Three‐Dimensional Numerical Investigation of Nanofluids Flow in Microtube with Different Values of Heat Flux

Forced convective laminar flow of different types of nanofluids such as Al₂O₃, CuO, SiO₂, and ZnO, with different nanoparticle size 25, 45, 65, and 80 nm, and different volume fractions which ranged from 1% to 4% using ethylene glycol as base fluids were used. A three‐dimensional microtube (MT) with 0.05 cm diameter and 10 cm in length with different values of heat fluxes at the wall is numerically investigated. This investigation covers Reynolds number (Re) in the range of 80 to 160. The results have shown that SiO₂‐EG nanofluid has the highest Nusselt number (Nu), followed by ZnO‐EG, CuO‐EG, Al₂O₃‐EG, and finally pure EG. The Nu for all cases increases with the volume fraction but it decreases with the rise in the diameter of nanoparticles. In all configurations, the Nu increases with Re. In addition, no effect of heat flux values on the Nu was found.     

MICROCHANNEL HEAT SINKS: AN OVERVIEW OF THE STATE-OF-THE-ART

Computers are rapidly becoming faster and more versatile, and as a result, high-powered integrated circuits have been produced in order to meet this need. However, these high-speed circuits are expected to generate heat fluxes that exceed the circuit's allowable operating temperature, and so an innovative cooling device is needed to solve this problem. Microchannel heat sinks were introduced in the early 1980s to be used as a means of cooling integrated circuits. Since then, many studies have been conducted in the field of these microchannel heat sinks. Earlier research used mainly single-phase coolants in their heat sinks, but two-phase coolants are now the focus of more recent research. The purpose of this article is to present a state-of-the art literature review of the progress of research in the field of microchannel heat sinks. This literature will focus mainly on the most recent research, starting with the latter half of the 1990s.     

内燃机冷却系统中应用纳米流体强化传热研究综述

内燃机工作时依赖冷却系统将多余热量及时带走以保证燃烧室核心部件及润滑油膜的正常工作温度。常规内燃机冷却介质导热系数偏低,而新一代强化传热工质纳米流体具有明显提升的传热性能,应用于内燃机冷却系统有利于强化内燃机传热及提高热管理性能。且由于纳米流体的传热性能受纳米粒子的种类、大小、浓度、形状等因素影响,可以通过改变这些因素控制内燃机冷却水腔的传热量。综述了国内外研究者针对纳米流体导热系数与对流换热性能开展的试验测试、理论分析和计算机模拟研究工作,以及纳米流体应用于内燃机冷却系统中强化传热的进展,最后指出当前研究工作的不足及未来工作方向。     

四氧化三铁纳米流体强化热管换热的实验研究

通过实验研究四氧化三铁(Fe3O4)纳米流体重力热管的传热性能.在不同输入功率、不同充液率、不同纳米流体质量分数的工况下测试重力热管的外壁温度,再理论计算其等效对流传热系数、热阻.结果表明:当充液率为50％、输入功率为40 W时,水基液重力热管和纳米流体重力热管都有最高的等效对流传热系数,并且纳米流体质量分数为1.0％...     

Multiwalled Carbon Nanotube Nanofluid for Thermal Management of High Heat Generating Computer Processor

Minichannel heat sink geometries with varying fin spacing were tested with de‐ionized water and MWCNT (1 wt %) nanofluid to evaluate their performance with flow components of a liquid cooling kit. Four heat sinks with fin spacing of 0.2 mm, 0.5 mm, 1.0 mm, and 1.5 mm were used in this investigation. Heat sink base temperature was analogous to processor operating temperature which was the prime parameter of interest in this investigation. The base temperature decreased by reducing the fin spacing and using multiwalled carbon nanotube (MWCNT) nanofluid. The lowest value of heat sink base temperature recorded was 49.7 ^°C at a heater power of 255 W by using a heat sink of 0.2 mm fin spacing and MWCNT nanofluid as a coolant. Moreover, as a result of reduced fin spacing and using MWCNT nanofluid as a coolant the value of overall heat transfer coefficient increased from 1200 W/m²K to 1498 W/m²K, translating to about a 15% increase. The value of thermal resistance also dropped by reducing the fin spacing and using MWCNT nanofluid. The most important aspect of the study is that the heat sinks and MWCNT nanofluid proved to be compatible with the pump and radiator of the commercial CPU liquid cooling kit. The pump was capable to handle the pressure drop which resulted by reducing the heat sink fin spacing and by using MWCNT nanofluid. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res, 43(7): 653–666, 2014; Published online 11 November 2013 in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21107     

DESIGN METHODOLOGY AND COOLING POTENTIAL OF THE ENVIRONMENTAL HEAT SINKS

The usefulness of defining the cooling potential and other related variables is a function of how these figures will help the designer in the decision taking process. Consequently, a rational approach to these definitions should start from a certain design methodology

The present paper proposes a design methodology which uses the concepts of gross cooling potential, net cooling potential, available cooling potential, usable cooling potential and natural cooling saving fraction. These concepts are expressed under common terms for three natural cooling techniques (NCT), radiative cooling via night sky radiators, ground cooling using buried ducts and evaporative cooling via mechanical evaporative coolers.

The objective is to develop a systematic procedure of producing information about the suitability of using a certain NCT in a given locality and for a certain building. The application of the methodology mentioned for a set of European localities has been included in the ATLAS about the potential of NCT which constitutes one of the final products of the PASCOOL project.     

纳米流体在电子冷却中的应用研究进展

随着新型电子元件产生热量的增加,纳米流体特性与微通道特性的结合成为研究热点。这种技术的发展可以使电子设备进一步小型化,并提高能源效率。从纳米颗粒材料和散热器几何结构两方面综述了近年来纳米流体在电子冷却中的应用研究进展,总结了这一领域未来的研究机会和存在的挑战。研究发现,将纳米流体作为新型冷却液应用于不同的散热器中可以提升电子冷却技术的工作效率。     

Effect of Serpentine Grooves on Heat Transfer Characteristics of Microchannel Heat Sink with Different Nanofluids

An experimental and numerical investigation of the thermal performance of three different nanofluids ethylene glycol‐based CuO, water‐based CuO, and Al₂O₃ is done in a serpentine‐shaped micorchannel heat sink. The microchannels considered ranged from 810 μm to 890 μm in hydraulic diameter and were made of copper material. The experiments were conducted with the Reynolds number ranging from approximately 100 to 1300. The forced convective heat transfer coefficient of nanofluids shows that there is an improved heat transfer rate compared to base fluids water and ethylene glycol. The experimental results also confirm that there is an earlier transition from laminar to turbulent flow in microchannels. The results prove that as the hydraulic diameter decreases there is increased pressure drop and the heat transfer coefficient increases for both the base fluids and nanofluids. The flow characteristics are discussed based on the pressure drop. While investigating the heat transfer coefficient of the three different nanofluids the nanofluid CuO/EG has the highest heat transfer coefficient as a result of the material's property. This research also will encourage young researchers to work on nanofluids of varying nanoparticle size and concentration to discover new results.     

Heat transfer and pressure loss characteristics of very compact heat sinks

High-performance compact heat sinks have been developed for the effective cooling of high-density LSI packaging. Heat transfer and pressure loss characteristics of the heat sinks in both air-cross-flow and air-jet cooling have been experimentally studied. The present heat sinks were of plate-fin and pin-fin arrays with a fin pitch of 0.7 mm. The plate-fin heat sinks had higher cooling performance than the pin-fin heat sinks in the range of large airflow rates both in air-cross-flow and air-jet cooling. The thermal conductance in cross-flow cooling was 20 or 40% larger than that in jet cooling. The correlation of Colburn j-factor/Fanning friction factor versus the Reynolds number for the present heat sinks was found to be very close to that of a conventional large-size heat exchanger. © Scripta Technica, Heat Trans Asian Res, 28(8): 687-705, 1999     

Heat Transfer Performance of Heat Pipe Radiator with SiO2/Water Nanofluids

The effect of nanofluids on thermal performance of the miniature heat pipe radiator which was assembled by two heat pipes containing 0.6 vol.% SiO₂/water nanofluids and 30 pieces of rectangular aluminum fins was investigated experimentally and theoretically. The wall temperatures of the miniature heat pipe and fin surface temperatures were measured. Results showed that the utilization of SiO₂/water nanofluids as a working fluid in the heat pipe enhanced the heat performance by reducing wall temperature differences. Compared with Deionized water (DI water), the thermal resistance of the miniature heat pipe with SiO₂/water nanofluids decreased by about 23% to 40%. Furthermore, the theoretical calculation on a basis of one dimension found that the fin heat dissipation in the miniature heat pipe radiator charged SiO₂/water nanofluids was about 1.17 times of that of the DI water radiator.     

Experimental Verification of Model for Liquid-Cooled Staggered Pin Fin Heat Sinks with Top Bypass Flow

Pressure drops and heat transfer over staggered pin fin heat sinks with top bypass flow were experimentally evaluated. The authors considered liquid-cooling applications because there were few data available comparing to air-cooling applications. Empirical equations to predict heat transfer on the endwall were developed by obtaining experimental data on the copper base plate with acrylic pins. A new model for predicting pressure drops and heat transfer over staggered pin fin heat sinks with top bypass flow based on mass, momentum, and energy conservation within the two control volumes is proposed. The first control volume in the model is located within the finned area, and the second is located in the gap between the tip of the pins and the flow channel. This model combines two conditions according to the boundary-layer thickness. A comparison between experimental and calculated results revealed that dimensionless pressure drops and the Nusselt number could be predicted within 30% error for the former and 50% error for the latter.     

叶片蒸汽冷却耦合换热特性的数值研究

为评估透平叶片的蒸汽冷却效果,以Mark II叶片为对象,采用热流固耦合的数值计算方法,通过与实验数据进行比较考察了不同湍流模型对计算结果的影响,对比分析了空气、过热蒸汽和湿蒸汽冷却效果的差异,研究了冷却蒸汽质量流量、进口湍动度和叶片表面粗糙度对蒸汽冷却效率的影响.结果表明:SST转捩湍流模型对于流动换热计算有较高的精度;与空气冷却相比,过热蒸汽冷却的效率更高,叶片壁面温度更低;与过热蒸汽冷却相比,湿蒸汽的冷却效率更高,叶片壁面温度更低,且随着蒸汽湿度的增加,冷却效率提高,叶片壁面温度降低;增加冷却蒸汽的质量流量可使冷却效率提高,但冷却蒸汽的温升减小;当湍流强度小于3%时,冷却效率随冷却蒸汽进口湍流强度的增大而提高;增加叶片粗糙度使得叶片冷却效率显著提高.