A THREE-DIMENSIONAL NUMERICAL STUDY ON HEAT TRANSFER CHARACTERISTICS OF BACKWARD FACING STEP FLOW IN A RECTANGULAR DUCT

摘要

Three-dimensional numerical simulation has been performed for flows over a backward-facing step in a rectangular duct. The calculations were carned out for several cases of different aspect ratios,AR, and Reynolds numbers,Re, based on the step height. The expansion ratio was kept constant at two. Reattachment points calculated in the present study agree well with experimental results obtained by other researchers. Close attention was paid to the distribution patterns of both the Nusselt number,Nu, and the skin friction coefficient,C_f, on the bottom wall. The numerical results for the case of Re = 250 showed that the values of both Nu and C_f remarkably changed along the spanwise direction in the case of AR < 16. The maximum Nusselt numbers on the bottom wall were obtained at the points near the side walls, not on the centedine. These tendencies depend on the three-dimensionality of the flow, which was confirmed by the observed secondary downward flow coming from the vicinity of the side walls toward the center of the duct.