Shaft failure and overspeed modelling: systems integration and overall engine response

Abstract

The consequences of a shaft failure can be severe if not controlled or contained, potentially leading to an uncontained engine failure. Engine certification requirements dictate that an engine must be capable of handling a shaft failure event, with no hazardous release of debris. Demonstrating that an engine will have no hazardous consequences requires either a full-scale engine test or a comprehensive set of simulations. Engine tests of this scale are prohibitively expensive; therefore manufacturers rely upon validated engine models to provide predictions. Following a shaft failure, the turbine is free to accelerate since it is free from load. The specific bearing arrangement dictates whether the turbine is constrained axially. Should the turbine be free to move axially, i.e. unlocated, then additional complexity is introduced through interactions between rotating and static components. The objective of this research is to develop a tool that is capable of modelling a shaft failure event, in particular a high pressure shaft failure, with the ability to capture the inter-connected relationship between engine systems. Such a tool can be applied early on in a design program, which is also applicable to intact engine post-stall predictions. The developed Whole Engine Simulation Tool, WEST, features a one-dimensional poststall model of the compression system, a quasi-steady turbine solver, a transient onedimensional secondary air system model, a friction model and a comprehensive combustion model suite. The compression system features a fully-upwind Roe solver (TRSS), modified to incorporate variable composition gas, source terms and an entropy fix. WEST was validated against steady-state flight envelope data, an intact low power fuel spike and compressor rig surge data. WEST was applied to a high pressure shaft failure case study, which demonstrated its ability to capture the multidisciplinary nature of shaft failure scenarios. A part of this study was the complete derivation of a set of turbine overspeed characteristics; since the conditions experienced by a turbine during a shaft failure modify its ability to extract power from the incoming flow.