Air Conditioning and Refrigeration Center. College of Engineering. University of Illinois at Urbana-Champaign.
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