Session: 01-04: Inlet Distortion and Engine Operability

Submission Number: 176018

High Fidelity Performance Modelling of a Turbofan Engine Under Windmilling Conditions 

Aircraft engines require to be certified for altitude relight, a safety requirement imposed by airworthiness authorities such as the European union Aviation Safety Agency (EASA) or the Federal Aviation Administration (FAA), to demonstrate satisfactory in-flight windmill relights. During in-flight windmill relight, windmilling or ground start-up, engines operate in a sub-idle/very-low-power regime, while other operating conditions require near-idle operation. An engine cycle is designed based on the requirements provided by the airframer regarding engine high-power conditions such as Top of Climb (ToC), CRuise (CR), and Maximum Take-Off (MTO), among others, representing critical conditions along the mission profile. Idle and sub-idle performance requirements are not of “high priority” for the airframer and are not accounted in the engine preliminary cycle design phase, also due to the difficulties and inaccuracies arising when modelling and simulating such engine far-off-design performance. Whole-engine performance models usually rely on isentropic efficiency-based maps and calculations for compressors ad turbines. The isentropic efficiency, by definition, becomes indeterminate when rotational speed approaches zero and physically meaningless for Pressure Ratio (PR) equal or below unity. For sub-idle performance modelling and simulation, the whole-engine model reliability rests on the accuracy of the low-speed component characteristics, which are often not available.  Sub-idle performance is difficult to capture experimentally, as components behave far from nominal conditions. Scenarios occurring at low-power settings such as stall and low Air-to-Fuel Ratio (AFR), resulting in unburnt fuel being released into the atmosphere, require specialised rigs, instrumentation and careful testing. Extrapolation is generally used to generate component sub-idle performance data, but this method however relies highly on engineering judgement. The lack of a comprehensive sub-idle performance methodology for the generation of sub-idle characteristics and elements that can handle non-efficiency-based calculations, make it difficult to include windmill relight considerations into the preliminary engine cycle design process to assist the altitude relight certification process. In this context, the need to model and simulate these regimes during an engine design phase demands the development of a whole-engine methodology for the prediction of engine sub-idle performance before any expensive testing takes place. In this work, a whole-engine methodology for modelling sub-idle performance for turbofan engines is presented and discussed. The methodology is validated against sub-idle performance of a 3-spool Very-High-Bypass-Ratio (VHBR) turbofan engine, for which altitude test facility data is available. In this study, the Numerical Propulsion System Simulation (NPSS) platform is used and adapted to handle specific requirements for sub-idle performance modelling such as torque-based component maps, which requires the development of several new NPSS classes. The generation of accurate component sub-idle characteristics is achieved with the use of in-house tools relying on a mean-line-method and CFD-derived sub-idle aerodynamic correlations to predict compressor and turbine sub-idle performance. This is executed to ease the engine altitude relight certification process, and to ensure that essential altitude relighting capabilities can be achieved when new and advanced technologies are introduced.

Presenting Author: Adriano Isoldi Cranfield University

Presenting Author Biography: Mr Adriano Isoldi is a member of the research staff at Cranfield University, affiliated with the Rolls-Royce University Technology Centre, the Centre for Propulsion and Thermal Power Engineering, and the Cranfield Air and Space Propulsion Institute. He has over five years of research experience in propulsion systems design, advanced modelling and simulation, and holds an MSc (1st Class Hons) in Thermal Power and Propulsion from Cranfield University. Current and past activities include technical contributions, management and delivery within the Innovate UK-funded LH2GT, PINES, COLIBRI and the EU-funded Clean Sky II PROTEUS and Horizon2020 C3HARME projects.

Authors:

Adriano Isoldi Cranfield University
Vassilios Pachidis Cranfield University
Ioannis Roumeliotis Cranfield University
Luis E. Ferrer-Vidal Cranfield University
Dimitrios Lamprakis Cranfield University
Richard Tunstall Rolls-Royce plc
Mark Stockwell Rolls-Royce plc
Marianne Britten Rolls-Royce plc