Liquid film sheet 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). 

On the basis of the experimental observations,l160 169 327 Liu[325] conceived a liquid film-sheet breakup model for atomization of liquid metals with close-coupled atomizers. In this atomization model, it was postulated that atomization of a liquid metal with a close-coupled atomizer may occur in the following sequence (1) formation of a liquid film, (2) conversion of the liquid film into a liquid sheet, (3) primary breakup of the liquid sheet into droplets, (4) droplet... 

In gas atomization via film or sheet breakup (Table 4.16), the mean droplet size is proportional to liquid density, liquid viscosity, liquid velocity, and film or sheet thickness, and inversely proportional to gas density and gas velocity, with different proportional power indices denoting the significance of each factor. In recent experimental studies on liquid sheet and film atomization processes using a close-coupled atomizer, Hespel et al. 32X concluded that the... 

The solution of the gas flow and temperature fields in the nearnozzle region (as described in the previous subsection), along with process parameters, thermophysical properties, and atomizer geometry parameters, were used as inputs for this liquid metal breakup model to calculate the liquid film and sheet characteristics, primary and secondary breakup, as well as droplet dynamics and cooling. The trajectories and temperatures of droplets were calculated until the onset of secondary breakup, the onset of solidification, or the attainment of the computational domain boundary. This procedure was repeated for all droplet size classes. Finally, the droplets were numerically sieved and the droplet size distribution was determined. 

The Linearized Instability Sheet Atomization (LISA) model has been developed by Schmidt et al. [3, 4] in order to model the ptimaiy breakup of the liquid film... 

Figure 3.8. Liquid sheet/film breakup modes Successive stages in the idealized breakup of (a) a sheet with a thick rim, (b) a wavy sheet, and (c) a perforated sheet.

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