Jet-ligament breakup

Droplet Formation in Gas Atomization. Experimental and modeling studiesl160 161 169] 318] 319] 321]- 325] have shown that gas atomization of liquid metals in spray forming and powder metallurgy processes may take place in two primary modes, i.e., liquid jet-ligament breakup and liquid film-sheet breakup. 

Figure 3.12. Schematic showing Liquid Jet-Ligament Breakup mode (left) and Liquid Film Sheet Breakup mode (right) in two-fluid atomization of melts.

The temperature of a liquid metal stream discharged from the delivery tube prior to primary breakup can be calculated by integrating the energy equation in time. The cooling rate can be estimated from a cylinder cooling relation for the liquid jet-ligament breakup mechanism (with free-fall atomizers), or from a laminar flat plate boundary layer relation for the liquid film-sheet breakup mechanism (with close-coupled atomizers). 

Shinjo J, Umemura A (2010) Simulation of liquid Jet primary breakup Dynamics of ligament and droplet formation, Int. J. Multiphase Flow 36 513-532... 

Current breakup models need to be extended to encompass the effects of liquid distortion, ligament and membrane formation, and stretching on the atomization process. The effects of nozzle internal flows and shear stresses due to gas viscosity on liquid breakup processes need to be ascertained. Experimental measurements and theoretical analyses are required to explore the mechanisms of breakup of liquid jets and sheets in dense (thick) spray regime. 

Satellite drops can also be eliminated by increasing the amplitude of the initial distorbance. This reduces the breakup time, and therefore, there is no time for the development of the satellite-forming liquid ligament. For a jet with Oh = 200 the initial disturbance amplitude has to be very large in order to eliminate the satellites. 

Droplets are initialized with a negative deformation velocity in order to avoid the almost immediate breakup of highly tmstable initial ligaments, and to extend their lifetime to levels comparable with experimentally observed jet breakup lengths [8]. 

Figures 25.3 and 25.4 show that liquid ligaments detach from the remaining liquid in the orifice with a conical tip. This is due to the abrupt stop of liquid motion inside the nozzle. This conical point quickly forms a round end, due to local sharp curvature. The ligaments formed in Fig. 25.3 contract into droplets without breakup, while the jet in Fig. 25.4 breaks into a ligament and satellite droplets.

Figure 29.6 shows an example of one of the 2D droplet deformation studies carried out specifically for the purpose of modeling the LnCT by Sarchami et al. [9]. The flow conditions for this case are more extreme than can be handled by the theoretical methods for droplet deformation. As the figure shows, not only can these types of results be used to obtain information about jet deformation and trajectory, but they also provide an estimate for the size of the droplets and ligaments that form due to shear breakup at the tips of the elongated cross section. They also show the pattern of the flow field around the droplet. Of particular interest are the double vortices that form behind the droplet and help stretching it out and into a thin shape. The formation of these vortices can play an important role in the secondary atomization processes of the droplets formed in the lee-side of the jet (2D drop). 

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