Experimental Studies of Ice Crystal Accretion on an Axisymmetric Body at Engine-Realistic Conditions

Abstract

It has been recognised in recent years that high altitude atmospheric ice crystals pose a threat to aircraft engines in flight. Instances of damage, surge and shutdown have been recorded at altitudes significantly greater than those associated with supercooled water icing. It is believed that ice particles can accrete inside the core compressor, although the exact mechanism by which this occurs remains poorly understood. In order to model ice crystal accretion, an estimate of the proportion of the impinging ice and water that sticks to a surface (the ‘sticking efficiency’) is required. This is believed to be dependent upon a number of parameters including particle melt ratio and diameter, and surface condition (rough or smooth, dry or wetted, warm or cold). This paper presents data from experiments undertaken in the National Research Council of Canada’s (NRC) Research Altitude Test Facility (RATFac). An axisymmetric test article, which featured three interchangeable cone ‘noses’ of varying half-angle, was used over a period of two weeks. A 35° half-angle nose was used for a parametric study of Mach number, Total Water Content (TWC), wet bulb temperature and particle size distribution (PSD). At selected test conditions, 20° and 45° half-angle noses were also tested. An assessment of the response of the Science Engineering Associates WCM-2000 multiwire probe in glaciated condition is presented, as a function of TWC, particle size and Mach number. A shadowgraphy technique was used to measure the ice accretion growth rate on the nose, with isometric camera views for qualitative assessments of spatial uniformity and build/shed events. The results show that sticking efficiency has a strong dependency on particle melt ratio, with maximum values attained when melt is typically between 9-13%. Erosion is shown to be correlated with particle size, Mach number and surface angle. New semi-empirical models are presented for sticking probability and erosion.