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获得较高的冷却热流。在这种情况下，需要加强沸腾传热，减少沸腾开始时的壁面过热度，并增加临界热流密度。在研究者之前的池沸腾实验中，通过真空等离子体喷涂可以在实验热流密度范围内获得恒定的传热强化，并且可以降低沸腾开始时的壁面过热度。而热喷涂涂层的临界热流密度与光滑表面基本相同。此外，在飞机抛物线轨迹飞行的微重力实验中，热喷涂涂层产生的沸腾换热强化效果与正常重力下几乎相同。可以说，由于涂层中存在复杂的微观子通道，使得涂层表面具有较高的润湿性。本文对不同材料和结构的热喷涂涂层进行了池沸换热和强制对流沸腾换热的研究。在池沸实验中，为了阐明难以分离的汽泡的影响，在水平管试验段上盖上间隙为1 ~ 2mm的圆筒，并考虑盖对沸腾传热的影响。结果表明，覆盖层改善了热传导，对光滑表面的效果大于热喷涂涂层，两种表面的临界热流密度均被覆盖层降低。另一方面，强制对流沸腾实验采用铝和铜窄通道，管道宽度为20 mm，高度为2 mm。对铝材试验段进行了入口过冷状态下的垂直向上流动试验，并对沸腾开始时的壁面过热度和稳定状态进行了评估。此外，用中子射线照相法测量了窄通道内空隙率的分布。结果表明，热喷雾表面降低了沸腾开始时的壁面过热度，提高了临界热流密度。在铜板上镀铜，水平沸腾流实验表明，镀铜表面的传热系数提高了2 ~ 4倍。增强因子随热通量的增大而增大。沸腾传热特性中存在加热过程的滞后现象。加热速度越快，传热系数越高。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