Direct numerical simulations were performed for air flows and related heat transfer in a developing region of the ribbed channel, where five ribs were attached to one wall. Three rib pitch-to-height ratios, Pi/H = 7, 4 and 2, were considered. The Reynolds number based on the bulk velocity and the channel width was 4,560. For the same parameters, simulations were also made for periodic fully developed cases. Although the largest rib pitch, Pi/H = 7, was the best choice for a heat transfer enhancement ratio in the fully developed cases, the enhancement ratio was larger for the smaller values of rib pitch, Pi/H = 4 or Pi/H = 2 in the initial region. Preference of smaller rib spacing in the initial region was found to come from: (1) the pressure drop was kept low due to the modest meandering of the stream lines; (2) downstream ribs were placed immediately behind the upstream ribs being exposed to the thin thermal layer; (3) the turbulent heat transport was active because of the fluctuation based on the Kelvin-Helmholtz type instability in Pi/H = 4 though fluctuation was weak in Pi/H = 2. Therefore, flow structures positively contributed the heat transfer enhancement of Pi/H = 4 and 2 in the developing region, and the smaller values of the rib pitch was recommended to be selected in such situations.

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