Reynolds-averaged Navier-Stokes simulations of opposing flow turbulent mixed convection heat transfer in a vertical tube

茂木 孝介  ; 柴本 泰照 ; 日引 俊詞*

Motegi, Kosuke; Shibamoto, Yasuteru; Hibiki, Takashi*

We performed Reynolds-averaged Navier-Stokes (RANS) simulations of a single-phase turbulent opposing flow mixed convection in a heated vertical circular tube. Previous research has indicated that the Launder-Sharma model (hereafter the LS model), one of the most popular RANS turbulence models, overestimates experimental heat transfer coefficients for opposing flows. Although the RANS models have been widely applied to opposing flows in various systems, the mechanism and conditions under which the predictive performance of the LS models fail remain unclear. This study aims to understand the model characteristics and their applicability under various mixed convection conditions. This article investigates the LS model, the LS model with the Yap correction, and the model. The LS model remarkably over predicts the Nusselt number and the friction coefficient under highly buoyant conditions. The error for the Nusselt number was more than 90% for , where is a controlling parameter. The conditions under which the prediction of the LS model fails are linked to those under which reverse flow occurs near the heated wall. The reverse flow condition is given by . This condition could be used where the LS model cannot be applied. The LS model with Yap correction and model can predict experimental data successfully from forced convection to mixed convection conditions . For natural convection-dominant conditions , the LS model with the Yap correction was numerically unstable and could not obtain a converged numerical solution; however, the model stably reproduced the experimental data. By optimizing the model constants included in the Yap correction, the stability and accuracy of the calculation can be improved under highly buoyant opposing flow conditions.