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

热喷涂改善受热面润湿性及强化沸腾传热的研究

基本信息

批准号：
17560187
负责人：
ASANO Hitoshi
金额：
$ 2.18万
依托单位：
Kobe University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2005
资助国家：
日本
起止时间：
2005 至 2006
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-17560187/
关键词：
Nucleate boiling Heat transfer enhancement Thermal spray coating Pool boiling Forced convective boiling Wall superheat at onset of boiling Critical heat flux Neutron radiography 核沸騰熱伝達沸騰開始過熱度ボイド率

项目摘要

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

需要利用潜热的两相流冷却系统来从电力或电力设备获得更高的冷却热通量。在这种情况下，需要增强沸腾传热、减少沸腾开始时的壁过热度以及增加临界热通量。在研究者之前的水池沸腾实验中，结果表明，等离子喷涂可以在实验热通量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，但临界热通量会降低。热喷涂涂层几乎而且，飞机抛物线飞行过程中的微重力实验表明，热喷涂涂层产生了与正常重力下几乎相同的沸腾传热增强效果。由于涂层中存在复杂的微观子通道，因此具有较高的润湿性。在这项研究中，对一些不同材料和结构的涂层进行了池沸腾和强制对流沸腾传热评估。为了明确难以脱离的蒸汽泡的影响，用间隙为1～2mm的圆筒覆盖水平管试验段，并考虑覆盖物对沸腾传热的影响。结果表明，覆盖物改善了传热，对光滑表面的影响大于热喷涂涂层，并且覆盖物降低了两个表面的临界热通量。另一方面，对于强制对流沸腾实验，铝和铜窄通道与导管宽度对于铝试验段，采用厚度为20毫米、高度为2毫米的材料，检查入口过冷条件下的垂直向上流动，并评估沸腾开始和稳定条件下的壁面过热度、窄通道内的空隙率分布。采用中子射线照相法测量结果表明，热喷涂表面使壁面开始沸腾时的过热度较低，临界热流密度较高。产生的表面 2传热系数随加热过程的增加而增加，加热速度越高，传热系数越小。