Impact of engine icing on jet engine compressor flow dynamics
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Core engine icing has been recognized to affect a wide variety of engines since the 1990's. This previously unrecognized form of icing occurs in flights through high altitude convective regions and vicinity of thunderstorms. Engine icing events involve power loss or damage associated to the engine core, namely instabilities such as compressor surge, stall, engine rollback and even combustor flameout events. The effects on compressor performance are significant in understanding the response of the engine to atmospheric ice ingestion. A one-dimensional axisymmetric flow model is used to simulate the continuous phase through the compressor. The steady state operation of dry air is validated with an industrial database. By changing an exit throttle, the point where the dry compressor mass flow rate slowly starts to drop, is predicted. The stage that is the first to locally collapse, causing the remaining stages and eventually the complete compressor failure, is determined. The continuous flow model is then coupled with a Lagrangian model for the discrete phase in a framework that conserves mass, momentum and energy. From numerical simulations of the coupled, continuous-discrete phase flow model, it is observed that a rematching of the stages across the compressor occurs with increasing ice flow rates to accommodate loss of energy to the ice flow. The migration of the operating point towards the stall point at the rear stage eventually causes the compressor to stall. The onset of stall is characterized by initial oscillations followed by a rapid decay of pressures of the last stage with the instability traveling quickly towards the front of the compressor. Effectively, a reduction in the compressor stall margin is observed as the ice flow rate increases. Further, the relevance of factors such as blockage due to discrete particles and break/splash semi-empirical models in the icing physics, are analyzed through parametric studies. Conclusions are drawn that underscore the influence of the assumptions and models in prediction of the flow behavior in the presence of ice ingestion. Smaller ice crystal diameters have a greater influence on the gas flow dynamics in terms of a higher reduction in surge margin. The break empirical model for ice crystals and splash model for the droplets that are used to calculate the secondary particle size upon impact with rotor blades have a significant influence on the gas flow predictions.