Internal cooling channels with differing aspect ratios are typically found in gas turbine blades due to the varying thickness of the blade from the leading to trailing edge. These serpentine passages often contain several 180 deg bends, which are sharp edged in the region of the blade tip. The 180 deg bend has a pronounced effect on the heat transfer characteristics in the outlet channel and tip wall, where a strong influence is seen due to the divider wall-to-tip wall distance in the bend. The present study investigates the effect of the divider wall-to-tip wall distance for a ribbed two-pass cooling channel with a 2:1 inlet and 1:1 outlet channel. Spatially resolved heat transfer measurements were made using the transient thermochromic liquid crystal technique for a smooth and a ribbed configuration using parallel 45 deg ribs. Effects of the 180 deg bend on heat transfer and rib-induced enhancements were identified separately and bend effects were found to dominate the heat transfer increase in the outlet channel near the bend. Pressure losses due to the bend and ribs were also independently evaluated for a range of tip wall distances. Results show that the smaller tip wall distances increase heat transfer on the tip wall and outlet channel but at the cost of an increased pressure loss. An optimum tip wall position is suggested, forming a compromise between heat transfer improvement and increased pressure losses.

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