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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]

Surface treatment is an effective method of enhancing the heat transfer performance of flow boiling. Diamond/Cu, as a particle‐reinforced composite, exhibits heterogeneous surface properties and surface microstructures that play distinct roles in heat transfer. To elucidate the synergistic effect of heterogeneous material surfaces and microstructures, pure copper surface (PC), untreated diamond/Cu surface (DC), and surface‐treated diamond/Cu surface (SDC) are prepared for this experiment. These surfaces demonstrate various combinations of diamond, copper, and their corresponding surface microstructures. DC, characterized by heterogeneous wetting properties, demonstrates a heat transfer coefficient (HTC) that is 9%–24% higher than that of PC. The surface treatment of SDC results in the formation of a superhydrophilic CuO nanosheet structure, significantly enhancing both surface roughness and energy. SDC develops microcracks on the surface of diamond. The increased roughness provides numerous nucleation areas for the hydrophobic diamond, enhancing the overall wettability of the surface when combined with the superhydrophilic CuO region. SDC demonstrates the advantage of enhanced flow boiling on the surface of heterogeneous materials. At the identical heat flux, SDC exhibits a reduction of 5–10 °C in wall superheat and an increase of 38%–47% in HTC. [ABSTRACT FROM AUTHOR]

To fill the research gaps in the study of flow boiling on composite surfaces, four heat exchange tubes with different structures were investigated: an enhanced tube with herringbone teeth (EHT_HB), a tube with a dimple structure (EHT_DIM), a tube with a composite herringbone tooth/dimple structure (EHT_HB/DIM), and a smooth tube as a baseline for comparison purposes. The experimental conditions were set as follows: saturation temperature 6 °C, mass flux 50-205 kg/(m²·s), and vapor quality 0.2-0.8. After confirming the validity of the experimental results, the effects of the mass flux on the flow boiling heat transfer and pressure drop characteristics were examined. Results showed that the flow boiling heat transfer coefficient and pressure drop increase with the increase of the mass flux and the vapor quality in the tested tubes. The EHT_HB/DIM tube was found to combine the advantages of the two structures and to have the highest heat transfer coefficient. The average value of the heat transfer coefficient was found to be 43.75% higher than that of the smooth tube. Also, the average flow boiling heat transfer coefficients of the EHT_HB and the EHT_DIM tubes were found to be 15% and 35% higher than that of the smooth tube, respectively. From the perspective of the frictional pressure drop, it was found that the EHT_HB/DIM tube exhibited the maximum frictional pressure drop. The optimum working conditions of the enhanced tubes were determined by introducing a performance factor that captures the combined effects of heat transfer and pressure drop. [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. [ABSTRACT FROM AUTHOR]

Copyright of Chemical Engineering (China) / Huaxue Gongcheng is the property of Hualu Engineering Science & Technology Co Ltd. and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The purpose of this study is to explore the boiling heat transfer performance of microchannel flow under the combination of two enhanced technologies of pin electrodes and phase separation structure. Therefore, a parallel bidirectional microchannel heat sink was proposed to realize the gas-phase separation by switching high and low pressure for the downstream and countercurrent microchannels. Ethanol was taken as an experimental working fluid, while the modified polyvinylidene fluoride (PVDF) was as a gas-liquid phase separation tool. Then the flow boiling experiment was carried out under the conditions of inlet supercooling at 10 °C and effective heat flux range of 48.08 -87.29 kW/m², where other experimental conditions remained the same. Furthermore, the heat transfer enhancement factor of saturated boiling heat transfer coefficient (Fht) was introduced to investigate the boiling heat transfer mechanism in the microchannel under the combination of the number of pores in different phase separation structures (4-hole PSS-1, 6-hole PSS-2, 10-hole PSS-3) (Phase Separation Structure, PSS) and different electric field strengths (200, 400 and 600 V). The results show that the heat transfer performance of the microchannel under the action of the single-phase separation structure and the pin electrodes was effectively improved under the same working conditions, compared with the ordinary microchannel (0 V without electric field and 0-hole PSS-0 without phase separation structure). The number of opening holes shared a strong correlation with the enhanced heat transfer of the phase separation structure. The maximum Fht of the phase separation structure was 1.14 at a heat flux of 80 kW/m², with an increase in the number of opening holes. There was the strengthening effect of the electric field on small or confined bubbles, with small lengths and diameters. An outstanding strengthening was found under the condition of low and medium heat flux, such as the maximum Fht under pin electrodes was 1.20 at a heat flux of 65 kW/m². The heat transfer performance of the microchannel was further improved under the combined role of phase separation structure and pin electrodes, in which the maximum Fht was 1.29. In addition, the electric field was introduced to improve the strengthening effect of the phase separation structure at low heat flux density. The phase separation structure also effectively enhanced the strengthening effect of the electric field at high heat flux density, indicating the synergistic effect. In addition, the comparison was made on the effects of phase separation structure and electric field enhancement on the heat transfer of micro-channel boiling under the condition of mass flow rate (86.11 and 172.11 kg/(m²·s) of the two groups. The results show that the strengthening effect of phase separation during the boiling heat transfer of the microchannel was weakened at the high mass flow rate, compared with the low one. The strengthening effect of the electric field was enhanced, according to the influence of fluid flow rate. And there was little difference in the enhanced effect of boiling heat transfer under the two mass flow rates. The finding can provide a strong reference for the phase separation structure combined with the electric field. [ABSTRACT FROM AUTHOR]

The development of new and more effective cooling technologies is required for several high thermal power dissipation applications such as in electronics cooling or high concentrated photovoltaic panels. The present paper addresses an experimental study on the development of a microchannel based heat sink to cool photovoltaic panels. Experiments focus on the test of a microchannel, with geometry and dimensions optimized from previous work. The analysis performed here emphasizes the experimental characterization of flow boiling in the microchannel under different working conditions. The results include pressure drop and heat flux maps, obtained combining pressure sensors with high-speed imaging and time resolved thermography. The analysis performed was able to identify where nucleation sites were formed. Slug flow interfacial heat transfer could be observed and accurately described in the heat flux maps. Overall, results show the high potential of combining high-speed imaging with time resolved infrared thermography to characterize complex flows. These results also show that there is a good potential for this microchannel based flow cooling in removing the required heat fluxes for the application considered here, when compared to other liquid and air-cooling technologies. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Xi'an Jiaotong University is the property of Editorial Office of Journal of Xi'an Jiaotong University and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The objective of this review article is to provide a comprehensive analysis of the latest research on microchannel heat sinks, with a particular focus on single-phase flow, flow boiling, and coating microchannels. The review aims to highlight the progress made in this field and identify the challenges that need to be addressed to promote the widespread adoption of microchannel heat sinks. The review article examines the research on microchannel heat sinks and analyzes the findings related to single-phase flow, flow boiling, and coating microchannels. The analysis of single-phase flow in microchannels covers the impact of various parameters, including channel size, fluid properties, and flow conditions, on the heat transfer rate. The analysis of flow boiling in microchannels includes the identification of the potential advantages and challenges associated with this technique, as well as the strategies developed to mitigate these issues. The analysis of coating microchannels examines the effects of surface coatings on flow behavior and heat transfer performance. The review finds that microchannel heat sinks have gained significant attention due to their potential for high heat transfer rates in a compact size. The analysis of single-phase flow reveals that the heat transfer rate is proportional to the channel diameter and inversely proportional to the fluid viscosity. Increasing the flow rate results in higher heat transfer rates and pressure drops. The analysis of flow boiling reveals that this technique can significantly increase the heat transfer rate compared to single-phase flow. However, it can also lead to flow instability, wall superheat, and premature drying. The review identifies strategies to mitigate these issues, including using different surface coatings, enhancing nucleation, and optimizing channel geometry. The analysis of coating microchannels shows that coatings can alter the surface energy, wettability, and roughness of the channel walls, affecting the flow behavior and heat transfer performance. Coatings made from materials such as metals, polymers, and self-assembled monolayers have been investigated, and their effects on the flow of boiling heat transfer have been analyzed. The review highlights the need for further research to address the challenges associated with microchannel heat sinks, including flow instability and premature drying. Additionally, the review suggests that there is a need for more comprehensive studies on the effects of different surface coatings on flow behavior and heat transfer performance. Future research can also explore the potential of other materials for coating microchannels and investigate new strategies to optimize channel geometry and enhance nucleation. The review article concludes that the progress made in understanding microchannel heat transfer is significant, but further research is necessary to realize the full potential of microchannel heat sinks.

• Flow boiling of R134a in a plate heat exchanger with wavy corrugations was studied. • Three major flow patterns were identified with the variation of vapor quality. • The variation of heat transfer coefficient and wall temperature were discussed. • A new piecewise Nu correlation was developed based on flow pattern transition. As a common component in various applications, the two-phase heat transfer of plate heat exchanger has been frequently studied. However, further investigations are still required on the flow pattern, especially for those with new plate structures brought with the emergence of new applications. This study presented an experimental investigation on the flow boiling of R134a in a plate heat exchanger with mini-wavy corrugations designed for thermal management system of electric vehicle (EV). The flow pattern transitions in the test section were captured and discussed, meanwhile the effects of vapor quality and mass flux on the variation of heat transfer coefficient (HTC) were analyzed. Based on the observation, the two-phase flow pattern developed from pulsating film flow, then rough stream flow and finally to thin stream flow with the increase of vapor quality. The flow pattern map was determined with the superficial momentum of the liquid and vapor phase refrigerant, while the empirical correlations were developed based on two-phase Weber number to predict the transitions between the flow patterns. The HTC showed an intermediate increase firstly under low vapor qualities, and rapidly decreased after reaching the peak value at the vapor quality of 0.45–0.6. In addition, the experimental data were compared with the present correlations, and a new Nu correlation was developed considering the transitions of flow patterns, showing an optimal prediction on the experimental data with the mean absolute error of 11.3%. [ABSTRACT FROM AUTHOR]

Excessive hydrofluorocarbons (HFCs) usage significantly contributes to the global warming. Therefore, there is a widespread shift toward natural refrigerants with low global warming potential (GWP). Among these alternative refrigerants, R290, R600a, and their mixtures have gained considerable attention. However, their flammability limits the charge. To decrease the equipment volume and reduce the refrigerant charge, internally threaded tubes have been introduced to enhance the heat transfer. Therefore, in this study, flow boiling experiments were conducted on R290, R600a, and mixtures inside an internally threaded tube, obtaining heat transfer data under a pressure of 0.216–0.416 MPa, heat flux of 10.5–73.0 kW m−2, mass flux of 70–190 kg m−2 s−1. To quantitatively discern the dominant heat transfer mechanism, a dimensionless number Nz was introduced. In the nucleate boiling dominant region, an increase in the heat flux markedly enhances the heat transfer, with pressure and mass flux playing minor roles. Furthermore, the heat transfer deterioration trend of the mixtures aligns with the variation trends of concentration slip and boiling range. In the convective heat transfer dominant region, a rise in the mass flux significantly increases the heat transfer coefficient, while the pressure and heat flux have minor effects. Additionally, at high R290 mole fraction and vapor quality, high surface tension maintains a continuous liquid film, enhancing the heat transfer. Furthermore, new heat transfer correlations for flow boiling inside the internally threaded tubes were established, which exhibit a mean relative deviation of −2.30 % and 8.80 % for the obtained experimental results and collected data, respectively. [ABSTRACT FROM AUTHOR]

Understanding the nanofluid flow boiling phenomena is essential for many engineering applications including atomic reactors, cooling of electronic devices, synthetic buildings and spacecraft. Hybrid nanofluids, which are commonly defined as mixtures of two or more nanoparticles, are yet unknown to have improved features. When the coupled nanoparticles are suitably coated with one another, one on top of the other, this would be a successful technique as the usage of such nanofluids is hardly reported. The current research aims to investigate the effects of various novel hybrid ZnO coated Cu (Cu@ZnO) nanofluid concentrations on the critical heat flux (CHF) and heat transfer coefficient (HTC) for their potential real-life application. The Cu@ZnO nanoparticles with hybrid properties were selected because Cu increased the conductivity of heat whereas ZnO improved chemical durability. Efficient spark discharge in liquid nitrogen was employed in three stages to produce robust Cu@ZnO nanoparticles with hybrid properties. In order to create durable nanofluids, the nanoparticles with mixed properties were dispersed using an ultrasonication process at four different volumetric concentrations: 0.025%, 0.050%, 0.075% and 0.1%. At 45°C, the nanofluids' thermal conduction was 31% higher than that of base fluid. The flow boiling (subcooled) tests are carried out in minichannel with novel nanofluids. When the flow boiling investigation was being conducted, a greater concentration improved the HTC and CHF. In comparison to water, 0.1% of nanofluid caused the maximum rises in CHF and HTC, which were found to be 168.09% and 186.9%, respectively. It is possible for the fluid to utilize latent heat, create bubbles, distinct and delay the commencement of CHF because the thick Cu@ZnO-coated surface possesses strong thermal conductivity. [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]

In the pursuit of enhancing thermal management for miniaturized electronic devices, our study delves into the impact of entry effects on Ledinegg instability and flow maldistribution within parallel microchannels. Utilizing a coupled model that incorporates phase change and pressure drop dynamics in boiling flow, we examine microchannels characterized by a 50 length-to-diameter ratio and a 200 μm hydraulic diameter. Our findings unveil a significant influence of entry effects, which narrow the total flow excursion interval, thereby bolstering system stability. Specifically, as the heat flux escalates from 5 W/cm2 to 120 W/cm2, the entry effects increasingly mitigate flow instability and maldistribution in parallel channels, diminishing the total flow rate range susceptible to flow instability by 4.73% and 47.52%, while narrowing the total flow rate range corresponding to uneven flow distribution by 4.70% and 46.75%, respectively. Furthermore, entry effects expand the inlet subcooling range necessary for stabilizing the parallel channel system by 38.89% and 1000%. This research not only underscores the importance of considering entry effects in microchannel design but also opens avenues for further exploration into enhancing thermal management solutions. [ABSTRACT FROM AUTHOR]

The article presents experimental results of boiling heat transfer during FC-770 flow in a group of five minichannels with a common heated wall. The flow orientation was changed from 0° to 180°, with a 15° increment. During the experiments, the temperature of its outer heated wall surface was measured by an infrared camera. At the same time, flow patterns were captured through the glass plate opposite the heated wall using a high-speed camera. The purpose of the calculations was to determine local heat transfer coefficients on the contact surface between the working fluid and the heated surface in the central minichannel, using a simplified 1D calculation method. The results in the form of dependences of the temperature of the heated wall and the heat transfer coefficient as a function of the distance from the channel inlet for various flow orientations were analysed. Furthermore, typical boiling curves and two-phase flow patterns were presented. The mean relative error of the heat transfer coefficient was determined for various flow orientation. The dependence of the void fraction as a function of heat flux was illustrated for various angles of minichannel inclination to the horizontal plane. It was observed that the void fraction increased with heat flux and with increasing angle of inclination of the minichannel to the horizontal plane. [ABSTRACT FROM AUTHOR]

Interconnected microchannels (IMCs) in flow boiling have the advantages of optimized heat transfer performance, energy savings and high efficiency, compact size, and strong customizability. They provide new solutions for thermal management and heat transfer at the microscale and have broad application prospects. To further investigate the effect of microchannels with different numbers of transverse sections on the flow boiling heat transfer, we performed numerical simulations on a rectangular microchannel (RMC) and IMCs with 3, 5, and 7 transverse microchannels at high and low mass flux. It was found that fluid experiences similar bubble and slug flow in different numbers of IMCs and the RMC at low mass flux. At a heat flux of q = 90 W/cm2, the downstream regions of the IMCs produce vapor films that span the channels, obstructing the cross-section and weakening the flow exchange between the channels, which lead the heat transfer performance factor of IMC-3, reaching 148.43%, 110.04%, and 116.92% of the RMC, IMC-5, and IMC-7. Under high-quality flux, as the heat flux increases, the heat transfer coefficient increases and the pressure drop decreases due to the existence of lateral microchannels introduced in the interconnected microchannels. Whether at high or low mass flux, structural reasons pertaining to the RMC can easily lead to the accumulation of bubbles and the occurrence of slugs, and the flow boiling instability increases with the increase of heat flux, which leads to a pressure drop and heat transfer performance generally lower than that of IMCs under the same conditions. At q = 120 W/cm2, IMC-7 showed the best heat transfer enhancement. Its heat transfer performance factor was 129.37%, 120.594% and 107.98% of the RMC, IMC-3, and IMC-5, respectively. This article provides theoretical support for the design of interconnected microchannels in thermal management. [ABSTRACT FROM AUTHOR]

To solve the problem of high heat flux heat dissipation in microelectronic devices, a manifold micro-channel heat sink with corrugated bottom (CB-MMC) is proposed on the basis of the manifold microchannel heat sink (MMC). The flow and heat transfer characteristics of HFE-7100 in MMC and CB-MMC are investigated numerically. The results show that CB-MMC reduces the pressure loss and enhances the heat transfer performance in single-phase flow. The orthogonal test method is used to obtain structural design solutions with optimal thermal performance. It is observed that the temperature reduction is always at the expense of the increase of the pressure drop. In addition, the optimization parameters combination obtained through comprehensive evaluation of temperature and pressure drop through weight matrix - optimized solution 19 (wavelength A = 800 µm, amplitude B = 40 µm, channel depth C = 180 µm, outlet width D = 300 µm, channel width E = 25 µm). Its Tave has decreased by 6.89 °C, ΔP decreased by 10.27 kPa. Moreover, the subcooled boiling flow and heat transfer performance in MMC and CB-MMC are comparatively studied. The results demonstrate that the dynamic behavior of vapor bubbles causes large pressure fluctuations, which further leads to small temperature fluctuations, and thus reduces the stability of the flow and boiling heat transfer. Compared with MMC, CB-MMC exhibits more stable two-phase flow and better boiling heat transfer. [ABSTRACT FROM AUTHOR]

An experimental investigation of R245fa flow boiling in the horizontal flow (θ fd = 0°), vertical upward flow (θ fd = 90°), and vertical downward flow (θ fd = –90°) was carried out using a bare stainless-steel tube (BT) with an inner diameter of 10.02 mm and an effective heating length of 820 mm. The experimental condition was performed under a saturation pressure of 0.6 MPa, a heat flux varying from 4.99–74.60 kW/m2, a mass flux range of 197.94–696.63 kg/(m2·s), and a vapour quality range from 0.01 to 0.9. Five flow patterns, stratified flow, intermittent flow, churn flow, annular flow, and drying flow, could be observed in the experiment. When the heat flux was relatively low (q ≤ 15 kW/m2)), the variation of the heat transfer coefficient with flow direction was relatively gradual. However, when q ≥ 30 kW/m2, a distinct 'U'-shaped trend in the heat transfer coefficient is observed with the flow direction transitioning from vertical downward to horizontal and then to vertical upward. Due to the effect of buoyancy force, the heat transfer performance was relatively better at θ fd = –90° The friction pressure drop in the horizontal flow was the smallest, and the maximum value always occurred at θ fd = 90° Meanwhile, the experimental data for heat transfer coefficient and pressure drop was compared to the well-known correlations from the literature. The predictive correlation by Fang et al. for the heat transfer coefficient was in good agreement with the experimental data. Regarding frictional pressure drop, the Filho, Muller-Steinhagen and Heck and Zakaria correlations were recommended for pressure drop in horizontal flow, upward flow, and downward flow, respectively. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Engineering for Thermal Energy & Power / Reneng Dongli Gongcheng is the property of Journal of Engineering for Thermal Energy & Power and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

This paper experimentally investigates the factors influencing the flow boiling heat transfer of R134a refrigerant within smooth and micro-fin tubes having an outer diameter of 9.52 mm. The tube has a peculiar micro-fin shape with a 46° apex angle and 22° helix angle, producing a surface area 1.62 times bigger than a smooth tube of identical diameter. The main goal of the study is to understand the influences of mass flux, heat flux, saturation temperature, and average vapor quality on heat transfer characteristics. The findings are initially compared with known flow boiling correlations; moreover, performance index, penalty factor, and enhancement factor considerations are also made. The heat transfer coefficient (HTC) relies on mass flux, with significant improvements at G = 200 kg m−2 s−1, despite dry-out difficulties at high vapor quality. Intensified flow velocity from higher mass flux causes a rise in frictional pressure drop (FPD), with micro-fin tubes demonstrating superior heat transfer, owing to their larger surface area and improved swirl formation. Compared to smooth tubes, FPD is significantly influenced by vapor quality for micro-fin tubes. The study analyses the importance of micro-fin tubes to improve heat transfer in industrial applications, as well as the effects of saturation temperature and vapor quality on HTC.

As a new thermal control technology, a two-phase mechanically pumped cooling loop (MPCL) holds promise in addressing cooling issues of avionics. The heat sinks in MPCL can remove heat from avionics to refrigerant. Since previous research focused primarily on conventional heat sinks, three novel pin fin heat sinks (PFHSs) were investigated based on an MPCL. With increasing heat flux, heat transfer coefficient (HTC) and frictional pressure drop (FPD) increase. With the inlet state from subcooled to saturated and then to two-phase, HTC and FPD increase. Increasing inlet saturation temperature yields an increase in HTC and heating wall temperature (Tw). An increase in flow rate inhibits heat transfer deterioration while inducing significant growth in FPD. When heat flux is below 150 kW/m2, the petaloid I PFHS has the best temperature control performance, while the honeycombed PFHS has the best FPD. When heat flux exceeds 150 kW/m2, HTC decreases rapidly after reaching the peak. Increasing average vapor quality leads to a slight decrease in Tw but an increase in FPD. When heating load is started, flow rate decreases and Tw and pressure drop increase significantly, but they can gradually stabilize. These findings have significant implications for optimizing the MPCL.

The investigation of staggered finned manifold microchannel (MMC) for addressing thermal management challenges in high heat flux electronic devices presents promising insights. The research indicates that staggered finned MMCs exhibit superior heat transfer characteristics in comparison to traditional rectangular microchannels, particularly under single-phase flow conditions. When staggered finned microchannels reduce the maximum temperature of the heating surface by around 10 K, the pressure drop is reduced by 1.3 kPa. Notably, as the heat flux escalates, the advantages of staggered finned MMCs become increasingly pronounced, while higher flow rates lead to a gradual diminishment of these advantages. Furthermore, in scenarios involving flow boiling with low heat flux and high flow conditions, staggered finned MMCs demonstrate enhanced heat transfer performance over traditional rectangular microchannels. When staggered finned microchannels reduce the maximum temperature of the heating surface by around 3 K, the pressure drop is improved by 1.2 kPa. Moreover, the study underscores that at lower flow rates, staggered finned MMCs experience a reduced pressure drop relative to traditional rectangular microchannels, whereas at higher flow rates, this trend is reversed. These findings carry significant implications for the optimization of thermal management strategies for electronic devices operating under high heat flux conditions.

Phase Change Materials (PCMs) demonstrate significant potential as latent heat storage systems for direct steam generation. This investigation explores the heat transfer dynamics of an upward‐flowing heat transfer fluid (HTF) boiling within a vertical tube, concurrently examining the solidification process of PCM outside the tube. A two‐dimensional model of a shell and tube heat storage device is established for this purpose. The simulation employs the Volume of Fluid (VOF) model coupled with the Lee model to simulate HTF boiling inside the tube. Concurrently, the enthalpy‐porosity model is utilized to analyze the PCM solidification process. The findings reveal that PCM solidification and heat discharge are influenced by the temperature and thermal conductivity of the HTF, leading to uneven solidified PCM thickness along the tube's length over time. Additionally, variations in tube length, inlet velocity, and inlet temperature impact both the vapor quality of HTF and the thickness of solidified PCM. Tube length correlates with boiling time, affecting the accumulation of steam quality throughout the coupling process. Inlet velocity dictates the heat transfer of HTF with PCM within a limited distance, and insufficient heat exchange resulting in the cessation of HTF boiling. Inlet temperature determines the appropriate device size, lower inlet temperatures necessitate longer tubes to extend the heat charging duration until boiling initiation. [ABSTRACT FROM AUTHOR]

The article aims to explore boiling heat transfer in mini-channels with a rectangular cross-section using various fluids (HFE-649, HFE-7000, HFE-7100, and HFE-7200). Temperature measurements were conducted using infrared thermography for the heated wall and K-type thermocouples for the working fluid. The 2D mathematical model for heat transfer in the test section was proposed. Local heat transfer coefficients between the heated wall and the working fluid were determined from the Robin condition. The problem was solved by means of the finite element method (FEM) with Trefftz functions. The values of the heat transfer coefficient that were obtained were compared with the results calculated from Newton's law of cooling. The average relative differences between the obtained results did not exceed 4%. The study included uncertainty analyses for temperature measurements with K- and T-type thermocouples. Expanded uncertainties were calculated using the uncertainty propagation and Monte Carlo methods. Precisely determining the uncertainties in contact temperature measurements is crucial to ensure accurate temperature data for subsequent heat transfer calculations. The results of the heat transfer investigations were compared in terms of fluid temperature, heat transfer coefficients, and boiling curves. HFE-7200 consistently exhibited the highest fluid temperature and temperature differences at boiling incipience, while HFE-7000 demonstrated the highest heat transfer coefficients. HFE-649 showed the lowest heat transfer coefficients. The boiling curves exhibited a typical shape, with a notable occurrence of 'nucleation hysteresis phenomena'. Upon the analysis of two-phase flow patterns, bubbly and bubbly-slug structures were observed. [ABSTRACT FROM AUTHOR]

It is widely established that flow boiling, being a direct cooling technique also employing the latent heat of the fluid, has the potential to be more efficient than being useful in single-phase conventional cooling methods. This results in considerable potential for thermal management in many fields like microelectronics, space technology, thermal power plants, etc. The increasing demand for heat dissipation, consequent to component miniaturization, has pushed the development of new strategies for enhancing heat transfer efficiency, such as employment of functionalized surfaces. This review aims to describe in detail the current status of technology related to flow boiling heat transfer enhancement via micro/nanoscale surface functionalization. Key objectives are an increased nucleation site density and enhanced bubble dynamics. The vast majority of findings show favorable heat transfer performance, evidenced by an earlier onset of boiling (ONB), an improved flow boiling heat transfer coefficient (HTC), and an ameliorated critical heat flux (CHF). Increased pressure drop is a serious concern in certain application cases. Nanoscale textures mainly enhance capillary wicking to nucleation sites, thus being more effective in combination with microscale textures that define fixed nucleation sites. Degradation effects need to be more thoroughly and systematically characterized for application cases. Extra effects related to the manufacturing process can be easily overlooked, but one should be aware of their possible existence when drawing conclusions. Finally, the implementation of enhanced surfaces in mainstream applications is hindered by the absence of general predictive design tools for different channel configurations/materials, fluids, and operating conditions. A more universal understanding of the basic mechanisms involving texture geometry is needed in this aspect. [ABSTRACT FROM AUTHOR]

• Flow boiling characteristics of the non-azeotropic mixture R448A in a microchannel tube were measured in one pass. • As mass flux or heat flux increased, HTC of R448A increased; when the saturation pressure increased, HTC of R448A decreased. • R448A had a higher HTC than the HFOs and R134a at low quality, but lower HTC at high quality. • The correlations for predicting pressure gradient and heat transfer coefficient were proposed or recommended. R448A is a mixture to replace R404A in the thermodynamic cycle applications. R448A has a temperature glide of about 5°C and might exhibit thermal non-equilibrium in the microchannel tube. This study investigated the flow boiling heat transfer and pressure drop of R448A, a five-component non-azeotropic mixture, in a microchannel tube. Six heat transfer coefficients (HTC) and pressure gradients (dP/dz) of R448A were measured in one pass in a 24-port microchannel tube. The experimental conditions covered mass fluxes from 100 to 200 kg-m−2s−1, heat fluxes from 2 to 8 kW- m−2, and saturation temperatures from 10 to 30 °C. The study compared the experimental results of R448A with four of its component fluids (R134a, R1234yf, R1234ze(E), and R32). R448A had higher HTC and lower dP/dz than its component fluids, except for R32 which had higher HTC and dP/dz than R448A. Muller-Steinhagen and Heck was the best correlation for predicting the dP/dz of R448A, with a mean absolute error (MAE) of 8.78 % and a mean error (ME) of 7.47 %. A new correlation for predicting the HTC of R448A, based on modifying Liu and Winterton (Müller-Steinhagen and Heck, 1986), was proposed with an MAE of 6.01 % and an ME of 0.588 %. [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]

Liquid hydrogen has attracted widespread attention due to industrial decarbonization as an excellent carrier of renewable energy accommodation. Accurate prediction of the heat transfer coefficient during flow boiling has become a fundamental prerequisite for enhancing the design rationality and safety of hydrogen devices since a slight heat leak will trigger flow boiling. A novel selection standard of the dataset for liquid hydrogen flow boiling is proposed, and an experimental database of liquid hydrogen is compiled, including 923 data points from 5 sources. The prediction performance of eight conventional empirical correlations and six machine learning models for liquid hydrogen flow boiling is compared and evaluated. Results show that the prediction ability of machine learning models for the heat transfer coefficient of liquid hydrogen flow boiling is much higher than that of traditional correlations. Fang correlation is recommended to use as the calculation benchmark due to simplicity and reliability if the inlet condition is obscure. The Extra tree model shows the most excellent evaluating performance among the six machine learning models with a mean absolute deviation of 11.44% and a fairly superior R 2 of 0.9543. The Froude number Fr exhibits an essential impact according to the feature importance analysis, which may need to be taken into account when developing new empirical correlations. An improved correlation for the flow boiling of liquid hydrogen is provided based on the Fang correlation, achieving a mean absolute deviation of 18.61% and a mean relative deviation of 16.49%. The results of this paper can offer some theoretical guidance for the design of a liquid hydrogen apparatus. [Display omitted] • A novel selection standard of data set for liquid hydrogen flow boiling is proposed. • Fang correlation is recommended as the calculation benchmark due to simplicity and reliability. • Extra tree model has the best prediction performance with MAD of 11.44% and R 2 of 0.9543. • Froude number Fr exhibits an essential impact on heat transfer. • An improved correlation is proposed with MAD of 18.61% and MRD of 16.49%. [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]

The development of new and more effective cooling technologies is required for several high thermal power dissipation applications such as in electronics cooling or high concentrated photovoltaic panels. The present paper addresses an experimental study on the development of a microchannel based heat sink to cool photovoltaic panels. Experiments focus on the test of a microchannel, with geometry and dimensions optimized from previous work. The analysis performed here emphasizes the experimental characterization of flow boiling in the microchannel under different working conditions. The results include pressure drop and heat flux maps, obtained combining pressure sensors with high-speed imaging and time resolved thermography. The analysis performed was able to identify where nucleation sites were formed. Slug flow interfacial heat transfer could be observed and accurately described in the heat flux maps. Overall, results show the high potential of combining high-speed imaging with time resolved infrared thermography to characterize complex flows. These results also show that there is a good potential for this microchannel based flow cooling in removing the required heat fluxes for the application considered here, when compared to other liquid and air-cooling technologies.

This experimental study examined the subcooled-flow boiling of water-alumina nanofluids with 0.1 vol % concentration. The study aimed to explore how surface roughness, fluid velocity, and surface orientation influenced the subcooled flow boiling heat transfer (SFBHT). A cross-sectional area of 2 × 3 cm2 and a length of 1.20 m are the dimensions of the plexiglass channel in the experimental setup. A copper heater was embedded at the bottom surface of the channel. The results of the experiment revealed that the surface roughness had a positive effect on SFBHT for horizontal channels, and it depended on the surface configuration. The heat flux increases by 233 % and 98.27 % at low and high fluid velocity (0.5 and 0.9 m/s) by changing the surface roughness from 0.65 to 4.4 μm, respectively. The fluid velocity did not have uniform impacts on SFBHT for higher and lower surface temperatures. When altering the fluid velocity from 0.5 m/s to 0.9 m/s, the heat flux experiences significant variations: The overall heat fluxes are 56.52 % and −35.86 % for rough surface at lower and higher surface temperature, respectively. Depending on whether the surfaces were smooth or rough, the SFBHT performance varied due to the surface configuration. The boiling heat transfer (BHT) performance increased for smooth surfaces but decreased for rough surfaces by changing the surface orientation in both negative and positive directions. The overall heat fluxes increase by about 72.84 % and 66.67 % at smooth surface for both positive and negative inclination, respectively. They are about −32.94 % and −29.82 % at rough surface for both positive and negative inclination, too. The rate of surface roughness’ effects on SFBHT reduces by enhancing the surface angles.

Flow boiling in the rod bundle channel is an important phenomenon in nuclear reactors. The local inhomogeneous void fraction distribution in the flow channel will affect the flow and heat transfer in two phase flow. A 5 × 5 rod bundle test section is designed and utilized in the experiments and a conductivity probe is utilized to measure the local void fraction of two phase flow. A remote-control stepping driving assembly is designed to drive the void probe. The local void fraction characteristics at the outlet of a 5 × 5 rod bundle channel during subcooled boiling and saturated boiling are experimentally studied when the system pressure is 2∼3 MPa. The outlet subcooling ranges from 0 °C to 18 °C. The mass flux is 2000 kg/m2s and the heat flux ranges from 186.3 kW/m2 to 320.8 kW/m2.The effects of thermal parameters on local phase interfacial parameters in subcooled boiling and saturated boiling are obtained. The research results are important for revealing the two phase flow mechanisms in rod bundle channels.

A crucial parameter remarkably influencing the reliability and accuracy of vapor quality measurements in flow boiling experiments is the measurement of heat loss. The latent heat loss estimation in two-phase flow boiling sounds more demanding than that of sensible heat loss in single-phase flow experiments. To accurately investigate the impact of vapor quality on the heat transfer coefficient of flow boiling, accurate estimates of heat loss and subsequent calibration of heat supplies are of a great significance. It is widely common that the single-phase flow tests are performed to estimate a fixed heat loss percentage and calibrate heat supplies for further measurements of vapor quality and heat transfer coefficient of flow boiling experiments. In the present work, an empirical methodology is first proposed to measure heat losses and calibrate latent heat supplies directly from flow boiling process. The proposed methodology is then compared and verified with the commonly applied methodology which uses single-phase flow experiments. The proposed methodology exhibited higher accuracy than the single-phase-based methodology to estimate heat losses and measure vapor quality within two-phase flow boiling under low-pressure low-flow (LPLF) operating conditions.

The study investigates the limitations posed by low-value heat transfer coefficients and higher wall superheat in conventional tube bundles, which adversely impact the performance of heat exchangers. To address this challenge, semi-closed microstructure tubes, created via deformational cutting at ribbing pitches of 300 µm and 400 µm, are proposed as a potential solution. The research centers on analyzing the two-phase boiling heat transfer performance of these microstructure tubes arranged in 2 × 3 staggered tube bundles. A comparative analysis is conducted, evaluating the thermal effectiveness of the microstructure tube bundles against a 2 × 3 smooth tube bundle and a copper-coated tube bundle under varying mass flux, heat flux, and pitch-to-diameter ratios. The experimental findings indicate the superior heat transfer performance of both microstructure tube bundles, with reduced wall superheat compared to other tube bundles. The findings further indicate that the 2 × 3 semi-closed microstructure tube bundle with a higher ribbing pitch outperforms the microstructure with a lower ribbing pitch. The 2 × 3 high-pitched microstructure tube bundle exhibits a maximum enhancement of 420 % over the 5 × 3 smooth tube bundle and 340 % over the 2 × 3 smooth tube bundle. Moreover, it surpasses the performance of copper-coated tube bundles by 230 % and outperforms its lower-pitch counterpart by 70 %. Particularly, the use of microstructure tube bundles allows for a reduction in the number of tubes required in the heat exchanger configuration without compromising its performance, promoting compactness and cost-effectiveness. Overall, the study highlights the feasibility of implementing these tubes in the two-phase heat exchanger owing to their superior performance compared to conventional tubes.

A proposed midterm substitute of R410A, mixture with 1924 GWP100 (Global Warming Potential over 100 years), is R32 (GWP100 = 677), which, although mildly flammable, is pure, relatively cheap, already available on the market, with different suppliers due to the absence of patents. Moreover, its thermo-physical properties are already well known. Increasingly stringent energy saving regulations are boosting the residential heat pump market, which typically requires reversible brazed plate heat exchangers to be used both as evaporators and condensers. This work reports an experimental analysis of the two refrigerants during flow boiling in brazed plate heat exchangers, varying the channel volumes. The number of plates of the heat exchangers ranges from 4 (1 refrigerant channel) to 120 (59 refrigerant channels in parallel). The heat transfer coefficient and the pressure drop have been measured for R410A and R32 at saturation temperature between 1.5 °C and 5.5 °C, outlet vapour superheating of 5 K, heat flux from 4 to 31 kW m−2, mass velocity from 8 to 87 kg m−2 s−1 and inlet channels vapour quality from 0.2 to 0.3. The number of plates affects the distribution of the liquid-vapour mixture in the channels and its effect is discussed by comparing the heat transfer coefficients. It is shown that the differences in performance between the two fluids increase with the mass velocity, suggesting the influence of a convective component on the heat transfer coefficient. This is further confirmed by comparing the data with the results of two different models from literature.

Zeotropic refrigerants possess the characteristic of temperature glide during phase-change process, making them suitable for temperature matching between cold and hot sources and reducing total temperature differences. In this manuscript, the applicability of two commonly used refrigerants, zeotropic refrigerant R407C and pure refrigerant R134a in data center cooling systems are investigated. Firstly, the heat transfer and flow characteristics of R407C and R134a are studied to obtain boiling heat transfer empirical correlations. Secondly, a data center cooling model is established, and modeling calculations are conducted based on experimental results. Finally, the influence of refrigerants on the energy saving effect are calculated based on the model. Experimental results demonstrate that R134a exhibits a 48.3% higher boiling heat transfer coefficient compared to R407C at heat flux 16500 W/m2. However, when considering a data center cooling system, using R407C as working fluid shows the advantage of temperature matching as heat transfer area increases. Based on modeling results, at large heat transfer area, the annual cooling load factor using R407C at low mass flow rate is 0.5% lower than that of using R134a at low mass flow rate, and 22.5% lower than that of using R134a at large mass flow rate for typical climate city Harbin.

The study aims to numerically investigate the flow boiling of Al2O3/H2O and CuO/H2O nanofluids and water in pipes filled with copper metal foams. Four different values of porosity and three values of pore density have been used. To perform numerical simulation, the mixture model has been developed. For the first time, the effects of nanoparticle deposition on the wettability of heating surfaces were considered with the help of user-defined functions. Besides, the effect of metal foams with different porosities on the onset of nucleate boiling was evaluated. The thermal performance of metal foam pipes has been compared with each other by comparing the increase in heat transfer and pressure drop. As a result, by reducing the porosity from 0.95 to 0.80, the heat transfer coefficient was increased by 59 %, while the pressure drop increased by 28 %. Finally, by comparing the increase in heat transfer and pressure drop, results show that the metal foam pipe with 80 % porosity and 5 pores per inch has the best thermal performance. The results of this study are expected to be used for the optimization of advanced phase change cooling technologies.

Recently, the thermal management of electronic devices has become paramount because the power density of semiconductor elements is rapidly increasing with the rapid miniaturisation of electronic devices. Boiling heat transfer (BHT) is an effective technique to achieve high cooling systems, and micro-structures are expected to enhance the BHT. In this study, copper, a hydrophobic surface, was used as a heat transfer surface, and microstructure was applied to the surface to investigate the effect of hydrophobic microstructured surfaces on the enhancement of BHT, and the effects of changing flow velocity from laminar (Reynolds number = 1,220) to the turbulent (Reynolds number = 6,640) were investigated. Comparing the boiling curves of smooth and microstructured surfaces, we confirmed that the heat flux on the microstructured surface was higher than that on the smooth surface at the same superheating, attributable to the increase in the number of bubble points caused by the cavities of the microstructured surface, which facilitated heat transfer. For the effect of changing the flow velocity, the critical heat flux (CHF) increased with the flow velocity for the smooth surface, but no significant improvement was observed for the microstructured surface. The prediction of the relationship between heat flux and bubble population density was found to be reasonable because the difference between the experimental values and the theoretical values obtained from a well-known prediction equation was small. Using this equation to predict the bubble population density on the microstructured surfaces, it was estimated that a small fraction of the number of cavities was activated, even near the CHF condition. In the future, it will be necessary to investigate the cause of the small number of activated cavities of the microstructured surfaces in detail.

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.

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]

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The use of a micro heat sink is an effective means of solving the problem of high-power chip heat dissipation. Diamond/Cu composites exhibit high thermal conductivity and a linear thermal expansion coefficient that is compatible with semiconductor materials, rendering them ideal micro heat sink materials. The aim of this study was to fabricate diamond/Cu and Cu separately as heat sinks and subject them to flow boiling heat transfer experiments. The results indicate that the diamond/Cu heat sink displayed a decrease in wall superheat of 10.2–14.5 °C and an improvement in heat transfer coefficient of 38–51% compared with the Cu heat sink under identical heat fluxes. The heat sink also exhibits enhanced thermal uniformity. Secondary diamond particles are incorporated into the gaps of the main diamonds, thereby constructing a three-dimensional heat conduction network within the composite material. The diamond network enhances the internal heat flux of the material while also creating more nucleation sites on the surface. These increase the boiling intensity of the diamond/Cu heat sink, leading to better heat transfer performance. By combining the transient thermal model with computational fluid dynamics, a heat transfer model based on the diamond/Cu heat sink is proposed. The efficient heat dissipation capability of diamond/Cu heat sinks can lower the working temperature of microelectronic devices, thereby improving device performance and reliability during operation. [ABSTRACT FROM AUTHOR]

• Flow boiling heat transfer and pressure drop data for R513A. • HTC shows both nucleative and convective behavior with operating conditions. • Pressure drop increases with mass flux and decreases with saturation temperature. • New composite prediction method (MAPE = 25.3%) for heat transfer coefficient. • Frictional pressure gradients well predicted by Friedel correlation (MAPE = 13.8%). This paper presents two-phase heat transfer coefficient and pressure drop data of refrigerant R513A, a new azeotropic mixture conceived as possible alternative to R134a for medium temperature small-size refrigeration systems. All the experiments were performed in a commercial horizontal stainless-steel tube having an internal diameter of 6.0 mm and an outer diameter of 8.0 mm. The channel heating was obtained through DC current and Joule effect. The effect of the main operating parameters in terms of mass flux (from 150 to 500 kg/m2s), heat flux (from 5.0 to 40 kW/m2), saturation temperature (from 30 to 50 °C) and vapor quality (from the onset of boiling up to the dry-out occurrence) was analyzed and discussed for the heat transfer coefficient values, finding that both convective and nucleate boiling were significant contributions. The same ranges of mass flux and saturation temperature were also applied to frictional pressure gradient trends with vapor quality, taken in adiabatic conditions. The thermo-hydraulic performances were then compared with those of R134a, obtaining lower heat transfer coefficients and very similar pressure gradients. Finally, the collected experiments were assessed with values predicted from the most quoted correlations available in literature. A new composite method for nucleate-dominant or convective-dominant mechanisms was proposed for the evaluation of the heat transfer coefficient, with a mean absolute error of 25.3%, whereas the frictional pressure gradient values were well predicted with the Friedel correlation, that provides a mean absolute error of 13.8%. [ABSTRACT FROM AUTHOR]

• Boiling flow pattern map of R32-oil is proposed based on the observation results. • A new heat transfer correlation reflecting the effect of flow patterns is developed. • The correlation could predict 82% of experimental data within a deviation of ±20%. R32 has been widely applied in room air-conditioners (ACs) because of its low GWP, and its heat transfer characteristics in refrigeration cycles may be affected by the lubricating oil mixed in R32. To quantitatively evaluate the effect of oil on the flow boiling heat transfer characteristics of R32, the flow pattern map and heat transfer correlation of R32-oil mixture are needed. In this study, an experimental rig for R32-oil mixture is established, in which the boiling flow patterns are observed and the heat transfer coefficients are measured. The experimental conditions cover the evaporation temperature ranging from -5 to 15 °C, mass flux ranging from 100 to 400 kg m −2 s −1, vapor quality ranging from 0.1 to 0.9 and oil concentration ranging from 0% to 5%. A total of 80 flow patterns and 336 heat transfer coefficients have been obtained under the above experimental conditions. The results show that the flow patterns of R32-oil mixture contain stratified wavy flow, intermittent flow, annular flow and dryout flow, and the presence of the oil could promote the transformation from stratified wavy flow to annular flow; the heat transfer would be enhanced by the oil under lower vapor qualities and be deteriorated under higher vapor qualities. The boiling flow pattern map is developed based on observation results, which could well predict the flow patterns under the experimental conditions. The heat transfer correlation is developed based on the flow patterns and could predict 82% of the experimental results within a deviation of ± 20%. [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]