A new turbulent mixing modeling approach for sub-channel analysis code

Sub-channel code is one of the well-applied numerical tools in nuclear reactor thermal-hydraulics analysis. It takes consideration of the lateral transfer between adjacent sub-channels, which is its distinct characteristic. In most sub-channel codes, only one turbulent mixing coefficient for energy is used to account for lateral turbulent exchange. This imperfect description of the turbulent mixing parameter in different equations (e.g. mass, momentum and energy) significantly affects the accuracy of the calculation result. Besides, the empirical correlations to get the value of this coefficient have limited parameter ranges. In this paper, CFD simulations of two sub-channels in bare rod were performed with large geometry and flow condition ranges. The SSG turbulent model was used to simulate the non-isotropic turbulence and the calculation result was verified with experimental data. Based on the phenomenological analysis and theoretical consideration, a new turbulent mixing modeling approach was developed by studying βm, βE and βM, the turbulent mixing coefficients for mass, energy and momentum, respectively. Three empirical correlations of these coefficients were suggested and compared with existing correlations.