Experimental and numerical studies on ice crystal icing of civil aircraft

Abstract Icing has been a severe weather hazard for aircraft operation since the beginning of civil aviation. In this context, the phenomenon of ice crystal icing has been identified as a risk for flight safety in the recent past. High concentrations of tiny ice crystals can particularly be found in the vicinity of convective cloud systems in tropical regions. In a warm air environment and upon impact on heated surfaces the ice crystals are forced to partially melt. So-called mixed phase conditions develop, enabling the particles to stick to solid surfaces and to form a cohesive ice accretion layer. Ice crystal icing generally occurs on heated stagnation pressure probes and engine stator compressor blades. As a result, biased pressure measurements can cause a false display of the airspeed and the loss of the autopilot. Engine icing can effect mechanical damages and may result in thrust losses or even a flame out. Comprehensive icing wind tunnel studies on ice crystal icing of generic aerodynamic test articles are the main focus of this thesis work. The experimental results are compared with complementary numerical simulations, performed by ONERA with the in-house icing code IGLOO2D. Moreover, high-speed sequences of ice particle impact during accretion growth have been recorded. In preparation of the experiments, the ice crystal simulation capabilities have been established at the Braunschweig Icing Wind Tunnel. Close replicates of natural ice crystals are grown by an innovative production system based on cloud chamber technology. The development of the system, as well as the icing wind tunnel calibration, are described with regard to the experimental icing studies. The ice accretion experiments have shown a strong influence of the ambient temperature on the icing process. In agreement with literature findings, the ice particle sticking ability can be correlated with the ice cloud composition. Correlations between accretion shape and growth rate have been identified. The experimental and numerical findings of this study can be considered to be complementary to the existing knowledge on ice crystal icing as they have been performed at relatively low Mach numbers and temperatures. Therefore, the experimental results have been provided to an international benchmark of test cases for icing code validation. The comparison between computational and experimental results indicates that erosion has a minor influence on the ice accretion shape due to low particle impact velocities. Moreover, high-speed imaging has shown that for the low Mach number conditions of the present study, ice particle sticking appears by mechanical interlocking of particle fragments with the irregular accretion surface.