Experimental Studies of Mixed-Phase Sticking Efficiency for Ice Crystal Accretion in Jet Engines

Abstract

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...

Eulerian method for ice crystal icing in turbofan engines

“…to a three-dimensional control volume equal to a grid cell with six faces this yields 25) where ∆t = t n+1 − t n and V is the volume of the grid cell. The approximation of the average flux…”

“…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].…”

“…Fully frozen particles impacting a cold surface will simply bounce and leave no or only a very thin layer of frost on the surface. Currie et al [22,25] have investigated the ice accretion on four different geometries: a wedge aerofoil with a chord of 221 mm and either a nose in the form of a hemisphere, cone or curved cylinder with 4.1 Particle impingement a (maximum) diameter of 44.5 mm attached to a streamlined afterbody. The icing surfaces of the geometries are made of a titanium alloy.…”

“…K d depends on the substrate temperature T s because the lower the substrate temperature the faster the droplets will freeze resulting in a smaller amount of remaining liquid to which the crystals can stick. The model in Equation (4.4) is based on the experimental observation from Currie et al [22,25], described in section 4.1.2, that the particle sticking efficiency strongly depends on the ratio of LWC i c and IWC. Furthermore, two separate efficiencies for mixed phase (ice crystals and droplets) and melting phase (partially melted ice crystals) conditions are defined because it was observed that for the same ratio LWC/TWC the sticking efficiency was higher for melted particles than for mixed conditions.…”