Investigation of the Influence of Aero-Thermal Non-equilibrium Conditions of an SLD Cloud on Airfoil Icing

This study examines the impact of slip in aero-thermal conditions of supercooled large droplets (SLD) produced in an Icing Wind Tunnel (IWT) on the ice accretion characteristics. The study identifies potential biases in the SLD model development based on IWT data and numerical predictions that assume the SLD to be in aerothermal equilibrium with the IWT airflow. To obtain realistic temperature and velocity data for each droplet size class in the test section of the Braunschweig Icing Wind Tunnel (BIWT), a Lagrangian droplet tracking solver was used within a Monte Carlo framework. Results showed that SLDs experience considerable slips in velocity and temperature due to their higher inertia and short residence time in the Braunschweig IWT. Large droplets were found to be warmer and slower than the flow in the test section, with larger droplets experiencing larger aerothermal slips. To examine the impact of these slips, numerical ice accretion simulations were performed on a NACA 0012 airfoil using in-house icing software PoliMIce with realistic droplet slip conditions and ideal equilibrium conditions. Results showed that an increase in velocity slips with larger droplets led to a reduction in the impingement water mass rate and impingement limits. Further, the velocity slip reduced the splashed mass rate estimated by NASA LEWICE and ONERA SLD models. The reduced supercooling of larger droplets reduces the freezing fraction and produces extended runback limits at warmer temperatures ([-1, -5] °C). However, the influence of thermal slip is negligible below -5°C. The study concludes that aero-thermal slips significantly alter the ice accretion characteristics of SLD. It emphasizes the need to consider these slips in IWT data-driven SLD icing model development and subsequent SLD ice accretion simulations.