Numerical investigation on forced, natural, and mixed convective heat transfer of n-decane in laminar flow at supercritical pressures

The present study numerically investigates the forced, natural, and mixed convective heat transfer characteristics of supercritical n-decane in a tube in laminar flow, focusing on the law of mutual coupling of forced and natural convection in mixed convection. Effects of heat flux (100 W/m2–8000 W/m2) and flow direction on heat transfer are investigated. For forced convection, deterioration occurs when the wall temperature reaches the pseudo-critical temperature and the local Nusselt number decreases by up to 17.4% compared to the theoretical value at the heat flux of 2400 W/m2. For natural convection, heat transfer is continuously enhanced with the increase of heat flux. For mixed convection in upward flow, heat transfer is enhanced by the buoyancy of increasing the fluid velocity gradient near the wall. As for the downward flow, buoyancy illustrates the opposite effects. Based on numerical simulation results, the Nusselt number correlations are proposed for forced and natural convection respectively and the correlation for mixed convection is modeled employing a function of the forced and natural convection.