With gas temperatures far exceeding the melting point of nickel-base alloys, advanced cooling schemes are essential to meet the desired mission life of turbine airfoils. Naturally, combustion systems produce gas-temperature non-uniformity in the exiting flowfield. Downstream turbine components must be tolerant to the maximum anticipated gas temperatures. On the other hand, excessive use of cooling air reduces engine efficiency and compromises combustor durability. Throughout gas turbine design history it has been the desire of Turbine Aerodynamicists to be able to compute combustor hot streak migration and mixing through multiple turbine airfoil stages. Typically, hot streak migration studies have been performed using (a) mixing-plane models between rotating and stationery domains or (b) unsteady simulations in which the flowpath annulus is represented by a segment containing airfoil counts that are integer multiples in each blade row or (c) Non-Linear Harmonic methods. With the development of highly-parallelized Computational Fluid Dynamic (CFD) codes driving high performance computer clusters simulation of combustor hot streak migration through multiple High Pressure (HP) turbine stages using an unsteady, 360° (full-annulus) model can be achieved. To this end, Honeywell, in collaboration with Numeca Corporation, has performed a study to evaluate the state-of the art for computation of the effect on second-stage HP turbine nozzle metal temperatures of combustor hot streaks migrated through the first-stage of a two-stage HP turbine.

Numerical calculations are performed to examine the merits of a novel cooling scheme proposed to improve the durability of heat shields in a gas turbine combustor. The novel scheme is comprised of a combination of convective and impingement heat transfer. The impingement scheme incorporates three rows of holes two of which target the bore of the heat-shield while the third row targets the OD of the heat-shield and are all staggered with respect to each other. The convective heat transfer is provided through annular collection of the spent impingement flow from the two rows targeting the bore. The impingement height-to-hole diameter spacing is in the range of 1.4 ≤ Z/d ≤ 2.3, with the jet Reynolds number 12050 ≤ Red ≤ 13770. The spent impingement flow from the inner-diameter holes however, acts as a cross-flow on the impingement jets targeting the outer diameter of the heat-shield which reduces the impingement heat transfer rate of these outer-diameter holes. U-shaped ribs are introduced on the surface of heat-shield with the objective of first reducing the detrimental effects of cross-flow on impingement heat transfer and secondly, to increase the convective heat transfer area due to annular collection of the spent flow. It is shown that, provided the U-ribs are clocked with respect to the 3rd (outer-diameter) impingement row, the maximum heat-shield metal temperature is reduced by as much as 80 °C. The thermal gradients in both radial and “through-the-wall” directions are also reduced which in turn reduce the hoop (tangential) stresses. The combination of heat-shield temperature and stress reduction, due to the introduction of U-ribs, increase the heat-shield life.