Meaning
Aerodynamic disturbances occur in high speed gas flows when the pressure and velocity fields fluctuate rapidly over time. Supersonic shock wave instability often develops in gas atomization nozzles when the operating pressure exceeds a critical threshold. These fluctuations disrupt the primary breakup of the molten metal stream and lead to a wider particle size distribution.
Understanding these instabilities is necessary for designing more efficient and stable atomization systems.
Gas Velocity
Flow of gas through a converging diverging nozzle reaches the speed of sound at the throat and becomes supersonic in the exit section. Supersonic shock wave instability arises from the interaction between the high speed gas and the surrounding atmosphere or the central melt tube. The position of the shock front can shift back and forth, creating a pulsing effect in the gas curtain.
This pulsing changes the local energy density available for atomizing the metal. Proper gas pressure control minimizes these fluctuations.
Breakup Mechanism
Fragmentation of the liquid metal into small droplets relies on the high shear forces provided by the gas stream. Supersonic shock wave instability interferes with the steady transfer of kinetic energy to the liquid. The metal stream breaks into discrete batches or ligaments under these conditions.
Large and irregular particles form as a result of this erratic breakup.
Powder Consistency
Industrial applications require metal powders with highly repeatable properties from one batch to the next. Supersonic shock wave instability introduces a source of variability that is difficult to control without specialized nozzle designs. Using sensors to detect the acoustic signature of the shock waves can provide an early warning of unstable flow.
Modern nozzle geometries are optimized to suppress these instabilities and produce a more uniform and spherical powder.
