真空条件下太阳能蒸馏小空间内的非均匀传热传质特性研究

Diffusion solar still is composed of multiple small-scale cavities. Due to the unsteady solar thermal power, processes of the distillation, diffusion and condensation of the vapor is not uniform inside the cavities. Understanding the characteristics of the non-uniform mass and heat transfer process inside the small-scale distillation cavity is the key to improve the stability of diffusion solar still. To overcome the failure problem and deterioration in heat transfer of the conventional diffusion solar still, the wick material will be replaced by water troughs in this project, and the operation performance of the cavity with the new structure will be studied under vacuum condition. In the experiments for a single cavity, Micro-PIV and high speed camera will be employed to study the evaporative process of the saturated water. The data of the pressure and temperature fluctuation arising from boiling process will be obtained for further study. CFD method will be used to obtain the flow field as well as the unsteady evaporation and non-uniform condensation processes of the vapor inside the single cavity. Both the experimental work and CFD research will significantly help understand the transient relationship between evaporation and vapor diffusion in small-scale cavity under vacuum condition. Based on the experimental results of a single cavity, performance of the equipment composed of multiple cavities will be tested under environment condition. Analysis will be also conducted on the response between the non-uniform temperature and evaporation rate on the wall of the cavity. The theoretical analysis model for the mass and heat transfer process will be obtained for further application of this technology.

扩散式太阳能海水淡化装置由多个结构紧凑的蒸馏小空间构成，在不稳定加热条件下，小空间内蒸汽的产生、扩散及凝结过程呈现动态非均匀特点。掌握小空间蒸馏的非均匀传热传质特性是提高扩散式海水淡化装置运行稳定性的关键。本项目采用多个壁面储水槽结构替代以往装置内部的多孔介质材料，以克服其在长期运行过程中易失效及传热恶化等问题，并对这种形式蒸馏小空间在真空条件下的运行特性进行研究。应用Micro-PIV示踪粒子法与高速摄像技术捕捉在压力饱和点附近单个空腔海水的蒸发过程，分析液面温度不均匀导致的局部沸腾引起的空腔压力和温度波动，结合CFD模拟计算得到单个空腔在不稳定蒸发和不均匀凝结条件下的蒸汽流场，研究单个小空间内部蒸汽生成与扩散过程的响应机制。在单个空腔研究基础上，进一步探讨蒸馏壁面温度不均匀分布和蒸发率之间的动态响应关系，建立描述空腔传热传质特性的理论计算模型，为该技术的推广应用提供科学依据和技术支持。

扩散式太阳能海水淡化装置由多个结构紧凑的蒸馏小空间构成，在不稳定加热条件下，小空间内蒸汽的产生、扩散及凝结过程呈现动态非均匀特点。掌握真空条件下小空间蒸馏的非均匀传热传质特性是提高扩散式海水淡化装置运行稳定性的关键。本项目执行过程中，通过采用储水小空间替代多孔介质材料，有效克服了以往太阳能蒸馏器在长期运行中失效和传热恶化等问题。通过理论分析、数值模拟和实验测试手段对该类蒸馏空腔处于真空条件下的运行特性进行了研究。（1）理论分析结果表明，当蒸馏空腔操作压力<60kPa或运行温度>60℃时，空腔内部传质过程取代温差，成为驱动该类空腔内部自然对流形成的主要原因；基于构建的理论计算模型分析，表明负压操作可使空腔能量效率从0.6提升到0.9左右。（2）数值模拟发现，随着空腔温度升高及操作压力降低，湿空气平均流速加快，40kPa下湿空气的平均流速将提高至常压时的1.5倍左右，水温70°C时可达0.064m/s。（3）稳态实验测试结果表明，其凝结壁面温度非均匀分布特性随着空腔压力降低而减弱，当压力从常压降低为20kPa时，在弧面间隔60mm（约45o夹角）测点的温差降低50%；内部运行温差随压力降低而减小，在加热水温为60℃的实验中，在30 kPa条件下运行时温差从常压下的8.3℃降低为4.9℃，该数据表明负压操作有利于装置布置更多蒸馏级数。（4）户外实地测试结果表明，蒸馏装置产水性能随操作压力降低呈现先升后降的趋势，本次实验中三级蒸馏装置分别对应操作压力为101、60、40、20kPa，其当日累计产水为3.27、6.323、7.056、4.287kg，性能系数为0.77、1.28、1.28、0.88，最佳操作压力范围为40-60kPa。整理相关数据后，已联系四川大学校团委和四川甘洛县尝试开展扶贫攻坚和科技转化事宜，期望利用该技术为当地居民改善饮水质量，将科研成果回馈社会。