Effects of tip-bleed holes on two-pass channel on heat transfer with various aspect ratios

The heat transfer characteristics in 180-degree turn region of two-pass channel with three different aspect ratios were investigated as coolant being extracted through tip-holes. The naphthalene sublimation technique was used for the heat/mass transfer measurements from the internal ceiling surface of the 180-degree turn region. The channel aspect-ratios of 1:1, 2:1, and 5:1 while keeping the same hydraulic diameter were studied. The inlet Reynolds number was set at 10000 that matches an actual land-based gas turbine. The tip-extraction mass flow rate was set at 10% of the total inlet mass flow rate. The experimental results showed that the heat/mass transfer is improved with the coolant extraction from tip-holes for all test cases. The improvement of thermal performance on tip surface is ranged from 10.8% to 67.0% compared to the no coolant extraction cases. The improvement was highest for the case of 1:5 aspect ratio as the extraction helped to minimize the region of local recirculation. The study proved that the tip-hole extraction could be effectively designed to improve the cooling of 180-degree turn region.