【作者】 强爱红；

【导师】 周明；

【作者基本信息】 天津大学 ， 化学工程， 2007， 博士

【摘要】 为了适应工业过程强化和发展高效传热设备的需要,近几十年来研究者提出了多种强化传热技术。随着纳米技术的飞速发展,一些学者开始尝试将纳米颗粒与流体混合,制成纳米颗粒悬浮液,强化对流传热,给强化传热技术带来了蓬勃生机。本文的研究主要是围绕纳米流体对流传热方面展开,以期为纳米流体对流传热的深层次研究奠定基础。将纳米颗粒和液体介质直接混合,并添加分散剂和超声振动悬浮液,制备了稳定的氧化铜纳米颗粒悬浮液。采用沉降实验、zeta电位测量、粒度分布测定及红外光谱等手段对悬浮液的稳定性进行了分析。分析了分散剂的种类、加入量及介质的pH等因素对纳米颗粒悬浮液稳定性的影响。自行设计并建立了一套纳米流体传热性能测试实验装置,测量了不同粒子体积分数的水-CuO纳米流体在不同流速下的管内对流传热系数并与水和分散剂溶液进行了对比。实验结果表明,在液体中添加纳米粒子增大了液体的管内对流传热系数,粒子的体积分数是影响纳米流体对流传热系数的主要因素之一,在相同雷诺准数下,纳米流体的对流传热系数随粒子体积分数的增加而增大。根据纳米颗粒强化流体对流传热的机理,应用有效连续介质理论,假设流体和颗粒相具有相等的运动速度,仅考虑颗粒质量通量的变化,建立了纳米流体对流传热微分方程组——颗粒迁移模型。颗粒迁移通量包括由剪切速率梯度和黏度梯度诱导、颗粒布朗运动及热泳引起的颗粒迁移通量。通过对模型进行参数分析发现:颗粒尺寸、K c/Kμ及颗粒平均浓度的大小影响颗粒在管道截面上的分布以及导热系数和黏度等物性的分布。随着颗粒的迁移,流体的速度分布变得更加平坦,影响了壁面附近的温度梯度,强化了传热。模拟结果与实验数据比较表明:模拟结果能够正确反映实验数据的变化趋势,且高于导热系数均匀分布时的模拟值。考虑热泳引起的颗粒迁移后,模拟结果改善了。颗粒迁移模型正确考虑了影响纳米流体对流传热的主要因素,具有一定的合理性。更多还原

【Abstract】 Responding to the need for industry process intensification and more efficient heat transfer systems, many efforts have been devoted to heat transfer enhancement techniques in the past few decades. With the rapid development of nanotechnology, some scholars attempted to enforce heat transfer process with nanoparticles, which introduced a new method into heat transfer enhancement. The purpose of this paper is to demonstrate experimentally and theoretically the convective heat transfer characteristics of nanofluids, so as to lay a foundation for the farther investigation on heat transfer of nanofluids.CuO nanoparticle suspension has been prepared by directly mixing nanoparticles and base fluids. Some auxiliary dispersants and ultrasonic oscillation were necessary to obtain even distributed and stabilized suspensions. Sedimentation experiment, zeta potential, granularity test and infrared spectrum were given to illustrate the stability of suspensions. Some factors that affect the stability and evenness of suspension, such as the property and concentration of dispersants, pH of base fluid were discussed.An experimental system was designed and built up to investigate convective heat transfer characteristics of CuO nanofluids flowing in a tube. The effects of such factors as the volume fraction of suspended nanoparticles and the Reynolds number on heat transfer were discussed in detail. The convective heat transfer performance of water and dispersant solution were also tested to compare with nanofluids. The experimental results show that nanoparticles remarkably increase the convective heat transfer coefficient of base fluid. The heat transfer feature of nanofluids increases with the volume fraction of nanoparticles.By considering factors affecting the convective heat transfer of nanofluids, a convective heat transfer model for nanofluids under laminar flow was established. It is according to effective continuum theory, which supposes a common velocity for two phases and merely considers the mass flux of particles migration. The tatal flux of particle migration takes into account the effects of shear-induced and viscosity-gradient-induced, Brownian motion, as well as thermophoresis-induced particle migration. With the model the effects of particle size, K c/Kμand mean concentration of particle on concentration distribution were analyzed. The migration of particles leads to non-uniform viscosity and thermal conductivity distribution of suspension. Increases in blunting of velocity profile influences the fluid temperature grads close to the wall and thus intensifies heat transfer of nanofluids.Comparison between experimental data and calculated results indicates that the model describes correctly the trend of experimental value and simulation results are higher than those of even thermal conductivity distribution. The capability of model is improved after considering the migration of thermophoresis. The particle migration model is reasonable for energy transport of nanoparticle suspension.更多还原