Development and Application of an Impedance-Based Instrument for Measuring the Liquid Fraction and Thickness of Ice Crystal Accretions

Ice crystals ingested by a jet engine at high altitude can partially melt and then accrete within the forward stages of the compressor, potentially causing performance loss, damage and/or flameout. Recent research into this ice crystal icing (ICI) phenomenon conducted at the National Research Council of Canada suggests that the liquid water content v_liq of an accretion significantly affects the accretion's susceptibility to erosion by ice crystals, and therefore accretion growth. This paper describes the development and application of an instrument for measuring v_liq, potentially providing a method for correlating erosion behavior (e.g. as ductile or brittle) and properties. The instrument measures the complex admittance Y* of a mixed-phase deposit bridging a pair of electrodes, which is modeled as a resistor and capacitor in parallel, and calculates the deposit's relative permittivity ε_r from the capacitance. Experiments and electrostatic simulations were conducted to correlate ε_r as a function of v_liq for v_liq up to ∼40%, using calorimetry to determine v_liq. These measurements were made at an excitation frequency of 2MHz, where the relative permittivities of ice and liquid water are insensitive to change of frequency. The paper also describes icing tests where the instrument was used to measure v_liq as a function of freestream liquid-to-total water content (LWC/TWC) for two test articles, a cone and a forward-facing cup. The measurements show that v_liq lies in the range 25-33% when LWC/TWC is in the range giving worst accretion (LWC/TWC∼10-20%). The instrument is also used to measure thickness growth rate for the cup.