仿生叶脉分形微结构表面的平板热管传热性能研究

基于植物叶片的蒸腾作用,以叶脉结构作为蓝本,设计了分形角度为50°、60°、70°、80°、90°的仿叶脉槽道结构,并将其应用于平板热管冷凝端。其中上一级槽道与下一级槽道的长度比、宽度比均为0.7,第一级槽道长度为16.0 mm、宽度为2.0 mm、深度为0.3 mm,并且设置一组未经表面处理的平板热管作为对照。实验结果表明:分形角为80°时性能最优,其冷凝热阻最小可达39.420℃/kW,相比未经表面处理平板热管的热阻降低了40.29%;相对于传统的直槽道结构,置于冷凝端的仿叶脉槽道结构能极大地降低工质流阻,有利于工质回流。     

Effect of surface wettability on boiling and evaporation

Titanium dioxide, TiO₂, is one of the photocatalysts that has a very unique characteristic. The surface coated with TiO₂ exhibits extremely high affinity for water by exposing the surface to UV light and the contact angle decreases nearly to zero. On the contrary, the contact angle increases when the surface is shielded from UV light. We applied this superhydrophilic nature to enhancement of boiling and evaporation heat transfer. Experiments of pool boiling and evaporation of single water droplet have been performed to manifest the effect of high wettability on heat transfer characteristics. Both of TiO₂-coated and non-coated surfaces were used for comparison in each experiment. It is found that (1) the critical heat flux (CHF) of TiO₂-coated surface is about two times larger than that of non-coated one, and (2) Leidenfrost temperature increases as the contact angle decreases. The superhydrophilic surface can be an ideal heat transfer surface.     

Effects of functional surface on performance of a micro heat pipe

The effect of a functional surface with the axial ladder contact angle distribution on the thermal performance of a triangular micro heat pipe has been analyzed based on a one-dimensional steady-state model. Compared with the traditional micro heat pipe (MHP) with a uniform contact angle distribution on its surface, the simulation results show that a MHP with a functional surface can remove a greater amount of heat under the same condition. The increase in thermal performance is more obvious with the increase in the ladder difference of the contact angles between the adjacent sections of the MHP. The increased thermal performance associated with the functional surface can be attributed to the increase of the liquid capillary force as well as the no obvious increase of the liquid shearing force provided by the functional surface, which also brings about the increase in condensate mass flow rate through the adiabatic section–evaporation section interface. It is also found that for the traditional MHP with uniform contact angle surface, there is an optimal contact angle leading to the maximum heat input. The deviation of the optimal value will decrease the capillary force and thermal performance of the MHP.     

An experimental investigation of a three-dimensional flat-plate oscillating heat pipe with staggered microchannels

Using water or acetone as the working fluid, the thermal performance of a three-dimensional flat-plate oscillating heat pipe (3D FP-OHP) with staggered microchannels was experimentally investigated by varying heating area, cooling temperature and operating orientation. It was found that when the heating area is larger at the same input power, the heat pipe is less orientation-dependent. When the heating area was decreased, to form a localized heating condition and higher heat flux, the thermal resistance and peak-to-peak amplitudes of temperature oscillations in the evaporator increased. The utilization of water as the working fluid generally provided the lowest thermal resistance for all experimental conditions investigated, but – unlike acetone – resulted in more severe temperature fluctuations in the evaporator during localized heating. The 3D FP-OHP, with overall dimensions of 130.18 × 38.10 × 2.86 mm³, demonstrated to efficiently manage heat fluxes as high as approximately 300 W/cm² at a total heat load of 300 W.     

Experimental investigation of the transient thermal performance of a bent heat pipe with grooved surface

A bent copper–water heat pipe with grooved inner surface has been investigated experimentally. A comparison between the bent and the straight heat pipes was performed at different inclination angle. Experimental results show that there is a small temperature difference between the condenser of the straight and that of the bent at the vertical orientation. The temperature difference increases as an inclination angle increases. Furthermore, the response time increases as the inclination angle increases. The thermal response of the straight to a sudden heat load is slightly faster than that of the bent. However, as the inclination angle increases to after the horizontal, the heat flux at the condensers decreases nonlinearly and the response time increases nonlinearly. A two-phase flow map has been proposed to explain the nonlinear performance of the thermal response and the heat flux, based on force balance among gravity, capillary, friction and buoyancy force acting on the working fluids. The nonlinear performance of the thermal response and the heat flux results from the capillary blocking due to formation of liquid bridge of two-phase flow. It was also found that the bent heat pipe is more sensitive to the change of the inclination angle than the straight in terms of the thermal response time and the heat flux of the condenser. The heat flux of the bent decreases faster than that of the straight after the horizontal orientation.     

Investigation into the performance of a micro gravitational heat pipe and a micro gravitational heat pipe with artery

The performance of a normal micro gravitational heat pipe was investigated using the analytical and numerical models previously developed. An innovative structure of the heat pipe, i.e. the micro gravitational heat pipe with artery, was then proposed in an attempt to overcome some of the drawbacks of the normal pipe. The thermal behaviour of the new type of heat pipe was simulated, and this was compared with that of a normal micro heat pipe. A performance estimation of both pipes was carried out based on the simulation results. Copyright © 2002 John Wiley & Sons, Ltd.     

Evaporation resistance measurement with visualization for sintered copper-powder evaporator in operating flat-plate heat pipes

The evaporation resistances of loosely-sintered copper-powder evaporators were measured in operating flat-plate heat pipes. The evaporation processes was also visualized through a top glass plate. Irregular or spherical powders of different size distributions were investigated. Uniform heating of 16–170 W/cm² was applied to the base plate near one end with a heated surface of 1.1 × 1.1 cm². At the other end was a cooling water jacket. The evaporation performance was first examined with the effect of liquid flow resistance minimized, i.e., the copper powders covered only the heated area with the remaining region covered with sintered copper wire screens. Similar to multi-layer mesh wicks, quiescent surface evaporation without nucleate boiling was observed for all test conditions, in spite of the abundant nucleation sites. The water film receded and the evaporation resistance reduced with increasing heat flux. Once partial dryout occurred, the evaporation resistance re-rose. The minimum evaporation resistances were about 0.08–0.09 W cm²/K for wicks containing fine powders. These values are similar with those for multi-layer-mesh wicks having a fine bottom screen. In the absence of fine powders, the minimum evaporation resistances were significantly larger. In the second part for homogeneous sintered-powder wicks, the large flow resistance tended to retard the condensed water from returning to the evaporator. However, this can be compensated by a larger charge and/or a thicker wick.     

Effect of nanofluids on the thermal performance of a flat micro heat pipe with a rectangular grooved wick

In this paper, the effect of water-based Al₂O₃ nanofluids as working fluid on the thermal performance of a flat micro-heat pipe with a rectangular grooved wick is investigated. For the purpose, the axial variations of the wall temperature, the evaporation and condensation rates are considered by solving the one-dimensional conduction equation for the wall and the augmented Young–Laplace equation for the phase change process. In particular, the thermophysical properties of nanofluids as well as the surface characteristics formed by nanoparticles such as a thin porous coating are considered. From the comparison of the thermal performance using both DI water and nanofluids, it is found that the thin porous coating layer formed by nanoparticles suspended in nanofluids is a key effect of the heat transfer enhancement for the heat pipe using nanofluids. Also, the effects of the volume fraction and the size of nanoparticles on the thermal performance are studied. The results shows the feasibility of enhancing the thermal performance up to 100% although water-based Al₂O₃ nanofluids with the concentration less than 1.0% is used as working fluid. Finally, it is shown that the thermal resistance of the nanofluid heat pipe tends to decrease with increasing the nanoparticle size, which corresponds to the previous experimental results.     

Effect of silver nano-fluid on pulsating heat pipe thermal performance

This paper presents preliminary experimental results on using copper tube having internal and external diameter with 2.4 mm and 3 mm, respectively, to carry out the experimental pulsating heat pipe. The working fluids include the silver nano-fluid water solution and pure water.In order to study and measure the efficiency, we compare with 20 nm silver nano-fluid at different concentration (100 ppm and 450 ppm) and various filled ratio (20%, 40%, 60%, 80%, respectively), also applying with different heating power (5 W, 15 W, 25 W, 35 W, 45 W, 55 W, 65 W, 75 W, 85 W, respectively). According to the experimental result in the midterm value (i.e. 40%, 60%) of filled ratio shows better. In the majority 60% of efficiency is considered much better. The heat dissipation effect is analogous in sensible heat exchange, 60% has more liquid slugs that will turn and carry more sensible heat, so in 60% of filled ratio, heat dissipation result is better than 40%, and the best filled fluid is 100 ppm in silver nano-fluid.Finally, we observed through the measurement comparison in thermal performance with pure water. When the heating power is 85 W, the average temperature difference and the thermal resistance of evaporator and condenser are decreased by 7.79 °C and 0.092 °C/W, respectively.     

Experimental investigation on thermal performance of thermosyphon flat-plate solar water heater with a mantle heat exchanger

The thermal performance of thermosyphon flat-plate solar water heater with a mantle heat exchanger was investigated to show its applicability in China. The effect on the performance of the collector of using a heat exchanger between the collector and the tank was analyzed. A “heat exchanger penalty factor” for the system was determined and energy balance equation in the system was presented. Outdoor tests of thermal performance of the thermosyphon flat-plate solar water heater with a mantle heat exchanger were taken in Kunming, China. Experimental results show that mean daily efficiency of the thermosyphon flat plate solar water heater with a mantle heat exchanger with 10 mm gap can reach up to 50%, which is lower than that of a thermosyphon flat-plate solar water heater without heat exchanger, but higher than that of a all-glass evacuated tubular solar water heater.     

倾角对环状热管蒸发器性能的影响

针对生产过程中低品位能量回收,设计了带有环状管蒸发器的不锈钢水工质重力型分离式热管,环状管由31.6 mm管径内管热水加热,环空间隙为15.0 mm,可视化地研究了26 kPa蒸发压力,0~90 °倾斜角度下多个充注率环状管蒸发器的壁温特性。结果表明：该类热管的环状管蒸发器运行时存在一高温区,随倾角增加而扩大;环状管内蒸发侧平均表面换热系数随倾角增大先增后减、再增大,与沸腾流型随角度发生转变有密切关联;与一些相似文献进行了对比,发现环状管蒸发器与普通重力型热管在换热性能均在10~20 °倾角达到极大值,而环状管蒸发器则在90 °时达到了另一极大值。     

复合热沉环路热管的热点散热性能模拟研究

针对芯片功耗与集成度提高而导致的局部热点问题,设计了一种用于芯片散热的复合热沉环路热管系统。建立了环路热管蒸发段模型,通过数值模拟的方法,证明了复合热沉环路热管系统能够降低热点温度,提高散热表面的温度均匀程度,且散热效果与热点的分布位置有关。当热点的热流密度为160W/cm2,热沉横向、纵向导热率分别为1500W/(m?K)、24W/(m?K)时,热点温度为88.88°C,相比于无热沉时降低了5.96°C。研究了不同热沉导热率下的热沉厚度对热点温度的影响,结果表明：若导热率各项同性,热点温度随热沉厚度的增加而降低,之后趋向不变;若为各项异性,存在最优的热沉厚度,使热点温度最低。     

微槽群平板热管散热器传热性能的研究

分别选择不同的翅片间距和高度,对一种新型微槽群平板热管散热器的翅片结构进行优化,得到了热管散热器的最佳整体结构。结果表明:翅片的间距为14mm、高度为60mm时,平板热管散热器的传热性能最好。将热管、管脚以及翅片的温度与实验结果进行对比,结果吻合良好。     

High speed flow visualization of a closed loop pulsating heat pipe

We provide the high speed flow visualization results for the closed loop pulsating heat pipes (PHPs). It is identified that there exists the bulk circulation flow which lasts longer and the local flow direction switch flow. The bubble displacements and velocities do display the sine oscillation waves but the local oscillation waves were superimposed with short periods and small oscillation amplitudes. Distinct with the methanol PHP, the water PHP has quasi-rectangular shape for the bubble displacements, behaving the periodic stationary-fast movement characteristics. Dispersed bubbles, vapor plugs and the transition flow patterns from the dispersed bubbles to the vapor plugs are the major flow patterns in PHP. Long vapor plugs are only observed for the methanol PHP, not observed in the water PHP, due to the vapor plug deformation and breakup mechanism, which was analyzed in the present paper. Bubble sizes have quasi-fixed distributions versus time over the entire PHP, but have unsymmetry distributions among various tubes. The complicated combined effects of bubble nucleation, coalescence and condensation are responsible for the oscillation flow in PHP.     

Experimental study on heat transfer performance of pulsating heat pipe with refrigerants

The effects of different refrigerants on heat transfer performance of pulsating heat pipe(PHP) are investigated experimentally.The working temperature of pulsating heat pipe is kept in the range of 20℃-50℃.The startup time of the pulsating heat pipe with refrigerants can be shorter than 4 min,when heating power is in the range of 10W-100W.The startup time decreases with heating power.Thermal resistances of PHP with filling ratio 20.55% were obviously larger than those with other filling ratios.Thermal resistance of the PHP with R134a is much smaller than that with R404A and R600a.It indicates that the heat transfer ability of R134a is better.In addition,a correlation to predict thermal resistance of PHP with refrigerants was suggested.     

相变面积对平行环路热管性能影响的研究

重力型环路热管作为一种高效的传热装置,其性能与运行时换热器的充注量及换热器倾斜角度密切相关。为了更深入研究在运行时换热器换热量、传热系数与充注量变化所引起的蒸发器、冷凝器相变换热面积之间的关系,设计了一种蒸发器与冷凝器平行放置的新型热管系统。分析实验结果表明:蒸发器在固定角度下换热量与相变有效面积的函数关系呈具有单波峰的曲线分布,其波峰随着倾斜角的增加而向浸湿面积变小的方向移动;倾斜角度越大达到最佳换热效果所需的换热面积越小,但最大换热强度会减小;在热管进口温差固定时,倾斜角度大于45°才能较为有效的通过提升进口温度来提升换热量;在蒸发器未被完全浸润时,冷凝器换热量处于下降阶段,可以增加冷凝器管长来提升换热;在蒸发器被完全浸润时,增加冷凝器管长无法提升换热;蒸发器与冷凝器的传热系数与相变有效换热面积成线性下降关系,下降速率与角度近乎无关。     

Interfacial heat transfer during microdroplet evaporation on a laser heated surface

A comprehensive experimental and numerical investigation on water microdroplet impingement and evaporation is presented from the standpoint of phase-change cooling technologies. The study investigates microdroplet impact and evaporation on a laser heated surface, outlining the experimental and numerical conditions necessary to quantify the interfacial thermal conductance (G) of liquid-metal interfaces during two-phase flow. To do this, continuum-level numerical simulations are conducted in parallel with experimental measurements facilitating high-speed photography and in-situ time-domain thermoreflectance (TDTR). During microdroplet evaporation on laser heated Al thin-films at room temperature, an effective interfacial thermal conductance of G_eff = 6.4 ± 0.4 MW/m² is measured with TDTR. This effective interfacial thermal conductance (G_eff) is interpreted as the high-frequency (ac) interfacial heat transfer coefficient measured at the microdroplet/Al interface. Also on a laser heated surface, fractal-like condensation patterns form on the Al surface surrounding the evaporating microdroplet. This is due to the temperature gradient in the Al surface layer and cyclic vapor/air convection patterns outside the contact line. Laser heating, however, does not significantly increase the evaporation rate beyond that expected for microdroplet evaporation on isothermal Al thin-film surfaces.     

Effect of low thermal conductivity wick on the performance of compact loop heat pipe

The existing work deals with the evaluation of compact loop heat pipe by means of a low thermal conductivity sintered chrysotile wick to avoid large heat leaks as of the evaporator to the compensation chamber. Accordingly, a wick with low thermal conductivity (0.068–0.098 W/mK) chrysotile powder of a mean particle diameter of 3.4 μm is fabricated through sintering. Nine chrysotile wicks are sintered with different compositions of binders (bentonite and dextrin) and pore-forming agent NaCl at sintering temperatures of 500°C, 600°C, and 700°C with a sintering time of 30 min. The wick properties, for instance, porosity, permeability, wettability, and capillary rise are studied owing to sintering temperature. Consequently, it is observed that a pure chrysotile powdered wick at a sintering temperature of 600°C exhibits a high porosity of 61.8% with permeability 1.04 × 10⁻¹³ m² and a capillary rise of 4.5 cm in 30 s and is considered optimal. This optimal wick is used for performance evaluation in compact loop heat pipe and a decrease of 36.1% in thermal resistance is found when compared with copper mesh wick in a loop heat pipe. The lowermost thermal resistance originates to be 0.147 K/W at 120 W with wall temperature 57.7°C. This indicates that loop heat pipe with sintered chrysotile wick can operate at lower heat loads efficiently when compared with copper mesh wick and as heat load increases a chance of dry out condition occurs. The highest evaporative heat transfer coefficient obtained is 65.7 kW/m² K at a minimum heat load.     

Study on heat transfer performance for loop heat pipe with circular flat evaporator

A loop heat pipe (LHP) with a circular flat evaporator was designed for cooling electronic devices. The flat evaporator with an outside diameter of 41 mm and a thickness of 15 mm was developed with a copper powder wick. The developed evaporator was examined to improve insufficient subcooling of liquid in a compensation chamber, which decreases an operating limitation of the LHP. Many different orientations of the elevation and direction of the evaporator were also considered during all of the experiments for this system. The active heating area was 3 cm × 3 cm, and water was used in the tests. This LHP generated a heat load in excess of 140 W with a total thermal resistance of 0.39 °C/W.     

Influence of wick characteristics on heat pipe performance

The performance of a heat pipe depends on several factors, one of which is the nature of the wick structure. Optimization of heat pipe performance requires wick structures that can provide high capillary pressure, and yet still offer low resistance to fluid flow. The current level of technology being developed in our laboratory makes it possible to engineer desired wick structures both micro‐ and macroscopically, especially tailored to optimize heat pipe performance. In order to assist the fabrication of unique wick structure, the influence of wick structure characteristics on heat pipe performance is numerically investigated in this work. Numerical methods are an effective tool to significantly reduce the number of experimental trials. Comparisons are made between performances of heat pipe with different wick structures; coarse pore sizes, fine pore sizes and a composite comprised of coarse and fine pore sizes. Results indicate superior performance, with a factor of up to 2, for heat pipe with composite structure combining coarse/fine wick. Validation of the simulation result presented using experimental data is being carried out. Copyright © 2005 John Wiley & Sons, Ltd.