Abstract
It has been recognised in recent years that high altitude atmospheric ice crystals pose a threat to aircraft engines. Instances of damage, surge and shutdown have been recorded at altitudes significantly greater than those associated with supercooled water icing. It is believed that solid ice particles can accrete inside the core compressor, although the exact mechanism by which this occurs remains poorly understood. Development of analytical and empirical models of the ice crystal icing phenomenon is necessary for both future engine design and this-generation engine certification. A comprehensive model will require the integration of a number of aerodynamic, thermodynamic and mechanical components. This paper studies one such component, specifically the thermodynamic and mechanical processes experienced by ice particles impinging on a warm surface. Results are presented from an experimental campaign using a heated and instrumented flat plate. The plate was installed in the Altitude Icing Wind Tunnel (AIWT) at the National Research Council of Canada (NRC). This facility is capable of replicating ice crystal conditions at altitudes up to 9 km and Mach numbers up to 0.55 [1]. The heated plate is designed to measure the heat flux from a surface at temperatures representative of the early core compressor, under varying convective and icing heat loads. Heat transfer enhancement was observed to rise approximately linearly with both total water content and particle diameter over the ranges tested. A Stokes number greater than unity proved to be a useful parameter in determining whether heat transfer enhancement would occur. A particle energy parameter was used to estimate the likelihood of fragmentation. Results showed that when particles were both ballistic and likely to fragment, heat transfer enhancement was independent of both Mach and Reynolds numbers over the ranges tested.
Highlights
Over 240 icing related engine events have been recorded since 1990
Mason et al theorised that partially melted ice particles may stick and refreeze on these surfaces, the heat transfer rate between particles and the surface are of great interest
A study was conducted by the National Research Council of Canada (NRC), Boeing and NASA Glenn Icing Research Branch in 2009 [3] using an S-shaped intercompressor duct with integral titanium strut
Summary
Over 240 icing related engine events have been recorded since 1990. Mason et al [2] analysed 46 of these events, GTP-17-1386 BUCKNELL many of which occurred at altitudes above 7000 m, the recognised upper limit at which water can exist in liquid form.Fig. 1 shows the fan and compressor of a generalised two shaft turbofan engine, with the low pressure compressor (LPC) stators and intercompressor duct highlighted as areas of potential ice crystal accretion. A study was conducted by the NRC, Boeing and NASA Glenn Icing Research Branch in 2009 [3] using an S-shaped intercompressor duct with integral titanium strut. The study concluded that the likelihood of accretion is maximised over a specific range of liquid-to-total water content ratios (‘melt ratios’). At ratios below this range, particles are more likely to bounce, whereas at high ratios the low ice quantity and water runback inhibit accretion. A wedge-shaped simplified aerofoil was used by NASA/NRC in the Research Altitude Test Facility (RATFac) in 2010 [4] to measure accretion growth rates for a range of aerodynamic and icing conditions, and to measure heat transfer in dry conditions
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