CFD analysis of thermal stratification and sensitivity study of model parameters for k–ɛ model in a cylindrical hot plenum

A sensitivity analysis of various numerical parameters on the prediction of thermal stratification in a 2D axi-symmetric domain has been carried out by using commercial code FLUENT 6.3. The analysis is carried out for both steady and unsteady flow conditions. The Reynolds number in all the cases has been considered as Re = 19,200. The effect of under relaxation parameters, numerical schemes on convergence while solving mass, momentum and turbulence equations was first studied for steady state conditions. An optimal range for under relaxation parameters was found which ensures the stability of the numerical solutions and ensures faster convergence. Further, 1st order upwind scheme for convective terms of turbulent equations (k and ɛ) is found to give a better convergence in comparison with higher order schemes. The study was then extended to transient flow conditions and the stratification characteristics like rising of the stratification interface was studied. The turbulence model parameters of the standard k–ɛ model were changed and predictions were observed. It is observed that the values used for turbulence model parameters (Cμ, C1, C2) while solving the standard k–ɛ model for studying stratification characteristics are different than the model values used in k–ɛ equation. In the lower part of reactor vessel, k–ɛ model with standard value of model parameters have a very good agreement with the experimental observations of Moriya et al. (1987) while in the upper part the model parameters take different values than the standard values.