Numerical Predictions of Impingement Heat Transfer with Obstacles as Turbulators: Conjugate Heat Transfer Computational Fluid Dynamics Procedures

Impingement heat transfer cooling investigations with obstacles in the gap that are applicable to gas turbine (GT) combustor and turbine blades walls were carried out, using conjugate heat transfer (CHT) and computational fluid dynamics (CFD) codes. The heat transfer enhancing obstacles (or Turbulators) investigated include: rib walls (cross and co-flows), rectangular-fins (cross and co-flows), zigzag (cross-flow), pin-fin(cross-flow) and dimples (direct-flow). All the obstacles were aligned transverse to the direction of the cross-flow on the impingement target surface. The CHT CFD analysis was carried out using ANSYS ICEM and Fluent CFD tools for meshing and numerical calculations, respectively. Only the computational geometries for the rib walls and that for the rectangular-fins were investigated experimentally and were validated elsewhere and the other obstacle geometries were not. But, the methods applied in carrying out the computational works are similar to that investigated experimentally. This work varied the shapes of the obstacles and the results obtained were compared, an indication that the current CHT CFD predictions are possible for future design optimization. A 10 × 10 row of impingement jet holes or air hole density, n of 4306 m-2 with ten rows of holes in the cross-flow direction was used for all the obstacles. The impingement hole pitch X to diameter D, X/D and gap Z to diameter, Z/D ratios were also kept constant at 4.66 (15.24/3.27) and 3.06 (10.00/3.27), respectively. The target wall made from Nimonic-75 materials has thickness of 6.35 mm for all geometries; also the obstacles have the same material. The rib and the zig-zag walls are 4.5 mm high and 3.0 mm thick, the pin-fin is cylindrical in shape of 8.6 mm diameter and 8.0 mm high and is the same to that used for the rectangular-fins with 3.0 mm thick, while the depth of the dimple obstacle is 4.5 mm with the centre-line diameter of 8.6 mm. The obstacles were equally spaced on the centre-line between each row of impingement jets transverse to the cross-flow. One heat transfer enhancing obstacle was used per impingement jet air hole for the rectangular, circular fins and the dimple, while for the rib wall, is one rib per row of the impingement jet air hole. The CFD calculations were carried out for air coolant mass flux G of 1.08, 1.48 and 1.94 kg/sm2, which are the high flow rates used for regenerative combustor wall cooling whereby, the outlet air-flow, would pass to a low NOx combustor flame stabilizer. The pressure loss ΔP and surface average heat transfer coefficient (HTC), h for all the G were predicted, with the rectangular pin in co-flow showing better performance in the results. The comparison showed that for low coolant G, a 10 % increase in the overall surface averaged HTC were predicted for the rectangular fin walls, as compared to the other predicted obstacle geometries and 5 % predicted for the high G. These are the reasons that the predicted thermal gradient for this obstacle indicated lower values, showing adequate cooling