Numerical study of non-uniform cooling heat transfer characteristics of supercritical CO2 in horizontal air-cooled finned tube

This study numerically investigated the heat transfer characteristics of the supercritical CO2 under the non-uniform cooling condition. The non-uniformity of heat flux distribution on a typical air-cooled exchanger is estimated at first and the heat transfer performances of supercritical CO2 under uniform and non-uniform cooling conditions are compared. The results demonstrated that the heat flux of the finned tube exhibited a significantly uneven distribution along the circumferential tube wall, and the unevenness increased with an increase in the wind speed. A clear distinction can be observed between uniform cooling and non-uniform cooling of supercritical CO2. Different from the non-uniformly heated CO2, when the bulk temperature is much higher than the pseudocritical point, the average heat transfer coefficient of non-uniformly cooled supercritical CO2 was 6.84% higher than that of the uniformly cooled supercritical CO2 and the situation is reversed when the bulk temperature was close to the pseudocritical point. The difference between non-uniform cooling and uniform cooling increased with an increase in the unevenness of the heat flux and fluid mass flux. The non-linear change of thermophysical properties and the buoyancy effect of CO2 near the pseudocritical point are the main reasons for the difference between uniform cooling and non-uniform cooling.