Thermal Characterization of Microscale Heat Convection under Rare Gas Condition by a Modified Hot Wire Method

Abstract

As power electronics shrinks down to sub-micron scale, the thermal transport from a solid surface to environment becomes significant. Under circumstances when the device works in rare gas environment, the scale for thermal transport is comparable to the mean free path of molecules, and is difficult to characterize. In this work, we present an experimental study about thermal transport around a microwire in rare gas environment by using a steady state hot wire method. Unlike conventional hot wire technique of using transient heat transfer process, this method considers both the heat conduction along the wire and convection effect from wire surface to surroundings. Convection heat transfer coefficient from a platinum wire in diameter 25 um to air is characterized under different heating power and air pressures to comprehend the effect of temperature and density of gas molecules. It is observed that convection heat transfer coefficient varies from 14 Wm-2K-1 at 7 Pa to 629 Wm-2K-1 at atmosphere pressure. In free molecule regime, Nusselt number has a linear relationship with inverse Knudsen number and the slope of 0.274 is employed to determined equivalent thermal dissipation boundary as 7.03E10-4 m. In transition regime, the equivalent thermal dissipation boundary is obtained as 5.02E10-4 m. Under a constant pressure, convection heat transfer coefficient decreases with increasing temperature, and this correlation is more sensitive to larger pressure. This work provides a pathway for studying both heat conduction and heat convection effect at micro/nanoscale under rare gas environment, the knowledge of which is essential for regulating heat dissipation in various industrial applications.