Aircraft jet engines operating at high altitudes in ice crystal clouds can experience operational problems and/or damage resulting from accretion of ingested ice crystals within the compressor. It is believed the ice crystals partially melt, allowing them to stick to internal components. A method for modelling such mixedphase accretion is required to de-risk new engine designs, modify existing designs with such icing issues and define critical operating points for scrutiny in proposed ice crystal certification tests. This paper presents preliminary results for a modelling approach which treats the accretion process as strictly a sticking phenomenon, largely ignoring heat transfer, phase change, runback and other location-dependent effects commonly used in the analysis of supercooled water icing. Ice-on-ice growth is described by a sticking efficiency, defined as the fraction of the mixed-phase impinging mass flux which remains on the surface (i.e. sticks). Experimental results are presented for 3 test articles tested in a small mixed-phase icing tunnel located in an altitude chamber (Research Altitude Test Facility or RATFac) at the National Research Council of Canada. These results show that the sticking efficiency is highly correlated with the ratio of liquid water content (LWC) to total water content (TWC) in the freestream, reaching a maximum value of 0.4-0.5 at melt (LWC/TWC) ratios in the approximate range 10-20%, as measured with a multi-element probe. It is shown that sticking efficiencies are largely independent of TWC, Mach number (M) and particle size at normal incidence (i.e. at the stagnation point) at these melt ratios, at least in the limited ranges of these variables investigated, but are strongly dependent on these parameters at oblique impingement angles. It is also shown that accretions can grow to a very large size at an almost constant rate at high levels of TWC. The experimental results are used to develop an erosion-based semi-empirical accretion model which at least partially explains this super-growth phenomenon and predicts most of the experimental results with acceptable fidelity. The model predicts that the almost unlimited growth observed in the experiments is possible at lower Mach numbers (e.g. 0.25) for TWC levels exceeding ~10g/m 3 , when the sticking efficiency remains finite at all particle impingement angles. The model also predicts that such growth is unlikely for higher Mach numbers (e.g. 0.4), at least for the 45μ (MVD) particles to which the model is applied. Smaller particles will likely extend the Mach number range over which the sticking efficiency remains finite. Nomenclature a, b = TWC-dependent coefficients in relation for t & a f = coefficient in expression for flux reduction factor f A,B = constants dependent on mechanical, physical properties of eroded material and erodent particles D = diameter D xx = particle diameter below which the cumulative particle volume is xx% of the total particle volume DSD = droplet size distribution E = erosion function (of γ) f = eros...

This paper describes experiments performed in an altitude chamber at the NationalResearch Council of Canada (NRC) as the first phase of a joint NRC/NASA program investigating ice crystal accretion in aero engines. The principal objective was to explore the effect of wet bulb temperature T wb (dependent on air temperature, humidity and pressure) on accretion behavior, since preliminary results published in an earlier paper indicated that well-adhered accretions are only possible at T wb <0°C, when water in an impinging mixedphase flow can freeze to a surface. To assess the accretion sensitivity to T wb , the symmetrical airfoil used in the previous work was tested at pressures of 44.8 kPa and 93kPa, usually at 0.25 Mach number, over a range of freestream liquid water and ice water concentrations, total air temperatures and humidity levels. T wb was typically maintained at +2°C or -2°C, based on dry total conditions (i.e. without ice or water injection). Total air temperature was >0°C in all tests. The limited test results confirmed that accretion behavior is very sensitive to T wb , which is in turn strongly related to pressure since evaporative cooling increases with decreasing pressure. Humidity and total temperature did not appear to have an independent effect on accretion behavior. Accretions, often resembling glaze ice, formed at T wb <0°C, when freestream water would freeze on the test airfoil without ice crystals present in the freestream. At T wb >0°C ice deposits were observed to be slushy, poorly adhered and shed frequently. The size of such deposits appeared to be a non-linear function of the freestream ice water content (IWC), becoming much larger at high IWC. NomenclatureAOA = airfoil angle of attack (°), positive for nose up C p = specific heat D = diffusion coefficient of water vapor in air h c , h m = heat, mass transfer coefficient k = thermal conductivity 1 2 L evap = latent heat of vaporization of water Le = Lewis number (= Pr/Sc) LE = wedge airfoil leading edge IW LW = mass flux C = Ice Water Content (g/m 3 ) C = Liquid Water Content (g/m 3 ) M = Mach number M wt = molecular weight MMD = median mass diameter of ice particles, micrometers NASA = National Aeronautics and Space Administration NRC = National Research Council of Canada Nu = Nusselt number (= h c x/k) p = pressure P P = heat flux a = Pascal (N/m 2 ) r = Prandtl number (= µC p /k) RATFac = Research Altitude Test Facility Re = Reynolds number (= ρUx/µ) RH = relative humidity, % R u = universal gas constant Sc = Schmidt number (= µ/ρD) SH = specific humidity (mass of water vapor/unit mass dry air) t = ice thickness T = temperature TWC = Total Water Content (= IWC + LWC) U = velocity x = distance (length) µ = dynamic viscosity ρ = density ρ acc = accretion density (≈( IWC* ρ ice + LWC* ρ liquid )/TWC, for ice and liquid densities ρ ice and ρ liquid respectively) Subscripts conv = convective evap = evaporative i = injected with the ice grinder or water spray nozzles. Ice or water mass flowrate divided by tunnel air volumetric flowrate to ...

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...

Due to numerous engine power-loss events associated with high-altitude convective weather, ice accretion within an engine due to ice-crystal ingestion is being investigated. The National Aeronautics and Space Administration (NASA) and the National Research Council (NRC) of Canada are starting to examine the physical mechanisms of ice accretion on surfaces exposed to ice-crystal and mixed-phase conditions. In November 2010, two weeks of testing occurred at the NRC Research Altitude Facility utilizing a single wedge-type airfoil designed to facilitate fundamental studies while retaining critical features of a compressor stator blade or guide vane. The airfoil was placed in the NRC cascade wind tunnel for both aerodynamic and icing tests. Aerodynamic testing showed excellent agreement compared with CFD data on the icing pressure surface and allowed calculation of heat transfer coefficients at various airfoil locations. Icing tests were performed at Mach numbers of 0.2 to 0.3, total pressures from 93 to 45 kPa, and total temperatures from 5 to 15°C. Ice and liquid water contents ranged up to 20 and 3 g/m 3 , respectively. The ice appeared well adhered to the surface in the lowest pressure tests (45 kPa) and, in a particular case, showed continuous leadingedge ice growth to a thickness greater than 15 millimeters in 3 minutes. Such widespread deposits were not observed in the highest pressure tests, where the accretions were limited to a small area around the leading edge. The suction surface was typically ice-free in the tests at high pressure, but not at low pressure. The icing behavior at high and low pressure appeared to be correlated with the wet-bulb temperature, which was estimated to be above 0°C in tests at 93 kPa and below 0°C in tests at lower pressure, the latter enhanced by more evaporative cooling of water. The authors believe that the large ice accretions observed in the low pressure tests would undoubtedly cause the aerodynamic performance of a compressor component such as a stator blade to degrade significantly, and could damage downstream components if shed.https://ntrs.nasa.gov/search.jsp?R=20120004044 2018-05-12T11:53:54+00:00Z

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