The Influence of Purge Flow Parameters on Heat Transfer and Film Cooling in Turbine Center Frames

Abstract

Due to stringent environmental legislation and increasing fuel costs, the efficiencies of modern turbofan engines have to be further improved. Commonly, this is facilitated by increasing the turbine inlet temperatures in excess of the melting point of the turbine components. This trend has reached a point where not only the high-pressure turbine has to be adequately cooled, but also components further downstream in the engine. Such a component is the turbine center frame (TCF), having a complex aerodynamic flow field that is also highly influenced by purge-mainstream interactions. The purge air, being injected through the wheelspace cavities of the upstream high-pressure turbine, bears a significant cooling potential for the TCF. Despite this, fundamental knowledge of the influencing parameters on heat transfer and film cooling in the TCF is still missing. This paper examines the influence of purge-to-mainstream blowing ratio, purge-to-mainstream density ratio and purge flow swirl angle on the convective heat transfer coefficient and the film cooling effectiveness in the TCF. The experiments are conducted in a sector-cascade test rig specifically designed for such heat transfer studies using infrared thermography and tailor-made flexible heating foils with constant heat flux. The inlet flow is characterized by radially traversing a five-hole-probe. Three purge-to-mainstream blowing ratios and an additional no purge case are investigated. The purge flow is injected without swirl and also with engine-similar swirl angles. The purge swirl and blowing ratio significantly impact the magnitude and the spread of film cooling in the TCF. Increasing blowing ratios lead to an intensification of heat transfer. By cooling the purge flow, a moderate variation in purge-to-mainstream density ratio is investigated, and the influence is found to be negligible.

Heat transfer and film cooling measurements on aerodynamic geometries relevant for turbomachinery

“…As shown in Jagerhofer [23], the heat transfer is enhanced for the purged undisturbed inflow condition (M = 0.21) due to a flow acceleration of the main flow caused by a mild blockage effect of the injected purge flow. After the transition, especially the no purge (M = 0) undisturbed inflow condition excellently agrees with the turbulent flat plate correlation, validating the measurement technique.…”

“…On this insulating surface, the heating foils were applied and spray-painted with high emissivity paint (Nextel Velvet 811-21) with an emissivity of 0.967 and a thermal conductivity of 0.197 W/m•K [22]. After spray-painting, the surface was sanded to obtain a hydraulically smooth surface finish [23]. The heating foils were powered by standard laboratory power supplies.…”

“…The shroud purge flow was not investigated in this paper and was switched off for the experiments presented in this work. The undisturbed inflow case corresponds to the "chilled high purge" case in Jagerhofer et al [23] and is explained more in detail there. The purpose of the inlet pegs, installed in the second test case, was to create disturbed inlet flow conditions with wakes and increased turbulence; a highly simplified abstraction of an upstream turbine stage.…”