Numerical study of indoor pollutant transport focused on the gradient-diffusion hypothesis

The majority of numerical studies of room airflow using Computational Fluid Dynamics (CFD)  are conducted with the steady Reynolds-averaged Navier-Stokes (RANS) approach. In this approach the averaged quantities are computed, and the effect of turbulence is modelled. Furthermore, the standard-gradient diffusion hypothesis is often used to model the turbulent mass transport, which relates the turbulent mass flux to the mean concentration derivative. In this paper, a CFD analysis of pollutant dispersion in an enclosure ventilated by a transitional wall jet (Re ≈ 2,500) is presented, using validated high-resolution RANS and Large Eddy Simulations (LES). Although the LES computations show that a counter-gradient turbulent mass flux is present, indicating that the standard gradient-diffusion hypothesis used in RANS is not valid in the entire flow domain, it is shown that the convective mass fluxes dominate over the turbulent mass fluxes, and that therefore the pollutant concentrations predicted by RANS do not differ significantly.