Meaning
Gas flow behavior in supersonic nozzles is heavily influenced by the intersection of pressure discontinuities and solid surfaces. This phenomenon is known as shock boundary layer interaction, and it alters the stability of the gas jet. The boundary layer can thicken or separate when it encounters the sudden pressure rise of a shock wave.
This interaction can cause pressure oscillations that disrupt the liquid metal delivery tube. Unstable gas jets also lead to asymmetric powder spray patterns.
Flow Separation
Severe interactions force the gas flow to detach from the internal walls of the nozzle. This separation creates a zone of recirculating gas that can draw molten metal droplets back onto the nozzle face. This recirculation leads to metal buildup, nozzle clogging, and eventually a premature halt to the run.
Energy Loss
Kinetic energy is converted into heat across the shock wave, reducing the velocity of the gas jet. This reduction in velocity decreases the shear forces available for droplet breakup. Designing nozzle profiles that minimize these interactions is essential to maintain high gas velocity.
Nozzle Performance
Computational fluid dynamics allows engineers to optimize the internal curves of the nozzle to prevent flow separation. By stabilizing the boundary layer, the gas jet retains its supersonic velocity over a longer distance. This stabilization improves the efficiency of the secondary breakup process.
