Altitude Scaling of Ice Crystal Accretion

Abstract

This paper describes experiments performed in an altitude chamber at the National ResearchCouncil of Canada (NRC) as the first step towards developing altitude scaling laws and procedures that will possibly allow aero-engines to be certified for operation in ice crystal clouds at high altitude by testing in sea level facilities. The principal objective was to test the hypothesis that accretion within a compressor due to ice crystal ingestion occurs when the local ratio of freestream liquid water content (LWC) to total water content (TWC) lies within a critical range at an accretionsusceptible location. If this hypothesis is correct, the local LWC/TWC ratio is the key parameter that must be matched in tests at low and high pressures to match accretions. Experiments were conducted in a small wind tunnel with an axisymmetric test article, consisting of a hemispherical nose attached to a conical afterbody, at a fixed TWC over a range of LWC/TWC ratios at (absolute) pressures of 34.5 kPa and 69 kPa to test the hypothesis. The LWC/TWC ratio was varied by changing the wet bulb temperature. Accretion steady-state volumes and growth rates measured at the two pressures were compared at conditions which were analytically predicted to produce matched LWC/TWC ratios. Good agreement was achieved in all cases. Accretion growth was greatest for LWC/TWC ratios in the range 10-25%. Additional tests demonstrated that wet bulb temperature, which was identified as an important variable in earlier studies, had little influence on accretion growth beyond its effect on LWC/TWC (i.e. ice particle melting). Tests were also conducted to determine whether accretion growth scales linearly with TWC at constant LWC/TWC. Those tests confirmed that not only does the accretion growth rate in the early growth phase scale in direct proportion to TWC , but so does the final size of the accretion. A simple semi-empirical model for predicting this behavior is described. While most of the tests were conducted with an ice particle median volumetric diameter of 45μ, some of the scaling tests were repeated with larger particles, which produced smaller accretions. Nomenclature& a, b = TWC-dependent coefficients in relation for t A = surface area C p = specific heat of air at constant pressure C ice = specific heat of ice d = diameter h c = heat transfer coefficient L evap = latent heat of vaporization of water L f = latent heat of fusion of water L subl = latent heat of sublimation of ice IWC = Ice Water Content (g/m 3 ) LWC = Liquid Water Content (g/m 3 ) m = mass = mass flow rate 1 = mass flux (mass flow rate per unit area) M = Mach number MVD = median volume diameter of ice particles, micrometer NRC = National Research Council of Canada Nu = Nusselt number (= h c d/k) p = pressure p ref = reference pressure (10 5 Pa) Pa = Pascal (N/m 2 ) Pr = Prandtl number (= µC p /k) = heat flux r = radius RATFac = Research Altitude Test Facility Re = Reynolds number (= ρUd/µ) t = accretion thickness t o = accretion thickness at stagnation point t & = thickness growth...

Eulerian method for ice crystal icing in turbofan engines

“…This conclusion has been based on the observation that adding ice crystals to a free stream with a wet bulb temperature below freezing led to an increase of ice growth on the leading edge, but to a decrease of ice growth further downstream. Currie et al [24,25] have also observed that at constant LWC i c /TWC ratio, the initial ice growth rate as well as the steady-state ice accretion increase proportionally with increasing TWC. At the steady-state the impinging mass flux is equal to the mass flux lost due to particle bouncing or re-impingement, phase change and erosion.…”

“…• erosion scales with the square of the tangential velocity [23,24,25], • erosion decreases for increasing IWC due to flux interference [24,25], • erosion increases for increasing melting ratio η i c = LWC i c /TWC [24,25], • erosion increases with increasing ice crystal diameter d i c [71,72], • erosion increases with increasing curvature of the surface of the ice layer [132].…”

“…The reference values, with subscript 0, and the constant parameters E and a through e are given in Table 5.3 and have been fitted to correspond with experimental data of Currie et al [23,24,25], Al-Khalil et al [2] and Lozowski et al [91]. For a detailed description of this fitting procedure the reader is referred to Trontin et al [132].…”

“…Examples of the obtained ice accretions are shown in Figure 5.6. Currie et al [24,25] have performed a study on ice accretion for two different geometries. One geometry consisted of an axi-symmetric hemispherical nose with a diameter of 44.5 mm attached to a streamlined afterbody.…”

“…The first three of these facilities are capable of simulating ice crystals under various altitude conditions. These upgrades have resulted into a number of experimental studies on glaciated and mixed-phase accretion for various geometries: an aerofoil shape by Struk et al [127], a NACA-0012 aerofoil by Al-Khalil et al [2] and Struk et al [126], a double-wedge aerofoil by Currie et al [23,22] and Struk et al [124,126], a streamlined body with an axi-symmetric hemispherical nose, with a cylindrical nose and with a conical nose by Currie et al [22,24,25], a bleed slot in a compressor stage by Knezevici et al [71,72] and an aerofoil in a compressor transition S-duct by Mason et al [96].…”