Numerical Prediction of Flow Structure and Heat Transfer in Square Channels with Dimples Combined with Secondary Half-Size Dimples/Protrusions

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The present study employs square cross-section dimpled channels with different arrangements of upstream secondary half-size dimples or protrusions to determine the optimal configurations for augmenting heat transfer rates with minimized pressure drop penalties. Five dimpled channels with and without upstream secondary dimples or protrusions are investigated (simple dimpled channel [case A]; dimpled channels with secondary dimples upstream each dimple [cases B1 and B2, respectively]; and dimpled channels with secondary protrusions upstream each dimple [cases C1 and C2, respectively]). All turbulent fluid flow and surface heat transfer results are obtained using computation fluid dynamics with a k-ϵ RNG turbulence model. Numerical results are qualified using grid-independent predictions of experimental data for one baseline dimple array arrangement. The channel inlet Reynolds number ranges from 8,000 to 24,000. From this study, secondary protrusions can bring forward flow separations and reduce the scope of recirculating flows in adjacent primary dimples and then greatly improve averaged local heat transfer of primary dimple surface. The result does not apply to secondary dimples which hinder flow reattachment in primary dimples and go against heat transfer enhancement. For averaged heat transfer on all the middle heated surfaces, heat transfer enhancement by secondary protrusions is not evident especially at high Reynolds numbers and the uniformity of roughness arrangements as dimples and protrusions makes a dominant role in the averaged heat transfer efficiency, while the dimple structure exhibits heat transfer advantage over protrusions at high Reynolds numbers. For the studied cases, case C1 obtains the best overall thermal performance at low Reynolds numbers, while case B2 is the best one at high Reynolds numbers. It is also recommend that case A can be effectively designed to exhibit the relatively good overall thermal performance with minimizing the blade weight and stress.