A Study on Converging-Diverging Nozzle Design for Supersonic Spraying of Liquid Droplets Toward Nanocoating Applications

Supersonic cold spray (CS) of functional nanomaterials from atomized droplets has attracted significant attention in advanced thin-film coating as it enables particle deposition with high-adhesion strength. In CS, optimum design of the supersonic nozzle (i.e., converging-diverging nozzle) is essential for accelerating particles to desired velocities. However, research on the nozzle design for supersonically spraying of "liquid droplets" for nanocoating applications is limited. To this end, we investigate the effect of nozzle geometrical parameters, including throat diameter, exit diameter, and divergent length on droplets impact velocity by numerical modeling and experimental validation, followed by a case study on nanocoating. The discrete-phase modeling was employed to study droplets' flow behavior in continuous gas flow for various nozzle geometries. The results reveal that the nozzle expansion ratio, defined as a function of throat and exit diameters, has a significant influence on droplet velocity, followed by divergent length. Noteworthy, to correctly accelerate "low-inertia liquid microdroplets," it was found that the optimum nozzle expansion ratio for axisymmetric convergent-divergent nozzles should be in a range of 1.5-2.5, which is different and way smaller than the recommended expansion ratio (i.e., 5-9) for CS of conventional micron-scale "metal" powders. Based on the simulation results, an optimum design of supersonic nozzle is established and prototyped for the experimental studies. Particle image velocimetry (PIV) was used to experimentally investigate the spray flow field and to validate the numerical modeling results. Moreover, coating experiments using the optimized nozzle confirmed the effective supersonic spraying of droplets containing nanoparticles, thereby showing the potential for advanced nanocoating applications.