CFD-Example 2: Underexpanded freejets in melt atomisation 

Disintegration of melts (steel, copper or aluminium) by twin fluid atomisation with inert gases is one of the first steps in processes like powder metallurgy and spray forming. The disintegration of the melt is attained by transferring kinetic energy from high speed gas jets (nitrogen, argon or helium) to the melt.

Fig. 1: 3D configuration of underexpanded freejets, Simulates density contours and shlieren photo

Conventionally underexpanded free jet configurations with sonic exit velocity occur within gas atomisers. Therefore, at first single underexpanded free jets and interactions of jet clusters from various nozzle configurations are analysed by Schlieren optics and numerical methods to investigate the typical shock structures. Fig. 1 shows the interactions of jet clusters expanding from a three hole nozzle configuration visualized by shlieren-optical system and simulated density contours. Due to the jet arrangement, the simulation has been carried out 3D, turbulent and fully compressible (strong shocks).

Fig. 2: Comparison numerical and experimental velocities

Gas velocities are measured in  underexpanded jets by laser Doppler anemometry  in comparison with numerical analysis. Due to the inertial forces of the particles in LDA-measurement, they are not able to follow the high velocity gradients. So here the numerical data matches the real velocity distribution.

Fig. 3: Freejet configuration in atomization nozzle

Figure 3 shows a underexpanded and ideal expanded freejet configuration for the atomisation nozzle. Using ideal expanded configurations from  convergent/divergent nozzles, higher velocities occur in the atomization region. Smaller particles occur by the same gas consumption. This result was also proven by experimental methods.   

( click on the images to enlarge)

Further information:

U. Heck

Zur Zerstäubung in Freifalldüsen, Dissertation, VDI-Verlag Reihe 7, Nr. 348, Düsseldorf, 1998

U. Heck, U. Fritsching,  K. Bauckhage

Gas flow effects on twin –fluid atomization of liquid metals, Atomization and Sprays, Vol. 11, p.25-46, 2000