Accurate prediction of pressure drop and heat transfer in convective cooling system is of importance to gas turbine industry. In the present paper, a detailed study and assessment of RANS method based on SST reattach turbulence model is performed on a convective cooling system in three kinds of U-duct: smooth, with 45 degree or 90 degree angled parallel ribs. Heat transfer and pressure drop distributions in the ducts are analyzed spatially at the Reynolds numbers of 15000, 30000 and 60000. The numerical results are compared with the experimental data and the empirical correlation from Han et al. It is found that the obtained pressure drop distribution based on RANS with SST reattach turbulence model matches the experimental data for all the U-ducts adequately. Meanwhile, the heat transfer is well predicted by the RANS method in the cases of smooth duct and 45 degree ribbed duct. A good agreement is obtained in the turn region of 45 degree ribbed duct, it owes to the strong secondary flow induced by ribs, which restrain the mainstream to separate and accelerate in the turn. But, the heat transfer is significantly under-predicted for 90 degree ribbed duct since the flow reattachment point between the ribs is predicted farther away from the upstream rib than that in the experiment. Therefore, it is suggested that the RANS method with a suitable turbulence model is valuable for the smooth and 45 degree ribbed U-duct with the acceptable engineering accuracy. But the prediction of heat transfer in 90 degree ribbed U-duct is still a challenge for the RANS to solve.

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