A study on converging-diverging nozzle design for supersonic spraying of liquid droplets toward nanocoating applications

Supersonic cold spraying of liquid droplets containing functional nanomaterials is of particular interest in advanced thin-film coating, that enabling high-adhesion strength particle deposition. In cold spraying, the optimum design of the supersonic nozzle is essential for accelerating particles to desired velocities. However, research on the supersonic nozzle design for liquid droplets is limited. Thus, we thoroughly investigate the influence of nozzle geometrical parameters (i.e., throat diameter, exit diameter, divergent length) on droplets acceleration by numerical modeling followed by experimental validation, and a case study on surface coating application. Two-phase flow modeling was used to predict droplets' behavior in continuous gas flow for different nozzle configurations. The results show that the nozzle expansion ratio - a function of throat and exit diameters - has a significant influence on droplet velocity, followed by divergent length. In particular, to correctly accelerate “low-inertia liquid droplets”, optimum nozzle expansion ratio for an axisymmetric convergent-divergent nozzle is found to be in a range of 1.5-2.5 for different sets of parameters, which is different than the recommended expansion ratio (i.e., 5-9) for cold spraying of conventional “metal” particles. Based on the simulation results, an optimal design of supersonic nozzle is selected and prototyped for the experimental studies. Numerical modeling results are validated by particle image velocimetry (PIV) measurements. Moreover, coating experiments confirm the adaptability of the optimized nozzle for supersonic cold spraying of droplets containing nanoparticles, which thereby has the potential for rapid production of advanced thin films.