Critical Heat Flux of CO2 in a Microchannel at Elevated Subcritical Pressures

Abstract

The connection between chlorofluorocarbons and ozone depletion has prompted a resurgence in the study of environmentally friendly refrigerants for heat exchanger applications. Recent studies have shown compact heat exchangers using a transcritical CO2 cycle optimized in a microchannel geometry are competitive with baseline units in terms of physical dimension and thermal performance. However, the critical heat flux may occur at lower-than-expected vapor qualities in the heat absorption portion of the cycle, significantly affecting heat exchanger performance. The purpose of this study is to determine the critical heat flux and associated vapor quality under conditions simulating compact heat exchanger conditions. Non-intrusive wall temperature measurements, in conjunction with standard transducer measurements and flow visualization, were used to detect critical heat flux phenomenon. The vapor qualities associated with the critical heat flux were determined to be lower than would be expected in larger-diameter channels, and the critical heat flux was lower than predicted by the Shah correlation.Air Conditioning and Refrigeration Project 10