Effects of large density variation on strongly heated internal air flows

Direct numerical simulation (DNS) is employed to examine the effects of large density variation of strongly heated air flowing in a vertical pipe on turbulent heat and momentum transfer. The predictions of the heat transfer and skin friction coefficients as well as the mean velocity and temperature profiles are in excellent agreement with the existing experimental data. Like some previous studies on heated air flows, the present study shows that the flow is laminarized with heat flux because a large reduction occurs in turbulence intensity. Unlike the velocity fluctuations, however, the thermal turbulence intensities such as the normalized density and temperature fluctuations remain as relatively insensitive to the heating conditions. Moreover, it is observed that the mean velocity and temperature profiles become dissimilar to each other during the process of laminarization with heating. In the present study we elucidate these anomalous behaviors of the strongly heated air flows with a notion of similarity breakdown between the mean velocity and temperature profiles, which occurs due to large density variations, and by examining the production rates of the turbulence kinetic energy and thermal turbulence fluctuation variance.