Heat Transfer Enhancement due to Combination of Dimples, Protrusions, and Ribs in Narrow Internal Passage of Gas Turbine Blade

Internal convective cooling of gas-turbine airfoil is essential because turbine inlet temperature becomes higher for pursuing higher thermal efficiency. For higher cooling performance, heat transfer is often enhanced by installing ribs and/or pin-fins in the internal passage. In this study, in order to enhance heat transfer, the combination of spherical dimples, cylindrical protrusions, and transverse square ribs was applied to one wall of a narrow passage. As for the cylindrical protrusions, two different diameter cases were examined. The heat transfer enhancement was measured by a transient infrared thermography method for the Reynolds number of 2,000, 6,000, and 10,000. The pressure loss was also measured in the experiments, and RANS simulation was performed to give a rationale for the experimental results. The present results clearly showed the spatial variation of the local Nusselt number: the high Nusselt number was observed on the rib top-surface and also near the leading edge on the protrusion top-surface. In addition, the areas around the dimple’s trailing-edge on the oblique line connecting the neighbor dimples showed moderately enhanced heat transfer. When two different protrusion-diameter cases were compared, both the mean Nusselt number and the friction factor were similarly higher in the larger protrusion case than the smaller protrusion case, and therefore the larger protrusion case was more effective in cooling even when the pressure loss was taken into account.