Flow analysis and nozzle-shape optimization for the cold-gas dynamic-spray process

Abstract

An isentropic, one-dimensional model is used to analyse the dynamics of dilute two-phase (feed powder particles plus the carrier gas) flow during the cold-spray process. While the physical foundation of the model is quite straightforward, the solution for the model can be obtained only numerically. The results obtained show that there is a particle-velocity-dependent, carrier-gas-invariant optimal value of the relative gas/particle Mach number that maximizes the drag force acting on feed powder particles and, hence, maximizes the acceleration of the particles. Furthermore, it is found that if the cold-spray nozzle is designed in such a way that at each axial location the acceleration of the particles is maximized, a significant increase in the average velocity of the particles at the nozzle exit can be obtained. For the optimum design of the nozzle, helium as the carrier gas is found to give rise to a substantially higher exit velocity of the particles than air. All these findings are in good agreement with experimental observations.