Maximum Heat Transfer Coefficient Research Articles

Hollow fiber polytetrafluoroethylene membrane heat exchanger with anti-corrosion properties

Thin hollow polytetrafluoroethylene (PTFE) fibers (thickness: 200 μm; outer diameter: 1.3 mm; inner diameter: 0.9 mm) were produced by a specially designed die via stretching and sintering, and assembled into a shell-and-tube membrane heat exchanger. The effects of the flow velocities on the heat transfer performance and thermal resistance were studied in a water-water system. The maximum overall heat transfer coefficient (OHTC) was up to 391 W/(m2‧K) (flow velocities: 0.24 m/s for hot water; 0.11 m/s for cold water). The OHTC became larger (increased from 229 to 391 W/(m2‧K)) at a higher cold water flow velocity (increased from 0.02 to 0.11 m/s) on the shell side when maintaining the hot water flows on the tube side. The OHTC increased from 119 to 391 W/(m2‧K) first and then decreased to 298 W/(m2‧K) with the increase of the hot water flow velocities (from 0.08 to 0.40 m/s) on the tube side, while maintaining the cold water flow velocity on the shell side. The thermal resistance results showed that the increased flow velocity (0.11 m/s) on the shell side significantly reduced the thermal resistance to 2.56 × 10−3 K m2/W on the shell side. Furthermore, the flow turbulence on the tube side is favorable for the heat transfer, and the optimum flow velocity on the tube side was 0.24 m/s. The hollow PTFE fibers were extremely stable and durable in corrosive solutions, with less than 1% changes in bursting strength, tensile strength at break, and elongation at break. This work opens new opportunities for the fabrication and application of hollow fiber PTFE membrane heat exchangers to reuse waste heat in high temperature corrosive environments.

Experimental and CFD investigation on heat transfer enhancement in evacuated tube solar collector with coiled wire inserts

ABSTRACT This article presents a novel design of a modified evacuated tube collector fabricated for air heating applications by fixing helical coiled inserts in the vacuum tube. The investigations on modified evacuated tube collector (ETSC-HI) were conducted experimentally and using Computational Fluid Dynamics (CFD) and assessed with the unmodified ETC solar air heater under analogous conditions. The working of the modified evacuated tube collector (ETSC-HI) has been scrutinized under different modes, geometrical parameters, flow parameters and different operating conditions. Experimentally, the influence of these variables on outlet air and thermal efficiency of the modified collector have been studied in detail. The CFD findings have been visualized by plotting graphical contours, vectors and streamlines for various cases. Optimal geometrical parameters for maximum heat transfer coefficient were calculated from the CFD investigations and were considered for further experimental analysis by fixing the coiled wire on the vacuum pipe. It was seen that ETSC-HIperforms better contrasted to the unmodified ETC, registering Nusselt number enhancement of 3.23 times corresponding to P/Dh = 1.5 e/Dh = 0.074 at m = 0.015 kg/s. The highest thermal efficiency for ETSC-HI and unmodified collector are obtained as 70.99% and 64.86% respectively showing significant improvement in the performance. On average, the enhancement of 5.85% in daily average efficiency was seen for the modified ETSC-HI.

Flow boiling of R1336mzz(Z) in tapered microgaps with asymmetric dual-V microchannels

The development of high heat flux cooling technologies has attracted great interest of investigations during the last decades, which motivated a series of investigations regarding flow boiling in microchannels. In a recent investigation, new heat sinks composed of a bottom surface milled with asymmetric Dual-V microchannels combined with a tapered microgap were presented, and the occurrence of boiling inversion was documented for the first time in flow boiling experiments using water as working fluid. It is noteworthy, however, that despite its great thermal properties the use of water as working fluid may not meet some cooling requirements, and the use of refrigerants is necessary. In addition, with the increasing concerns with environmental impacts, novel refrigerants with low environmental impacts have been presented to the market, but limited results are found in the literature for their flow boiling performances. Hence, the present work is focused on flow boiling experiments with the Hydro-Fluoroolefin R1336mzz(Z) in copper heat sinks combined to a tapered manifold. Tests were conducted using three different surfaces, with and without asymmetric Dual-V microchannels, the selected saturation temperature was 41 °C, and inlet mass fluxes varied from 400 to 1000 kg/m2s. Effects of various parameters were evaluated and put in context to the flow patterns observed with a high-speed camera. The best surface promoted the dissipation of up to 93 W/cm2 without reaching the critical heat flux, with wall temperature lower than 74 °C, pressure drop lower than 10 kPa, and maximum heat transfer coefficient reaching 22 kW/m2K.

Experimental and numerical analysis of effect of combined drop-shape pin fins and plate fins type heat sink under natural convection

As the junction temperature of a light-emitting diode (LED) is inversely proportional to its lifespan and efficiency. This study investigates the thermal performance of heat sink-based model with a drop-shaped pin fin for LED cooling under natural convection, experimentally, and numerically. The study used three distinct drop form pin fin arrays with variable vertical fin spacing (Sv = 25, 50, and 75 mm), with the best array being used in subsequent studies with variations in drop-shaped pin fin size (diameter DD and apex length LD ). The heat transfer coefficient and Nusselt number were obtained in the quantitative analysis of thermal performance. A numerical model with Boussinesq approximation for natural convection is used for this study and well-validated with experimental results. A qualitative investigation was also carried out utilizing numerical research to investigate velocity streamlines and their impacts on heat transfer enhancement in various configurations. An experimental test with conventional plate fin, circular pin fins were done for the verification of the test setup results and their results were compared with results calculated from the standard correlations presented in previous well-known literature. The maximum heat transfer coefficient was found to be 12.80 W/m2K for drop form pin fins with vertical fin spacing (Sv ) equal to 75 mm, a diameter of 6 mm, and a length of 7.2 mm, and the corresponding Nusselt number was 94.85. Also, the maximum enhancement in Nusselt number was found to be (Nu/Nu plate fin) 3.11 corresponding to the conventional plate-fin heat sink. This comparison will help the industrialists determine innovative passive cooling technology for electronic devices such as LED street lights.

Effect of particle size distribution on heat transfer in bubbling fluidized beds applied in thermochemical energy storage

The aim of the present work is to characterize the effect of particle size changes from cycling of the thermochemical energy storage material CaO/Ca(OH)2 on fluidizability and wall-to-bed heat transfer coefficients in the fluidized bed. Materials replicated from previous storage cyclization experiments are experimentally investigated in an ambient fluidization test rig. Differential pressure measurement and a horizontally immersed heat transfer probe are used to characterize fluidizability and wall-to-bed heat transfer between immersed cylinder and bed material. Sauter mean diameters of the replicated particle size distributions range from 27µm to 282µm. Particle size distributions with Sauter mean diameters significantly smaller than approximately 50µm were found to be difficult to fluidize due to excessive channeling, resulting in low heat transfer coefficients. In this case, fluidization quality and heat transfer is enhanced for higher gas velocities. Fluidizable particle size distributions ranged from Sauter mean diameters of 48µm to 282µm. Measured heat transfer coefficients show little dependency on the particle size. Observed maximum heat transfer coefficients range from 344Wm−2K−1 to 350Wm−2K−1.

Numerical study on heat transfer characteris tics of supercritical water in straight and helical tubes

The subcritical steam generators in high-temperature gas-cooled reactor pebble-bed modules can be replaced by supercritical steam generators to improve thermal efficiency. The axial and circumferential nonuniform flow and heat transfer characteristics of supercritical water in straight and helical tubes with varying inclinations (0–90°) are simulated to investigate the combined effects of tube structure, flow direction, thermophysical properties, property variations, buoyancy, and centrifugal forces. With heat fluxes of 233–930 kW/m2 and inlet mass fluxes of 225 or 1260 kg/(m2 s) under a working pressure of 24.5 MPa, the friction coefficients, turbulent kinetic energy, secondary flow, temperature, and heat transfer coefficients are studied in the cross section of variable structure tubes. The wall temperature and heat transfer coefficients of four regions are compared in straight and helical tubes; the maximum heat transfer coefficient appeared before the pseudo-critical point. The heat transfer coefficients are the greatest at the bottom of the straight tubes, where fluidity was high, and at the outer of the helical tubes, where the fluid velocity was high under centrifugal force. The secondary flow in the helical tubes causes a nonuniformity in the wall temperature, causing the circumferential maximum wall temperature to move.

An experimental investigation of subcooled pool boiling on downward-facing surfaces with microchannels

Upward-facing pooling boiling enhancement employing micro-structure has been extensively studied, yet it is rarely applied in the cooling system in microgravity or In-Vessel Retention (IVR) in nuclear industry. The main challenge lies in effective removal of the vapor columns and replenishment of the fresh liquid without the aid of the buoyancy force. To solve the issue, additional pressure induced by microchannel is introduced to yield spontaneous bubble removal and liquid replenishment in the present work. Downward-facing pool boiling of subcooled FC-72 are conducted on Plain, Straight- and Expanding-Channeled Surfaces (PS, SCS and ECS) with visualization experiments. The superheat at the onset of nucleate boiling (ONB) on channeled surfaces was decreased about 3 K compared to that of the PS, and the maximum heat transfer coefficient (HTC) of the SCS and ECS were improved by 149 % and 217 %, respectively. Compared to PS, the channeled surfaces are more independent from saturated pressure, displaying high and relatively consistent heat transfer coefficients under various pressure conditions. Additionally, with the increasing subcooling degree, channeled surfaces showed greater enhancement in heat transfer performance. Proved by the bubble dynamics, the ECS enabled a directional movement and effective removal of vapor bubbles even though the buoyancy force cannot be utilized under downward-facing condition. The proposed enhancement mechanism attributed to separated vapor–liquid flow paths is independent on gravity and achieves a satisfied pool boiling performance, thus is expected to provide a promising solution for thermal management systems in aerospace and nuclear industry.

Reliability of Different Nanofluids and Different Micro-Channel Configurations on the Heat Transfer Augmentation

Nanofluid, the fluid suspensions of a metallic nanoparticle, became a coolant fluid that is used when a promising enhancement in heat transfer is required. In the current study, the characteristics of fluid flow and heat transfer are numerically investigated using different nanofluids (Al2O3–H2O, TiO2–H2O, and SiO2–H2O) and different micro-channel heat sink (MCHS) configurations (rectangular, triangular, trapezoidal, and circular). In this numerical investigation, the effect of Re number ranged from 890 to 1500, and the effect of nanoparticle concentration ranged from 1% to 7% at constant heat flux q = 106 W/m2, and constant fluid inlet temperature of 288 K, were studied. The average heat transfer coefficient, h, and pressure drop, Δp, are used to quantify the fluid flow and heat transfer characteristics in each MCHS configuration and for each nanoparticle concentration. It is revealed that a better heat transfer coefficient is obtained for Al2O–H2O compared with other types of nanoparticles and pure water, such as 8.58% heat transfer coefficient improvement obtained at Re = 1500 and φ=7% more than that of pure water. It is also inferred that the maximum heat transfer coefficient is obtained by the triangular MCHS; however, it has the highest pressure drop because of the lowest hydraulic diameter.

Micro pin fins with topologically optimized configurations enhance flow boiling heat transfer in manifold microchannel heat sinks

The manifold microchannel heat sink (MMCHS) is a promising design for heat dissipation in high power electronics. However, the configuration of the microchannels in MMCHS is generally rectangular straight channels, which would limit the further improvement of heat transfer performance. Topology optimization has been proven to be an effective approach to design single-phase heat sinks, which only depends on the iterative algorithm. In this work, we first design three single-phase heat transfer structures (2D) by a density-based topology optimization method, and then systematically investigate the two-phase flow and heat transfer characteristics of MMCHS (3D configurations that are converted based on 2D designs). The thermohydraulic performances of three topologically optimized MMCHS obtained by varying the constrained pressure drop (named TOC-I, TOC-II and TOC-III for simplicity) and the baseline rectangular MMCHS are compared. The results show that the flow paths in the TOCs exhibit root-like configurations and the number of pin fins increases with the increase in constrained pressure drop (i.e., TOC-III has the most complex flow path). Due to the differences in velocity distribution and channel width among various configurations, churn flow and confined bubbly flow are observed in BC and TOCs, respectively. Compared to BC, the maximum average heat transfer coefficients of TOC-I, TOC-II and TOC-III are increased by 40%, 62% and 87%, respectively. The TOC-I has the lowest pressure drop due to the minimum number of pin fins and the relatively large channel width. The maximum values of Figure of Merit (FOM), a parameter usually employed to analyze the comprehensive performance of MMCHS, for TOC-I, TOC-II and TOC-III are 1.72, 1.77 and 1.8 at Qb= 1000 W/cm2, respectively. Moreover, TOC-I and TOC-III have the best comprehensive performance at 300 W/cm2≤ Qb ≤ 800 W/cm2 and 900 W/cm2 ≤ Qb ≤ 1000 W/cm2, respectively.

On the Hydrothermal Behavior of Fluid Flow and Heat Transfer in a Helical Double-Tube Heat Exchanger with Curved Swirl Generator; Impacts of Length and Position

The hydrothermal behavior in a helical double-tube heat exchanger is numerically estimated. A new type of swirl generator with two sections, including; outer curved blades and a semi-conical section with two holes in the inner section, is employed. Two geometrical factors, containing the length (L1) and the position of the swirl generator (S), are used for investigation. The calculations were performed by a commercial FVM code, ANSYS FLUENT 18.2. The numerical outcomes show that a shorter length of the swirl generator leads to a better hydrothermal behavior. Accordingly, the model with L1 = 100 mm at m˙ = 0.008 kg/s achieves the maximum thermal performance by about 17.65, 53.85, and 100% enhancement compared to the models L1 = 200, 300 mm, and without swirl generator. Among the different studied positions of the swirl generator, the maximum heat transfer coefficient and average Nusselt number in entire mass flow rates belong to the case with position S = 0.3π mm. Moreover, the thermal performance of the case with S = 0.3π mm is higher than cases with S = 0.1π mm, S = 0.5π mm, and without swirl generator by about 11.11, 53.84, and 100%, respectively.

Effect of orthogonal channel structure on the heat transfer in pool boiling and its heat flux prediction model

Pool boiling has a heat transfer performance far exceeding that of traditional heat transfer methods, so it has broad application prospects. At present, the mainstream means to improve the boiling performance are mainly to change the heat transfer surface and to change the liquid working medium. Previously, a T-shaped microchannel structure has been designed, whose maximum heat flux was 338% of that of a smooth surface, and whose maximum boiling heat transfer coefficient was 184.7% of that of a smooth surface. The structure of this paper is further developed on the basis of this structure. It is found that the orthogonal channel structure can better improve the heat transfer effect. Specifically, within a certain range, the promotion effect of heat transfer is positively correlated with the increase of fin length. When the wall superheat is 12.3 K, the heat flux of the best orthogonal channel structure S4 is 358% of that of the smooth surface. The maximum heat transfer coefficient of S4 is 11.9 W/(cm2·K), which is 281.3% of that of the smooth surface. In addition, a prediction model is proposed for the structure of this paper, and its prediction error is within 25% under medium and high heat flux.

Study on heat transfer performance of immersion system based on SiC/white mineral oil composite nanofluids

With the rapid development of the global digital economy, the number and size of data centers are also expanding. The high energy consumption of data centers has become a highly concerned issue in the industry. Immersion cooling is a thermal strategy for servers with high heat flux density in data centers. The low thermal conductivity of its working medium has become the main bottleneck hindering its application. In this study, the performance of heat transfer of the SiC/white mineral oil-based nanofluids as new coolant and oil as traditional coolant under different heat loads are presented. The heat transfer capacity in different application scenarios is explored. The results of the simulations show that with the new coolant, the maximum surface heat transfer coefficient of bare heat source is increased by 11.3% at 100 W. With heatsink and heat pipe auxiliary cooling, the maximum increase is 11.7% and 10.7%, respectively, at 300 W. The contour pictures reveal a considerable decrease of surface and internal high-temperature area using heatsink and heat pipe auxiliary cooling. The analysis of the economy and reliability of the new working fluid concludes that the PUE (Power usage effectiveness) of the new working fluid only increases by 0.07–0.03%, which is negligible. Lastly, after three months of repeated experiments, the average increase in temperature is 0.4–2.3%. It means that the new coolant has good stability and reliability, which proves it has good application prospect in submerged cooling system.

Influence of particle deposition on heat transfer characteristics for nanofluid free impinging jet: A numerical study

Based on the two-body collision model, a nanoparticle collision, deposition and peeling model is established to describe the nanoparticle deposition process during SiO2-water nanofluid jet impinging on a heated copper column. The model is loaded with the Euler-Lagrange multiphase model in Ansys Fluent 19.1, and its accuracy is verified by comparing with the experimental data. Then the nanoparticle deposition processes during nanofluid jet impinging are studied with different inlet Reynolds numbers (Re), nanofluid volume fractions and impact heights (H/D). Result shows that the amount of nanoparticle deposition increases with the increasing nanofluid volume fraction, and it has a tendency of first increasing and then decreasing with the increasing inlet Reynolds number and the impact height, which reaches a maximum value when Re = 8849 and H/D = 4. Result also shows that it is easier to deposit at the junction between the impact zone and the wall jet zone with a deposition amount approximately 2 times of that at the outlet. A nanoparticle thermal resistance layer and a high-density nanofluid layer are formed in the near-wall region, and velocity slip between phases in the layer could reach 2.824m/s, which significantly enhance the base-fluid’s micro-flow intensity and the particles’ movement, thus strengthening the momentum exchange and energy exchange between phases and wall. The maximum heat transfer coefficient could reach 27857.4 W/(K·m2), and the average heat transfer coefficient has a 67.9% increment with 3% SiO2-water nanofluid volume fraction.

Saturated pool boiling heat transfer enhancement of R245fa based on the surface covered by sintered copper powder with and without nanostructure

The pool boiling characteristics on the enhanced surfaces with sintered copper powder coatings with and without nano-treatment were experimentally investigated. Three kinds of enchanted surfaces formed by sintered porous layers with copper powder particles and their further nano-treated surfaces were adopted as the test surfaces. The thermal oxidation process was used to form nanostructures with super hydrophilic characteristic on the surface of sintered copper particles. Refrigerant R245fa was selected as the test fluid due to the potential the application in electronic cooling field. The saturation temperature of boiling experiments was maintained about 20 °C. The result showed that the boiling heat transfer performance was effectively improved by coating sintered copper powder porous layer on the plain copper surface, and the maximum heat transfer coefficient and CHF value enhancement could reach 151% and 160% of the plain one, respectively. The enhanced surfaces no matter with or without nano-treatment could trigger the nucleation process at the wall superheats lower than 12 °C rather than the value as high as 30 °C for plain copper surface. After nano-treatment, the heat transfer enhancement for sintered porous surfaces was further improved at medium and low heat flux, where the bubble nucleation or detachment remained robust. The CHF peaked as 140% of untreated surface, while the heat transfer coefficient increased to 108% at most.

Synergistic effect of coating copper foam with carbon nanotubes on pool boiling heat transfer performance

Pool boiling heat transfer is a commonly used method for efficient phase change heat transfer. This paper studied the synergistic enhancement of heat transfer performance of pool boiling in deionized water by introducing an MWCNT (multi-walled nanotube) coating on a foam copper frame, and the influence of MWCNT content was explored. The results showed that the MWCNT coating strengthened the critical heat flux (CHF) and heat transfer coefficient (HTC) compared to a polished Cu surface. When 2.4 mg of MWCNTs were deposited, the maximum CHF was reached (785.2 kW·m−2), 3.41 times that of the polished Cu surface; and when 2.8 mg were deposited, the maximum HTC was reached (33.41 kW·m−2· k−1), 2.50 times that of the polished Cu surface. The results show that this heat transfer surface improves the performance of pool boiling heat transfer. A high-speed camera was used to record the bubble state during the boiling process of the pool. A simulation of the physical model in ANSYS Workbench returned the same results as the experiments.

Enhancement of heat transfer on micro- and macro- structural surfaces in close-loop R410A flashing spray cooling system for heat dissipation of high-power electronics

High effective spray cooling system is urgently required for the thermal management of high-power electronics in small areas. These electronic devices need to be cooled at functional operating temperature, and it is particularly lacking in current spray cooling studies. For this purpose, a hybrid structural surface that integrates micro- roughness and macro- structure is investigated in close-loop R410A flash spray cooling system. The heat transfer performance is tremendously enhanced as the increase of roughness on flat surface until it reaches the value of 3.5 μm. Meanwhile, a novel tetrahedral fin surface is proposed and compared with modified surfaces including flat, wavy, square, and pyramid fins. It performs high heat flux of 411.2 W cm−2, a 101.5 % enhancement over the flat surface. What’s more, the hybrid structure with 3.5 μm roughness can further improve the maximum critical heat flux and heat transfer coefficient by up to 424 W cm−2 and 814 kW K−1 m−2, respectively, while keeping the surface temperature below 50 °C. Besides, correlations and energy efficiency are studied to explore the performance of structural surfaces. These findings in a wider range of roughness and macrostructure will provide insights to guide the surface modification in flash spray cooling.

Experimental investigations on heat transfer enhancement of enhanced surfaces in spray cooling using HFE-649

The present study utilizes commercial HFE-649 fluid to analyze the effect of enhanced surfaces on spray cooling. Two pressure swirl nozzles, N1 and N2 with an orifice diameter of 0.51 mm and 1.19 mm respectively, have been used to generate micro-sized droplets and to diffuse these droplets on copper specimen of 10-mm2 cross-section. The volumetric flux range of the fluid varies from 1.00 to 21.67 cm3/cm2s. Three enhanced surfaces, namely, two micro-coated surfaces having coating thickness of 100 µm and 200 µm, and a micro-finned (1 mm fin height) surface, have been used in the present study. Heat transfer enhancement of these enhanced surfaces has been compared with a reference plain surface. With micro-coated surface having thickness 100 µm, the maximum heat flux and heat transfer coefficient of 115 W/cm2 and 9.87 W/cm2K with N1, and 143.48 W/cm2 and 10.31 W/cm2K with N2 has been achieved, respectively. When compared to plain surface, coating thickness of 100 µm resulted in a maximum enhancement in heat flux of 61.54% with N1 and 52.91% with N2, and enhancement in heat transfer coefficient of 352% with N1 and 326% with N2. Effect of subcooling at two fluid inlet temperatures, 25℃ and 49℃, on heat transfer performance of enhanced surfaces has also been investigated. The critical heat flux also increased by up 97.1% for the micro-coated surface having 100 µm thickness at near saturated condition when compared to the plain surface, depending upon the volumetric flux. A new correlation for calculating HTC and CHF has been proposed which is based on the earlier correlation proposed by Estes and Mudawar for plain surface. In addition, a correlation to estimate CHF for enhanced surfaces has also been proposed in terms of density ratio, Weber number and Jakob number.

Dynamic thermal management of flashing spray cooling by the frequency conversion of compressor

Dynamic thermal management of electronic equipment is of practical importance to adapt to the quickly changed heat load at local spot. A closed-loop experimental system was constructed to study the heat transfer mechanism and cooling performance of flashing spray cooling by R410A on a smooth flat copper surface in steady-state and varying operating conditions, especially the effect of frequency conversion of compressor. The results indicated that the superheat of 4 ℃ and 22 ℃ can be regarded as the transition points between three heat transfer stages of steady R410A flashing spray cooling: single-phase heat transfer, nucleate boiling, and transition boiling. To achieve the best cooling performance, superheat should be between 4 ℃ and 22 ℃ to keep heat transfer in stage II. The increase of compressor frequency can improve the cooling performance of spray system effectively. When the compressor frequencies are 10 Hz, 15 Hz and 20 Hz, CHF can reach 162.9, 168.3 and 175.0 W/cm2, and the maximum heat transfer coefficient is around 60.5, 80.6, and 90.4 kW/(m2·K), separately, with surface temperature below 35 ℃, 30 ℃ and 25 ℃. At the same heat flux, the power consumptions for the system to reach steady-state at 15 Hz and 20 Hz are 12.2% lower but 5.9% higher than that at 10 Hz, which provides a feasible energy-saving strategy for spray cooling system in the practical thermal management of electronic equipment.

Flow Boiling Heat Transfer of R141b in Sintered Porous Tubes

In this paper, the high flux tubes of 25 mm outer diameter with the content of 8% and 12% nanoparticles were experimentally studied with the refrigerant R141b under atmospheric pressure. The liquid mass flux was controlled from 128.30 to 252.03 kg/m2·s, the maximum heat flux provided by the electric heating rod was 80 kW/m2, and the range of the superheated degree was from −7 to 4 °C. The results indicate that the porous media promotes the phase change at a low superheated degree, and the heat transfer capacity is 1.88 times that of the smooth tube. Additionally, the vapor quality increases significantly with the increase of nanoparticle concentration. Moreover, the heat transfer coefficient decreases as the outer wall temperature increases due to the formation of the gas film but increases with the increase of nano-powder content. Under the same superheated degree condition, the maximum heat transfer coefficient of the sintered porous tube with 12% nano-powder content is 1.3 times that of the tube with 8% nano-powder content.

Pool boiling heat transfer enhancement on the hybrid surfaces coupling capillary wick and minichannels

Pool boiling heat transfer can fully utilize the latent heat of the vaporization, to meet the heat dissipation needs of high-power electronic components. In this study, several enhanced hybrid surfaces were designed and processed by sintering spherical copper powder on the minichannels in different ways. The coupled effects of various parameters of capillary wick and minichannels were considered to enhance the pool boiling heat transfer performance of these hybrid surfaces using HFE-7100 as the working fluid. The bubble dynamic behaviors were captured and analyzed by the high-speed camera. The results indicate that the ring groove around the minichannels and the capillary wick have a significant impact on the boiling heat transfer of these hybrid surfaces. This is because the capillary wick provide a large number of nucleation sites and large specific surface area, which can greatly reduce the contact thermal resistance between the capillary wick and the minichannels. At ΔTsub = 30 K, the critical heat flux (CHF) of hybrid surfaces coupling capillary wick and minichannels is up to 147.4 W/cm2, and its maximum heat transfer coefficient (HTC) is 2.69 W/(cm2·K). Compared with minichannels without capillary wick covered, the CHF and HTC are increase by 86.8% and 192.0%, respectively. Furthermore, a correlation for predicting CHF on these hybrid surfaces covered capillary-wick is proposed. Dimensionless parameters such as the subcooled Jakob coefficient (Jasub), the outer surface area enhancement ratio (AER), the particle distribution coefficient (PDC) and the ratio of inner to outer surface area of capillary wick (IOR) are defined to estimate the effect of different factors on the CHF.