Natural convection is the dominant flow regime inside containments following the short-term blowdown phase. To simplify CFD computations, thermal radiation has traditionally been ignored due to the expected modest temperatures. Recently however, some reduced scale experiments have shown that radiation has profound effects on the flow field even at relatively low temperature levels and differences.
We will summarize two series of computations conducted with CFD tools. The first exercise consists of simulating turbulent air flow inside the DIANA cubical differentially heated cavity at PSI (Switzerland). A large eddy simulation (LES) is performed with and without wall-to-wall radiation. Including radiation significantly improves the prediction of the flow field, correctly displaying a reduced level of thermal stratification and an enhanced level of turbulence.
Secondly, the technical scale THAI (Becker Technologies, Germany) natural circulation test TH24 is analyzed. In this experiment, a stratified air-steam atmosphere is remobilized due to a natural circulation flow, induced by wall heating in the lower part of the facility and steam condensation inside the stratified cloud. An unsteady RANS model with and without consideration of the gas radiation heat transfer within the steam rich atmosphere is applied. The comparative evaluation of both simulations clearly highlights the effect of gas radiation on the overall gas energy balance and thus also the natural circulation and mixing process. In comparison to the experimental data, a significantly improved consistency is obtained if gas radiation is considered.