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The Effects of Retraction on Primary Atomization in a Pulsating Injector
Abstract
Despite its industrial relevance, the exploration of primary atomization within a transonic self-generating pulsatile three-stream injector has been minimal. Our prior experimental and computational work was centered around compressible axisymmetric (AS) models and incompressible 3-D models for the purpose of obtaining spectral content and preliminary droplet size distributions. Here, the emphasis shifts to compressible 3-D computational models involving a non-Newtonian slurry and a much more inclusive computational domain in order to further elucidate droplet size information. Effects of numerics, turbulence model, and geometric parameters are revisited. In addition, a surrogate measure for injector face erosion is introduced. Lastly, links are discovered between responses in Sauter mean diameter and trends in AS modeling metrics. As with prior air-water work and incompressible slurry simulations, higher gas inner flow rate reduced droplet size measurably. While the temporal mean droplet length scale was relatively insensitive to numerics, turbulence model, compressibility, and modeled domain size, droplet size temporal variability responded very strongly to some of these effects; compressibility dampened the droplet variability, while increased inner gas flow augmented variability, and the use of a more rigorous turbulence model showed a mixed effect. It was found that designs with less retraction (smaller pre-filming region) produced smaller droplets and allowed increased process throughputs. Newly discovered correlation equations are provided and followed similar trends as some from the earlier AS work. Interestingly, it was also shown that droplet size can be correlated with spectral information from prior companion AS studies.
... The slurry waves at time t are very different than those at t + 0.003 s. This was further explored in [42], as direct links can be made between retraction and its effect on droplet size mean and variation. ...
... Table 5 (herein and in [22]) shows, just like with the equivalent AS models discussed in prior sections of this document, that the SH feed materials show lower mean pressures, astoundingly lower pressure COVs, higher pressure tones, and higher pressure magnitudes, consistently for IG and OG. Credence continues to mount for the value of using AS models as surrogates for 3D models as was introduced in [22,42]. ...
The effects of geometry, numerics, gas flow rate, and superimposed flow modulation on the self-generating pulsatile spray produced by an industrial scale three-stream coaxial airblast reactor injector have been studied for a non-Newtonian slurry and high-pressure gas (SH) system. A fully retracted design showed the most inner gas pulsation, and the spray character changed significantly between a flushed and retracted design; the flushed design showing more radially synchronized and focused pulsations. Pressure drop was found to be linearly proportional to retraction, and new correlations were introduced. Higher inner gas flows typically widened sprays for the base geometry only and lowered the droplet length scales, indicating that the lower droplet size limit was not set by viscosity limitations. Modulation of the inner gas at its dominant tone did not strongly affect many metrics, except that the inner gas pulsations substantially increased. Slurry video analyses provided spray angle directional trends so that a subset of the domain could be simulated to save computational time.
Relative to prior air-water (AW) studies, SH flow patterns and acoustics typically differed significantly, with the exception of the base geometry spray profiles at the higher inner gas flows, along with the droplet length scale. In general, SH simulations showed lower pressure drop, astoundingly lower pressure temporal variability, higher dominant tones, and less periodicity (more diffused spectra). Furthermore, the relationship between 3D SH droplet size and distance was of the form constant/distance; the constant was the same for both feed materials. It appears that acoustics cannot be linked between the two feed materials, but there is some connection in mean droplet size.
High pressure gas atomization (HPGA) is a technique for the manufacture of fine spherical metal powders. Supersonic gas jets are used to disrupt a stream of liquid metal into droplets which then solidify in flight. We construct a CFD model for gas flow within a gas atomizer which is compared against high-speed video footage of a research scale atomizer. Good agreement between simulation and experiment is found for the presence of Prandtl-Meyer waves on the surface of the spray cone. The simulation also reproduces the location of the recirculation zone below the melt delivery nozzle. Simulation results indicate a radially outward flow from the bore of the melt nozzle to the circumferential edge which would facilitate pre-filming.
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