Experimental and numerical investigation of accretion inception and heat transfer physics in ice crystal icing

Abstract

Earlier research highlighted severe detrimental effects related to aircraft performance due to ice crystal icing. As part of current research impact of glaciated icing cloud on heated substrates is investigated. To that end, a series of experiments were carried out at the icing wind tunnel of Technische Universitt Braunschweig to investigate the physics of ice crystal accretion on heated substrates for various operating conditions. Dedicated transparent and metallic heatable substrates were designed for macro and microscopic investigation of ice accretion from a qualitative visualization and heat transfer physics perspective. In addition to that numerical simulations were performed at ONERA. Qualitative observations from dedicated experiments on ice accretion initiation phase resulted in advancement of numerical model to accurately capture the different stages leading up to ice accretion as well as the necessary conditions for growth of slushy and glaciated ice layers. Furthermore, a dedicated test matrix was defined to explicitly study the influence of dominant parameters such as flow velocity, heat flux, wet bulb temperature and ice water content on ice accretion process. The experiments showcased a strong influence of increasing flow velocity and ice water content yielding shorter duration required to accrete an ice layer on a heatable substrate. Further experimental investigation reflected that upon increasing the heating power of the test article the icing cloud had to overcome a larger temperature gradient resulting in longer duration required for accretion. It was found that on one hand, the test run with negative wet bulb temperature required a heating source from the substrate as the necessary condition for ice accretion resulting in glaciated ice layers. On the other hand, for positive wet bulb temperature cases natural melting of ice layer was sufficient to induce ice accretion generating slushy ice layers and the heating source from the substrate had little to no influence on the overall ice accretion growth. Numerical simulations were also performed for same conditions and were able to correctly capture the trends and orders of magnitude in comparison with experimental results. The experimental findings presented in this paper lead to development of an accretion solver based on enthalpy approach which considers accretion process as a homogeneous mixture of crystals and liquid water and not by a mere superposition of ice layer and liquid water film. The findings helped calibrate, validate and advance the numerical model for ice crystal icing and found to be more representative of accretion process than unsteady triple layer approach. The findings ensure better predictive capability resulting in improved flight safety and performance criterion.