【作者】 魏葳；

【导师】 倪明江； 骆仲泱； 余春江；

【作者基本信息】 浙江大学 ， 工程热物理， 2013， 博士

【摘要】 随着能源和环境问题的日益严重,传统化石能源的大量使用所带来的环境危害也得到了越来越多的关注。太阳能作为可再生清洁能源蕴藏着巨大能量,是最丰富的可再生资源,并且分布广泛,清洁无污染。因而对太阳能的开发利用,太阳能材料及相关技术的开发在全世界范围都引起了广泛的重视。太阳能热发电技术即以太阳能作为热源产生蒸汽驱动汽轮机的热发电技术。纳米流体是指将尺度在100nm以下的微小颗粒均匀稳定的分散在基液中形成的稳定的悬浊液。关于纳米流体强化传热特性研究者们已开展了大量的相关研究工作,而本文主要着眼于纳米流体的辐射特性及其在太阳能热利用中的应用,尤其是在聚光式太阳能集热器中的应用。本文首先采用两步法制备了多种油基纳米流体样品,通过机械搅拌、超声震荡,以及添加分散剂等方法改善流体中颗粒的分散性和流体的稳定性。采用马尔文激光粒度仪对流体中团聚体的等效粒径进行测量以表征颗粒的分散程度。在制备得到较为稳定的纳米流体样品基础上,对流体样品的辐射特性开展了实验和理论的研究工作。试验方面,本文采用分光光度计测试了不同材料、不同尺度、不同浓度纳米颗粒的添加,以及不同厚度液层等参数对流体样品表征透射率的影响。理论方面,本文通过比对独立散射近似(Isotropic scattering approximation),有效场近似(Effective field approximation),半晶体近似(Quasicrystaline Approximation), Fedorov and Viskanta’s基于米氏理论的优化模型(FV),以及Yin and Pilon’s基于近场近似和远场近似的优化模型(YP)等五个近似模型对计算掺杂介质的有效散射因数和有效吸收因数的适用性,在考虑颗粒对辐射的吸收和散射的同时,考虑基液对辐射的部分吸收所带来的影响,并为了适用于散射波之间存在干涉效应的浓度较高的掺杂介质辐射特性计算,本文优化并提出了QCA’模型用以计算纳米流体工质的辐射特性参数。依据直接吸收概念,本文设计并搭建了纳米流体直接吸收式太阳能集热器模型实验台。在计算得到纳米流体工质的辐射特性参数基础上,求解辐射传递方程,分析了纳米流体样品中的辐射传递。通过耦合介质内的热传导,以及边界上的热对流,本文建立了纳米流体直接吸收式太阳能集热器的能量平衡模型。并依据模型分析了采用不同循环工质、不同入口流速、不同入射辐射强度,以及不同几何尺寸等不同工况参数对集热器内温度分布、出口温度,及光热转换效率的影响。在模型分析的同时,通过实验验证了采用纳米流体工质对集热器光热转换效率的提升,采用纳米流体直接吸收式太阳能集热器获得了品位相对更高的中低温热源。更多还原

【Abstract】 Alternative energy is attracting a growing interest in solving the problems of global energy and environment, including nuclear, wind and solar energy. Solar energy is clean, abundant, and widely distributed, and solar power technologies have a great progress in recent years.Nanofluid is an engineered colloidal suspension of nanoparticles in a base fluid. Convective transport and effective thermophysical properties of nanofluids have been extensively studied in the last decade. As to the radiative properties, it is shown that nanofluids can strongly absorb or spectral selectively absorb solar radiations. Nanofluids are expected to apply in fields of automotive industry, disease treatment, cooling of electrical devices, solar energy and etc.The nanofluids were prepared though a two-step method in this thesis, e.g. TiO2, Al2O3, Ag, Cu, SiOa, graphite and carbon nanotubes, were added directly into Texaherm oil for preparing stable suspension colloids. In preparation, the mixtures were mechanically stirred, sonication, and oleic acid (OA), cetyltrimethyl ammonium bromide(CTAB) and hexamethyl disiloxane (HMDSO) were added as dispersing agents with SN ratio0.1-1. The equivalent diameters of the agglomerated clusters were detected by Malvern Zetasizer Nano ZS90to evaluate the stabilities of the nanofluids samples.The radiative properties of nanofluids are numerically and experimentally investigated. For nanofluids, scattering and absorbing of electromagnetic waves by nanoparticles, as well as light absorption by the matrix/fluid wherein the nanoparticles are suspended, should be considered. Five models for predicting apparent radiative properties of nanoparticulate media were compared and evaluated their applicability. Using spectral absorption and scattering coefficients predicted by different models, the apparent transmittance of a nanofluid layer was computed, including multiple reflecting interfaces bounding the layer, and compare the model predictions with experimental results in literatures and measured using SHIMADZU UV3150ultraviolet and visible spectrophotometer. Finally, a new method to calculate the spectral radiative properties of dense nanofluids were proposed for the radiative properties of nanofluids, which shows a quantitatively good agreement with the experimental results.Hunt introduced the small particle heat exchange receiver (SPHER) for solar-thermal electric power generation in1978, by dispersing small absorbing particles into a gaseous working fluid, and showed that this device offered an effective improvement. A nanofluid solar collector experimental model based on direct absorption collection (DAC) concepts was built to analyze influences of working fluids and operation conditions on system performances. By solving the radiative transfer equation (RTE) combined with conduction and convection heat transfer equations, system efficiencies and temperature distributions of the collector were modeled, whose results were well agreed with those of experiments. The study indicated that nanofluids were almost "black" on absorbing solar radiation even for low-content nanofluids, providing high outlet temperatures and system efficiencies.更多还原