Evidence for importance of scaling viscous effects in the water film in glaze icing tests

Abstract

Nomenclature It would be very desirable to be able to carry out valid reduced scale glaze icing tests in icing tunnels. In glaze icing some liquid water is present on the surface of the ice accretion; this paper reports on icing-tunnel experiments which explored the influence of surface tension and viscosity of this water on icing behaviour and scaling laws. Icing tests were done with 45 mm and 20 mm diameter cylinders, the 'reference' case and the 'subscale' case, respectively. The reference and some subscale runs were done using pure demineralized spray water; other sub-scale runs were done with waterpropanol solutions. In the latter runs, 1-propanol was added to the spray water in concentrations of either 1 % or 5% in order to reduce the surface tension of the water. In some of the sub-scale runs the Weber number based on cylinder diameter was matched to the reference value while in other runs a parameter involving water viscosity was matched instead. All other accepted similarity parameters were matched in all cases. The results indicate that water viscosity is important and that the relevant Weber number is the one based on thickness of the water film or rivulets present on the ice surface. Both of these are new hypotheses. If these hypotheses are correct, the relevant Weber number would 'automatically' be matched if the water-viscosity parameter and other similarity parameters are matched and there would be no incentive to manipulate the surface tension of the spray water. d mean volume diameter of droplets D cylinder diameter hc convective heat transfer coefficient hfs latent heat of fusion of the spray water solution J a length scale; thickness of water film or rivulet size LWC liquid water content (g/m) qc convective heat loss rate q,. heat loss rate due to evaporative mass transfer t time Ts surface temperature T. free stream total temperature V air velocity in free stream H air viscosity 1 viscosity of liquid water p air density pdr density of liquid droplets a surface tension