Numerical investigation on heat transfer of supercritical carbon dioxide in the microtube heat exchanger at low reynolds numbers

Abstract

A numerical study on the heat transfer between supercritical carbon dioxide and water in the microtube heat exchanger at low mass fluxes of CO2 is reported in this paper. The inner diameter of the tube used in the heat exchanger was 1.6 mm. Four sections with different heat transfer performance are found during the heat exchange process between CO2 and water. The operating pressure, mass fluxes of CO2 and water, buoyancy effect are considered to analyze the heat transfer characteristics of sCO2. The heat transfer coefficient is found to be the maximum when the temperature is near pseudocritical point and the peak value increases when the pressure is getting closer to the critical point. The heat transfer coefficient increases with the increase of mass flux of CO2. The buoyancy effect is found to have a significant influence on the heat transfer at the different locations of the tube. The secondary flow of CO2 results in the enhancement of heat transfer on the top side of the tube and weakens the heat transfer on the bottom side. Different flow patterns of water in the shell side are compared in this study. The counter-current flow achieves the best heat transfer effect, while the downward flow is slightly better than the upward flow on the total heat transfer due to the different intensity of secondary flow. Existing different correlations should be used to get a better prediction of heat transfer depending on the state of CO2 in the microtube heat exchanger, especially when CO2 flows at a low mass flux.