【作者】 于峰；

【导师】 李强；

【作者基本信息】 南京理工大学 ， 热能工程， 2010， 硕士

【摘要】 冲击射流冷却技术因能在局部产生极高的换热系数被认为是解决未来局部高热流密度冷却问题的首选技术之一。但目前应用于射流技术中的传统换热工质的热导率都较低,无法满足高热流密度电子器件冷却的需要,因此提高工质热导率成为强化冲击射流换热的关键。提高液体导热系数的一种有效方式是在液体中添加纳米级的金属、非金属或聚合物固体粒子,即纳米流体。本文尝试将纳米流体技术引入冲击射流强化传热技术,提高射流工质自身的传热性能,以期进一步强化冲击射流冷却系统的传热效率,为提高冲击射流冷却效果提供一种新的手段。主要工作包括以下几个方面：首先,制备纳米流体并进行悬浮稳定性实验。本文采用两步法,将纳米粒子与基液直接混合,进行一定时间的超声震荡和强烈的机械搅拌,得到不同体积份额的Cu-水纳米流体。悬浮稳定性实验结果表明,此方法制备得到的纳米流体具有较好的稳定性。其次,研究纳米流体冲击射流的流动和换热特性。建立了测试纳米流体冲击射流冷却性能的实验系统,测试了不同体积份额的Cu-水纳米流体作为换热工质时的换热效果,分析了出口Re数、射流冲击高度、射流冲击角、纳米流体粒子体积份额、粒子粒径以及纳米流体温度对系统换热性能的影响。实验结果表明,纳米粒子的加入增大了基液的冲击射流换热系数,增强了射流换热系统的换热性能。另外,实验结果也表明,由于纳米粒子的小尺寸效应,纳米流体的流动阻力并没有增加,也不会像毫米或微米级粒子易产生磨损、堵塞等不良结果。再次,综合考虑影响纳米流体冲击射流换热的多种因素,如粒子的微对流和微扩散、流体的流动状态、冲击射流结构参数等,提出了计算悬浮有金属纳米粒子的纳米流体冲击射流换热系数的关联式。关联式的计算结果与实验数据相近,表明关联式正确地描述了纳米流体射流换热过程,可以用来计算纳米流体的冲击射流换热系数。更多还原

【Abstract】 With the high local heat transfer capacity, jet impingement cooling technique is considered as one of the most powerful cooling solutions for high-heat-flux removal. Although many investigations have been carried out in the area of jet impingement heat transfer over years, the applications in high-density electronic components are limited because of the low thermal conductivity of jet working fluids. The improvement of the thermal properties of jet working fluid may become a trick of augmenting heat transfer performances for the impinging jet system. An effective way of improving the thermal conductivity of fluids is to suspend nanometer solid particles in the carrier fluids, namely nanofluids. The purpose of this paper is to introduce the nanofluids into jet impingement cooling system as the working fluid. It is expected to improve jet impingement performance by the way of improving the thermal conductivity of fluids. The main research contents are the following.Firstly, nanofluids are prepared and the suspension stability is tested. In this paper, Cu-water nanofluids are prepared by classical two-phase method, in which Cu particles are directly joined into distilled water. In order to obtain the stabilized suspension, mixture is consumingly whisked and vibrated in an ultrasonic vibrator. With this method, several sampled nanofluids with different volume fraction have been prepared. The results of suspension stability tests show that the nanofluids prepared by this method have favorable decentralization property.Secondly, the heat transfer performance and flow features of nanofluids in jet impingement are studied. An experimental system has been build up to investigate the heat transfer coefficient and the pressure drop of nanofluids in a submerged impinging jet. The effects of such factors as Reynolds number, the distance between the jet outlet and the impingement surface, oblique angle, the volume fraction and dimensions of the suspended nanoparticles are discussed in detail. The experimental results show that the suspended nanoparticles remarkably increase the heat transfer coefficient of the base fluid. In addition, the pressure drop experiments indicate that nanofluids with smack particle volume fraction will not cause significant augmentation in pump power and it is suitable for practical application.Thirdly, considering both the effects of the suspended nanoparticles and the condition of impinging jet, a new type of the heat transfer correlation for nanofluids in a confined and submerged impinging jet has been proposed The good coincidence between the experimental data and the calculated values show that the correlation correctly describes the energy transport and can be used to predict the impinging jet heat transfer coefficient with nanofluids.更多还原