Pool Boiling Characteristics of Heat Transfer Surface With Micro Structures Created by Using MEMS Technology

Abstract

Pool nucleate boiling heat transfer experiments were performed for water by using well-controlled and -defined heat transfer surfaces. The silicon wafers of 0.200 mm thickness were used as the heat transfer surfaces. Artificial-cylindrical cavities, micro-straight-line grooves or micro-crossing-straight-line grooves (square pillars) were created on the silicon plate by utilizing the Micro-Electro Mechanical System (MEMS) technology. In the case of the straight-line grooves and the crossing-straight-line grooves, the grooves were wetted after the heat transfer surface experienced subcooling. Once the grooves were wetted, only small diameter cavities which were formed during the MEMS processing at the bottom of the grooves functioned at the inception of boiling. Thus, a large overshooting of the wall superheat at the inception of boiling was observed. In this point, the micro grooves and micro pillars are not advantageous to cooling a body that periodically generates heat such as MPUs and electro devices. In the fully developed nucleate boiling region, the general trend was similar to that of the usual heat transfer surface. In the case of the artificial-cylindrical cavities, nuclei were well preserved in cavities even after the heat transfer surface experienced subcooling. Thus, no overshooting of the wall superheat at the inception of boiling was observed. As the number of the artificial-cylindrical cavities was increased, the wall superheat shifted to a low wall superheat side. The boiling heat transfer coefficient of the heat transfer surface that had the artificial-cylindrical cavities of the 1 mm pitch was better than that of a usual copper heat transfer surface. The artificial-cylindrical cavities are advantageous to get reliable and better cooling efficiency.