Numerical Studies on the Effects of Stagnation Pressure and Temperature on Supersonic Flow Characteristics in Cold Spray
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Min-Wook Lee, Jung-Jae Park, Do-Yeon Kim, Sam S. Yoon, Ho-Young Kim, Scott C. James, Sanjeev Chandra, and Thomas Coyle (Submitted August 30, 2010; in revised form January 24, 2011) 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 < Po < 45 bar and 300 < To < 1500 K, respectively. The exit Mach number, Me, and velocity, Ve, range from 0.6 to 3.5 and 200 to 1400 m/s, respectively. The effects of stagnation pressure (Po) and stagnation temperature (To) 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 Po and To. Po is decreased because of friction (loss ranges from 40 to 60%) while To is nearly conserved. To realize the nozzle exit condition of Pe = Pamb, we demonstrate that Po should be adjusted rather than To, as To has little effect on exit pressures. On the other hand, To is more influential than Po 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.
Keywords
cold spray, diamond shock structure, supersonic nozzle, thin-film coating
1. Introduction Cold spray (Ref 1-4) is an attractive deposition technique because it does not require heating of the working fluid by plasma or substrate as do thermal spray coating techniques (Ref 5). The proposed relatively low-temperature process has advantages, over other techniques, such as energy saving and prevention of thermal damage to the coating materials and substrates. As shown in Fig. 1, gas is supplied through a converging-diverging de Laval nozzle that accelerates the flow to
Min-Wook Lee, Jung-Jae Park, Do-Yeon Kim, Sam S. Yoon, and Ho-Young Kim, Department of Mechanical Engineering, Korea University, Seoul 136-713, Korea; Scott C. James, Thermal/Fluid Science & Engineering, Sandia National Laboratories, Livermore, CA; and Sanjeev Chandra and Thomas Coyle, Department of Mechanical and Industrial Engineering, University of Toronto, 5 KingÕs College Road, Toronto, ON, Canada. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
supersonic. Particles may be injected into the flow, which eventually stagnates at the substra
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