Experimental frossling numbers for ice-roughened NACA 0021 Airfoils

Experimental Frossling numbers are presented for two aluminum castings of ice-roughened NACA 0012 airfoil surfaces at 0-deg angle of attack for chord Reynolds number ranging from 4.00 OE 105 to 1.54 OE 106. The castings were meticulously obtained from actual ice accretions representing mildly rough glaze and rough glaze ice with horns. A modified investment casting technique was used to capture all of the surface roughness details. The rough glaze ice with horns produced higher heat-transfer rates than those for the mildly rough glaze ice, especially at the horns. Immediately downstream of the horns, stagnation air gaps resulted and caused lower heat-transfer coefficients. For both types of ice, higher Reynolds numbers in general produced higher heat-transfer coefficients. For the same chord Reynolds number and at the same position on the airfoil, the Frossling numbers were generally higher than those for the smooth case and those for the hemispherical roughness elements of previous studies. A maximum increase of approximately 306 percent over the smooth case and 192 percent over the dense hemispherical roughness case was recorded at one rough glaze ice horn. This work provides some directly measured values of the Frossling number needed to improve the prediction of some icing codes. Such icing codes help in the effective design of some deicing systems of aircraft.