Analysis of heat transfer of supercritical water by direct numerical simulation of heated upward pipe flows

Abstract

Direct numerical simulation (DNS) was performed to investigate the turbulent heat transfer of supercritical water in an upward pipe with and without buoyancy. The heat transfer deterioration mechanisms were comprehensively studied by analyzing the effects of large thermophysical property variations, flow acceleration and buoyancy. It is shown that the large property variations near the wall severely impair the heat transfer in supercritical water especially for forced convection. The flow acceleration induced by thermal expansion shows an obvious suppression on turbulence but does not affect the heat transfer significantly due to the quick developing of thermal boundary layer. A small buoyancy first impairs the heat transfer and then improves the heat transfer due to the turbulence attenuation and recovery. With buoyancy increasing, the turbulence recovery gets earlier and stronger, and the wall temperature is significantly reduced due to the large buoyancy production of turbulent kinetic energy (TKE) in supercritical water flows. The instantaneous vortical structures based on Q criterion also clearly show the process of turbulence attenuation and recovery. In addition, the simulation using two eddy-viscosity turbulent models is compared to DNS data. The results show that the failed prediction of wall temperature and Nusselt number by those turbulent models is possibly due to the bad results of TKE, production of TKE and turbulent Prandtl number (Prt).