Direct numerical simulation was performed for the heat transfer of airflow in the entrance region of a channel with repeated rib protrusions. The rib-pitch to rib-height ratio (Pi/H) was increased from 2.0 to 16.0 by four steps. The rib-height ratio (H/δ) was maintained constant at 0.20. The distribution of heat transfer coefficient numerically simulated agreed with the experiment by Kattchee and Mackewicz (1963, “Effects of Boundary Layer Turbulence Promoters on the Local Film Coefficients of ML-1 Fuel Elements,” Nucl. Sci. Eng., 16, pp. 31–38). The enhancement parameter was used to evaluate the heat transfer performance by a ribbed channel. This parameter was defined as the ratio of the mean Nusselt number for the ribbed channel against the smooth channel consuming the same pumping power. The simulation result revealed that the enhancement parameter was maximized for Pi/H = 2 to 4 over the upstream ribs (x/δ < 2) and was remained high for Pi/H = 4, 8, and 16 over the downstream ribs (x/δ > 4). Therefore, the optimal rib pitch was smaller for the upstream ribs, and increased to the developed region. The mechanisms underlying this trend were discussed through observation of the streamlines, mean temperature, turbulence statistics, and instantaneous structures. The turbulence was increased over the ribbed wall for the cases of medium to wide rib pitch (Pi/H = 4, 8, and 16), whereas the turbulence increase appeared only over the upstream ribs (x/δ < 2) for the cases of narrow rib pitch (Pi/H = 2). The excellent performance of the wider rib pitch (Pi/H = 4, 8, and 16) at the downstream ribs (x/δ > 2) was resulted from the turbulence increase activating the turbulent heat transport. Whereas, the superiority by the narrower rib pitch (Pi/H = 2, 4) comes from the turbulence activation, and the renewed thin boundary layer which continues due to the densely allocated ribs.

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