Numerical Investigation on Convective Heat Transfer of Supercritical Carbon Dioxide in a Mini Tube Considering Entrance Effect

There are more and more researches on heat transfer characteristics and prediction of supercritical CO₂. The method of adding adiabatic section before and after heating section is usually adopted in these researches to ensure that the fluid entering the heating section is no longer affected by boundary layer, but the appropriate length range of adiabatic section and the influence of entrance effect are not discussed. However, some studies show that the entrance effect would affect the heat transfer in mini tubes. This paper uses the commercial CFD code FLUENT 19.0 to numerically study the heat transfer of supercritical CO₂ in a mini tube under different working conditions (such as Re_in, P_in, q_w and flow direction) and the lengths of the adiabatic section (l_as/d). The entrance effects on heat transfer is more pronounced when Re_in is within the transition state and wall heat flux is relatively high, the resulting heat transfer deterioration causes T_w,x and h_w,x to rise sharply. As the adiabatic section increases, the location at which the heat exchange deteriorates moves to the entrance of the heating section and eventually leaves. The buoyancy effect and flow acceleration effect caused by the sharp change of physical properties are analyzed, and the dimensionless velocity distribution at the inlet of the heating section in different adiabatic sections is compared. It is proved that the entrance effect has an influence on the convection heat transfer of supercritical CO₂ in mini tubes. The interaction reflected by wall shear stress between boundary layer development and drastic changes in physical properties is the cause of heat transfer deterioration.