Transient heat transfer measurements of surface roughness due to ice accretion

A high-resolution, non-intrusive heat transfer measurement method based on Infra-Red thermography and transient heat transfer analysis is presented. This technique was successfully applied to NACA 0012 airfoils with castings of natural accreted ice surface roughness. A total of 11 representative icing conditions were reproduced at the Adverse Environment Rotor Test Stand (AERTS) facility at the Pennsylvania State University. The ice roughness was retained using ice molding and casting techniques. Roughness on casting models was categorized into a smooth zone and a rough zone. Smooth to rough zone transition location and ice limit of each case was recorded. Arithmetic averages of roughness height (R_a) were calculated for 8 locations on the chordwise direction for each of the casting models. Heat transfer measurement was conducted in a low-speed wind tunnel using thermal transient infra-red measurement techniques as well as multiple embedded heat flux and thermocouple sensors. The measured heat transfer coefficient was non-dimensionalized using Frossling numbers to account for the Reynolds variations between the compared tests. The Frossling number and roughness data were compared to LEWICE predictions in a parametric study. The effects of temperature, velocity, droplet size, liquid water content, and icing time on the heat transfer coefficient and Frossling number were examined. An over-prediction of LEWICE roughness height and corresponding heat transfer rate was quantified. A 200% to 391% over-prediction of the peak heat transfer value was identified from the comparison between LEWICE prediction and experimental measurements.