The Influence of the Relative Number of Impingement and Effusion Cooling Holes on Gas Turbine Combustor Wall Heat Transfer

The optimization of cooling hole geometry for combined impingement and effusion cooling of gas turbine combustors was investigated. The most common design is for there to be equal number of impingement and effusion holes. However, the number of holes is usually set by the requirement of the effusion wall to have good film cooling and a large number of holes is used with small diameter and a low pressure loss or small pitch to diameter ratio, X/D. The impingement wall does not need a large number of holes. This work compared impingement/effusion wall designs with equal number of holes for three hole numbers: 4306/m2, 9688/m2 and 26910/m2. Each of these effusion designs was investigated with a 1076/m2 impingement wall. The internal wall heat transfer for impingement/effusion cooling was measured and predicted using conjugate heat transfer (CHT) computational fluid dynamics (CFD). The work was only concerned with the internal wall heat transfer and not with the effusion film cooling and there was no hot gas crossflow. The CHT/CFD predictions showed good agreement with measured data and the highest number of effusion holes for the 1/25 hole ratio gave the highest hx. However, comparison with the predicted and experimental results for equal number of impingement and effusion holes for the same Z, showed that there was little advantage of decreasing the number of impingement holes, apart from that of decreasing the Z/D significantly for the 1/15 hole ratio, which increased the heat transfer. The largest number of effusion holes had the highest heat transfer due to the greater internal surface area of the holes and their closer spacing.