非均匀热流下槽式集热管内有机工质相变过程中的流动换热特性研究

Direct vapor generation (DVG) in parabolic trough solar collectors, featured by vapor generated directly from organic working fluids in the absorber tube, is advantageous with a higher collector efficiency, a lower investment cost and a better performance of power cycle. However, the harm done by non-uniform heat flux distribution would be more serious in this kind of system because of the existence of gas-liquid two-phase flow. Few studies on this topic can be found currently. To decrease the non-uniformity of wall temperature distribution, gas-liquid two-phase flow and boiling heat transfer of organic working fluids in horizontal tube are studied in this project..The DVG technology will be studied in the following aspects: 1) By experiments and numerical simulation, flow pattern distribution and transition under different heat flux conditions (namely varied intensity, position and non-uniformity) will be investigated; 2) Boiling heat transfer under typical flow patterns will be studied and effects of heat flux conditions on boiling heat transfer coefficients will be clarified. Heat transfer correlation will be obtained as well; 3) The variation of pressure drop with heat flux conditions will be investigated and flow excursion of organic working fluids in the case of changed heat flux conditions will be explored; 4) Effects of properties of organic working fluids on phase change process will be studied and selection criterions of organic working fluids will be developed. The method to optimize flow pattern distribution in tube will be proposed to decrease the non-uniformity of wall temperature distribution. .These researches can provide a fundamental knowledge of the application of solar-driven DVG technology. Besides, a further understanding on mechanisms of heat transfer under non-uniform heat flux distribution, phase change process of organic working fluids and two-phase flow in horizontal tubes would be provided.

有机工质在集热管内直接相变成为气体的太阳能槽式直膨系统，在集热效率、设备投资以及动力循环性能等方面具有优势，但由于集热管内气液两相流的存在，管外非均匀热流带来的挑战更加严峻，目前相关研究较为有限。为减小管壁温度场的非均匀性，本项目对有机工质在非均匀热流条件下水平管内的两相流动和沸腾换热进行研究。研究内容如下：1）通过实验和数值模拟，研究不同热流条件（强度、位置和不均匀程度）下流型分布和转变的规律；2）研究典型流型下管内的沸腾换热特性，明确热流条件对沸腾换热系数的影响，获得相应的换热关联式；3）研究管路压降与热流条件的关系，探索热流条件变化时有机工质两相流的流量漂移规律；4）研究有机工质物性对流动换热过程的影响，提出有机工质的选用原则，优化流型分布以减小管壁温度场的非均匀程度。该研究可为有机工质槽式直膨系统的应用提供参考，丰富和发展非均匀热流换热、有机工质相变和水平管两相流等领域的研究。

有机工质在集热管内直接相变成为气体的太阳能槽式直膨系统，在集热效率、设备投资以及动力循环性能等方面具有优势，但由于集热管内气液两相流的存在，管外非均匀热流带来的挑战更加严峻。为减小管壁温度场的非均匀性，本项目对有机工质在非均匀热流条件下水平管内的两相流动和沸腾换热进行研究。.主要研究成果如下：（1）提出了一种能够量化物理量空间分布均匀程度的评价指标，该指标全面考虑了物理量的数值特征和空间分布特征，为非均匀分布的量化提供了技术方案；（2）通过实验手段，研究了有机工质在均匀热流边界和非均匀热流边界下的流型分布和转变规律，并对比了纯工质和混合工质流型分布的区别。结果表明：在非均匀边界条件下，环状流会在干度更低的条件下出现。（3）研究了非均匀热流边界条件下，纯工质和混合工质在非均匀热流边界下的沸腾换热规律。结果表明：在均匀热流边界下得出的换热关联式无法对非均匀热流边界下的沸腾换热系数进行准确的预测，因此本研究开发了适用于非均匀热流边界下的沸腾换热关联式。（4）建立了有机工质管内流动沸腾过程中的压降模型，提出了表征流量漂移发生可能性的量化参数，并研究了设计参数和工质物性参数对流量漂移发生可能性的影响。（4）研究了工质物性参数和流型对管壁温度场均匀性的影响，结果表明：采用混合工质并不能有效改善管壁温度场的非均匀程度。当管内为环状流时，管壁温度场的均匀性更高。.本研究为有机工质槽式直膨系统的应用提供了理论支撑，丰富和发展了非均匀热流换热、有机工质相变和水平管两相流等领域的研究。