Influence of thermal gradient on gas turbine combustor wall using impingement/effusion cooling techniques: CHT CFD predictions

Internal wall heat transfer relevant to impingement/effusion cooling techniques was investigated using conjugate heat transfer (CHT) computational fluid dynamics (CFD) with ANSYS Fluent and ICEM commercial software. This work concentrates on the development of CHT CFD design procedures that are applicable to combustor wall and turbine blade heat transfer optimisation in gas turbine (GT). It specifically modelled and compares two configuration which are specifically relevant to the impingement and effusion holes density n (m-2) and is the ratio of the hole pitch X2. The configurations investigated are equal and unequal impingement and effusion holes density n (m-2), respectively, whereby in each case the variation in the number of cooling holes were carried out. The ratio of impingement and effusion number of holes/m2 (or hole density) n, investigated were impingement/effusion: 4306/4306 and 1076/4306, respectively. The geometries were for impingement wall, hole pitch X to diameter D, X/D ratio of ~ 11 but different number of holes N for both n geometries, at a constant offset effusion wall, hole X/D of 4.7 of the same N for both the two configurations. The model geometries have a constant impingement gap of 8 mm with both impingement and effusion walls at 6.35 mm thick Nimonic - 75 material and were computed for varied air mass flux G from 0.1 - 0.94 kg/sm2. Symmetrical applications were employed in modelling each of the geometry, whereby for the impingement hole, only quarter of one hole was modelled, while for the effusion side the holes were either quarter or half modelled. The two n geometries were computed with k - ɛ turbulence model using standard wall functions, which also applies to all G. The predicted locally surface X2 (or hole square area) average heat transfer coefficient (HTC) h values compared with with previously published experimental data showed good agreement. The reduced internal gap flow recirculation with reduced heat transfer to the impingement wall caused by increased number of effusion holes for unequal n impingement/effusion cooling design, shows that wall thermal gradient are higher than that found for equal n.