Numerical Investigation of Buoyancy Effect on Mixed Convection Heat Transfer Deterioration of Supercritical Pressure Carbon Dioxide

For supercritical pressure fluid upward pipe flow, turbulent mixed convection heat transfer deterioration, which is generally considered to be caused by buoyancy, is often put a deep concern for safety issues. The deterioration is typically characterized by a localized wall temperature peak. Sometimes, there will be another moderate temperature peak after the first one. However, due to the lack of reliable measure method, the understanding of the flow structure for these two localized temperature peaks were still limited. In order to investigate the detailed mechanism for these two peaks and further understand the effect of buoyancy, a numerical study of supercritical pressure carbon dioxide pipe flow mixed convection heat transfer deterioration was conducted in this paper. The SST k-omega model was selected as turbulence model. A variable turbulent Prandtl number model was adopted in the study to improve simulation accuracy. The variation of flow field and turbulence behavior were carefully analyzed. The results show that, the localized wall temperature rise is due to the suppressed turbulence in the near wall region. For the first localized temperature peak, the suppressed turbulence is due to the acceleration of near wall fluid. While for the second one, the restrained turbulence is due to the acceleration of core flow fluid.