Modification of RANS turbulence model for the prediction of mixed convection heat transfer inside a rectangular riser

To assess the heat removal performance of the reactor cavity cooling system, the passive safety system of a very-high temperature gas-cooled reactor, it is important to predict mixed convection heat transfer phenomena in rectangular risers accurately. In this paper, quadrant analysis is performed with flow visualization experiment data to investigate the heating effect on the near-wall turbulence characteristics. The analysis suggests that a vortex motion independent of the wall-bounded vortices, which we term ‘heating-induced vortices’, is generated near the heated wall. This insight is used to supplement the explanation of the heat transfer changes in mixed convection. Including the turbulence characteristics near corners from the authors’ previous work and the near-wall heating effect, a Reynolds-Averaged Navier-Stokes turbulence model is modified to predict the mixed convection heat transfer phenomena inside a rectangular riser. The modified model is validated by comparing the calculation results with experimental data for rectangular risers with different geometric configurations.