【作者】 吕伦春；

【导师】 刘振华；

【作者基本信息】 上海交通大学 ， 动力工程及工程热物理， 2009， 博士

【摘要】 随着微电子技术的迅速发展,电子元器件的微型化已成为现代电子设备发展的主流趋势。电子元器件的特征尺寸不断减小,同时芯片的集成度、封装密度以及工作频率不断提高,这使得芯片的热流密度也迅速提高。因此,对电子设备进行有效的散热是当前电子设备的关键技术。热管作为一种高效的传热元件已经被应用到散热领域中。但是,普通的热管不能实现长距离传输热量,其使用受到一定限制。而毛细泵回路(CPL)可以解决这一问题。它可以实现小温差、长距离、无附加动力的热量传递。纳米流体作为一种新型传热介质,由于其具有优良的传热性能,已经被人们应用到热管的研究上。研究发现,同以纯液体为工质的热管相比,以纳米流体为工质的热管的壁面温度降低了,热管的总体性能也得到了提高。因此,在电子设备散热研究领域中展开以纳米流体为工质的CPL的研究具有重要意义。本文以小型带有平板式蒸发器的CPL为研究对象,以纳米流体为工质,利用自行搭建的小型毛细泵回路实验台,研究了纳米颗粒种类、平均粒径、纳米颗粒质量浓度以及CPL工作压力对CPL换热性能的影响;并结合理论分析了以纳米流体为工质的毛细泵回路的最大传热能力,得出了如下结论:1:借助竖直细管内纳米流体的沸腾换热特性实验,研究扩散剂对纳米流体沸腾换热特性的影响。实验结果表明扩散剂对纳米流体的沸腾特性有重大影响。同以纯水为工质的管内沸腾相比较,扩散剂的加入使沸腾特性恶化了,而没有添加扩散剂的纳米流体的沸腾换热特性则强化了。因而在CPL实验中纳米流体没有添加扩散剂。2:对以纯水为基液,以平均粒径为20nm、50nm的铜纳米颗粒和平均粒径为50nm的氧化铜纳米颗粒的纳米流体的物理性质进行了测量。测量结果表明,在液体中添加纳米颗粒,液体的粘度稍稍增大了,纳米颗粒的质量浓度是影响纳米流体粘度的因素;在液体中添加纳米颗粒,能增大液体的导热系数。纳米流体的导热系数与纳米颗粒的质量浓度、平均粒径以及温度有关;在液体中添加纳米颗粒,能减小液体流体的表面张力,纳米颗粒的质量浓度越大,纳米流体的表面张力也就越小;在液体中添加纳米颗粒,能减小液体与不锈钢的固液接触角。3:在采用底部加热方式和运行压力不变的条件下对CPL进行了实验研究。实验结果表明,本CPL实验装置存在一个最佳充液率,在该充液率的条件下,整个系统的热阻最小;CPL热管中加入纳米颗粒能够强化蒸发换热特性,增加CPL最大功率,降低系统热阻,从而提高热管整体性能;纳米流体质量浓度对CPL蒸发器换热系数强化有明显影响,存在一个最佳质量浓度,此最佳质量浓度与粒子种类有关,与粒径无关。铜-水纳米流体的最佳质量浓度为1.0%,而氧化铜-水的纳米流体的最佳质量浓度为0.5%;纳米颗粒种类和粒径对蒸发换热系数强化率和CPL最大热流密度强化率有明显影响。对于采用粒径相同的纳米流体为工质的CPL来说,采用金属纳米颗粒的CPL的蒸发换热强化效果和最大热流密度强化率比采用金属氧化物纳米颗粒的CPL的要高;对于采用同种材质的纳米颗粒组成的纳米流体的CPL来说,采用小粒径的CPL的蒸发换热强化效果和热流密度强化率要比采用大颗粒的CPL的要高。随着CPL的运行压力的提高,纳米流体对蒸发器的换热特性的强化作用越显著。4:在冷却条件不变的条件下,通过实验研究了以纳米流体为工质的平板型CPL的启动及其静置后重新启动问题。实验结果表明,当CPL采用底部加热方式加热启动的时候,随着纳米颗粒的加入,系统启动时间减小,蒸发器壁面温度降低,CPL的启动性能得到了提高;与此同时,随着工质中纳米颗粒浓度的增加,CPL启动所花费的时间更小;工质中添加纳米颗粒后对CPL的启动所需的最小和最大功率没有影响;当CPL采用上部加热方式加热启动的时候,随着纳米颗粒的加入,整个系统启动成功的时间变长,当系统稳定时,蒸发器壁面温度和以纯水为工质的CPL稳定时的蒸发器壁面温度相差不大,整个CPL的性能没有改变;以纳米流体为工质的CPL在长时间静置后重新启动时,当采用底部加热方式加热的时候,其启动曲线和静置前的启动曲线相似,而采用上部加热方式加热时,其启动曲线和在该条件下纯水的启动曲线基本相同。5:对以纳米流体为工质的CPL在冷却条件不变的情况下的换热性能进行了实验研究。实验结果表明,以纳米流体为工质的CPL在实际应用中具有可用性。6:通过理论计算了以纳米流体为工质的CPL的最大热流密度,并把计算结果和实验结果进行了对比,发现二者的趋势吻合。更多还原

【Abstract】 With the rapid development of the microelectronic technology, the miniaturization of electronic components in modern electronic devices has become the mainstream of the development trend. The decrease of the feature size of electronic components, the rise of the chip integration, packaging density and the working frequency make the chip produce more heat per unit volume than before. Therefore, how to cool the electronic equipment effectively becomes the key technology at moment. The heat pipe has been used to the cooling of the electronic devices because of its highly efficient heat transfer. The common heat pipe could not transport heat across long-distance, which causes its application to some certain restrictions. The capillary pumped loop (CPL) is a two-phase control system, which could transport heat over long distance with minimal temperature difference and need not pumping power. Because of these, the CPL is used as the cooling solution to high load/flux problem of advanced electronic packaging . Nanofluid, which is a colloidal suspension with nanoparticles dispersing uniformly in a base liquid, has anomalous heat transfer characteristics and has been used in the heat pipe. All researches, which carried out the investigations on the heat pipe using nanofluid as the working liquid, proved that the addition of nanoparticles in the working liquid could lead to a decrease in the wall temperature and an increase in the heat transport capability. Therefore, the research on the CPL with nanofluid as the working liquid is of great significance in the thermal design of the electronic equipment.This paper aimed at a fundamental understanding of the application of nanofluid in the CPL with a flat evaporator. The small capillary pumped performance analysis platform was built for the investigation. The influences of types of nanoparticles, the sizes of nanoparticles, nanoparticles mass concentrations and operational pressures on the heat transfer coefficient and the maximum heat flux of the CPL were investigated. At the same time, a theoretical study was carried out on the calculation of the maximum heat flux of the CPL. Main results and findings are summarized as following.1: The influence of surfactant on the nanoluid nucleate boiling characteristics was carried out in a vertical small heated tube with a closed bottom (thermosyphon). The results showed that the boiling heat transfer rates of nanofluids with surfactant are poorer than that of the base liquid. However, the boiling heat transfer rates of nanofluid without surfactant are better than that of the base liquid.2. The physical properties of three types of nanoluid were measured. These nanofluids include the 20 nm Cu-water nanofluid, 50 nm Cu-water nanofluid and 50 nm CuO-water nanofluid. The experimental results showed that the viscosity of nanofluid is slightly greater than that of the base fluid and the nanoparticles mass concentration is one of the factors that influence the viscosity of nanofluid. The thermal conductivity of the fluid is increased with the addition of nanoparticles. The nanoparticles mass concentration, the type of nanoparticles, the size of nanoparticles and the temperature are the key factors that influence the thermal conductivity of the nanofluid. The surface tension of the fluid is decreased to some extent with the addition of nanoparticles in the fluid. The smaller of the surface tension of nanofluid with the increasing of nanoparticles mass concentration. And the solid-liquid contact angles between the stainless steel and the fluid is also decreased with the addition of nanoparticles in the fluid.2. The experiments were carried out at a steady operational pressure, which was 5.81 kPa, 9.86 kPa and 16.14 kPa, respectively. The results showed that there exists an optimal filling ratio to the CPL and the thermal resistance is at the minimum when the optimum filling ratio is taken. The experimental results indicated that the thermal performance of the CPL is improved with the addition of the nanoparticles. There exists an optimal nanoparticles mass concentration corresponding to the maximum heat transfer enhancement. And the optimal nanoparticles mass concentrations are 1.0 % and 0.5 % for Cu nanofluid and CuO nanofluid, respectively. The nanoparticle type and the nanoparticle size influence the thermal performance of the CPL. The maximum heat flux is increased with the addition of nanoparticles in the working liquid. The influence of the operational pressure on both the heat transfer coefficient and the maximum heat flux of the evaporator are remarkable. The heat transfer coefficient and the maximum heat flux are increased with the increasing of the operational pressure.4. The startup and re-startup of the CPL with the nanofluid as the working liquid were investigated at the condition that the cooling condition unchanged. Two types of heating location were investigated, which included the evaporator was heated from the top and the evaporator was heated from the bottom. To the evaporator heated from the bottom, the results of experiments showed that the startup time and the evaporator wall temperature are reduced with the addition of nanoparticles in the working fluid. With the increasing of nanoparticles mass concentration, the smaller of the startup time f of the CPL. However, the startup time is increased with the addition of nanoparticles in the working fluid when the evaporator is heated from the top. The wall temperature of the evaporator is similar with that of evaporator with the pure water as the working liquid when the whole system is stable. Comparing the CPL with the pure water as the working liquid, the entire performance of the CPL with nanofluid as the working fluid has not changed. The whole system was stopped to work for 72 hours after the tests. Then the system was restarted. The results showed that the startup process for the CPL heated from the bottom with nanofluid as the working liquid is similar with that for the CPL heated from the bottom with nanofluid as the working liquid. However, the startup process for the CPL heated from the top with nanofluid as the working liquid is similar with that for the CPL heated form the top with pure water as the working liquid.5. An experiment was carried out to understand the heat exchanger performance of the CPL with nanofluid as the working liquid when the cooling condition was unchanged. The experimental results showed that the CPL with nanofluid as the working liquid is applicable in application.6. The maximum heat flux of the CPL with nanofluid as the working liquid is gotten through theoretical analysis. It is found that the trends are similar between the calculated results and the experimental results.更多还原