Meaning
Supersonic gas jets used in powder production exhibit complex internal shock wave structures that affect gas expansion. A prominent feature of these flows is the Mach disk, which is a flat, normal shock wave that forms in the jet core. This disk alters the local gas velocity and pressure, directly influencing the energy available for droplet breakup.
Understanding this structure allows nozzle designers to optimize gas usage.
Shock Formation
Overexpanded or underexpanded conditions at the nozzle exit generate expansion waves that reflect inward from the jet boundary. These waves coalesce to form an oblique shock wave system that terminates in a normal shock. This normal shock is the disk itself, and it abruptly slows the gas to subsonic speeds.
This deceleration represents a significant loss of kinetic energy for the atomization process.
Atomization Impact
Liquid metal streams should ideally interact with the gas before this normal shock occurs to maximize energy transfer. If the melt hits the subsonic flow behind the disk, the primary breakup becomes much less efficient. This inefficiency produces coarser powders and increases gas consumption per kilogram of powder.
Process Optimization
Nozzle position and operating pressure must be adjusted to control the location of the shock structure. Advanced simulation tools model the gas flow to predict where the disk will form under different pressures.
