Ice crystal icing in gas turbine engines

High altitude ice particles can accrete inside the core compression system of turbofan engines in cruise and descent. This can lead to severe in-flight events including blade damage, surge and flameout. This thesis describes the development and validation of a new comprehensive computational model to aid prediction of ice crystal icing in turbofan compressors. The Ice Crystal Icing ComputationaL Environment (ICICLE) delivers a step change in modelling of the phenomenon compared to the first generation of models in the open literature. Modelling of this multi-faceted problem is broadly divided into three strands: first, modelling of the ice particles in flight; second their interactions with solid surfaces; and third the thermodynamics of ice accretion. To aid development of models and provide validation data, three different experiments were also undertaken. Treatment of particle size and shape distribution is considered first, and a particle trajectory model based on Lagrangian tracking is presented. A Nusselt number correlation for non-spherical particles is used to develop a phase change model for the particle in flight, incorporating sublimation, evaporation and melting. The model is then validated against measured particle melt data in an ice crystal facility. A model for the change in enthalpy and humidity of the airflow as a result of the particle phase change is proposed. Existing icing codes do not attempt to model these affects, but evidence from engine encounters with ice crystals indicate that they are significant. It was assessed that experimentation was required to develop modelling capability in three areas: particle sticking, erosion and heat transfer. Two experimental campaigns were performed at the ice crystal wind tunnels of the National Research Council of Canada (NRC) using simple geometries (an inclined flat plate and a cone). Data was presented for the first time on heat transfer from a warm substrate under ice crystal conditions, and a method to predict the change in particle melt during surface impacts was proposed. New semi-empirical models were developed for sticking and erosion, with a substantially wider range of applicability than achieved in previous studies. A new thermodynamic ice crystal accretion model was developed. A literature model for supercooled water icing was adapted to ice crystal and mixed phase conditions, and to substrates either above or below freezing. In the former case, an entirely novel three-layer accretion model was developed, which is a substantial advancement in modelling ice crystal growth on initially warm engine surfaces. Finally, the complete model is validated against experimental accretions on the case of a compressor stator test article, also tested at the NRC. Agreement is seen generally to be good, with the transient behaviour of growth rates well predicted, typically within 20% of experimental measurements. It is shown that a substantial improvement in prediction accuracy may be attained by updating the fluid domain at discrete time points. This accounts for the influence of the growing accretion on the flowfield. The successful application of a quantitative code to a more complex, engine-realistic geometry is a significant step forward for the literature, as existing ice crystal codes have only been validated against simpler geometries.