A Numerical Ice-Accretion Model

Abstract

AbstractA numerical model is developed for calculating the rate and total amount of ice accretion under atmospheric conditions. The principal application of the numerical approach is to aircraft icing and more specifically, helicopter icing problems. These problems are best solved using numerical techniques because of three factors: (1) the dependence of the ice accretion rate on the amount of ice previously deposited, (2) the existence of two different ice growth regimes, the “dry" and “wet" growth regimes, determined by the surface temperature of the accreting surface, and (3) variable velocities (e.g. along rotor blades) which affect the rate of capture of swept-out water droplets and the amount of heat generated by the flow on the accreting surface. These three factors cause feedback in the two governing equations for determining the mass rate of ice accumulation.The first of these equations is for the mass rate of water captured, and the second equation is for the heat balance of the accreting interface. For the numerical calculation, the object, such as a helicopter rotor blade, is broken down into elements of constant velocity, and for each time step the resulting ice thickness is used to recompute new cross-sectional and surface areas which are then used as input to the next time step. Changes in the cross-sectional and surface areas caused by ice build-up affect both the mass rate (directly through the cross-section and indirectly through a change in collection efficiency) and the heat balance (directly through the cross-sectional and surface areas and indirectly through changes in the collection efficiency and Reynolds number). An additional instability in the ice growth rate develops when the transition between wet and dry growth occurs, enhancing the feed-back that already exists between the mass rate of ice accumulation and the thickness previously deposited. Numerical icing simulations using various helicopter configurations and the icing conditions they typically encounter are presented.