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.</p

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.</p

Experimental study and analysis of ice crystal accretion on a gas turbine compressor stator vane

A significant number of historical engine powerloss events have recently been attributed to ingestion of high altitude ice crystals, prompting regulators to expand engine certification envelopes to incorporate 'ice crystal icing' conditions. There has been a resulting effort by OEMs and academia to develop analytical and semi-empirical models for the phenomenon, partly through use of rig testing. The current study presents results and analysis of experiments conducted in the National Research Council's Research Altitude Test Facility (RATFac). The experiments used a simplified compressor stator vane test article, designed to produce data to build semi-empirical models and validate an existing ice crystal icing code. Accretion growth rates, extracted from backlit shadowgraphy, are presented as a function of test condition, and the algorithm of a new image processing technique using Canny filtering is discussed. Wet bulb temperature, Mach number, particle size and test article angle of attack were systematically varied. In line with previous experiments, the accretion growth rate was observed to be strongly dependent upon bulk particle melt ratio, with a peak growth rate at approximately 10% melt ratio. If leading edge accretions shed during the test, the growth rate of the second accretion would be greater than the first, regardless of test condition, due to the cooling of the substrate surface during the first accretion. The rate of erosion was found to correlate with bulk particle kinetic energy. The highest growth rates were observed for positive angles of attack, at both the leading edge and pressure surface. In contrast, at negative angles of attack growth rates were minimized, attributed to unfavourable accretion conditions on the suction surface. Finally, a qualitative assessment of the accretion quality and build/shed behavior as a function of test condition is presented