基于纳米流体的菲涅尔聚光系统微通道散热器传热特性研究

发布时间：2018-03-03 17:37

本文选题：聚光光伏　切入点：纳米流体　出处：《内蒙古工业大学》2015年硕士论文　论文类型：学位论文

【摘要】：采用聚光器提高光伏电池表面的热流密度,是提高光伏系统效率、降低系统发电成本的有效途径。但砷化镓电池的工作效率会随着电池温度的升高迅速下降,且长期热应力的作用会使组件结构永久破坏,所以,针对聚光光伏发电系统热流密度分布不均匀以及光伏电池温度过高等问题,本文提出了一种螺旋型微通道冷却结构并进行优化,且以纳米流体为冷却工质进行散热。本文的主要研究内容包括:应用高压微射流纳米分散仪分别制备质量分数为5%、3%和1%的Si O2-H2O纳米流体,并对纳米流体的粒径分布、导热系数和辐射特性进行测试和分析。针对纳米流体集热管换热特性,模拟计算了蒸馏水和纳米流体的温度场和速度场分布,同时,对以Si O2-H2O纳米流体和蒸馏水为工质的集热管进行了闷晒试验研究。结果表明,随着质量分数的增大、温度的升高,Si O2-H2O纳米流体导热系数逐渐变大;且Si O2-H2O纳米流体对太阳辐射具有选择透过性,质量分数是其重要影响因素。纳米流体具有比蒸馏水高的换热特性且随其质量分数的增加而增强;随着工作时间的增加,纳米颗粒易团聚、且纳米流体不稳定等特征会导致其换热特性降低。基于微通道冷却技术提出螺旋式高倍聚光砷化镓电池冷却结构,并通过Ansys Fluent软件分析了PCB板面积、流道长度和入口流速对冷却结构换热特性的影响,同时,引入强化换热因子对其进行结构优化,结果表明,强化传热因子随着微通道长度的增加而降低、流体入口速度的增加而升高,在此基础上,获得可提供膜蒸馏热源且强化传热因子最高时的冷却结构,即4圈微通道,流速为0.58m/s的模型。采用优化后的冷却结构,以纳米流体作为冷却工质,并应用两相流模型从纳米流体的质量分数、颗粒粒径及类型三方面对纳米流体强化换热机理进行探讨。结果表明在相同的Re下,Al2O3-H2O纳米流体的传热特性高于Si O2-H2O纳米流体的传热特性;Al2O3-H2O纳米流体的换热特性随纳米粒径的减小而升高,且纳米粒子的高导热性及由黏度引起的流动阻力共同影响纳米流体的换热特性。最后,通过中间程序将光学软件所得的非均匀热流密度作为边界条件读入FLUENT中,并改变太阳直射辐照度对非均匀热流密度条件下冷却结构的换热特性进行分析,结果表明电池上表面热流密度分布越均匀,冷却结构的换热能力越强;不同太阳直射辐照度存在对应的最佳入口流速,在实际应用中,若使不同辐照度下冷却结构的强化换热因子达到最大,需注意调节工质流速与太阳直射辐照度相匹配。
[Abstract]:It is an effective way to increase the efficiency of photovoltaic system and reduce the cost of power generation by using concentrator to increase the heat flux on the surface of photovoltaic cell, but the efficiency of GaAs cell will decrease rapidly with the increase of cell temperature. And the long-term thermal stress will make the module structure permanent damage, so, in order to solve the problems such as uneven heat flux distribution and excessive temperature of photovoltaic cells, the heat flux distribution of concentrated photovoltaic power system is not uniform, and the temperature of photovoltaic cells is too high. In this paper, a spiral microchannel cooling structure is proposed and optimized. The main contents of this paper are as follows: 5% Sio _ 2-H _ 2O nano-fluids with mass fraction of 5% and 1%% were prepared by using high-pressure micro-jet nano-dispersion instrument, and the particle size distribution of nano-fluids was obtained. The thermal conductivity and radiation characteristics are measured and analyzed. The temperature and velocity distributions of distilled water and nanofluids are simulated and calculated according to the heat transfer characteristics of nanofluid collector tubes. The sun-stuffing experiment of the collector tube with Sio _ 2-H _ 2O nano-fluid and distilled water as working medium was carried out. The results showed that the thermal conductivity of Sio _ 2-H _ 2O nano-fluid increased with the increase of mass fraction. Furthermore, Sio _ 2-H _ 2O nanofluids have selective transmittance to solar radiation, and mass fraction is an important influence factor. The nano-fluids have higher heat transfer characteristics than distilled water and increase with the increase of mass fraction, and with the increase of working time, The characteristics of nano-particles are easy to agglomerate, and the characteristics of nano-fluid instability will reduce their heat transfer characteristics. Based on microchannel cooling technology, the cooling structure of spiral high-power gallium arsenide cells is proposed, and the area of PCB plate is analyzed by Ansys Fluent software. The effects of channel length and inlet velocity on the heat transfer characteristics of cooling structure were studied. At the same time, the enhanced heat transfer factor was introduced to optimize the structure. The results show that the enhanced heat transfer factor decreases with the increase of the length of the microchannel. On the basis of the increase of the inlet velocity of the fluid, the cooling structure with the highest heat source and enhanced heat transfer factor for membrane distillation was obtained, that is, a four-circle microchannel model with a flow rate of 0.58 m / s. The optimized cooling structure was adopted. The nano-fluid was used as the cooling medium, and the two-phase flow model was used to calculate the mass fraction of the nano-fluid. The enhanced heat transfer mechanism of nano-fluid is discussed from three aspects of particle size and type. The results show that the heat transfer characteristics of Al _ 2O _ 3-H _ 2O nanofluids are higher than those of Sio _ 2-H _ 2O nanofluids under the same re. The heat transfer characteristics of Al _ 2O _ 3-H _ 2O nanofluids follow the same conditions. With the decrease of nanometer particle size, The high thermal conductivity of the nanoparticles and the flow resistance caused by the viscosity affect the heat transfer characteristics of the nano-fluids. Finally, the non-uniform heat flux obtained by the optical software is read into the FLUENT as the boundary condition by the intermediate program. The heat transfer characteristics of the cooling structure under the condition of non-uniform heat flux are analyzed by changing the direct solar irradiance. The results show that the more uniform the heat flux distribution on the surface of the cell, the stronger the heat transfer ability of the cooling structure. The optimum inlet velocity exists for different direct irradiance of the sun. In practical application, if the enhanced heat transfer factor of cooling structure under different irradiance is maximized, it is necessary to adjust the velocity of working medium to match the irradiance of direct solar radiation.
【学位授予单位】：内蒙古工业大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TM914;TK124

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