Effect of Bleed air on Performance of Turbofan Engines

Abstract

Present and future wide-body airliners (such as the Airbus 350, A380 and Boeing 787) are powered by high bypass ratio (HBPR) turbofan engines. To maximize its thrust force and performance parameters, turbine cooling is a necessity. Air bled from compressor is used for turbine cooling. Such bleed ratio is examined in his work to analyze both its effects on turbine and the overall engine performance. As a case study, three spool mixed and unmixed engine (similar to Trent 700) is examined. Operating conditions are flight Mach of 0.82, altitude 12,000 meters and the maximum temperature ranging from 1500 to 1700 Kelvin. A comparison between two cases of mixed and unmixed turbofan engines is performed. Matlab-7 is used to simulate and analyze thermal cycle of both cases. A case of 8% air bleed is employed for both engines. For uncooled mixed turbofan engine bleed air decreased specific thrust by 0.84% which is negligible compared with the increased maximum temperature of the engine from 1200 to 1700 Kelvin, which led to an increase specific thrust by 25%. Uncooled unmixed turbofan engine bleed air decreases specific thrust by 56% but this depletion increases the temperature of the turbine from 1200 to 1700 K, which increases the thrust by more than two folds. The present study determines the relationships between engine performance parameters and design parameters for HBPR Trent 700 unmixed turbofan engine. Comparison between mixed and unmixed turbofan designs is next performed. The effect of bleed air from high pressure compressor to cool high and intermediate pressure turbines on engine performance is finally examined. The paper ends with a brief summary of conclusions that include, but not limited, to the following points: