Large-eddy simulation of mixed convection-radiation heat transfer in a vertical channel

A mixed convection boundary layer in a vertical channel is studied using large-eddy simulation. Two different cases were considered by applying constant heat flux boundary condition to one of the channel walls while the other wall was kept insulated. The Grashof to Reynolds number ratios based on the wall heat flux and the channel width are GrW/ReW2=9.4×107/50802=3.6 (Case I) and 2.5×108/95752=2.8 (Case II), which suggest that the buoyancy forces are comparable to the inertial forces in both cases. Owing to the large temperature differences between the hot wall and the inlet air, the governing equations were solved using two different approaches. Boussinesq approximation considering constant properties was employed in the first approach. In the other, fluid properties were assumed to be temperature dependent. A consequence of the wall high temperature is the radiation heat transfer, which is also taken into account. It is shown that an average of about 11% of the heat in Case I and 15% in Case II is transferred to the insulated wall via radiation. Differences between the results of the constant and variable property computations are rather high, which suggest that Boussinesq approximation is not accurate for very large temperature differences. The differences between the results of the variable and constant property computations are expectedly larger in Case II, where the wall temperature is higher than Case I. Large discrepancies can be observed between the numerical and experimental results. The discrepancies are larger in Case II than in Case I. The possible reasons for these discrepancies are discussed.