Comparative Study on Gas-Particle Acceleration Behavior in Straight Divergent and Curved Divergent Cold Spray Nozzles: Numerical Simulation and Experimental Validation

This study combined three-dimensional CFD numerical simulations with Lagrangian particle tracking and experimental deposition to compare straight divergent and curved divergent nozzles for cold spray under identical stagnation conditions (3 MPa, 600 °C). Importantly, the two nozzle geometries were designed to have the same expansion ratio and divergent-section length, enabling an isolated assessment of downstream profile effects. The curved divergent nozzle accelerated the gas more rapidly: Peak axial velocity reached 1003 m/s at about 114 mm downstream versus 937 m/s at the exit (220 mm downstream) for the straight divergent nozzle. The curved divergent geometry produced a faster pressure decay and earlier temperature reduction in the downstream region while also producing a stronger exit shock. Lagrangian particle tracking predicted higher impact velocities for Al, Ti, and Cu particles across 10-50 μm; mean increases (curved divergent versus straight divergent) ranged roughly from 11 to 44 m/s depending on material and diameter. Furthermore, for the investigated parameters, particles of 10-30 μm were recommended to maximize curved divergent nozzle benefits. Experimental Cu coatings validated the simulations: Curved divergent nozzle coatings were more uniform and thicker (1730 μm) than straight divergent coatings (1120 μm), corresponding to 1.5 times higher deposition efficiency. These results quantify the influence of the nozzle downstream profile on gas acceleration efficiency and demonstrate that changes in the gas field produced by different profiles consequently modify particle acceleration and deposition behavior.