A study on improvement of wettability on heating surface and boiling heat transfer enhancement by thermal spray coating

利用潜热的两相流冷却系统需要从电力设备中获得更高的冷却热流密度。在这种情况下，需要加强沸腾换热，减少沸腾开始时的壁面过热度，并增加临界热流密度。在前人的池沸腾实验中，研究人员发现，在实验热流密度范围内，真空等离子喷涂层可以获得恒定的强化换热，并且可以降低沸腾开始时的壁面过热度。而热喷涂层的临界热流密度与光滑表面的临界热流密度基本相同。此外，飞机抛物线飞行过程中的微重力实验表明，热喷涂涂层对沸腾换热的强化效果与正常重力下几乎相同。可以说，由于涂层中复杂的微观子通道，表面具有很高的润湿性。在本研究中，Pool boi…对不同材料和结构的热喷涂涂层进行了LING和强迫对流沸腾换热的对比研究。在池沸腾实验中，水平管段被间隙为1~2 mm的圆柱体覆盖，以阐明难以分离的汽泡的影响，并考虑了盖子对沸腾换热的影响。结果表明，盖板能改善换热，对光滑表面的影响大于热喷涂层，并降低了两种表面的临界热流密度。在强迫对流沸腾实验中，采用了管宽20 mm、高度2 mm的铝和铜窄缝通道。对于铝材试验段，考察了入口过冷状态下的垂直向上流动，并评估了沸腾开始和稳定状态下的壁面过热度。此外，还用中子照相法测量了窄缝通道内的空泡率分布。结果表明，热喷涂表面降低了壁面沸腾过热度，提高了临界热流密度；水平沸腾流动实验表明，对于铜板上的铜涂层，涂层表面的换热系数提高了2~4倍。增强因子随热流密度的增大而增大。沸腾换热特性存在加热过程的滞后现象。加热速度越高，换热系数越高。较少

Two-phase flow cooling system utilizing latent heat is required to obtain higher cooling heat flux from electro or power equipments. In this case, enhancement of boiling heat transfer, reduction in wall superheat at the onset of boiling, and increase critical heat flux are desired. In the investigator's previous work on pool boiling experiments, it was shown that constant heat transfer enhancement could be obtained by vacuum plasma spray coating over the experimental heat flux range, and wall superheat at the onset of boiling could be reduced. However, critical heat flux on thermal spray coating was almost the same as that on smooth surface. Moreover, from a microgravity experiment during a parabolic trajectory flight of airplane, it was shown that thermal spray coating produced almost the same boiling heat transfer enhancement as that under normal gravity. It could be said that the surface has high wettability due to complex microscopic sub-channels in coatings.In this study, pool boi … More ling and forced convective boiling heat transfer on thermal spray coatings were evaluated for some coatings with different material and structure. In the pool boiling experiments, a horizontal tube test section was covered by a cylinder with a clearance of 1 to 2 mm in order to clarify the effect of vapor bubble that is hard to depart, and the effect of the cover on the boiling heat transfer was considered. As the result, it was shown that heat transfer was improved by the cover, the effect for smooth surface was larger than for thermal spray coating, and the critical heat flux was decreased by the cover for both surfaces.On the other hand, for the forced convective boiling experiment, aluminum and copper narrow channels with the conduit width of 20 mm and height of 2 mm were used. For the aluminum test section, vertical upward flows under inlet subcool condition were examined, and wall superheat at the onset of boiling and steady condition were evaluated. Moreover, void fraction distributions in the narrow channel were measured by neutron radiography method. As the results, the thermal spray surface made the wall superheat at the onset of boiling lower and the critical heat flux higher.For copper coating on copper plate, it was shown from horizontal boiling flow experiments that the coating surface produced 2 to 4 times higher heat transfer coefficient. The enhancement factor increased with increasing heat flux. Hysteresis in heating procedure existed in boiling heat transfer characteristics. Higher heating velocity lead to higher heat transfer coefficient. Less