We propose a new numerical method based on the usual incompressible scheme which is taking into account the density variation to realize the turbulent heat transfer prediction accurately. For the assessment of the applicability of this method for the flow field where the compressibility is not negligible even in the case of low Mach number, the direct numerical simulations (DNS) of turbulent flow in a horizontal channel of different wall temperatures are carried out by using the numerical method and the results for the turbulent flow under some conditions of unstable stratification are compared. By the 4-quadrant analysis of turbulent shear stress, it is found that the ejection events in the vicinity of the walls are particularly affected by the density variation. In addition, the global balance of the temperature profile are also affected. The results suggest that the present method is potent in simulating the change in heat transfer rate due to density fluctuation in more practical flow fields.

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