Investigation of the Impact and Melting Process of Ice Particles

Since 2006, it is known that ice particles at high altitudes in the vicinity of deep convective clouds can pose a threat to aviation safety. When flying through regions containing ice particles, ice particles can fragment upon impact onto the aircraft’s engine and probe surfaces. These fragments can (partially) melt in the warm environment within the engine’s compressor or heated probe and can stick to warm surfaces (partially) covered with a water film. In such conditions, more incoming ice particles can cool down the surfaces and can cause significant ice accretion. Engine core or aircraft probe icing potentially leads to performance loss and false air data indications. Icing due to ice particles is a complex problem which includes ice particle impact onto dry and wet surfaces, particle melting, and ice accretion. Fundamental knowledge of the physical mechanisms governing these processes is limited. The physics are not yet fully understood and adequate models are very scarce. To advance the understanding and prediction of icing due to ice particles, several experimental and theoretical investigations were conducted in this work. To understand the physics of ice particle impact onto a dry surface better, impact experiments were conducted within an icing wind tunnel. Four different ice particle fragmentation modes were defined. Velocity scales and probability distributions for different fragmentation modes were successfully derived based on a model for the impact of semi-brittle spherical impactors onto a flat, rigid target. The restitution coefficients and post-impact angles of the fragments were observed to decrease with increasing particle diameter and impact velocity. The derived scaling laws agreed well with the restitution coefficients and post-impact angles of the fragments of large hail particles. To predict the melting process of ice particles with higher accuracy, melting experiments of suspended ice particles were conducted in a controlled airflow using an acoustic levitator. The melting processes of individual spherical and non-spherical ice particles were observed. A melting model for ice particles was introduced and adapted using two different approaches to approximate the particle surface area (i.e. the sphericity) of non-spherical ice particles. The model was successfully validated with spherical ice particles. The predicted melting times of non-spherical ice particles agreed very well with the experimental data. To expand the knowledge of ice particle impacts onto wetted surfaces, an experimental test apparatus was built which allowed the investigation of ice particle impacts onto a thin, controlled water film. The film thickness was between 130 and 600 µm. Sticking, bouncing, and fragmentation impacts of spherical ice particles were observed. It was determined that ice particles can impact a thin water film with nearly double velocity – compared to a dry wall – before fragmentation occurs. The mechanisms initiating ice accretion on a surface in a stream of fully frozen ice particles were experimentally observed on a microscale level. It was determined that target surface temperatures above freezing generated meltwater droplets by melting tiny ice fragments which deposited on the warm surface. These droplets allowed larger ice particles to stick to the surface due to capillary forces, potentially resulting in macroscopic ice accretion. It was also observed that meltwater covering partially melted ice particles can deposit on the target surface upon impact and initiate ice accretion as well. The investigations conducted in this thesis allow a better prediction of the fragmentation modes of ice particles upon impact onto dry and wet surfaces. The knowledge of the initial post-impact trajectories of the fragments allows a better prediction of particles’ and fragments’ trajectories and so of potential ice accretion locations within aircraft engines and probes. Applying the melting model for ice particles, the melt ratios of the ice particles upon impact - which determine icing severity - can be calculated more accurately. The main mechanisms which initiate ice accretion were identified, allowing for an efficient search for adequate countermeasures, like using superhydrophic, smooth surfaces, to reduce or delay ice accretion in future engines or probes