High Temperature Microscale Reactor Analysis Using a Counterflow Heat Exchanger Model

Abstract

A microscale reactor model, with axial conduction and radiation heat loss, has been developed for predicting the thermal performance of high temperature systems. The model considers: 1.) flow loss due to non-unity effectiveness, 2.) thermal conduction along the axial direction, and 3.) radiation surface loss to the environment. A system of three coupled differential equations were developed where two of the equations modeled the temperature variation in the fluid streams and the third equation gave the temperature of the wall. The wall equation contained a highly non-linear term linked to radiation surface loss. This study is unique in several ways. First, the boundary conditions for the problem modeled a micro reactor attached to a substrate at ambient temperature while the hot end was free to assume a wall temperature half way between the two fluid temperatures. Next, surface radiation was treated explicitly as a heat loss term. At elevated temperatures, the overall thermal performance of the micro reactor was significantly impacted by this loss mechanism. Finally, an implicit method is described capable of solving the non-linear coupled differential equations. The results of the study are presented in the form of normalized total heat loss curves for each of the three loss mechanisms. A scaling study is presented showing what contributions to heat loss are important as the characteristic length scale of the device is reduced. This study demonstrates that both conduction and surface radiation losses are significant in high temperature micro reactors. Furthermore, the heat loss (in normalized form) by radiation is significant for larger scale devices but the ultimate size limits for a micro reactor will be governed by conduction losses through the structure. For high temperature micro reactor technology to be practical, this study demonstrates that devices must be designed with low thermal conductivity materials, high aspect ratio geometries, and low effective surface emissivities.