Development of Practical TiO_2 Heat Transfer Surface by Advanced Coating Technique

二氧化钛作为光催化剂之一，在紫外光的作用下对有害物质具有很强的氧化能力，一直受到人们的关注。在这个研究项目中，我们试图利用火焰涂层技术开发一种实用的TiO2传热表面。进行了四种实验，结果总结如下：1)在池沸腾过程中，亲水表面的传热系数和临界热流密度大于法向表面。2)降膜蒸发时，表面可形成薄而稳定的水膜，最大情况下合成换热系数比正常表面提高了40倍。3)在浸没式冷却中，具有亲水性涂层的热金属比普通表面冷却得更快。冷却所需的时间大大缩短。4)在水滴蒸发实验中，水滴在超亲水性表面上的寿命明显短于常规表面，特别是在低温区域。超亲水表面的润湿极限温度升高。采用火焰涂层表面进行了池沸腾实验。为了比较，我们还对溅射表面和镜面进行了实验，得到了以下结果：a)火焰涂层的临界热流密度比正常表面提高1.5倍。而在高热流密度下，换热系数减小。b)在高热流密度区域，涂层厚度对传热特性影响较大。我们还对火焰镀膜、溅射和镜面进行了落膜蒸发实验，得到了以下结果：a)在较低温区，三种表面中火焰涂层表面效果最好。b)在较高温度区域，涂层厚度及其结构影响传热特性。

Titanium dioxide, one of photocatalysts, has been in spotlight since it has a strong oxidation power to harmful substances with the aid of UV light. In this research project, we tried to develop a practical TiO2 heat transfer surface by making use of flame coating technique. The four kinds of experiments have been performed and the results are summarized as follows.1) In pool boiling, heat transfer coefficient and critical heat flux for hydrophilic surface become larger than those for normal surface.2) In falling film evaporation, thin stable water film can be realized on the surface and resultant heat transfer coefficient increased by 40 times as the normal surface at its maximum case.3) In immersion cooling, hot metal with hydrophilic coating is cooled down more rapidly than normal surface. The time required to cool down is tremendously shorten.4) In water drop evaporation experiment, life time of water drop on superhydrophilic surface becomes much shorter than that on normal surface especially in lower temperature region. The wetting limit temperature increased for superhydrophilic surface.Pool boiling experiment using flame coating surface was conducted. For comparison, experiments using sputtered and mirror surfaces were also tested and the following results were obtained.a) Critical heat flux for flame coating increased by 1.5 times compared with the normal surface. However, heat transfer coefficient at higher heat flux decreased.b) In higher heat flux region, heat transfer characteristic is much influenced by the thickness of coating layer.We also conducted experiment of falling film evaporation using flame coating, sputtering and mirror surfaces and found the followings.a) In lower temperature region, flame coating surface is the best among three surfaces.b) In higher temperature region, thickness of coating layer and its structure influence the heat transfer characteristics.