Ice Crystal Icing Physics Study using a NACA 0012 Airfoil at the National Research Council of Canada’s Research Altitude Test Facility

Abstract

This paper presents results from a study of the fundamental physics of ice-crystal ice accretion using a NACA 0012 airfoil at the National Research Council of Canada (NRC) Research Altitude Test Facility in August 2017. These tests were a continuation of work which began in 2010 as part of a joint collaboration between NASA and NRC. The research seeks to generate icing conditions representative of those that occur inside a jet engine when ingesting ice crystals. In this test, an airfoil was exposed to mixed-phase icing conditions and the resulting ice accretions were recorded and analyzed. This paper details the specific objectives, procedures, and measurements which included the aero-thermal and cloud measurements. The objectives were built upon observations and hypothesis generated from several previous test campaigns regarding mixed-phase ice-crystal icing. The specific objectives included (A) ice accretions under different wet-bulb temperatures, (B) investigations of steady-state ice shapes previously reported in the literature, (C) total water content variations in search of a threshold for accretion, and (D) probe characterization related to measuring melt fraction which is important to characterize the mixed-phase condition. The resulting ice accretions and conditions leading to such accretions are intended to help extend NASA’s predictive ice-accretion codes to include conditions occurring in engine ice-crystal icing. National Aeronautics and Space Administration (NASA) and the National Research Council (NRC) of Canada. The investigations focus on the fundamental physics associated with ice accretion. This investigation examines ice accretions on an airfoil test article exposed to ice-crystal and mixed phase conditions similar to those believed to exist in core compressor regions of jet engines. The conditions were generated using the NRC’s Research Altitude Test Facility (RATFac) which can introduce ice particles (and/or supplemental liquid water droplets) into an airflow in warmer than freezing conditions and various pressures. The partially melted ice particles and supplemental liquid water, if used, produce a mixed-phase condition which impinges on the airfoil test article. Under certain aero-thermal conditions and melt ratios The test section is a plane just upstream of the airfoil leading edge. The aero-thermal conditions are the total pressure ( P 0 ), total temperature ( T 0 ), Mach number, and humidity using the mass mixing ratio which is also referred to as the specific humidity ( SH ). The wet bulb temperature is adjusted by varying the humidity of the flowing air for a fixed T 0 and P 0 . Lower humidity results in lower wet-bulb temperatures but also more evaporation. The target conditions referred to “cloud-off” conditions, and use the subscript ‘ off ’, since these do not account for changes due to thermal interaction with the cloud. During cloud-on testing, the changes in aerothermal conditions, specifically temperature and humidity, were measured and referred to as the “cloud-on” conditions using the subscript ‘ on ’. The cloud parameters are the bulk ice water content ( IWC i ), ice particle volumetric diameter percentiles ( Dv 10 , Dv 50 , and Dv 90 ), bulk supplemental liquid water content ( LWC i ), and supplemental liquid water content median volumetric diameter ( MVD i ). The IWC i and LWC i are defined based on the ice and water feed rate, respectively, using the test section area assuming a uniform water distribution and no evaporation. The target ice particle sizes were based on prior NRC characterizations for a given grinder setting although recent changes to the grinder system necessitated measurement of the actual PSD . Similarly, the target droplet sizes for the LWCi were based on prior NRC characterizations and actual PSD measurements were made when able.