Supercritical n-decane heat transfer in a vertical tube subjected to high-temperature crossflow air

The study presents a numerical investigation of high-temperature supercritical n-decane flowing upward in a tube subjected to non-uniform heating. The aim of the study is to evaluate the heat transfer mechanisms due to the non-uniform heating condition where it can be observed that the radial inner wall temperatures exhibit a typical pendulum-type distribution. In particular, between 140° < θ < 220° (near the cold side of the tube), the temperature at the inner wall is at the lowest and remains essentially constant, which indicates that the n-decane between the hot and cold sides in the tube exhibits poor mixing. Also, the higher the inlet temperature, the more likely heat transfer deterioration (HTD) will occur closer to the tube inlet. Moreover, detailed information on wall temperatures, velocity, and secondary flow of the supercritical n-decane are evaluated and discussed. The results show that Re is affected by the density and kinetic viscosity of n-decane near the pseudocritical temperature so that Re near the wall fluctuates and has an influence on the wall heat transfer in the HTD region. The Pr at the center of the fluid decreases along the axial direction and the fluid boundary layer becomes thinner, resulting in a more pronounced non-uniformity of Pr in the radial direction. Meanwhile, the stability of the turbulent kinetic energy (TKE) distribution and the high intensity of TKE favor the heat transfer of n-decane in the tube.