Flow Boiling Heat Transfer Performance Research Articles

Analysis of subcooled flows boiling in horizontal rectangular mini-channels under microgravity conditions

In the aerospace field, microgravity significantly impacts flow boiling heat transfer performance and the bubble behavior. This study uses water as the working fluid and employs numerical methods to investigate flow boiling heat transfer in rectangular mini-channels under microgravity conditions. A coupled Volume-of-Fluid and Level-Set method (VOSET) is utilized for precise tracking of vapor–liquid interfaces. The study reproduces flow patterns and heat transfer characteristics under various heat fluxes and flow velocities, revealing the effects of microgravity on flow boiling heat transfer. At low mass flux density of 100 kg/(m2·s) and heat fluxes of 300 kW/m2, the absence of buoyancy in microgravity causes bubbles to adhere to the heating wall, forming dry patches. Conversely, under normal gravity, buoyancy promotes bubble detachment from the wall, leading to lower pressure drop and better heat transfer. As the heat flux increases to 400 kW/m2, bubbles rapidly grow into large bubbles, eventually reaching a size comparable to the mini-channel dimensions. Under both gravitational conditions, bubbles become confined by the mini-channel walls, resulting in similar dry patches areas, thus diminishing the impact of microgravity on heat transfer performance and pressure drop. Subsequently, the study explored the impact of microgravity on heat transfer performance, with mass flux densities varying from 100 kg/(m2·s) to 1000 kg/(m2·s) and heat fluxes ranging from 300 kW/m2 to 500 kW/m2. As mass flux density increases, inertial forces become more dominant. Notably, when mass flux density exceeding 500 kg/(m2·s), the heat transfer performance and pressure drop under different gravitational conditions becomes consistent, diminishing the influence of microgravity on heat transfer.

Experimental study on flow boiling heat transfer performance of nanowire-printed substrates with porous-like structures

In this study, ZnO nanowires (NWs) were demonstrated to have considerable potential for heat transfer enhancement in flow boiling. Unlike the conventional NW fabrication method using metal-assisted chemical etching, we use the microcontact printing (µCP) method comprising solution-based growing methodologies to create ZnO NWs on different substrates, including bare Si-, Cu-, and Al-coated surfaces. This ZnO NW fabrication approach successfully overcomes the substrate limitation of typical NWs, such as Si- and Cu-NWs, which can be only formed on its intrinsic material following the high-cost fabrication approaches required by the metal catalyst. Additionally, a high pore-connected structure was observed on the interfacial area of the ZnO NWs, with the nanocore distribution area considerably enhanced by lengthening the NW length, beneficial to bubble nucleation and surface coolant supply during nucleate boiling. The length of the ZnO NW substrates is controllable by varying the growing period during the hydrothermal growth procedure. Further, the ZnO NW substrates reveal a remarkable enhancement in the flow boiling heat transfer regardless of the substrate variation, thereby exhibiting high variability in the substrate selection and potential of being used as the cooling approach for the thermal management of the high heat flux applications. We conducted a series of experiments related to the surface wettability, wicking, and bubble characteristics for different test substrates under saturated flow boiling conditions to explicitly determine the underlying mechanism in the heat transfer enhancement of the ZnO NWs.

Flow boiling heat transfer performance of R245fa in a vertical enhanced evaporator tube with sintering and electroplating composite coats

The R245fa flow boiling performance was investigated in a vertical evaporator tube of which inner wall surface is coated by a composite sintering and electroplating technology. The effects of vapor quality, heat flux, mass flux, and flow direction were compared and analyzed in the bare tube (BT) and the sintering and electroplating treated tube (SET). The results showed that the heat transfer coefficient increased and then decreased with the rise of vapor quality and heat flux. The heat transfer performance in downward flow was better than that in upward flow, while an opposite trend was observed for frictional pressure drop. With the increased of vapor quality, the frictional pressure drop increased and then decreased, but it monotonically increased with the increasing mass flux. The heat transfer enhancement mechanism was explored through flow pattern visualization. The annular flow pattern conversion line of SET was earlier than that of BT. Due to the strong wettability of SET, the thickness of liquid film was thinner in downward and the mother bubble was bigger in upward, which had a positive effect on flow boiling heat transfer. The maximum the heat transfer enhancement factor (EF) and the performance evaluation parameter (PEC) were 1.83 and 1.75, respectively.

Optimizing stability and enhancing flow boiling heat transfer performance of Al2O3-TiO2/water hybrid nanofluids

This study investigates the impact of stability on the flow boiling heat transfer performance of hybrid nanofluids comprising Al2O3-TiO2 nanoparticles dispersed in water, referred to as Al2O3-TiO2/W. The stability optimization of the Al2O3-TiO2/W hybrid nanofluid was carried out using the response surface methodology (RSM). Additionally, the boiling heat transfer performance of the Al2O3-TiO2/W hybrid nanofluid, along with its mono nanofluids (0.01, 0.02, and 0.03 vol%), was examined within a flow rate range of 30–90 L/h. The results revealed that the optimal stability of the initially prepared hybrid nanofluids was achieved with a surfactant SDS mass ratio of 0.554 and an ultrasonic time of 44.79 min. Furthermore, as the flow rate increased, the boiling heat transfer coefficient (h) exhibited an increasing trend followed by a decrease, reaching its maximum at 70 L/h. The h value also increased with higher heat flux and volume fraction. Compared to pure water, the Al2O3-TiO2/W hybrid nanofluid (0.03 vol%) exhibited a maximum enhancement of 61.9 % in h at a heat flux of 93.4 kW/m2. Moreover, the h of the Al2O3-TiO2/W hybrid nanofluid demonstrated an average enhancement of 6.4 % compared to its TiO2/W mono nanofluid counterpart. Additionally, the nano-influence factor (Fenh) displayed a pattern of increase followed by a decrease with the rise in flow rate, reaching its peak value of 1.62 at 40 L/h. Lastly, the thermal conductivity of the hybrid nanofluids exhibited a significant influence on the boiling heat transfer performance, showing a strong linear correlation coefficient (r) of 0.943. This correlation suggests a strong relationship between thermal conductivity and boiling heat transfer performance.

An experimental investigation of R245fa flow boiling heat transfer performance in horizontal and vertical tubes

An experimental investigation of R245fa flow boiling in the horizontal flow (θfd = 0°), vertical upward flow (θfd = 90°), and vertical downward flow (θfd = –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 θfd = –90° The friction pressure drop in the horizontal flow was the smallest, and the maximum value always occurred at θfd = 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.

FLOW BOILING HEAT TRANSFER IN MICROCHANNEL HEAT EXCHANGERS WITH MICRO POROUS COATING SURFACE

This study experimentally investigated the heat transfer and pressure drop performance of refrigerant HFC-245fa flow boiling in microchannel heat exchangers with and without microporous coating. The flow boiling heat transfer performance at various mass fluxes, heating rate, exit vapor qualities, and surface coating thickness was compared. The test results showed that the 52 μm coating thickness porous surface exhibited 65-148% higher heat transfer performance than the smooth surface, while the 98 μm microporous coating surface showed a lower improvement of 41-90% at various mass fluxes. Partial dry out was observed at high and moderate mass fluxes on both smooth and porous coating channels. It happened at lower exit vapor qualities in microporous coating channels than that in smooth channels. The partial dry-out exit vapor qualities increased with decreasing mass fluxes. For the lowest mass flux, because of the low heat flux and low nucleation suppression, no significant partial dry out was investigated. The pressure drops in microporous coating channels were around 25-47% higher than those in smooth channels. There was no significant influence of microporous coating layer thickness on flow boiling pressures drops.

Experimental investigation on flow boiling heat transfer characteristics of R245fa/oil mixture in horizontal smooth tube

During the operation of the Organic Rankine Cycle (ORC) system, it's inevitable that the lubricating oil in the expander will mix with the working fluid, leading to a noticeable impact on its heat transfer performance. In this investigation, we conducted experimental research on the flow boiling heat transfer characteristics of a mixture of R245fa and POE (Polyolester) in a smooth horizontal tube. We explored the effects of various parameters, including oil concentration (ranging from 0% to 4%), heat flux (6 to 20 kW/m²), mass flux (ranging from 150 to 410 kg/(m²·s)), and vapor quality (ranging from 0 to 0.8). The results revealed that the heat transfer coefficient (HTC) for R245fa/POE increased with higher vapor quality and mass flux for each oil concentration. However, as the oil concentration increased, the rate of increase in HTC gradually diminished. Notably, an optimal oil concentration of 2.3% was identified to achieve a maximum HTC of 6 kW/(m²·K) in our experiments. Furthermore, our investigation into frictional pressure drop (FPD) indicated that higher vapor quality, mass flux, and heat flux all led to an increase in FPD for the R245fa/POE mixture. Additionally, we evaluated the impact of lubricants on the HTC and FPD of R245fa/POE mixtures using enhancement factor (EF) and penalty factor (PF), respectively. The EFs for HTC in the R245fa/POE mix ranged mainly between 0.95 and 1.6, while the PFs for FPD were concentrated between 0.75 and 1.4. The lubricating oil was found to enhance the flow boiling heat transfer performance of R245fa, with a comprehensive evaluation index range of 1.22 to 1.31. In terms of prediction accuracy, the Schlager-R22/150SUS correlation provided the most reliable estimates for the enhancement factor of HTC and the penalty factor of FPD, with mean absolute deviations (MAD) of 9.79% for EF and 14.03% for PF.

A Comparative Study of Flow Boiling Heat Transfer and Pressure Drop Characteristics in a Pin-Finned Heat Sink at Horizontal/Vertical Upward Flow Orientations

Abstract With the continuous development of power electronic devices toward miniaturization and compactness, it is necessary to develop more efficient flow boiling heat transfer technologies. In this work, the flow boiling heat transfer and pressure drop characteristics of Novec649 in a pin finned channel under two kinds of flow orientations (horizontal and vertical upward) are experimentally investigated. Heat flux, inlet flow velocity, and inlet subcooling are considered as the variable parameters. The results show that among all boiling operating conditions, the heat transfer performances between two orientations are basically consistent, while the pressure drop of vertical upward pin finned channel is relatively lower, indicating that the comprehensive flow boiling heat transfer performance of vertical oriented channel is better. Subsequently, a series of flow visualization experiments are performed in vertical upward pin finned channel. With the increase of heat flux, four kinds of flow pattern are discovered in the order of dispersed bubble flow, bubble flow, homogeneous flow, and annular flow. In the region of annular flow, although a vapor flow has already formed in the channel, there is still a large amount of liquid phase surrounding the wall and pin fins. Therefore, no obvious heat transfer deterioration was observed in the pin finned channel. Along the flow direction, the diameter of bubbles will increase first, and then present obvious oscillation. As the heat flux increases, both the average bubble detachment diameter and the frequency increase correspondingly. As the fluid velocity increases, the average bubble detachment diameter presents a downward trend, while the average bubble detachment frequency presents an upward trend.

Enhanced single-phase and flow boiling heat transfer performance in saw-tooth copper microchannels with high L/Dh ratio

Owing to its potential of high heat dissipation capability, flow boiling in copper microchannels is one of the most effective cooling techniques for high power density thermal components. Normally, it is difficult to substantially enhance the HTC and CHF in high L/Dh (channel length relative to hydraulic diameter) microchannels due to bubble confinement and boiling crisis. To date, various functional geometries and surface modifications have been extensively studied to enhance flow boiling performances. In this study, the effect of extensive two-phase mixing in the entire channel length on flow boiling performance is investigated in this saw-tooth copper microchannel with L/Dh ratio up to 75 on DI-water ranging from 50 to 300 kgm-2s−1. The results show that HTC and CHF are dramatically enhanced owing to the intensified two-phase mixing and continuous interruption of thermal boundary layers. For example, CHF and HTC are up to 280 Wcm−2 and 141.8 kWm-2K−1, respectively, at G = 300 kgm-2s−1. These enhancements are ∼ 87% and ∼ 110.7%, respectively, compared to that of plain microchannel. Furthermore, a correlation of CHF is developed to predict CHF.

Channel flow boiling on hybrid wettability surface with lattice Boltzmann method

Channel flow boiling on the hybrid wettability surface is studied numerically using a two-dimensional pseudo potential lattice Boltzmann method (LBM) coupled with a thermal phase transition model. Heat transfer performance between single and hybrid wettability surfaces is compared first, and it is shown that the heat flux on the hybrid surface is higher than that of its single wettability competitor. After analyzing 36 groups of numerical results, it is found that the impacts brought by contact angle and geometrical parameters of the hybrid surface on heat flux is complex. By fixing the hydrophilic contact angle, the heat flux increases first but then decreases with the increase of hydrophobic contact angle. However, the heat flux decreases monotonously with increasing the hydrophilic contact angle under a fixed hydrophobic contact angle. The 30°/120° hybrid wettability surface shows the best heat transfer performance due to its better three-phase contact line spreading and accelerated bubble departure frequency. Aside from the contact angle, the flow boiling heat transfer performance dominated by bubble dynamics is also closely related to the widths of hydrophilic/hydrophobic regions, and there exist optimal widths under various wall superheats. Finally, the pressure drop inside channel is investigated, and it is shown that the hybrid surface is favorable to reduce pressure drop fluctuation on the premise of providing high vapor yield.

Parametric Evaluation of the Flow Boiling Heat Transfer Properties of R22 and R407c inside Horizontal Microfin Tube

The flow boiling heat transfer performance of R22 and R407c, in a microfin tube with a helix angle of 22° and an apex angle of 48°, was evaluated in a study. The study is aimed at investigating the impact of heat flux, mass flux, saturation temperature, and vapor quality on the heat transfer coefficient during flow boiling. Two different saturation temperatures, 293.15 K and 313.15 K, were used in the experiments, with heat fluxes ranging from 25 to 85 kW.m-2 and mass fluxes ranging from 150 to 350 kg.m-2.s-1. To validate the experimental data, the results were compared with existing correlations for microfin tubes. The calculated error margin among all correlations with the experimental dataset, which ±15% and ±30%, was also matched by 85% and 95% of the datasets, respectively. Findings reveal that at lower saturation temperatures, the average heat transfer coefficients augmented with increasing mass flux. The study also found that R22 has a higher heat transfer coefficient than R407c at low saturation temperatures due to its stronger thermal conductivity and lower viscosity.

Effect of electric field, mass flow rate, and subcooling degree on subcooled flow boiling in a micro-slit channel

This study investigated the design and cooling performance of a micro-slit-channel heat sink for electrical device cooling. The subcooled flow-boiling performance of a dielectric liquid, HFE-7000, was experimentally characterized. A flat electrode made from stainless steel with 10 narrow slits, each 1 mm width and 17 mm long, was placed at 600 μm above the heated surface to generate a high electric field. The flow boiling heat transfer performance of an electroplated diamond surface was evaluated. Four electric field intensities from 0 to −5 kV/mm, three mass flow rates from 1.83 to 4.8 g/s, and four inlet subcooling from 5 to 25 K were tested at an atmospheric system pressure. Four flow boiling patterns were categorized using a high-speed camera: single-phase flow, subcooled nucleate boiling, saturated nucleate boiling, and local dry out boiling. The maximum critical heat flux (CHF) and heat transfer coefficient were 95 W/cm2 and 30 kW/m2K, respectively, for mass flow rate of 4.83 g/s, electric field of −5 kV/mm, subcooling degree of 9 K, and mass flow rate of 4.83 g/s. A semi-empirical prediction of the CHF exhibited good agreement with the experimental results within ±18 %.

Effects of inlet subcooling on the flow boiling heat transfer performance of bi-porous mini-channels

Micro/mini-channel heat sinks have been viewed to be a potential option for the heat removal of electronics with rising thermal flux. A bi-porous mini-channel, fabricated through sintering copper woven tapes on channel surfaces, is developed in this paper for the sake of improving the flow boiling heat transfer. Two different pore structures exist in the copper woven tape: one is cavity constituted via copper strands, another one is crevice constituted via copper wires. The bi-porous mini-channel is multi-channel type, and the dimension of each single channel is 1.2 × 1.5 mm. Experimental studies,with various inlet subcoolings of 10.7, 13.7 and 16.7 °C, on the flow boiling performance of the bi-porous mini-channel were conducted, and a traditional mini-channel possessing planar surfaces was employed for comparison experiments. Working fluid R141b, inlet mass flux at 255 kg/m2·s and outlet pressure at 50 kPa (gage pressure) are adopted in the experiments. Boiling curve, heat transfer coefficient, temperature distribution and performance evaluation were investigated, combining with the visualization of bubble behaviors. The result demonstrates that the bi-porous mini-channel triggers the onset of nucleate boiling in the case of smaller heat flux conditions than the mini-channel with planar surface under different inlet subcoolings. The boiling hysteresis phenomenon is more obvious at higher subcooling for the mini-channel with planar surface, but it is eliminated in the bi-porous mini-channel under the same conditions. Under high heat flux conditions, the obvious inlet backflow phenomenon is observed and may deteriorate the heat transfer performance. The present study indicates that the bi-porous mini-channel possesses high heat transfer performance and temperature uniformity, which may meet the high demands of thermal management in fields such as battery cooling of electric vehicles.

Experimental study on subcooled flow boiling heat transfer of water-SDS mixtures in a vertical macrochannel

Flow boiling heat transfer performance of surfactant solutions is studied at a basis of deionized water. The tested fluids are aqueous solutions of SDS at concentrations ranging from 100 to 12500 ppm. Tests are performed at vertical, upward, 3 × 40 mm cross-section channel at 75 °C subcooling, atmospheric pressure, flow rates 200–300 kg/m2s (Re: 1400–2600) and heat fluxes 200–1000 kW/m2. The present experimental data show that the trend of heat transfer enhancement follows the opposite direction of surface tension trend when surfactant concentration increases. At CMC (Critical Micellar Concentration) heat transfer coefficient and bubble density take their maximum value, while bubble diameter takes its minimum value. Concentrations above CMC show minimal or no further effect on flow boiling heat transfer characteristics. The tendency of the heat transfer coefficient experimental data with respect to changes in surfactant concertation allows the development of a modified version of the well-known two phase model heat transfer of Shah, by incorporating a surfactant concentration dependent correction factor that predicts this work’s values at a ±10% margin and a mean absolute percentage error of 3.6%.

Flow boiling heat transfer characteristics over horizontal smooth and microfin tubes: An empirical investigation utilizing R407c

The flow boiling heat transfer performance of horizontal smooth and microfin tubes was investigated using the refrigerant R407c. The evaporator test tube contains an outside, inner diameter and length of 9.52, 8.52, and 1000 mm, respectively. The investigations were performed out at 283.15 and 303.15 K saturation temperatures with hot water circulation heat flux of 75 kW.m-2, refrigerant heat flux varying from 5 to 85 kW.m-2, and mass flux varied from 15 to 250 kg.m-2.s-1, keeping the area ratio of microfin tube 1.68%. The experimental results are compared to several previously recognised correlations. Experimental outcomes demonstrated that the heat transfer rate of the microfin tube is superior to the smooth tube. The percentage of heat transfer enhancement was considered to exceed in microfin tube by 28–280%. The pressure drop in regard to R407c was found to be augmented than smooth tube according to the experimental results and enhancement was achieved in microfin tube by 68–110%. Finally, a sensitivity analysis is carried out in flow boiling heat transfer phenomenon via the smooth tube.

Theoretical and experimental research on flow boiling heat transfer in microchannels for IGBT modules

Two-phase microchannel heat sink has received extensive attention due to its small size, high heat transfer efficiency, good temperature uniformity, and low working fluid charge. Heat flux and mass flow rate are important parameters that affect the heat transfer performance of two-phase microchannel heat sink. For different heat fluxes, if the mass flow rate is not reasonable, dry-out may occur, which will significantly reduce the heat transfer performance. In order to further explore the influence of mass flow rate and heat flux on heat transfer characteristics, the flow boiling heat transfer performance and flow regimes under different mass flow rates and heat fluxes were studied for a two-phase microchannel heat sink using R245fa as the working fluid. The results showed that when the mass flow rate was 100 kg/(m2·s), the boiling heat transfer coefficient of R245fa showed a trend of increasing firstly and then decreasing with the increase of the heat flux. At a higher heat flux, the local dry-out in the microchannels began to appear, and the heat transfer performance decreased significantly. And the higher the heat flux was, the more serious the dry-out was. There was no obvious dry-out phenomenon when the mass flow rate was 150–300 kg/(m2·s) and the heat flux was below 500 kW/m2. The research provided a valuable reference for exploring the influence of the mass flow rate and the heat flux on the flow boiling heat transfer characteristics of the microchannel heat sink.

Heat transfer coefficient and pressure drop of water flow boiling in porous open microchannels heat sink

• Porous open microchannels (POM) heat sink was proposed and fabricated. • Heat transfer coefficient was greatly improved by porous wall at high heat fluxes. • Periodic slug-stratified flow was newly observed in POM heat sink. • No local dryout was noted due to the enhanced capillary actions in porous wall. • Two-phase flow could maintain stable under all operating conditions tested. The open microchannel is attracting more attention due to the reduced pressure drop and the improved flow stability, but the local dryout easily occurs at the downstream of the microchannels and subsequently deteriorates the flow boiling heat transfer performance. In this study, two open microchannels heat sinks were fabricated utilizing solid copper substrate and the porous substrate sintered by copper powder particles, respectively. Flow boiling experiments were conducted using deionized water as the working fluid with the inlet temperate of 87 °C at different mass fluxes ranging from 151.5 to 551.1 kg/m 2 s over a heat flux range of 29.6-1871.1 kW/m 2 in the porous open microchannels (POM) and the solid copper open microchannels (SOM). Experimental results showed that the heat transfer coefficients (HTC) were greatly improved in POM compared with that in SOM at moderate (300 kW/m 2 < q eff < 600 kW/m 2 ) and high ( q eff > 600 kW/m 2 ) heat fluxes. No local dryout was observed even at the highest heat fluxes in POM heat sink. Therefore, the flow boiling HTC in POM no longer deteriorated with increment in heat flux at high heat flux region, unlike in SOM. A new periodic flow pattern named slug-stratified flow was found in POM heat sink during which the slug flow and the stratified flow alternated temporally and spatially. Besides, although the two-phase pressure drop of POM was larger than that of SOM due to the increased friction and acceleration loss, the flow could still maintain stable under all operating conditions tested here.

Influence of surface wettability on heat transfer and pressure drop characteristics of R1234ze(E) flow boiling in metal foam filled tubes

The influence of surface wettability on the flow boiling heat transfer and pressure drop characteristics of R1234ze(E) in metal foam filled tubes was experimentally investigated. The working conditions include metal foam pore densities from 10 to 40 PPI and porosities from 90% to 95%, mass fluxes from 90 to 180 kg·m−2·s−1, and heat fluxes from 12.4 to 18.6 kW·m−2. The experimental results show that the hydrophilic modification deteriorates the flow boiling heat transfer coefficient in metal foam filled tubes by 7%-13%, and reduces the pressure drop by 9%-20%, while the hydrophobic modification improves the flow boiling heat transfer performance of metal foam filled tubes by 3%-8% and increases the two-phase pressure drop by 8%-15%. Heat transfer and pressure drop correlations were proposed for flow boiling in metal foam filled tubes with different surface wettability.

Preparation and Flow Boiling Heat Transfer Performance of Concave Cr Coating on Stainless Steel Surface

To improve the heat transfer performance (HTP) of stainless steel, the micro-nano concave chromium (Cr) coating was fabricated on the surface of 304 stainless steel by the combination technology of electrodeposition and anodic oxidation. The effects of electroplating and anodizing time on the morphology, contact angle (CA) and heat transfer coefficient (HTC) of the coating were studied. The results show that the samples with large surface area and good wettability have higher HTC. Besides, the Cr coating obtained after 2 h electroplating has silver white appearance, smooth surface and a thickness of about 37 μm. The 2-h electroplated Cr coating after anodizing for 30 min presents a concave pore structure with a pore diameter of 7.4 μm and the best wettability (a CA of 41.8°), which is 20° lower than that of the stainless steel substrate. The two-phase heat transfer efficiency of the sample with the concave Cr coating was increased by 22.84 % when the fluid mass flux is 33.3 kg m−2 s−1 and the inlet temperature is 40 °C and the fluctuations of pressure drop in the micro-channel was effectively reduced. The enhanced HTP of concave Cr coating is owing to the larger specific surface area and better wettability caused by the unique rough concave structure.

A remarkable CHF of 345W/cm2 is achieved in a wicked-microchannel using HFE-7100

Boiling heat transfer of dielectric fluids is a promising cooling technique for thermal management of microelectronic systems. However, the critical heat flux achievable is generally low, because of the poor thermophysical properties of these fluids. To address this dilemma, we propose a new cooling concept to substantially enhance global liquid supply during phase-change process using enhanced capillary-driven force. Additionally, dedicated vapor pathways are designed among a bank of micro-pillars to facilitate vapor removal. In this work, new wicks comprised of silicon micro-pinfin arrays are explored to significantly enhance the flow boiling heat transfer performance. To examine the functionalities of this wick, experiments on HFE-7100 were carried out with mass velocities varying from 247 to 3,465 kg/m2s. To explore the enhancement mechanisms and to analyze the capillary-assisted flow boiling process, visualization studies were conducted. The results indicate that sustainable evaporation induced by wick microstructures and efficient liquid supply are the enhancement mechanisms compared to parallel microchannels with solid walls. It is found that the overall heat transfer coefficient is substantially increased up to 75%. Remarkably, a high critical heat flux (CHF) of approximately 345 W/cm2 is recorded at G = 3,465 kg/m2s at coolant inlet temperature of ∼20 °C. Equally importantly, this noticeable enhancement of CHF value is associated with drastically decreased pressure drops compared to microchannels decorated with μ-pinfin fences.