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. For droplets sufficiently small, we observe that accurate 2D collection efficiency predictions require a correct pressure distribution on the section. When the sectional Stokes number Stksec (formula Presented) 1, the droplet trajectories become ballistic. The full 3D solution shows a transition regime between the 2D solutions, i.e., from particles in equilibrium with to particles unaffected by the induced velocity, before reaching the geometric ballistic regime. We relate this regime to an induction Stokes number. For in-flight icing, the transition regime can occur under Appendix C or Appendix O conditions, limiting the application range of 2D simulations.