Consideration of heat transfer enhancement mechanism using nano- and micro-scale porous layer

A convective heat transfer enhancement using nano- and micro-scale porous layer surface was discovered by Kunugi et al. The heat transfer experiments, analytical considerations, flow visualization near the porous layer, and the porous layer surface observation were performed to grasp the heat transfer characteristics and the heat transfer enhancement mechanism. The heat transfer experiments revealed the porous layers were capable to enhance heat transfer by 20-25% in net energy compared to the bare plate, independently of substrate materials. The heat transfer experiment changing the Reynolds number showed the Reynolds number dependency of heat transfer performance. One-dimensional unsteady heat conduction analysis showed the temperature recovery of the porous layer was incapable to catch up with the very fast temperature fluctuation, so that the porous layer might be a thermal-resistance when the main flow was strongly turbulent. The vestige visualized by the tracer-particles of around 0.85 mum in diameter showed a fluid behavior like "squirt" from the porous layer. From observation of the porous-layer surface, the porous layer has some micron-scale bubbles inside its own pore-connecting structure in spite of the good wetting feature. The expansion and contraction of the bubble-foam in the layer was observed and these behaviors may be considered as the main contribution to the mechanism of the heat transport.