Enhancement of heat transfer with liquid-vapor phase change by photo-induced hydrophilicity
Enhancement of heat transfer with liquid-vapor phase change by photo-induced hydrophilicity
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发表时间：
1999-07
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通讯作者：
Y. Takata;Katsunori Tanaka;K. Kaijima;Takehiro Ito;T. Watanabe;M. Shimohigoshi
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作者：
Y. Takata;Katsunori Tanaka;K. Kaijima;Takehiro Ito;T. Watanabe;M. Shimohigoshi
TiO{sub 2}是具有非常独特特性的光催化剂之一。用TiO{sub 2}涂覆的表面通过将表面暴露于紫外线而显示出对水的极高亲和力，并且接触角减小到几乎为零。这种超亲水性赋予了涂层表面自清洁的效果，并已被应用于一些建筑材料，汽车涂料等。作者将这种超亲水性应用于从降膜蒸发传热的增强。通过使用TiO{sub 2}涂覆的表面，即使在极低的流速下也可以实现非常薄的稳定的液膜流动。在传热实验之前，作者测量了放置在玻璃和铜板上的水滴的接触角。在紫外光照射下，接触角随时间逐渐减小，最大可达0{approximately}2{deg}。他们得到了温度为27- 70 ℃时膜厚和水流速之间的关系，在最小流速时，相应的膜雷诺数为7.6。测得的膜厚略低于Nusselt理论计算的膜厚。作者测量了非涂层和涂层表面的传热系数，并进行了比较。结果表明，涂层表面的努塞尔数大于未涂层表面的努塞尔数。努塞尔数随膜雷诺数和热流密度的减小而增大。在最低Re{sub f}时，涂层表面的Nu{sub m}是未涂层表面的40倍。这种超亲水表面是一种新的强化技术，将适用于其他与液膜共存的传热系统。«少
The TiO{sub 2} is one of the photocatalysts that has very unique characteristics. The surface coated with TiO{sub 2} exhibits extremely high affinity for water by exposing the surface to ultraviolet rays and the contact angle decreases nearly to zero. This super-hydrophilic nature gives the self-cleaning effect of the coated surface and has already been applied to some construction materials, car coatings and so on. The authors applied this super-hydrophilic nature to enhancement of evaporation heat transfer from a falling film. By using TiO{sub 2}-coated surface very thin stable liquid film flow can be realized even in extremely low flow rate. Prior to heat transfer experiments, the authors measured contact angle of water droplet placed on glass and copper plate. The contact angle decreased gradually with time under UV illumination and reached to 0{approximately}2{degree}. They obtained the relation between film thickness and water flow rate for temperatures of 27--70 C and in the minimum flow rate the corresponding film Reynolds number was 7.6. The film thickness measured is a little bit lower than those calculated by Nusselt's theory. The authors measured heat transfer coefficients both for non-coated and coated surfaces and compared with each other. The results show that themore » Nusselt number of coated surface is larger than that of non-coated surface. The Nusselt number increases as the film Reynolds number and heat flux decrease. In the lowest Re{sub f}, Nu{sub m} for coated surface is 40 times as large as that for non-coated one. This super-hydrophilic surface is a new enhancement technique and will be applicable to other heat transfer systems accompanied with liquid film.« less