Numerical investigation of the gas and particle dynamics during the cold spray process.

Abstract

The present numerical investigation of the cold spray process examines the gas and particle dynamics both inside and outside the nozzle in the context of improving particle deposition. In the first part of the investigation, the one-dimensional isentropic relations coupled with the drag force acting on a particle is employed in a parametric study of the nozzle performance. Next, the addition of a barrel section to the end of the nozzle is proposed and shown to be more efficient in some cases than using the diverging section of the nozzle for particle acceleration. Finally, the effect of nozzle wall friction is incorporated to assess the amount of deviation from the isentropic model. The gas and particle dynamics between the nozzle and substrate during the steady cold spray process is the focus of the second part of the investigation. An underexpanded and overexpanded nozzle is employed to accelerate the particles and the operating conditions were set to those used in the validation cases. The particle impact statistics are extracted to provide information on the particle impact speed, angle and location. The particles are also tracked during their flight between the nozzle and substrate to characterise their response to changes in the gas flow. It was found that the variation in particle speed across the embedded shock structures became minimal as the diameter increased. For particles with a Stokes number greater than one, the nozzle exit velocity may be used as an approximation of the impact speed. A theoretical model is also proposed for calculating the particle impact speed using the nozzle exit conditions. The deployment of the shock tube in cold spray is a recent innovation in which a planar shock is released into the ambient air towards a substrate while the particles are injected across the wake of the shock. Although the process suffers from a number of practical limitations, monodisperse particles are used to compare the impact speed produced by steady and unsteady cold spray processes. It was found that this process also offers a mean of studying the shock formation process in underexpanded impinging jets. The shock speeds were selected such that either a shock diamond or Mach disk is reproduced in the impingement region characteristic of a moderately and highly underexpanded jet. Although the unsteady cold spray process requires a much lower driving pressure to produce the impact speeds found during the steady process, there are several practical limitations associated with it.