Shaping supersonic contoured nozzles for cold spraying metallic particles

Cold sprayed metallic coats can provide an appropriate surface finish for a range of applications, including marine propellers, boat keels, and ablating surfaces in aerospace. Cold spraying can coat surfaces at a comparatively lower environmental impact with respect to plasma spray or chemically bonded sprays. In a metal cold spray, the kinetic energy imparted to the metal particles by a high speed gas provides the means for particles to plastically deform and deposit on the target substrate. Therefore, a uniform particle distribution of uniform velocity is typically desirable to achieve a good quality deposition rate and to reduce wastage from particles failing to splatter on the target substrate. This work revisits the method of characteristics technique for designing axisymmetric nozzles for generating lightly laden jets. By Computational Fluid Dynamics, the performance of current commercial cold spray nozzles is compared with new nozzle profiles, designed with a smooth throat and for a parallel (axial) outflow. The two-phase flow is modelled by the compressible Reynolds-Averaged Navier-Stokes equations with k-omega turbulence closure, for the primary (gas) phase, and a Lagrangian discrete phase model is solved for the particle transport. Two-way coupling renders the kinetic interaction between the two phases. Particle dispersion is modelled by a discrete random walk model. Preliminary results indicate that the best performing shapes differ from the classic minimum-length bell-shaped nozzle for space propulsion and are closer to the contoured walls of supersonic wind tunnels. Plans for testing prototype nozzles are under development in collaboration with The Welding Institute (TWI), Cambridge, under the auspices of the EPSRC Doctoral Training Programme IMPaCT.