Experimental assessment of a water-film-thickness Weber number for scaling of glaze icing

Abstract

The evidence indicates that current practices for scaling of glaze icing tests do not recognize one or more important parameters. A Weber number based on a measure of the thickness of the liquid water film formed on the surface of the accreting ice has been proposed in another paper as the required additional scaling parameter. The present paper reports on experiments specifically designed to assess this proposal. Icing wind tunnel tests were done on 45 mm and 20 mm circular cylinders; the former constituted the reference cases and the latter, the sub-scale cases. In all sub-scale tests the accumulation parameter, the droplet inertia parameter and the calculated freezing fraction were made equal to the corresponding reference values. Freestream velocity for sub-scale test runs was chosen using several scaling parameters, including the newly proposed one. It was found that reasonably good similarity of ice accretion shapes was obtained for all of the sub-scale velocities that were tried, provided that the freestream static temperature was the same as that in the reference case. Possible explanations are suggested. When sub-scale freestream static temperature was the same as the reference value, the freestream velocities determined using Weber numbers based on water-film thickness and on droplet size were approximately equal, and this velocity gave marginally better similarity of ice shapes than velocities chosen on other bases. Solid aluminum and solid Plexiglas models gave essentially the same ice shapes for corresponding conditions. Most of the findings of the work are very preliminary and much more work is required to explore them. Nomenclature Ac accumulation parameter Ca capillary number, |awV/a d, D droplet diameter, cylinder diameter hc convective heat transfer coefficient K droplet inertia parameter L length scale of the overall air flow field (L = D in the present case) LWC liquid water content MVD mean volume diameter of water spray droplets n freezing fraction p., freestream static pressure qe/qc ratio of evaporative to convective heat transfer rates Re Reynolds number, pVD/|-i ^ water-film thickness T, freestream static temperature V freestream velocity Wed, Wer,7 We. Weber number based on droplet OLameter, body diameter and water-film thickness; e.g. Wed = pVM/a P local collection efficiency |i , j^ viscosity of air, viscosity of liquid water * Professor, Dept. of Mechanical and Aerospace Engineering, Assoc. Fellow AIAA. Senior Research Officer, Aerodynamics Laboratory, Institute for Aerospace Research Copyright © 2000 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved 1 American Institute of Aeronautics and Astronautics (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.