Impact of Blowing and Density Ratio on the Film Cooling and Heat Transfer in an Aggressive Turbine Center Frame

It was shown in the previous study that the aft purge flows from the last stage of the high-pressure turbine (HPT) have a significant film cooling potential in the downstream turbine center frame (TCF) [1]. The TCF is a stationary duct that guides the flow from the high-pressure turbine (HPT) outlet to the low-pressure turbine (LPT) inlet and is the third most thermally loaded engine component after the combustion chamber and HPT. This paper investigates the impact of different purge-to-mainstream blowing ratios and density ratios on the film cooling effectiveness and the heat transfer coefficient in the TCF. The experiments were conducted in a product-representative 1.5-stage HPT-TCF-LPT vane configuration under Mach-similarity in the transonic test turbine facility (TTTF) at Graz University of Technology. The blowing ratio has been demonstrated to be the dominant parameter for purge film cooling in TCFs since HPT purge flows typically have very low momentum. Even at twice the nominal blowing ratio, no cooling film detachment was observed on the TCF hub or shroud surface. Varying the density ratio in the experimentally possible range delivered no significant differences in the results. With increasing purge blowing ratios, the film cooling effectiveness in the TCF increases as expected, but the heat transfer also intensifies due to the purge injection. The gain in film cooling effectiveness, however, outweighed the heat transfer intensification as the heat transfer reduction on the hub monotonically increases with the blowing ratio. The circumferentially averaged film cooling on the hub scales relatively well with an offset version of the well-known Hartnett correlation [2] that takes into account the ingress-induced premixing of the purge flow in the cavities.