纳米流体非均匀不稳定集热对流形态及传热特性研究

Medium-temperature solar heat utilization, such as solar thermal power generation, solar thermal refrigeration and air conditioning, as well as solar energy based industrial process heating, has great potential for application. A previously proposed solar collector employs uncoated collector tube in which nanofluids with strong absorption properties acts as working fluid. As it could improve the absorption and thermo-physical properties, and exhibit a relatively higher main fluid temperature of the nanofluids than the tube wall temperature under certain condition. Therefore, a novel nanofluids based direct absorption solar collection/steam generation system is proposed in this project. The key characteristics of the flow and heat transfer involved in such novel system, such as the transition of convection pattern, the convective heat transfer coupled with solar absorption, will be investigated. Based on the analysis of physical properties of the prepared nanofluids, including the absorption coefficient, the thermal conductivity as well as the viscosity, the effect on the flow of nanoparticles by the thermophoretic and photophoretic velocity and forces incorporated with the Brownian motion will be studied theoretically. The law of the flow pattern transition caused by non-uniform heat absorption and unsteady flow in the nanofluids will be discussed. The model of the convective heat transfer with “non-uniform internal heat resource” will be developed, in the modeling, the factors of the particle flow and two phase condition will be considered. Then, under different temperature and radiation intensity conditions, the dynamic performance of the solar collection –heat transfer in the nanofluids with various mass fractions will be tested by theoretical simulation and experiment. The findings and results in this project will provide theoretical supports for the developments of the proposed novel mid-temperature solar collector/steam generator.

太阳能中温热利用节能潜力巨大。采用纳米流体在无涂层透明管内直接吸收太阳辐射进行中温集热，具有优异的光吸收和传热性能，避免了涂层耐久性差和传统集热管周向温差大导致可靠性降低等缺陷。鉴于纳米流体吸收辐射热具有 “内热源”效应，并呈现出集热主流温度高于管壁的特征，本项目提出并探讨一种纳米流体直接吸收式太阳能集热/蒸汽发生系统，对非均匀不稳定聚光集热-传热过程所涉及的纳米流体对流形态、光吸收与对流传热耦合特性等问题开展研究。在纳米流体制备和对吸光系数、热导率和粘度等物性分析基础上，探讨光泳、热泳和布朗运动引起的纳米颗粒迁移特性，以及纳米流体因吸热不均匀与流动不稳定导致的对流形态变化规律；建立涉及纳米颗粒迁移影响和两相特征的“非均匀内热源”集热-对流传热模型；通过理论和实验研究，获得不同温度与辐射强度工况下，不同纳米流体的集热-传热动态特性，为这种新型太阳能中温集热/蒸发发生装置研发奠定理论基础。

太阳能中温蒸汽应用于驱动制冷空调、有机朗肯循环热发电、工业加热等领域的潜力巨大。项目提出了一种纳米流体集热的新型太阳能蒸汽发生装置（NSVG），在内设蒸发套管的无涂层集热管内，纳米流体径向逐层吸收透过的太阳辐射能，形成环形空间的非均匀体积内热源，通过自然对流换热将热量传递给套管内蒸发工质。围绕聚光工况下纳米流体因吸热不均匀与流动不稳定导致对流形态变化及其耦合传热特性，完成了性能分析与优化，主要研究内容和结果如下：1）制备并测试了不同浓度石墨烯/导热油纳米流体的热物性和吸光系数，分析了光谱吸收与散射特性及颗粒泳输运特性；2）建立了聚光辐射能流分布边界条件下纳米流体非均匀集热-对流传热数学模型，揭示了纳米流体中颗粒光吸收所呈现的“内热源”特性下的集热机理与对流形态转捩规律，分析了纳米流体浓度、太阳辐射强度、环形空间结构尺寸等因素对流动形态和对流强度的影响规律；3）研究了纳米流体速度场与温度场的耦合特性及其对集热传热性能的影响，NSVG呈现出高流速和高温区均位于环形空间流体中心的对流形态与温度分布特征，强化了自然对流以及向蒸发介质的传热，同时可快速将聚光区收集的热量传递到非聚光区，提高了温度分布的均匀性，有利于提升集热效率和装置可靠性；4）搭建了纳米流体集热的太阳能蒸汽发生实验系统，模拟和实验相结合，得到了80~135℃工况下的NSVG集热和蒸汽发生性能，给出了纳米流体浓度、结构尺寸的优化方案。环形空间纳米流体层厚度为12.5mm和10.5mm时的最佳颗粒浓度分别约为0.06mg/ml和0.1mg/ml。研究结果为这种新型太阳能集热蒸汽发生系统的优化设计与应用提供理论支撑。

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