Assessment of Full-3D and 2D Sectional Collection Efficiency on a Rotor in Axial Flow Conditions

We analyze the behaviour of droplets impinging on the section of a propeller in axial flow conditions by comparing the collection efficiency in 2D and 3D simulations, and assess the range of validity of sectional simulations. For sectional simulations, we use both the aerodynamic angle of attack, including the rotor induced velocity, and the geometric velocity without accounting for induction effects. We observe that for droplets with a small sectional Stokes number (Stk_sec < 1), accurate collection efficiency predictions require a correct pressure distribution on the section. When Stk_sec ≫ 1, the droplet trajectories become ballistic. The full 3D solution can fall between the two 2D solutions before reaching the geometric ballistic regime, showing a transition regime from particles in equilibrium with the induced velocity to particles unaffected by it. Concerning in-flight icing, the transition regime can occur under Appendix C or Appendix O conditions, limiting the application range of 2D simulations.