Comparison of Freeze-Out versus Grind-Out Ice Crystals for Generating Ice Accretion Using the ICE-MACR

Since the introduction of ice crystal icing certification requirements [1], icing facilities have played an important role in demonstrating compliance of aircraft air data probes, engine probes, and increasingly, of turbine engines. Most sea level engine icing facilities use the freezing-out of a water spray to simulate ice crystal icing conditions encountered at altitude by an aircraft in flight. However, there are notable differences in the ice particles created by freeze-out versus those observed at altitude [2, 3, 4]. Freeze-out crystals are generally spherical as compared to altitude crystals which have variable crystalline shapes. Additionally, freeze-out particles may not completely freeze in their centres, creating a combination of super-cooled liquid and ice impacting engine hardware. An alternative method for generating ice crystals in a test facility is the grinding of ice blocks or cubes to create irregular shaped crystals. These grind-out particles have a different morphology to atmospheric crystals. but are fully glaciated and their irregular shapes may better approximate the fracture dynamics of atmospheric crystals when impacting engine hardware. The National Research Council (NRC), in collaboration with Transport Canada Civil Aviation (TCCA), have studied the differences between using freeze-out generated ice crystals and grind-out ice crystals to generate ice accretion in a compressor rig: the ice-crystal environment-modular axial compressor rig (ICE-MACR) in the NRC’s altitude icing wind tunnel (AIWT). Comparison of the freestream ice crystal morphologies is presented as well as the fractured particle characteristics downstream of a two-stage compressor within the compressor annulus. Qualitative and quantitative comparisons are made of the accretion behaviour resulting from the two ice-crystal generating methods. It was found that while particle morphology differs considerably between freeze-out and grind-out before rotor impact, fractured particle size and accretion within the rig was similar for both methods for the limited range of overlapping conditions that could be produced in the test facility.