Consideration of Heat Transfer Enhancement Mechanism of Nano- and Micro-Scale Porous Layer via Flow Visualization

Abstract

A convective heat transfer enhancement using nano- and micro-scale porous layer surface was discovered by the authors in 2004. Heat transfer experiments, analytical considerations, and flow visualization near the porous layer were performed to grasp the heat transfer enhancement mechanism. The heat transfer experiments revealed the porous layers were able to enhance heat transfer by 20–25% in net energy compared to the bare plate, independent of substrate materials. In order to understand the mechanism, one-dimensional unsteady heat conduction analysis was performed for a liquid column in the pore. It was found that the temperature recovery of the porous layer was incapable of catching up with the very fast 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 μm in diameter showed a fluid behavior like “squirt” from the porous layer. From the observation of the porous layer surface, the porous layer has some micro-scale bubbles inside its own pore-connecting structure in spite of the good wetting feature. These bubbles could be a main contributor to this heat transfer enhancement. To discuss this postulation, observations of bubble behavior in a microchannel have been carried out.

Acknowledgments

This work was partly supported by the “Energy Science in the Age of Global Warming” of Global Center of Excellence (G-COE) program (J-051) of the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

Haomin Sun is a graduate student in the Department of Nuclear Engineering, Kyoto University, Japan, under the supervision of Prof. Kunugi. He received his B.E. degree from Kyoto University in 2009 and studied heat transfer enhancement mechanism of nano- and micro-scale porous layers. He is currently working on condensation of water-vapor two-phase flow behavior through a rectangular tube.

Zensaku Kawara received the B.E. (1982), M.E. (1984), and D.E. (1991) degrees in nuclear engineering from Kyoto University. He is a lecturer in the Department of Nuclear Engineering, Kyoto University. His current interests include thermal hydraulics in energy systems, multiphase flow, natural convection, and nuclear energy systems.

Yoshitaka Ueki is a doctoral student in the Department of Nuclear Engineering, Kyoto University, Japan, under the supervision of Prof. Kunugi. He received his B.E. and M.E. degrees from Kyoto University in 2007 and 2009, respectively. He was previously in charge of the study on heat transfer enhancement mechanism of nano- and micro-scale porous layer for his bachelor's degree. He is currently working on liquid metal magnetohydrodynamic thermofluid under fusion reactor-relevant conditions for his Ph.D. degree.

Tetsuo Naritomi is a graduate student in the Department of Nuclear Engineering, Kyoto University, Japan, under the supervision of Prof. Kunugi. He received his B.E. degrees from Kyoto University in 2008 and studied heat transfer enhancement mechanism of nano- and micro-scale porous layer. He is currently working on condensation enhancement on nano- and micro-scale porous layesr.

Tomoaki Kunugi is a professor in the Department of Nuclear Engineering, Kyoto University. He received his B.E. and M.E. degrees from Keio University in Japan, and his Ph.D. degree from the University of Tokyo. He worked at Japan Atomic Energy Research Institute from 1979 to 1998 and spent 2 years as a visiting researcher at the University of California, Los Angeles (1989–1991). He became a professor of Tokai University in 1998, and has been teaching at Kyoto University from 1999 to the present. His interests are very wide such as thermal science and engineering including nano- and micro-scale problems, computational fluid dynamics, multiphase flow physics, including boiling phenomena, reactor engineering for various nuclear reactors (HTGR, BWR, PWR, and FBR) and fusion reactor-relevant technologies (first wall, diverter, blanket, and safety).