Numerical Investigation of Convective Heat Transfer and Pressure Drop for Ribbed Surfaces in the Bend Part of a U-Duct

This work concerns two-dimensional numerical simulations of the flow and temperature fields inside smooth and ribbed bend (turn) parts of a U-duct with relevance for internal tip cooling of gas turbine blades. The ribs are placed internally on the outermost bend surface. The renormalization group (RNG) k-epsilon turbulence model was used to solve the momentum and energy equations inside the bend (turn) part as well in the supply and return straight parts of the U-duct. For the ribbed surface three different rib configurations were simulated, namely (a) single rib at three different rib positions, i.e., inlet, middle and outlet, (b) two ribs for three different configurations, i.e., at the inlet and middle, at the middle and outlet as well as at the inlet and outlet, and (c) three ribs. The rib height-to-hydraulic diameter ratio, e/Dh, was 0.1, the pitch ratios were 13.5 and 27 and the Reynolds number was 20000. The details of the duct geometry were as follows: the cross section area of the straight part was 50x50 mm2, the inside length of the bend part was 240 mm. The results were compared with experimental data obtained at similar conditions. The numerical results were closer to the experimental ones for those cases with the rib at the inlet position than for the cases with the rib at the middle position. The case of two ribs at the inlet and middle gave the highest heat transfer coefficients while the case of a single rib at the middle gave the highest local pressure coefficient of all cases.