The Relative Performance of External Casing Impingement Cooling Arrangements for Thermal Control of Blade Tip Clearance

As a key way of improving jet engine performance, a thermal tip clearance control system provides a robust means of manipulating the closure between the casing and the rotating blade tips, reducing undesirable tip leakage flows. This may be achieved using an impingement cooling scheme on the external casing. Such systems can be optimized to increase the contraction capability for a given casing cooling flow. Typically this is achieved by changing the cooled area, local casing features such as the external flanges, or the external cooling geometry. This paper reports the effectiveness of a range of impingement cooling arrangements in typical engine casing closure system. The effects of jet-to-jet pitch, number of jets, inline and staggered alignment of jets, on an engine representative casing geometry are assessed through comparison of the convective heat transfer coefficient distributions as well as the thermal closure at the point of the casing liner attachment. The investigation is primarily numerical, however, a baseline case has been validated experimentally in tests using a transient liquid crystal technique. Steady numerical simulations using the realizable k-ε, k-ω SST and EARSM turbulence models were conducted to understand the variation in the predicted local heat transfer coefficient distribution. Constant mass flow rate was used as a constraint at each engine condition, this approximately pertaining to a constant feed pressure when the manifold exit area is constant. Sets of local heat transfer coefficient data generated using a consistent modelling approach were then used to create reduced order distributions of the local cooling. These were used in a thermo-mechanical model to predict the casing closure at engine representative operating conditions.