Modelling Unsteady Ice Accretion and Water Runback in Aero Engines

High altitude ice crystals have led to instances of ice accretion on stationary compressor surfaces in aeroengines. Rollback, surge and stall events are known to have been instigated through such accretions due to aerodynamic losses related to ice growth, damage and flameout due to ice shedding. The prevalence of these events has led to a change in certification requirements for icing conditions. Development of accurate numerical models allows the costs of certification and testing to be minimised. Ice crystal icing (ICI) accretion is modelled as a coupled heat transfer and phase change continuity and energy balance at the surface. The Extended Messinger Model (EMM) incorporates a temperature gradient across the ice and water layers. This model was updated previously to model mixed phase ICI with an additional water layer at the surface to generate a temperature gradient from a warmed surface to the ice interface (>0°C), the updated model is EMM-Crystals (EMM-C). Within this paper, the EMM-C is modified to generate unsteady ice accretion. This update is relevant due to the intrinsic 3D flows present in a compressor stage and documented testing of engine stages and flight tests showing the complex 3D shapes of mixed phase warmed surface ICI accretion. A substrate heat transfer model is implemented for turbomachinery specific flow cases. This work has been validated against ICI experiments comprising of a cantilevered prismatic stator test piece, a swan neck duct linear cascade and existing experimental results from test campaigns performed at the Research Altitude Test Facility (RATFac) in Canada.