Entrainment by a Refrigerated Air Curtain Down a Wall

Thermal entrainment is important as it adversely affects energy consumption and evaporator humidity levels of refrigerated air curtain display cases, often at transitional Reynolds numbers. In order to get a more fundamental understanding of the mean and unsteady thermal entrainment processes, the shelf structure of a display case has been idealized to that of a plane, adiabatic wall subjected to refrigerated wall jets at laminar and transitional flow conditions. The wall jets are studied at different inflow profiles, Reynolds numbers, and Richardson numbers to investigate the effect on thermal entrainment rates. The primary simulation technique was direct numerical simulation of the Navier–Stokes equations in two dimensions for the low and moderate Reynolds numbers (though three-dimensional simulations were also conducted). At higher Reynolds numbers, a conventional Reynolds averaged Navier–Stokes approach was employed, which was found to give reasonable agreement with the above approach at a wall jet (early-transitional) Reynolds number of 2000. In general, the results yielded a significant variation in entrainment as a function of Reynolds number, with a minimum occurring at flow speeds immediately prior to transition. The entrainment rates were also sensitive to the initial velocity distribution, whereby a constant gradient profile (where any local velocity-gradient peaks were minimized) provided the least entrainment. Entrainment was also found to decrease with increasing Richardson number.