Direct numerical simulation of a turbulent channel flow with heat transfer is performed at very low Reynolds numbers. Two different thermal boundary conditions are adopted, and temperature is considered as a passive scalar. The computations have been carried out with huge computational boxes (up to 327.7 d × 2 d × 128 d). The emphasis of this paper is to investigate the large-scale structure (puff) in the intermittent-turbulent flow including the scalar fields and to provide the values of the transitional and critical Reynolds numbers, below which the turbulent flow becomes intermittent and laminar. The statistics, such as the skin friction and the Stanton number, are also examined; they suggest that the puff should be effective in sustaining turbulence and in heat transfer enhancement.
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