Transient performance simulation of an ultra high bypass ratio turbofan engine
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Date
2026-06-27
Authors
Καλαβρής, Κωνσταντίνος
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Abstract
The constant demand for reductions in fuel consumption and environmental emissions has driven the aviation toward ultra-high bypass ratio turbofan architectures. Due to their enlarged fan diameter, increased rotor inertia and the complicated component coupling, the transient performance prediction is more demanding. This diploma thesis develops a framework to simulate and analyse the transient performance of these engines. A direct comparative assessment of two-spool versus three-spool configuration is conducted. Following design-point optimization and extensive off-design parametric studies – covering the effects of altitude, flight speed, ambient conditions and overboard bleed – transient simulations are conducted for both acceleration and deceleration manoeuvres. Three progressive modelling levels are investigated and compared: a simplified model considering only spool inertia, a heat soakage model, and a full thermal model integrating spool inertia, heat soakage and tip clearance variations. During transient operation, the heat transfer between the working fluid and the engine’s metal parts causes phenomena, such as heat soakage – the thermal energy absorbed by structural components – and tip clearance changes, which represent the changes in blade to casing gaps due to differential thermal expansion. The heat soakage effect creates a time lag in the parameters’ response, while the tip clearance variations have longer influence as they alter slightly the steady-state values. The impact of heat transfer is illustrated in the generated transient operating lines on the compressor maps. Furthermore, the sensitivity of the transient response to fuel flow schedules and shaft inertia is examined. Μore aggressive fuel flow schedules and higher inertia reduce the compressors’ surge margin.
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Keywords
Ultra-high bypass ratio turbofans, Baseline models, Off-design simulation, Transient performance simulation, Rotor inertia, Heat soakage, Tip clearance variations