Heat Transfer and Pressure Drop Measurements in a High Solidity Pin Fin Array With Variable Hole Size Incremental Impingement

Abstract

Leading edge heat loads on turbine vanes diminish relative to fully turbulent heat loads with increasing Reynolds number. Leading edge regions of turbine nozzles are often cooled using showerhead arrays while the near pressure surface is often protected with rows of shaped holes. However, in environments with impurities in the fuel or air cooling holes are susceptible to clogging and constitute sites where deposition can begin. Showerhead film cooling can be disruptive to downstream boundary layer development and film cooling. Also, high turbulence levels which normally exist in these regions quickly mix away film cooling protection. Consequently, internal cooling has many advantages over showerhead cooling and pressure surface film protection. Internal cooling produces higher levels of internal effectiveness and spent cooling air can be subsequently directed to near optimum discharge geometries for film protection. Conventional cooling methods have disadvantages when trying to cool leading edge regions and near pressure surfaces. Cooling air in pin fin arrays quickly heats up developing a lower cooling potential. Impingement arrays have issues due to increasing crossflows which deflect impingement jets and insulate the surfaces needing cooling. Incremental impingement overcomes these disadvantages by incrementally adding cooling air where needed and overcoming crossflows by hiding impingement jets behind high solidity pedestals. This paper presents heat transfer and pressure drop results for an incremental impingement array with variable hole size. The experimental measurements were acquired using a bench scale test rig. The array Reynolds numbers tested ranged from 5000 to 60,000 based on the average velocity of the accumulated flow through the minimum array flow area. The array consisted of an initial impingement row between a row of elongated pedestals followed by 7 additional high solidity round pedestal rows in a staggered arrangement. Impingement holes of variable sizes were placed behind even rows. Generally, the array consisted of rows of round pins spaced at 1.625 diameters in the spanwise direction, 1.074 diameters in the streamwise direction with a channel height to diameter ratio of 0.5. Impingement hole to pin diameter ratios used included d/D of 0.295, 0.351, and 0.417. Hole configurations were limited to arrays where the hole area upstream from the last row of holes was no more than 109% of the minimum array flow area. Heat transfer measurements were acquired at a constant temperature within the array and are reported on a row averaged basis in terms of the local internal effectiveness and the cooling parameter.