Investigations relating to factors influencing the effectiveness of an aero-engine intake thermal anti-icing system
Thermal anti-icing systems are commonly used to protect aircraft leading edges from a potentially hazardous build-up of ice. Such systems have proven reliable in service and are relatively cheap and efficient. Typically, hot air is tapped from the engine compressor and ducted (via a regulation and control system) to the surface to be protected. Ideally an optimisation process should be employed at the design stage in order to ensure adequate anti-icing capability with minimal use of engine bleed air, since the latter represents a performance penalty. Following submission by the author of an MSc thesis concerning thermal modelling of a hot air anti-icing system for a civil turbofan intake (Wade 67 ), it became clear that extension of the studies was necessary to enable systematic accounting of the factors which limit ice accretion. An experimental programme was therefore carried out to investigate primarily: various exhaust geometries (through which spent anti-icing air is emitted to join the main engine inlet airflow and provide heating of the downstream surface; various cowl internal configurations on a full-scale model section of a large civil turbofan Nose Cowl. The internal geometry affects the effectiveness of the cowl lipskin heating, and the spent anti-icing exhaust air limits the quantity of unevaporated water which runs back along the intake acoustic surface downstream of the directly heated area and freezes. The Computational Fluid Dynamics package PACE (Prediction of Aerodynamics and Combustor Emissions) was used to model the internal, freestream and exhaust airf lows to determine the program's potential and usefulness for predictive purposes in this type of application. PACE is capable of modelling two or three dimensional, recirculating or non-recirculating flows for simple rectangular or polar geometry. It encompasses a suite of sub-programs to generate meshes and to create and solve the set of coupled linear equations representing the fluid flow. Various parameters, including heat transfer coefficients, were predicted in two regions: downstream of the exhaust plane to model the mixing of the spent anti-icing air and the freestream main engine inlet flow; inside and outside the Nose Cowl highlight area to predict skin temperature distributions for the three internal geometry configurations tested. This thesis describes the experimental work and compares the results with the Computational Fluid Dynamics predictions. Agreement was generally found to be good, and it was concluded that PACE may provide a useful modelling (design) tool, albeit with some reservations.