Heat transfer performance of supercritical carbon dioxide in microchannels

This paper provides an experimental investigation of heat transfer performance and pressure drop of supercritical carbon dioxide cooling in microchannel heat exchanger. An extruded flat aluminum tube with 37 parallel channels and each channel of 0.5 mm × 0.5 mm cross section was used as the test section. Super critical carbon dioxide at pressure of 7.5 MPa and inlet temperature varied from 55 to 25 °C was tested. The temperature drops of CO₂ cooled inside the test section was controlled at 2, 4 and 8 °C separately for each test to investigate the effect of properties change on the friction and heat transfer performance at various temperature cooling ranges near the critical point. The test results showed that while the test conditions were away from (approximately 5 °C higher or lower) the critical point, both heat transfer and pressure drop performance agreed very well with those predicted by convention correlations. However, while the test conditions near the critical point, the difference between the present test results and the prediction values is very high. From the experiment results of various temperature change range inside the test section, we can find that both heat transfer and pressure drop were strongly affected by the temperature cooling ranges near the critical point. Since there is a drastic peak of the properties change near the critical point, neither fluid properties at the average temperature nor the average properties at the inlet and exit temperatures may appropriately present the actual properties change in the test process. If we use the properties integrated but not averaged from inlet to the exit temperatures, we may obtain the results that agree well with the values predicted by conventional correlations. The heat transfer and pressure drop performance of super critical carbon dioxide are indeed similar to these at normal conditions if its properties were appropriately evaluated.