Free-fall atomizer

Figure 2.14. Schematic of a free-fall atomizer for atomization of melts.

Figure 2.17. Different geometry arrangements of free-fall atomizers for 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). 

Figure 5.2. Calculated gas flow fields in the near-nozzle region of free-fall atomizers. Primary gas pressure 0.140 MPa secondary gas pressure 0.189 MPa angle of secondary gas nozzle relative to the spray centerline 10° angle of primary gas nozzle relative to the spray centerline (a) 0°, (b) 22.5°, and (c) 30° designed for minimizing recirculation gas flow. (Reprinted from Ref. 612.)...

Figure 18.19 (left) exhibits the calculated gas flow field from a discrete jet nozzle at atomization pressure po = 0.5 MPa. The nozzle consists of 24 inclined straight bored holes (inclination angle 10°) with a 3 mm diameter at the nozzle exit. This kind of gas nozzle has been employed as part of the free-fall atomization (FFA) system. The simulation is conducted in a 3D computational domain by taking into account the circumferential symmetry of the flow field (see Fig. 18.16). As shown in Fig. 18.19 (left), after the first significant shock cell, the velocity-gradient downstream turns smoothing. The gas jet in 3D simulation contains fewer shock... 

Fig. 18.26 Droplet-size distributions for different materials at atomization-gas pressure Pg = 0.5 MPa, free-fall atomization...

Fig. 18.27 Mass median diameters (MMDs) and distribution spans fm different matraials at different atomization-gas pressures, free-fall atomization, comparison between experimental data [6] and simulation results...

On the basis of a force balance model, Tomberg 486 derived a semi-empirical correlation for free-fall type of 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 ... 

Calculations by Gryzinski and Kowalski (1993) for inner shell ionization by positrons also confirmed the general trend. Theirs was essentially a classical formulation based upon the binary-encounter approximation and a so-called atomic free-fall model, the latter representing the internal structure of the atom. The model allowed for the change in kinetic energy experienced by the positrons and electrons during their interactions with the screened field of the nucleus. 

The ideal conditions for studying an atom is to have it at rest in free space, or in free fall as in a "fountain" experiment. Any process which confines an atom perturbs it However, as has been shown, at ultra low temperatures the perturbations of hydrogen due to a magnetic trap are small. Furthermore, the trap provides an enormous advantage in density compared to atomic beams or fountains density of 10u - 1012cm-3 is readily available. Thus, the trap is particularly attractive from the point of view of signal to noise ratio. 

Figure 13. Candidates of tori in the initial condition space for the free-fall problem helium atom, He. The painted region near the corner (x, y) = (1,0) represents the initial conditions with positive total energy.

In gas-liquid spray towers the liquid is atomized and enters as a fine spray at the top and the gas is introduced at the bottom. The gas flow rate has to be kept sufficiently low to permit the liquid to fall. It is generally chosen in such way that the liquid drops of mean diameter fall at 20 percent of their free-fall velocity, as calculated from Stokes law. An efficient dispersion of the liquid requires the openings of the distributor to be small and the pressure high. Thereby a fraction of the drops hits the wall and flows down the wall as a film. Furthermore, a certain degree of coalescence of the drops is inevitable, so that the drop size, velocity, and therefore residence time vary strongly with position. A rigorous hydrodynamic analysis of such a situation is extremely complicated so that only the overall behavior has been studied. 

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