Development of Heat Transfer Surface using Superhydrophilic Photocatalyst

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The purpose of this research project is to develop an enhanced heat transfer surface making use of photo-induced superhydrophilicity of titanium dioxide. This heat transfer surface is expected to have excellent characteristics in liquid-vapor phase change heat transfer process because of extremely high wettability.We have examined the heat transfer characteristics of TiO_2-coated surfaces by the following experiments;(1) falling film evaporation around the horizontal cylinder(2) pool boiling: effect of wettability on critical heat flux (CHF) and minimum heat flux (MHF)(3) evaporation of water drop on hot surface irradiated by plasma(4) change in contact angle of plasma-re formed aluminum surfaceHigh wettability of surface enhances heat transfer in above all experiments. In experiment of water drop evaporation, the lifetime of water drop on hot surface decreases as the contact angle of the surface decreases. This indicates that the heat transfer can be enhanced significantly in spray cooling process. In addition, the wetting limit temperature and the Leidenfrost temperature increase by about 20K compared with the normal surface.The plasma-reformed surfaces have a surface structure of fractal configuration. It initially gives the nature of spreading wetting, but the contact angle gradually increases with time and finally reaches more than 150°, a super-water-repellent state. When the surface is irradiated by plasma at a certain contact angle, the surface returns to spreading wetting state. This phenomenon is unusual and the mechanism has not yet been clear.