Numerical studies on the effects of stagnation pressure and temperature on supersonic flow characteristics in cold spray applications

Low-temperature particle coating requires supersonic flow. The characteristics of this supersonic flow are investigated using a nonlinear turbulence model. The low-temperature, supersonic particle deposition technique is valuable because its rapid and dense coating minimizes thermal damage to both particles and substrate. It has excellent potential for industrial production of low-cost thin films. Stagnation pressures and temperatures of the supersonic nozzle range from 4 < P _o < 45 bar and 300 < T _o < 1500 K, respectively. The exit Mach number, M _e, and velocity, V _e, range from 0.6 to 3.5 and 200 to 1400 m/s, respectively. The effects of stagnation pressure (P _o) and stagnation temperature (T _o) on supersonic flow impinging upon a substrate are described. In other words, the energy loss through shockwaves and shear interactions between the streaming jet and surrounding gas are quantified as functions of P _o and T _o. P _o is decreased because of friction (loss ranges from 40 to 60%) while T _o is nearly conserved. To realize the nozzle exit condition of P _e = P _amb, we demonstrate that P _o should be adjusted rather than T _o, as T _o has little effect on exit pressures. On the other hand, T _o is more influential than P _o for varying the exit velocity. Various nozzle geometries yielding different flow characteristics, and hence, different operating conditions and coating performances are investigated. The corresponding supersonic flows for three types of nozzles (under-, correctly, and over-expanded) are simulated, and their correctly expanded (or shock-free) operating conditions are identified. Diamond shock structures induced by the pressure imbalance between the exiting gas and the surrounding atmosphere are captured.