Direct Numerical Simulation of Heat Transfer Over Riblets

Riblets are well-known as a passive mean for drag reduction in turbulent flow conditions, but their effectiveness for heat transfer is quite controversial. In this paper we present the numerical results for fully developed laminar and turbulent flow and heat transfer in a channel with triangular riblets. The turbulent study is performed by means of direct numerical simulation at a Reynolds number Re_\tau =180 based on the wall-shear velocity, for a fluid with a Prandtl number Pr=0.71. Four different ribbed channels are considered, under a constant heat flux boundary condition, and correspond to ridge angle a \alpha = 45 and 60 degrees, and riblet spacing s^+ = 20 and s^+ = 40. The results obtained, for the flow and turbulent quantities, are in good agreement with past experimental and numerical studies, and correctly reproduce drag reduction over the smaller s^+ = 20 riblets and drag increase over the larger s^+ = 40 riblets. The predicted heat transfer efficiency of riblets do not agree with some experimental results, and is below that of a flat plate for all the configurations. The conditions for heat transfer enhancement are discussed.