纳米流体在太阳能中温集热过程的辐射吸收特性与传热机理研究

As the special radiation properties of nanoparticles and the excellent heat transfer performance of nanofluids, it is proposed that use a kind of heat transfer oil-based nanofluid as both the heat-absorbing and heat-carrying fluid for medium temperature solar heat collection. Nanoparticles dispersed in the liquid directly absorb the solar radiation and then transport the heat to the surrounding oil. It differs from conventional solar collecting process which rely on the tube with selective coating to absorb solar radiation and transport heat to the working fluid inside the tube. This project aims at investigating the oil-based nanofluids' radiation properties and heat transfer mechanism inside the absorbing tube. A variety of oil-based nanoparticle suspensions were prepared, the effects of nanoparticles' shape, size and volume fraction on the stability, as well as the absorptivity, effective thermal conductivity, viscosity of suspensions were theoretically analyzed and experimentally tested respectively. Consequently, one or more suitable nanofluids may be summed up for the solar collection. The mechanism of heat transfer between the nonoparticles and the surrounding base liquid was investigated, and its mathematical model be developed. Base on the model, heat transfer characteristics on different temperature and heat flux conditions were simulated and experimented, providing theoretical supports for the future development of this new type of medium temperature solar heat collecting method.

鉴于纳米颗粒特殊的光吸收性能和纳米流体优异的热传输性能，本项目提出一种以导热油为基液的纳米流体作为吸热和载热工质的太阳能中温集热方法。与常规集热方法依靠管壁吸收太阳辐射并传热到管内流体不同的是，太阳辐射直接被透明管内分散在流体中的纳米颗粒吸收，并很快将热能传递给周围的导热油。项目对该集热过程所涉及的纳米流体辐射吸收特性、管内传热机理等基础问题进行研究。实验制备导热油为基液的多种纳米颗粒悬浮液，理论分析并实验测试纳米颗粒的形状、尺寸和添加体积份数分别对悬浮液的稳定性以及吸收率、有效热导率、粘度等热物性的影响，分析归纳出一种或几种适宜该集热过程的纳米流体工质；探讨管内纳米颗粒吸收辐射后与周围导热油基液的热量传递机理，建立传热数学模型，模拟和实验研究不同温度和辐射热流密度工况下的传热特性,为这种直接吸收式新型太阳能中温集热系统的探索和开发提供理论支撑。

太阳能中温热利用在制冷空调、工业加热等领域的需求巨大。目前常规中温集热器因涂层高温稳定性和耐久性差，导致制造和维护成本高，成为其应用的主要障碍。本项目提出并探讨了一种采用透明无涂层集热管的纳米流体直接吸收式太阳能集热方法（NDASC），太阳辐射直接被透明管内分散在流体中的纳米颗粒吸收，并很快将热能传递给周围的导热油。对这种新型集热技术所涉及的纳米流体稳定性、辐射吸收特性，及其在太阳能中温集热过程中热量传递机理、集热特性与适用性等科学问题进行了研究。首先，制备了添加CuO、Fe3O4、Al2O3和石墨等导热油基液纳米流体，对其稳定性进行分析，获得了制备过程中用于改性金属氧化物纳米颗粒的最佳油酸量；其次，测试了不同纳米流体的全光谱辐射吸收特性，分析了颗粒种类和添加质量分数对纳米流体消光系数、吸光系数的影响规律，结果显示：添加纳米CuO、Co3O4、Fe3O4和石墨颗粒的导热油对太阳辐射的吸收强化效果显著，纳米颗粒的质量分数越大，吸光系数越大，综合考虑强化吸收和稳定性，纳米流体的质量分数在（0.05-0.1）%范围内较适宜。研究了纳米流体直接吸收辐射的管内热量传递机理，建立了NDASC和传统采用吸收涂层的间接吸收太阳能集热器（IASC）的集热数学模型，对两种集热器的管内温度场和集热特性进行了CFD数值模拟与对比。搭建了单根集热管以及槽式聚光型太阳能中温集热实验系统，开展了25-170℃不同工况下NDASC和IASC的集热性能对比实验。结果显示：相比传统IASC, NDASC管内纳米流体吸收辐射热的速度更快，管壁周向温差显著降低，且呈现出纳米流体温度高于管壁温度的特征。采用7.36倍槽式抛物面聚光的中温NDASC效率高于IASC的转换温度实验值为128℃，理论值为139℃。项目研究揭示了NDASC的集热特性规律，验证了其在太阳能中温集热领域的应用可行性，并且分析得到了适用温度范围，为这种新型纳米流体直接吸收式太阳能中温集热技术的应用提供了理论支撑。