Development of Practical TiO_2 Heat Transfer Surface by Advanced Coating Technique

二氧化钛是一种光催化剂，在紫外光的作用下对有害物质具有很强的氧化能力，一直是人们关注的焦点。在本研究项目中，我们试图利用火焰涂覆技术来开发一种实用的二氧化钛换热表面。实验结果表明：1)在池式沸腾中，亲水表面的换热系数和临界热流密度比正常表面大；2)在降膜蒸发过程中，表面可以形成较薄的稳定水膜，其换热系数最大时是正常表面的40倍；3)浸泡冷却时，具有亲水涂层的铁水冷却速度比正常表面快。4)在水滴蒸发实验中，超亲水表面的水滴寿命比正常表面的水滴寿命要短得多，尤其是在低温区域。超亲水表面的润湿极限温度升高，采用火焰涂层表面进行池沸腾实验。作为对比，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了1.5倍。在热流密度较高时，换热系数下降。b)在较高热流密度区，涂层厚度对换热特性影响较大。我们还利用火焰涂层、溅射涂层和镜面进行了降膜蒸发实验，发现：a)在较低温度区，火焰涂层表面是三种表面中最好的；b)在较高温度区域，涂层厚度和涂层结构影响换热特性。

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.