Enhanced Impingement Heat Transfer: The Influence of Impingement X/D for Interrupted Rib Obstacles (Rectangular Pin Fins)

Impingement flat wall cooling, with 15.2 mm pitch square hole arrays, was investigated in the presence of an array of interrupted rib obstacles. These ribs took the form of rectangular pin-fins with a 50% blockage to the cross flow. One side exit of the air was used, and there was no initial cross flow. Three hole diameters were investigated, which allowed the impingement wall pressure loss to be varied at constant coolant mass flow rate. Combustor wall cooling was the main application of the work, where a low wall cooling pressure loss is required if the air is subsequently to be fed to a low $NOx$ combustor. The results showed that the increase in surface average impingement heat transfer, relative to that for a smooth wall, was small and greatest for an $X∕D$ of 3.06 at 15%. The main effect of the interrupted ribs was to change the influence of cross flow, which produced a deterioration in the heat transfer with distance compared to a smooth impingement wall. With the interrupted ribs the heat transfer increased with distance. If the heat transfer was compared at the trailing edge of the test section, where the upstream cross flow was at a maximum, then at high coolant flow rates the increase in heat transfer was 21%, 47%, and 25% for $X∕D$ of 4.66, 3.06, and 1.86, respectively.