Effects of radiation on turbulent natural convection in channel flows

A computational study has been carried out to analyse complex interaction of radiation with turbulent natural convective flow of dry and humid air in open-ended channels. Transient flow simulations are undertaken in the channel with one uniformly heated wall and adiabatic side walls for different values of emissivity of active walls with and without participating medium. To adequately present turbulence and radiation, a computational model included large eddy simulations for the turbulent flow coupled with discrete ordinates method for radiation transfer. Spectral line-based weighted-sum-of-grey-gases for the absorption properties of water vapour has been adopted. Complex three-dimensional vortical structures are identified which directly affect the temperature distribution on the heated wall. Including wall to wall radiation resulted in significant changes in the heat transfer, reaching 14 °C temperature drop at the hot wall with wall emissivity of 0.9. Mixing and cooling rates in this case were increased by up to 25%. Including gas radiation for the humid air with the water vapour molar fraction of 0.02 corresponding to saturated conditions at inlet temperature of 25 °C did not have a significant effect on the mean flow and temperature values comparing with wall to wall radiation. However, turbulent statistics have changed significantly resulting in a delayed transition to turbulence near the active wall of the channel and increased turbulent activity near the cold wall. The model developed in the present study is also applicable in fire management, where the aim is to reduce the damage that occurs when a PV module is exposed to high temperatures.