Aircraft Engine Performance Improvement by Active Clearance Control in Low Pressure Turbines

Abstract

In order to minimize fuel consumption, resulting in reduced operating costs and lower environmental impact, turbofan engines must be of high overall efficiency. The design of the low pressure turbine (LPT) plays a significant role in the development of such engines. During a flight mission changing operating conditions (spool speeds, temperatures, pressures, etc.) cause altering magnitudes of the LPT tip clearance, leading to a decrease in LPT performance. As minimum clearances usually do not occur in steady state cruise condition — the major flight condition concerning fuel consumption — active measures to minimize radial tip clearance (ACC – active clearance control) must be incorporated to achieve a considerable reduction in fuel consumption over the whole flight mission. Actively minimizing radial tip clearance by manipulating the turbine casing requires energy in terms of cooling air (thermal ACC), electrical or hydraulical power (mechanical ACC). The cooling air or the power respectively must be provided by the engine itself, thus partly compensating the benefit gained through the improved LPT behavior. This paper investigates the potential of ACC systems from a whole engine perspective. The approach uses a performance model of a state-of-the-art high bypass turbofan engine with a thermal LPT-ACC system to assess the different benefits and detriments of an enhanced ACC. The overall benefit in TSFC for the simulated engine is compared to measured data of other engines indicating an increase of ACC effectiveness with increasing bypass ratios. To compensate deterioration losses due to single rub-in events, closed-loop controls are required. A tip clearance sensor allows the ACC to adapt to an individual engine. As thermal ACC systems show an optimum benefit with a corresponding optimum ACC cooling air flow, the additional TSFC benefit by compensating deterioration is limited. The achievable overall performance improvement is evaluated for different control loops. Mechanical ACC systems bear the highest potential of eliminating clearance losses, while only minor improvements can be made for thermal ACC systems.