Heat transfer augmentation with nano-and micro-scale porous layer structure formed on the surface

本研究旨在探讨平板表面形成纳/微孔层结构的强化换热机理。我们证实，使用基本设备，通过这种多孔表面的净输入能量比没有任何多孔层的裸板增加了20%-30%。考察了板厚对输入能量的影响，得到了相同的结果：板厚对换热强化没有影响。这意味着纳米/微孔层的热阻占主导地位。然而，由于纳米/微孔多孔层的厚度约为100微米，一般可以忽略它作为热阻。此外，由于这一厚度与层流边界层厚度相比也可以很小，因此它也可以忽略为流动搅拌器(即湍流促进器)。另一方面，从扫描电子显微镜的图像来看，这种多孔层由许多颗粒组成，并且相互接触。这意味着这种多孔层可以被认为是绝热介质。此外，多孔层与金属板(即换热面)之间的腐蚀层结合非常紧密，该层的厚度最小为5~10 nm。这个厚度太薄了，不能作为绝热层。因此，热流可以很容易地通过蚀刻层的这一薄部分。最终，热流可以在那个薄的部分增加。多孔层内的水可以通过这种增加的热流来加热，温度立即上升，最终达到壁温。这意味着这种充满水的纳/微多孔层结构可能起到类流体壁面的作用。这可能是表面形成的纳米/微孔层结构强化传热的一种可能机制。

This study is to investigate the mechanism of high heat transfer augmentation with nano/micro porous layer structure formed on the plate surface. We confirmed that the net amount of input energy through this porous surface was 20-30% increase compared to the bare plate without any porous layer by using the fundamental equipment. We examined the plate thickness effect on the input energy and then obtained the results showed all the same: no effect of the plate thickness on the heat transfer augmentation. This means that the thermal resistance is predominant at the nano/micro-porous layer. However, since the thickness of the nano/micro-porous layer is about 100 micron meter, in general it can neglect as a thermal resistance. Moreover, since this thickness can also be so small compared to the laminar boundary layer thickness, it can also neglect as a flow agitator (i.e., turbulence promoter). On the other hand, from the SEM image, this porous layer consisted of many particles and contacted with each other. This means that this porous layer can be considered as an adiabatic medium. Moreover, the etching layer between porous layer and the metal plate (i.e., heat transfer surface) shows very tight binding and this layer thickness is around minimum 5~10 nm. This thickness is too thin to act as an adiabatic layer. Therefore, the heat flow can easily pass through this thin part of the etching layer. Eventually, the heat flow can increase at that thin part. The water inside the porous layer could heat up by this increased heat flow and the temperature immediately increases and finally reaches to the wall temperature. This means that this nano/micro porous layer structure filling with water might act as a fluid-like wall. This might be a possible mechanism of this heat transfer augmentation by the nano/micro porous layer structure formed on the surface.