heat fluxes ranging from 2 W/cm2 to 297 W/cm2 are tested. The heat transfer, pressure drop, and flow instability characteristics are analyzed. Generally, the heat transfer in FFMCs follows the "three-zone" model. The heat transfer coefficient first increases with increasing heat flux and then decreases. A "synchronous backflow" phenomenon is observed in FFMCs, which is the direct cause of heat transfer deterioration. Due to the interaction of heat transfer area and the transverse flow resistance through the fins, the heat transfer coefficient increases with decreasing porosity in the low-quality region, while the opposite trend is observed in the high-quality region. The utilization efficiency of the fin area decreases as the fin width increases. An optimal ratio of fin width to channel width exists corresponding to the best heat transfer performance, which equals one in this work. A correlation is developed based on 583 data points considering heat transfer mechanisms in channels and fins. The new correlation predicts the experimental values with a mean absolute percentage error of 9 %, and over 99 % of the data are predicted within a 30 % deviation. A performance evaluation criterion is defined to comprehensively compare the FFMCs, and the FFMC with the porosity of 0.78 demonstrates the best flow boiling performance due to higher heat transfer coefficients and lower pumping powers when the ratio of fin to channel width is equal to one. [ABSTRACT FROM AUTHOR]

Pumped two-phase loop (P2PL) capable of handling high heat fluxes at low thermal resistance while consuming minimal pumping power offers a promising cooling solution for emerging high-power electronics. However, existing research primarily focuses on standalone evaporators with fixed boundary conditions, neglecting the interactions between the evaporator and other components within the P2PL. To address this gap, in this study, high-speed visualization with simultaneous temperature and pressure measurements is used to investigate the flow boiling regimes and their impact on the thermal characteristics of R134a in parallel microchannels. The ranges of parameters are: mass flux from 135.7 to 339.3 kg/m2-s, heat flux from 0 to 33.7 W/cm2, a fixed inlet temperature of 10 °C, inlet subcooling from 0.6 to 3.5 °C, and vapor quality from subcooled to dryout. Flow separation at the microchannel inlet induced both pressure-driven flashing of the subcooled liquid and thermal-driven nucleation on the heated walls, leading to rapid bubble formation. At low heat fluxes, bubbly flow dominates the inlet, swiftly transitioning to slug flow. Increased heat fluxes induce a unique jet flow regime featuring a wavy vapor jet. The local heat transfer coefficient was used to track the influence of flow regime transitions on microchannel thermal characteristics. • Flow separation at the channel inlet created an ideal condition for nucleation. • Strong nucleation of side walls with no nucleation on bottom walls was observed. • High-speed flow visualization reveals a unique jet flow regime at high heat fluxes. • An early transition to annular flow regime with enhanced heat transfer was observed. • Local heat transfer coefficient captures the effect of flow regime transitions. [ABSTRACT FROM AUTHOR]

Efficient thermal management is crucial for enhancing the performance and reliability of electronic devices. In this regard, flow boiling with microchannels promises high heat dissipation capability for a nominal temperature budget as it inherently takes advantage of high latent heat of vaporization along with high surface-area-to-volume ratio. However, the promise of high heat dissipation in flow boiling microchannels is limited by two-phase flow instabilities which cause large-amplitude temperature and pressure fluctuations. The current state-of-the-art thermal management techniques for enhancing heat transfer during flow boiling in microchannels operate in steady-state condition and provide sufficient continuous cooling to address the peak thermal loads. While such approaches can manage the thermal challenge, they result in significant cooling overdesign. To eliminate the overdesign, we introduce an adaptive vapor venting strategy which continuously adjusts to the varying vapor generation across different heat fluxes to completely mitigate two-phase fluctuations. Our results demonstrate that significant transient temperature fluctuations (±5 °C) in the conventional microchannels were completely eliminated with the incorporation of the adaptive vapor venting (within the experimental uncertainty). Furthermore, our results show that this strategy has the capability to handle transience and accordingly avoid the need for cooling overdesign. [ABSTRACT FROM AUTHOR]

Flow boiling provides an effective means of heat removal but can suffer from thermal and hydrodynamic transients that compromise heat transfer performance and trigger device failure. In this study, the transient flow boiling characteristics in two thermally isolated, hydrodynamically coupled parallel microchannels are investigated experimentally. High-speed flow visualization is synchronized to high-frequency heat flux, wall temperature, pressure drop, and mass flux measurements to provide time-resolved characterization. Two constant and two transient heating conditions are presented. For a constant heat flux of 63 kW/m2 into each channel, boiling occurs continuously in both channels and the parallel channel instability is observed to occur at 15 Hz. Time-periodic oscillations in the pressure drop and average mass flux are observed, but corresponding oscillations in the wall temperatures are virtually nonexistent at this condition. At a slightly lower constant heat flux of 60 kW/m2, boiling remains continuous in one of the channels, but the other channel experiences time-periodic flow regime oscillations between single-phase and two-phase flow. At this condition, extreme time-periodic wall temperature oscillations are observed in both channels with a long period (~7 s) due to oscillations in the severity of the flow maldistribution. For the transient heating conditions, square-wave heating profiles oscillating between different heat flux levels are applied to the channels. Because of their relatively high frequency, the heating transients are attenuated by the microchannel walls, resulting in effectively constant heating conditions and flow boiling characteristics like that of the aforementioned constant heating conditions. This study illustrates the susceptibility of parallel two-phase heat sinks to flow maldistribution, particularly when undergoing transient flow regime oscillations. [ABSTRACT FROM AUTHOR]

This study quantitatively compares the finite element (FE) and lumped modeling techniques to analyze phase-change and multiphase flow in a microchannel evaporator. We compare the one- and two-zone models in the lumped modeling approach by considering both pressure drop and phase change in the evaporator. The study investigates the deviation in results from the lumped model relative to the FE model for various evaporator heat loads. For the one-zone model, the relative errors in predicting the average density, pressure, temperature, and vapor quality increase with the evaporator heat load. This increase is mainly due to the nonlinear variation in the heat transfer coefficient and pressure drop, which are calculated with higher inaccuracies at larger evaporator heat loads. On the other hand, the liquid-phase region and the exit vapor quality are predicted more accurately. The overall errors using one- and two-zone lumped models were less than 30% when the exit qualities do not exceed 0.5. Since strategies involving flow boiling in evaporators typically avoid high exit vapor qualities to circumvent dry-out and critical heat flux, the use of lumped models under these conditions is favorable. This approach also avoids the computational costs associated with FE models while achieving sufficient accuracy to predict the evaporator’s performance. [ABSTRACT FROM AUTHOR]

Flow boiling in microchannel heat sinks is capable of providing the high-heat-flux dissipation required for thermal management of next-generation wide bandgap power electronics at low pumping power and uniform surface temperatures. One of the primary issues preventing implementation of these technologies is the presence of flow boiling instabilities, which may reduce the heat transfer performance. However, the effect of individual instabilities, such as the parallel channel instability or pressure drop oscillations, on the overall heat transfer coefficient and critical heat flux in microchannel heat sinks has not been fully quantified. The primary cause of these dynamic flow boiling instabilities is the interaction between the inertia of a two-phase mixture in a heated channel and sources of compressibility located upstream of the inlet. In order to isolate the effect of pressure drop oscillations on flow boiling heat transfer performance, experiments are performed in a single-square microchannel cut into a copper heat sink, with a controlled level of upstream compressibility. The impact of pressure drop oscillations on the heat transfer coefficient and critical heat flux is characterized through analysis of both time-averaged steady-state data as well as high-frequency pressure signals synchronized with high-speed visualization. The dielectric working fluid HFE-7100 is used in all experiments with a saturation temperature of 60 °C at the channel outlet pressure. The occurrence and effect of pressure drop oscillations in 20-mm-long microchannels of three different channel widths (0.5, 0.75, and 1 mm) are related to mass flux, the degree of two-phase flow confinement, and the severity of pressure drop oscillations. [ABSTRACT FROM AUTHOR]

Flow boiling of degassed double-distilled water in a single 50 × 50 μm and 100 × 50 μm microchannel was investigated on the basis of experimental measurements and high-speed visualization. The visualized events during boiling were analyzed in terms of the bubble frequencies and boiling front oscillations in microchannels. A digital image sequence analysis algorithm was composed to determine the time dependence of bubble and meniscus locations. The results show (i) the dynamic characteristics of boiling in microchannels, (ii) the increase of fundamental oscillation frequencies with increasing heat flux and temperature of the microchannel bottom, (iii) the amplitudes of the flow boiling oscillations are inversely proportional to the fundamental frequencies. The outcomes of the study are important as the oscillations during boiling in single microchannels are experimentally confirmed to be predictable in terms of oscillation frequencies and amplitudes trends and dependencies. This knowledge is especially significant at constructing efficient two-phase micro heat exchangers, micro mixers or micro reactors, as the cross section and the length of the channel become exceedingly important design parameters in micro devices with boiling.

An ultrasonic vibration field can be used to enhance the heat transfer efficiency in microchannels. This study aims to investigate the flow-boiling heat transfer characteristics of nanofluids in the microchannels with or without ultrasonic wave. A microchannel test was designed for the section that can be placed in an ultrasonic transducer. An ultrasonic vibration method was selected to prepare the uniform and stable TiO2/R141b nano-refrigerant with the mass fraction of 0.1%, 0.2% and 0.3%. The flow-boiling parameters of nanofluid were measured in the microchannels under ultrasonic wave. A flow boiling experiment was performed on a rectangular microchannel with a cross-sectional width of 2 mm, where the design system pressure of 152 kPa, the effective heat flux density ranged from 10.8 to 22.7 kW/m², the ultrasonic power of 50 W, ultrasonic frequency of 23 kHz, mass flow rate of 121.1 kg/(m²·s), and inlet temperature of 35 ℃. Different enhancement effects of heat transfer can be achieved under the nanofluids with different mass fractions. The reason is that the nanoparticles can be used to enhance heat transfer, while increase the thermal resistance avoided by heat transfer, and thereby the increase of thermal resistance can reduce the heat transfer efficiency. The results show that the heat transfer coefficient reached the highest, when the mass fraction of nanoparticles was 0.2%, where the heat transfer enhancement effect can be the best. The nanofluid with a mass fraction of 0.2% under the action of ultrasound indicated the optimal enhancement effect of heat transfer, compared with the case of no ultrasound, where the average saturation boiling heat transfer coefficient of R141b increased by 89.9%. The heat flux posed a great influence on the enhanced heat transfer effect of ultrasonic. There was a significant difference in the enhancement effect under different heat fluxes. The average saturated boiling heat transfer coefficient of nano-refrigerant under the action of sound field increased first and then decreased, with the increase of effective heat flux density. The sound field of vapor-liquid interface in the microchannel was also simulated by COMSOL software. The simulation results show that the propagation of ultrasonic waves in the bubble was weak. When the effective heat flux density below 15.2 kW/m², the ultrasonic wave can enhance the heat transfer via the increase in the breakaway frequency of the bubble, whereas, the average saturated boiling heat transfer coefficient increased, as the effective heat flux density increased. After the effective heat flux density was 15.2 kW/m², the strengthening effect of ultrasound began to weaken, due to the increase of bubbles in the microchannel. When the effective heat flow density reached 19.8 kW/m², the change of flow pattern can lead to an uniform heat transfer, due mainly to the elongation of bubble flow in the microchannel. In the nano-refrigerant with a mass fraction of 0.2%, the enhanced heat transfer effect increased successively for the imported ultrasonic wave, and the ultrasonic wave of the inlet and outlet. When the ultrasonic wave was applied to the inlet, the average saturated boiling heat transfer coefficient increased by 26%, whereas, increased by 46% under the action of ultrasonic import and export. The findings can provide new ideas to improve the heat transfer performance of microchannels when applying ultrasonic waves. [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]

Phase change heat transfer within microchannels is considered one of the most promising cooling methods for the efficient cooling of high-performance electronic devices. However, there are still fundamental parameters, such as the effect of channel hydraulic diameter Dh whose effects on fluid flow and heat transfer characteristics are not clearly defined yet. The objective of the present work is to numerically investigate the first transient flow boiling characteristics from the bubble inception up to the first stages of the flow boiling regime development, in rectangular microchannels of varying hydraulic diameters, utilising an enhanced custom VOF-based solver. The solver accounts for conjugate heat transfer effects, implemented in OpenFOAM and validated in the literature through experimental results and analytical solutions. The numerical study was conducted through two different sets of simulations. In the first set, flow boiling characteristics in four single microchannels of Dh = 50, 100, 150, and 200 μm with constant channel aspect ratio of 0.5 and length of 2.4 mm were examined. Due to the different Dh, the applied heat and mass flux values varied between 20 to 200 kW/m2 and 150 to 2400 kg/m2s, respectively. The results of the two-phase simulations were compared with the corresponding initial single-phase stage of the simulations, and an increase of up to 37.4% on the global Nu number Nuglob was revealed. In the second set of simulations, the effectiveness of having microchannel evaporators of single versus multiple parallel microchannels was investigated by performing and comparing simulations of a single rectangular microchannel with Dh of 200 μm and four-parallel rectangular microchannels, each having a hydraulic diameter Dh of 50 μm. By comparing the local time-averaged thermal resistance along the channels, it is found that the parallel microchannels configuration resulted in a 23.3% decrease in the average thermal resistance R¯l compared to the corresponding single-phase simulation stage, while the flow boiling process reduced the R¯l by only 5.4% for the single microchannel case. As for the developed flow regimes, churn and slug flow dominated, whereas liquid film evaporation and, for some cases, contact line evaporation were the main contributing flow boiling mechanisms.

• Mitigation of flow boiling instabilities by components external to microchannel heat sink. • Bubble slug ebullition cycle in three different flow configurations has been investigated. • Temperature and pressure oscillations in three different configurations have been investigated. • Loop components external to microchannel heat sink significantly affect the flow instabilities. Present work reports an experimental investigation of flow boiling instabilities in three different configurations of a flow boiling loop. The three flow configurations i.e. loop having flexible tubing and connections, loop having stiffed tubing and loop having stiffed tubing with a throttling valve have been investigated and their performances are compared. Experiments are performed with segmented finned microchannels (SMC) to investigate the roles of above mentioned components in flow boiling instabilities. The array of segmented finned microchannels are fabricated on a copper substrate with a footprint area of 10 × 10 mm2 and experiments have been performed using deionized water at 20 °C subcooling. It has been observed that loop components external to the heat sink affect the heat transfer, pressure drop characteristic and assist in mitigating flow boiling instabilities. Based on the flow patterns, the slug ebullition cycle has been categorized into three distinct stages (a) slug formation (b) upstream compression and (c) flooded flushing. Cycle time and number of oscillations during upstream compression stage were reduced in the stiffed connections and throttling valve flow configurations. Heat transfer performance of the throttling valve configuration has been found to be the best among all three, followed by stiffed tubing configuration. The loop with flexible tubing connections demonstrated largest time period of slug ebullition cycle and also experienced large amplitude wall temperature and pressure oscillations. The combination of stiffed tube materials surrounding the heat sink and throttling valve at pump exit effectively reduce the flow boiling instabilities resulting in better thermal and hydraulic performance. [ABSTRACT FROM AUTHOR]

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Highlights • Five parallel microchannels are interconnected by micro-slots. • Micro-slots can enable high frequency nucleate boiling. • Highly coordinated, rapid and sustainable nucleate boiling are realized. • The HTC and CHF are significantly enhanced on the present design. • The enhancements of HTC and CHF are ∼175% and ∼76%, respectively. Abstract Flow boiling in microchannels using dielectric fluids is one of the most desirable cooling solutions for high power electronics. However, it is challenging to promote the flow boiling performances, particularly critical heat flux (CHF), due to their unfavorable thermophysical properties. Flow boiling in parallel and isolated microchannels has been extensively studied. In this study, five parallel microchannels (W = 200 µm, H = 250 µm, L = 10 mm) are interconnected by 4 × 28 micro-slots (20 µm wide and 250 µm deep) starting from the middle section to the channel outlet. Our visualization study shows that these micro-slots designed as artificial nucleation sites can enable high frequency nucleate boiling by drastically reducing the bubble waiting time and remaining fully activated, simultaneously. More importantly, such rapid switch on–off harmonically coordinated nucleate boiling in the neighboring channels creates a highly desirable periodic rewetting mechanism to substantially delay CHF conditions and enhance heat transfer rate. Flow boiling in this innovative microchannel configuration has been systematically characterized with mass flux ranging from 462 kg/m2∙s to 1617 kg/m2∙s. Compared to plain-wall microchannels with inlet restrictors (IRs), flow boiling heat transfer coefficient (HTC) is enhanced up to ∼172% at a mass flux of 462 kg/m2·s primarily owing to the enhanced latent heat transfer including nucleate boiling and thin film evaporation. The peak value of effective HTC is ~60 kW/m2·K in the fully developed boiling regime. Moreover, CHF is substantially enhanced by ∼76% at a mass flux of 1155 kg/m2·s owing to the rapid and periodic rewetting enabled by these micro-slots. Such drastic enhancements have been achieved without compromising two-phase pressure drop. [ABSTRACT FROM AUTHOR]

Flow boiling of degassed double-distilled water in a single 50 × 50 μm and 100 × 50 μm microchannel was investigated on the basis of experimental measurements and high-speed visualization. The visualized events during boiling were analyzed in terms of the bubble frequencies and boiling front oscillations in microchannels. A digital image sequence analysis algorithm was composed to determine the time dependence of bubble and meniscus locations. The results show (i) the dynamic characteristics of boiling in microchannels, (ii) the increase of fundamental oscillation frequencies with increasing heat flux and temperature of the microchannel bottom, (iii) the amplitudes of the flow boiling oscillations are inversely proportional to the fundamental frequencies. The outcomes of the study are important as the oscillations during boiling in single microchannels are experimentally confirmed to be predictable in terms of oscillation frequencies and amplitudes trends and dependencies. This knowledge is especially significant at constructing efficient two-phase micro heat exchangers, micro mixers or micro reactors, as the cross section and the length of the channel become exceedingly important design parameters in micro devices with boiling. [ABSTRACT FROM AUTHOR]

• Pressure drop oscillations in parallel channel heat sinks are investigated. • Experiments are performed with controlled system compressibility. • Heat sinks with large and small thermal capacitance are tested. • Temperature oscillations are found to increase with lower thermal capacitance. • Critical heat flux is found to decrease due to pressure drop oscillations. The efficient heat transfer resulting from flow boiling in microchannel heat sinks can help dissipate high heat fluxes from high-density electronic devices across a small temperature difference. However, practical implementation challenges unique to two-phase flow boiling, as compared to single-phase liquid cooling, have prevented its widespread adoption. A primary challenge is the occurrence of dynamic two-phase flow instabilities, such as pressure drop oscillations (PDO), that have the potential to degrade heat transfer performance or trigger premature critical heat flux (CHF) under some conditions. Under other conditions, PDOs are observed to have little to no impact on performance. One factor proposed by modeling studies to be responsible for this discrepancy in observations of the effect of dynamic instabilities on performance is the thermal capacitance of the heat sink, though this has not been confirmed by experiments. In this study, the effect of thermal capacitance on the transient thermal response of a heat sink experiencing PDOs is examined through use of a dynamic two-phase flow model and experiments. Flow boiling experiments are performed with a controlled compressible volume upstream of parallel-microchannel heat sinks having either a large or a small thermal capacitance. In accordance with the behavior predicted by our model, when thermal capacitance is reduced, the pressure drop oscillation frequency is found to decrease and temperature swings in the heat sink become more severe. Additionally, the experimentally measured CHF limit is diminished in the heat sink at smaller thermal capacitance. These results reveal thermal capacitance as a critical parameter that determines how much dynamic instabilities degrade flow boiling performance in a microchannel heat sink. [ABSTRACT FROM AUTHOR]

Flow boiling of dielectric fluids in microchannels is one of the most desirable cooling solutions for high power electronics. However, the flow boiling of dielectric fluids is hindered by their unfavorable thermophysical properties. Specifically, without precooling dielectric fluids, it is challenging to promote critical heat flux (CHF) due to its high vapor density, low surface tension and the resulted superior wettability. In this study, each side wall of a five-parallel silicon microchannel array was structured with an array of microscale reentry cavities and four micronozzles bypassed by an auxiliary channel. The present microchannel configuration aims to significantly enhance CHF of HFE-7100 flow boiling by improving global liquid supply using auxiliary channels and micrononozzles as well as by sustaining liquid film using capillarity induced by reentry cavity array. Equally important, these structures can promote nucleate boiling at low heat flux, generate intense mixing, and promote thin film evaporation at high heat flux, resulting in high flow boiling heat transfer rate. Flow boiling of HFE-7100 in the present microchannel configuration is characterized with mass flux ranging from 231 kg/m 2  s to 1155 kg/m 2  s. The effective two-phase heat transfer coefficients (HTCs) are ranging from 6 kW/m 2  K to 117 kW/m 2  K. Compared to the four-nozzle plain-wall microchannels, for example, the effective HTC and CHF can be substantially enhanced up to 208% and 37%, respectively, without escalating pressure drop at a mass flux of 462 kg/m 2  s. Compared to plain microchannels with inlet restrictors, CHF is considerably enhanced up to 70% with a reduction of pressure drop ∼82% at a mass flux of 1155 kg/m 2  s. Significantly reduced pressure drop is achieved by integrating bypass and the enhanced confined bubble removal. A peak CHF value of 216 W/cm 2 is achieved at mass flux of 2772 kg/m 2  s in the present microchannel configuration with inlet temperature at room temperature. [ABSTRACT FROM AUTHOR]

Flow boiling in parallel microchannels can be dramatically enhanced through inducing self-excited and self-sustained high frequency two-phase oscillations as demonstrated in our previous studies using a two-nozzle microchannel configuration. Two-phase mixing induced by the rapid bubble collapse is shown to be the major enhancement mechanism. However, in the two-nozzle configuration microchannels, the mixing effect is limited to the downstream of the microchannels, meaning that only half length of the entire microchannel is functionalized as designated. In this study, a four-nozzle microchannel configuration is developed with an aim at extending the highly desirable mixing effect to the entire channel. Flow boiling in the four-nozzle configuration microchannels is experimentally studied with deionized water and the mass flux ranging from 120 kg/m 2 s to 600 kg/m 2 s. The onset of nucleate boiling temperature is considerably reduced by ∼14% because of more nucleation sites created by the multiple nozzles. Equally important, the improved microchannel configuration successfully extends the mixing to the entire channel as validated by the enhanced heat transfer rate and visualization study. Compared to the previous two-nozzle configuration, the overall heat transfer coefficient (HTC) is significantly improved primarily owing to the enhanced nucleate boiling. For example, the peak overall HTC of 262 kW/m 2 K is achieved at a mass flux of 150 kg/m 2 s, accounting for ∼83.7% enhancement. Additionally, the peak effective HTC reaches 97.6 kW/m 2 K, accounting for ∼67% enhancement. [ABSTRACT FROM AUTHOR]

Most of the microchannel flow boiling experiments are performed using electrical heaters to promote the boiling process. However, there are many heat exchangers applications (e.g. automotive, refrigeration, air-conditioning) in which the vaporization heat is provided by a secondary fluid (water, air…). The paper presents a comparative analysis between two experimental techniques that can be adopted when the heat flux is not imposed by Joule effect heating and hence its direct measurement from electrical parameters is not possible. The two methods are applied to the measurement of the local heat transfer coefficient during flow boiling inside a 0.96 mm diameter microchannel. The first method is the one proposed by Del Col et al. (2013), the second one is presented here for the first time. A key advantage of the new technique is the fact that the heat transfer coefficients can be measured in the dryout zone as well. The two methods are used to reduce new experimental data of flow boiling of R1234ze(E), which is a fluorinated propene isomer and a possible low-GWP refrigerant to replace R134a in some air conditioning and refrigeration applications. The flow boiling heat transfer coefficient strongly depends on the heat flux, with little effect of vapor quality and mass velocity. Only the adoption of the new technique allows to investigate the post dryout region as well. [ABSTRACT FROM AUTHOR]

• Dynamic flow boiling instabilities in microchannel heat sinks are investigated. • Experiments are performed with controlled system compressibility. • Effects of pressure drop oscillations and parallel channel instabilities are characterized. • These instabilities are found to have minimal impact on surface temperatures. • Critical heat flux is found to be insensitive to the occurrence of instabilities. Microchannel flow boiling heat sinks that leverage the highly efficient heat transfer mechanisms associated with phase change are a primary candidate for cooling next-generation electronics in electric vehicles. In order to design flow boiling heat sinks for such practical applications, one key obstacle is an understanding of the conditions for occurrence of dynamic two-phase flow instabilities, to which microscale flow boiling is particularly susceptible, as well as their impact on heat transfer performance. While mapping the operational regimes of these instabilities has been well-studied, with numerous stability criteria available, their impact on the heat transfer performance of heat sinks in practical applications is not understood. This work seeks to assess the impact of pressure drop oscillations and parallel channel instabilities on the surface temperature and critical heat flux in parallel microchannel heat sinks. This is achieved through measurement of time-averaged steady-state temperatures and pressures, combined with high-frequency pressure signals and high-speed flow visualization. These data are compared across three controlled flow configurations that comprise a condition of stable flow boiling, a condition where only parallel channel instabilities can occur, and a third where both pressure drop oscillations and parallel channel instabilities can occur. Experiments are performed using the dielectric refrigerant HFE-7100 in 2 cm long parallel microchannel heat sinks with square-cross-section channels (0.25, 0.5, 0.75, and 1 mm widths) at three mass fluxes (100, 400, and 1600 kg/m2s). Across this range of conditions, the time-averaged surface temperature and critical heat flux were remarkably insensitive to the occurrence of these instabilities despite the significant hydrodynamic events and transient flow patterns observed. [ABSTRACT FROM AUTHOR]

Pressure drop is experimentally investigated inside parallel microchannels during subcooled flow boiling of R134a in horizontal orientation. The test conditions included the inlet pressure, the inlet subcooled degree, the heat flux, the vapor quality, and the mass velocity, ranging from 600 to 900 kPa, 1 to 20 K, 5 to 220 kW m −2 , 0 to 95% and 250 to 1000 kg m −2  s −1 , respectively. The effect of the mass velocity and the inlet pressure were investigated. The relative weight of the pressure drop due to two-phase flow acceleration and the friction pressure drop for single phase and two-phase flows were considered. The experimental results for the pressure drop were compared with those predicted by the homogenous model and five other semi-empirical models. [ABSTRACT FROM AUTHOR]

Flow boiling in microchannels has been researched for decades due to its potential for heat transfer applications in micro structured devices. The large number of existing experimental datasets and fitted correlations show considerable scatter and lack a mechanistic basis. The computational fluid dynamics (CFD) model developed here applies to boiling in the annular flow regime and provides insights into the underlying behaviour. Analysis of the heat transfer augmentation by interfacial waves is studied. Parametric investigations are performed using the model to understand the impact of important flow and system parameters. The calculated heat transfer coefficients are consistent with those from existing correlations, showing no more scatter than the data used to create them. [ABSTRACT FROM AUTHOR]

Research efforts on flow boiling in microchannels were focused on stabilizing the flow during the early part of the last decade. After achieving that goal through inlet restrictors and distributed nucleation sites, the focus has now shifted on improving its performance for high heat flux dissipation. The recent worldwide efforts described in this paper are aimed at increasing the critical heat flux (CHF) and reducing the pressure drop, with an implicit goal of dissipating 1 kW/cm2 for meeting the high-end target in electronics cooling application. The underlying mechanisms in these studies are identified and critically evaluated for their potential in meeting the high heat flux dissipation goals. Future need to simultaneously increase the CHF and the heat transfer coefficient (HTC) has been identified and hierarchical integration of nanoscale and microscale technologies is deemed necessary for developing integrated pathways toward meeting this objective. [ABSTRACT FROM AUTHOR]

Increasing critical heat flux (CHF) and heat transfer coefficient (HTC), and reducing pressure drop (Δp) are highly desirable during flow boiling. An asymmetric Dual-V groove microchannels geometry was developed and combined with a tapered open manifold to achieve significant performance enhancements in all three aspects while reporting boiling inversion for the first time in flow boiling. Evaporation momentum force is utilized to direct bubbles along a central region of the doubly-finned structure, while inertia force is utilized to create a two-phase flow in the microchannel flow passages formed by the fins. The microchannel passages are contoured as adjoining pairs of asymmetric Dual-V grooves. Bubbles nucleate at the corners of the V-grooves and the evaporation momentum force modulates their traverse over the inclined surfaces of the microchannels. High-speed video images reveal that the growing bubbles flow rapidly over the microchannel walls and emerge into the microgap region a certain distance away from their respective nucleation sites. This leads to a self-feeding mechanism that causes boiling inversion in which wall superheat drops with increase in heat flux. Early departure from the nucleation site before fully growing improves the CHF and the bubble traverse normal to flow direction improves HTC. Furthermore, the momentum of the growing bubbles in the tapered open microgap configuration reduces the pressure drop. Using water as working fluid, we have reached a heat dissipation of 508.1 W/cm2 without reaching CHF at a wall superheat of 12.3 °C with a pressure drop of only 3 kPa. The resulting HTC was 412.2 kW/(m2 K). [ABSTRACT FROM AUTHOR]

Critical heat flux ( CHF ) during flow boiling in silicon microchannels ( H = 250 μm, W = 200 μm, L = 10 mm) using self-excited and self-sustained high frequency two-phase oscillations is studied both experimentally and theoretically. Tests are performed on deionized water over a mass flux range of 200–1350 kg/m 2 s. An enhanced CHF of 1020 W/cm 2 is achieved experimentally at a mass flux of 1350 kg/m 2 s in the present study. Since no existing CHF models and correlations on parallel mini/microchannels considered high frequency two-phase oscillations, hence are not applicable to predict CHF in the present microchannel configuration. Adopting Helmholtz and Rayleigh instability theories and based on experimental study of liquid thin film dry-out phenomena in two-phase oscillations, a semi-theoretical CHF model is proposed. The proposed theoretical predictions show satisfactory agreement with experimental data with a reasonable low mean absolute error ( MAE ) of 25–32%. [ABSTRACT FROM AUTHOR]

This paper focuses on flow boiling performance of a type of $\Omega $ -shaped re-entrant porous microchannels for efficient thermal management of high heat-flux devices. The porous-based microchannels are totally constructed by sintered irregular shaped copper powders that featured microchannel walls with structured surface. Flow boiling experiments are conducted utilizing two coolants (deionized water and ethanol) at inlet subcoolings of 10 °C and 40 °C, mass fluxes of 125–300 kg /\mathrm {m} ^{2}\,\cdot \, \mathrm {s} , and a wide range of heat fluxes. Flow boiling heat transfer, pressure drop, and two-phase flow instabilities are comprehensively evaluated to access the parametric effects, i.e., heat flux, mass flux, inlet subcooling, and coolants, on the flow boiling performance of re-entrant porous microchannels with structured surface (RPM-SS). It is found that for both water and ethanol, the two-phase heat transfer coefficients of RPM-SS generally decreased, while the pressure drop increased with the increase in heat flux and vapor quality. Though severe pressure drop instabilities with large magnitude of flow oscillations occurred for RPM-SS in the ethanol tests at large inlet subcooling cases, the RPM-SS can sustain fairly stable boiling at small inlet subcooling tests without the installation of inlet restrictors. [ABSTRACT FROM PUBLISHER]

In this work, an experimental investigation has been made to compare the flow boiling characteristics of deionized water in three different configurations of microchannels. The investigated channel configurations are uniform cross-section, diverging cross-section and segmented finned microchannels. In each configuration, an array consisting of twelve numbers of microchannels with rectangular cross-section has been fabricated on a copper block with footprint area of 25.7 × 12.02 mm 2 . Experiments have been conducted with subcooled liquid state at the entry with coolant mass and heat fluxes vary in the range 100–350 kg/m 2 s and 10–350 kW/m 2 , respectively. Depending upon the heat flux and coolant flow rate different regimes of two phase boiling have been observed. The comparison of three configurations has been made in terms of heat transfer coefficient, pressure drop characteristics and affinity towards backflow or flow reversal in the channels. Bubbles dynamics and their role in flow reversal and flow instability have been discussed for all three types of channels. For entire operating conditions, segmented finned channels demonstrate the highest heat transfer coefficient with negligible higher pressure drop compared to other two configurations of channels. The performance of diverging cross-section channels is better than the uniform cross-section channels. However, they underperform compared with segmented finned channels. At higher heat flux, bubble clogging and flow reversal problem is worst in uniform cross-section channels. The problem is partially solved in diverging cross-section channels. Segmented channels completely relieve the problem of bubble clogging allowing smooth and easy passage of growing bubbles. Moreover flow reversal is not observed in segmented channels for entire operating range of heat flux and coolant mass flux. [ABSTRACT FROM AUTHOR]

Flow boiling within advanced microchannel heat sinks provides efficient and attractive solutions for the cooling of heat-heat-flux devices. In this study, a type of porous-based microchannels with reentrant configurations is developed and tested in heat sink cooling systems. The reentrant porous microchannels (RPMs), which are constructed by porous copper powder, are fabricated by solid-state sintering method under the replication of specially designed sintering modules. Three reentrant porous microchannels with hydraulic diameters of 671 μm,786 μm and 871 μm are tested under boiling deionized water conditions at inlet temperatures of 90 °C, mass flux of 160–300 kg/m 2 s. The effects of heat flux, mass flux and channel size on the flow boiling performance of reentrant porous microchannels are examined. Test results show that the two-phase heat transfer performance of RPMs was strongly dependent on the heat flux, but showed a weak dependence on the mass flux. Reentrant porous microchannels with the medium size were found to enhance two-phase heat transfer rate compared to the other two samples at moderate to high heat fluxes. [ABSTRACT FROM AUTHOR]

• Silicon-based microchannels were fabricated with porous-structured sidewalls. • Porous walls sustain annular flow, promote film evaporation, and delay dryout. • HTC and CHF were significantly enhanced without compromising pressure drop. • 205% CHF enhancement was achieved compared with plain wall microchannels. Silicon microchannels with microporous decorated sidewalls were applied to enhance water flow boiling heat transfer, especially the critical heat flux (CHF). In experiments, a CHF of 473 W/cm2 based on the heater areas (210 W/cm2 if based on the effective heating areas) was achieved in the proposed microchannels at the mass flux of 389 kg/cm2, accounting for a 205% CHF enhancement compared with that in plain-wall microchannels without compromising the pressure drop. In addition, an improvement of 228% in effective heat transfer coefficient was achieved in the early nucleate boiling regime at the mass flux of 113 kg/cm2 compared with the plain-wall microchannels owing to the increased nucleation site density on the porous walls. Moreover, a visualization study via a microscope-linked high-speed camera was conducted to investigate the mechanism of liquid distributions and wetting phenomena in porous wall microchannels. The sustainable liquid film induced by the microporous structure greatly delays the local dryout and promotes more efficient heat transfer mechanisms such as thin-film evaporation. The exit vapor quality of the proposed microchannels has been increased above 0.3 at the tested mass flux range 113–389 kg/m2s, two times higher than that in the plain-wall microchannels (∼0.1). [ABSTRACT FROM AUTHOR]

Flow boiling in microchannels is a promising technique to achieve large cooling rates and high heat transfer coefficients for electronic cooling. This work reports an effective approach to enhance flow boiling heat transfer in microchannels through expansion areas. Flow boiling experiments were conducted on plain microchannels and microchannels with single and three expansion areas for comparison. Experimental results show that the flow boiling heat transfer coefficient can be effectively enhanced by up to 43.3% through adding three expansion areas in microchannels, while the pressure drop variation is within 3 kPa. Expansion areas significantly reduce the inlet temperature fluctuation, indicating the suppression of flow boiling instability in microchannels. As disclosed by the visualization of flow patterns, the improved heat transfer coefficient can be attributed to the enhanced bubble nucleation and formation of a nonuniform liquid layer near the corners of expansion areas. • Report an effective approach to improve flow boiling heat transfer in microchannels. • Enhance the HTC by 43.3% through adding expansion areas in microchannels. • Expansion areas effectively suppress flow reversal in microchannels. • The increase in pressure drop is less than 3 kPa. [ABSTRACT FROM AUTHOR]

An experimental study of flow boiling heat transfer and pressure drop was conducted using R245fa in stainless steel, brass and copper tubes of 1.1 mm internal diameter. Experimental conditions include: mass flux range 100–400 kg/m 2 s, heat flux range 10–60 kW/m 2 , pressure of 1.8 bar and exit vapour quality range 0–0.95. The tube surfaces were compared using scanning electron microscopy (SEM) and surface data acquired from confocal laser microscopy (CFLM), both showing differences between the surfaces. The heat transfer coefficient is similar in magnitude for all three materials but with a slight variation in trend. The heat transfer coefficient is seen to peak at high vapour qualities for stainless steel and brass, which is less evident with copper. The results were compared with past heat transfer correlations. These results showed better agreement with stainless steel compared to copper and brass. The pressure drop was shown to differ with surface characteristics, with the pressure drop for brass having a much steeper increase with heat flux. The pressure drop correlations tested did not show good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

New flow boiling experimental results for R407C in a microchannel based heat spreader are presented. Boiling curves were obtained for heat fluxes up to 310 kW/m 2 (based on the footprint area), mass velocities from 400 to 1500 kg/m 2 s, liquid subcoolings at the test section inlet of 5, 10 and 15 °C and saturation temperatures referred to the pressure at the heat sink inlet of approximately 25 °C. Based on these results, heat sink averaged heat transfer coefficients during convective boiling were estimated. The heat sink evaluated in the present study is comprised of fifty parallel rectangular channels with cross-section dimensions of 100 × 500 μm, and total length of 15 mm. Average heat transfer coefficients up to 30 kW/m 2 °C were obtained. It was also found that the boiling curve moves to the left hand side with decreasing the mass velocity and liquid subcooling at the heat-sink inlet. Moreover, for a fixed heat-sink averaged vapor quality, the average heat transfer coefficient increases with increasing mass velocity. Under similar experimental conditions, the refrigerant R134a provided higher heat transfer coefficients than R407C. Additionally, during flow boiling of R407C, pressure oscillations with lower amplitude and frequency were observed compared to R134a. No one of the heat transfer predictive methods evaluated in the present study was accurate enough to predict the present R407C database. [ABSTRACT FROM AUTHOR]

This investigation explores the possibilities to reduce the pressure drop of a single-channel micro-evaporator. The availability of micro-technology to create three-dimensional structures at a micro-meter scale opens opportunities to better control process conditions in once-through boilers. However, process miniaturization possesses some inherent drawbacks as well. Among others, the relatively large pressure drop in a micro-system makes it rather unsuitable for low-pressure applications. Especially in phase-change processes, the pressure drop may become large due to the expansion in small-sized channels. To address this drawback, flow boiling relations for small diameter tubes are first studied. These relations show a general form of the empirical correlations. Using this formulation, reduction factors could be deduced for the momentum pressure drop and friction pressure drop in case of a conical channel. These theoretically derived reduction factors show that the total pressure drop can be reduced significantly. The momentum pressure drop completely vanishes for outflow/inlet diameter ratios of 6.3 in the case of water. The friction pressure drop is reduced by a factor of ten at an outflow/inlet diameter ratio larger than four. An experimental comparison using a five-times diameter increase shows that the estimated reduction factor approaches the theoretically derived value for higher water supplies. [ABSTRACT FROM PUBLISHER]

Formation of the first bubble at nucieation site is an inception of the two phase flow in pool boiling and flow boiling. Bubble dynamics (bubble nucieation, growth, and departure) plays an important role in heat transfer and pressure drop characteristics during rv,'o phase flow in microchanneis. In this paper, a simplified model has been developed for predicting bubble growth rate at nucieation cavity in microchannel. It is assumed that heat supplied at nucieation site is divided between the liquid phase and the vapor phase as per instantaneous void fraction value. The energy consumed by the vapor phase is utilized in bubble growth and overcoming resistive effects; surface tension, inertia, shear, gravity, and change in momentum due to evaporation. Proposed model shows a good agreement with available experimental works. In addition, the bubble waiting time phenomenon for flow boiling is also addressed using proposed model. Waiting time predicted by the model is also close to that obtained from experimental data [ABSTRACT FROM AUTHOR]

In an effort to increase processor speeds, 3D IC architecture is being aggressively pursued by researchers and chip manufacturers. This architecture allows extremely high level of integration with enhanced electrical performance and expanded functionality, and facilitates realization of VLSI and ULSI technologies. However, utilizing the third dimension to provide additional device layers poses thermal challenges due to the increased heat dissipation and complex electrical interconnects among different layers. The conflicting needs of the cooling system requiring larger flow passage dimensions to limit the pressure drop, and the IC architecture necessitating short interconnect distances to reduce signal latency warrant paradigm shiffs in both of their design approach. Additional considerations include the effects due to temperature nonuniformity, localized hot spots, complex fluidic connections, and mechanical design. This paper reviews the advances in 3D IC cooling in the last decade and provides a vision for codesigning 3D IC architecture and integrated cooling systems. For heat fluxes of 50-100 Wlcm on each side of a chip in a 3D IC package, the current single-phase cooling technology is projected to provide adequate cooling, albeit with high pressure drops. For future applications with coolant surface heat fluxes from 100 to 500 Wlcm, significant changes need to be made in both electrical and cooling technologies through a new level of codesign. Effectively mitigating the high temperatures surrounding local hot spots remains a challenging issue. The codesign approach with circuit, software and thermal designers working together is seen as essential. The through silicon vias (TSVs) in the current designs place a stringent limit on the channel height in the cooling layer. It is projected that integration of wireless network on chip architecture could alleviate these height restrictions since the data bandwidth is independent of the communication lengths. Microchannels that are 200 ßm or larger in depth are expected to allow dissipation of large heat fluxes with significantly lower pressure drops. [ABSTRACT FROM AUTHOR]

Abstract: A novel porous heat sink with reentrant microchannels has been developed by traditional sintering fabrication method. It features 14 parallel Ω-shaped reentrant microchannels with a hydraulic diameter of 786μm. Two-phase boiling heat transfer performance of the reentrant porous microchannels (RPM) was evaluated to explore the feasibility of the enhancement and application in heat sink cooling. Using deionized water as coolant, flow boiling tests were conducted at the inlet temperature of 33, 60 and 90°C, mass flux of 125–300kg/m2 s. Comparisons with solid copper microchannels of the same reentrant configuration were performed, and the test results show that the reentrant porous microchannels presented a significant decrease of the wall superheat for the boiling incipience, delay and mitigation of two-phase flow instabilities, and 2–5 folds enhancement in two-phase heat transfer coefficient at low to moderate heat fluxes. It was also found that during the flow boiling of the reentrant porous microchannels, nucleation boiling governed the heat transfer mechanism at low heat fluxes and vapor quality, while forced convective boiling associated with thin film evaporation dominated at moderate to high heat fluxes. Furthermore, the two-phase flow instabilities characteristics were also accessed for the reentrant porous microchannels. [Copyright &y& Elsevier]