Close-coupled atomizer

Gas Atomization 50-300 Standard deviation 1.9-2.5 <10-50 at high gas pressures with close-coupled atomizer Solder materials. Precious metals, Cu, Fe, Al, Mg, Co, Ti, Zn, Al-6Cr-3Fe-2Zr alloy. Low-alloy steels. High speed steels. Stainless steels, Specialty alloys, Ni-base superalloys, Alumina, Intermetallics io3-.o5 1-70 Spherical smooth particles. Cleanliness, Rapidly-solidified structures, Acceptable production rates High cost, Low 1 volume, Low energy efficiency (EE), Gas-filled porosity in particles H... 

Figure 2.15. Schematic of a close-coupled atomizer for atomization of melts.

Figure 2.16. Design variations of close-coupled atomizers for atomization of melts.

Scale-up of gas atomizers is difficult and it requires the use of higher gas-to-melt mass flow rate ratio to maintain the same droplet size. The scale-up may also cause some complex phenomena to occur, such as the disappearance of the prefilming effect in close-coupled atomizers, the generation of turbulence in melt flow within delivery nozzle, and change in atomization mechanisms. 

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 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). 

For the delivery of atomization gas, different types of nozzles have been employed, such as straight, converging, and converging-diverging nozzles. Two major types of atomizers, i.e., free-fall and close-coupled atomizers, have been used, in which gas flows may be subsonic, sonic, or supersonic, depending on process parameters and gas nozzle designs. In sonic or supersonic flows, the mass flow rate of atomization gas can be calculated with the following equation based on the compressible fluid dynamics ... 

Figure 5.3. Atomizer geometry (top),computational grid mesh (left) and calculated Mach number contour (right) near a close-coupled atomizer (Atomization gas Ar, Ma = 1 at nozzle exit). (Reprinted with permission from Ref. 325.)...

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... 

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