Simulation of aero engine pre- and post-stall transient behaviour

Abstract

The commercial and operational attractiveness of improving the specific thrust, whilst minimising the weight, of an aero gas turbine engine, has inevitably led to the compressor operating as near to stall as is considered safe. However, it is acknowledged that under extreme circumstances a compressor will stall, and so effecting a quick recovery is important with respect to safety and mechanical integrity. Because of these ever increasing demands made on the performance of civil and military aero engines alike, it has become increasingly clear that a means of simulating pre- and post-stall transient behaviour is now a computer modelling requirement of the major engine manufacturers. Computer modelling is essential because a thorough experimental evaluation of the pre- and post-stall transient behaviour of an engine during a development programme, is prohibitively expensive. Further, such à capability will find applications in the stability assessment and fault diagnosis of existing production engines. Conventional transient performance computer codes operated within the aerospace industry, are limited to the simulation of engine performance during acceleration and deceleration. Performance prediction is good because the transient is predominately governed by spool inertia. However, account is not taken of the temporary accumulation of fluid mass, momentum and energy of flow disturbances as they pass through the engine; these effects become increasingly significant beyond frequencies of approximately 5 Hz. These codes are therefore incapable of predicting engine response, and in particular compressor stability, during high frequency flow transients such as a reheat mislight. Further, the simulation of any subsequent post-stall flow transients which typically occur at frequencies up to 40 Hz, is similarly not possible. This thesis marks the introduction of a high frequency modelling capability into just such a performance code, the Rolls-Royce Aero Engine Performance system, RRAP; and the extension of this capability to the simulation of whole engine performance during post-stall events. The high frequency response of an engine working fluid was achieved by splitting the engine into component volumes and solving time dependent equations conserving mass, momentum and energy across each. An extended version of the equations was used to predict radial flow, and hence further the modelling capability to multi-spool engines. The development of post-stall methods required extension of present compressor performance calculations as well as the introduction of post-stall performance characteristics. In addition, a whole new combustor model was developed to operate during low or reverse flow conditions commensurate with post-stall events. Together these modelling methods successfully simulated surge, cyclic surge and the 'descent' into rotating-stall. The simulations agreed favourably with test results. Development of these methods using the RRAP system has ensured that the work marked the beginning of a truely generic pre- and post-stall engine modelling capability; and that the ultimate objective of creating a computer model sufficiently reliable and accurate so that it may be used in engine design and development, is one step nearer.