The Water Film Weber Number in Glaze Icing Scaling 2007-01-3295

Surface water film dynamics is recognized as playing an important role in the glaze ice accretion process. The solution, in general scaling problems, requires the match of all the relevant non-dimensional parameters to take place individually. However, in water film dynamics similarity, we encounter that is not possible for both Reynolds and Weber numbers to be matched simultaneously, and the strict individual parameter similarity condition has to be relaxed and combination of parameters accepted as a reasonable compromise. In this paper, a film Weber number will be defined and then proposed as a similarity parameter along with the non-dimensional film thickness as well as others that have been long recognized as well established, to form the set of equations that determine the scaled variables in terms of the reference ones. The water film Weber number (Wef) is defined for the airfoil leading edge region and based on the water film thickness and the water film edge velocity, besides the water properties of density and surface tension (water/surface). In order that this parameter can be easily handled in practice, the edge film velocity is written in terms of the external flow velocity through the air/water interface shear equality. This permits finally to write Wef in terms of the non-dimensional water film thickness followed by the product of Reynolds and Weber numbers, both based on external velocity and body dimensions.

A first group of icing evaluation examples is included for the case of small droplets (Appendix C) and relatively high LWC, where the film thickness is experimentally measured at room temperature and the similarity scale results evaluated in icing conditions, through comparison with other methods that offer values for the scale variables that are close to the ones obtained by the present method. The second group of examples considered is for large droplets (SLD) and the corresponding set of scale variables obtained are evaluated in icing conditions, using the same procedure as for the Appendix C cases.