Conference Paper

Pulsatile Primary Slurry Atomization: Effects of Viscosity, Circumferential Domain, and Annular Slurry Thickness

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A central theme of our prior experimental and computational work on a transonic self-sustaining pulsatile three-stream coaxial airblast injector involved obtaining spectral content from compressible 2-D models and preliminary droplet size distributions from incompressible 3-D models. The three streams entail an inner low-speed gas, and outer high-speed gas, and an annular liquid sheet. Local Mach numbers in the pre-filming region exceed unity due to gas flow blockage by the liquid. Liquid bridging at somewhat regular intervals creates resonance in the feed streams. The effects of numerical decisions and geometry permutations were elucidated. The focus now shifts to compressible 3-D computational models so that geometric parameters, modeled domain size, and non-Newtonian slurry viscosity can be more elaborately explored. While companion studies considered circumferential angles less than 45°, specific attention in this work is given to the circumferential angles larger than 45°, the slurry annular dimension, and how this annular dimension interacts with inner nozzle retraction (pre-filming distance). Additional metrics, including velocity point spectral analyses, are investigated. Two-stream experimental studies are also computationally studied. Multiple conclusions were drawn. Narrower annular slurry passageways yielded a thinner slurry sheet and increased injector throughput, but the resulting droplets were actually larger. Unfortunately the effect of slurry sheet thickness could not be decoupled from another important geometric permutation; injector geometry physical constraints mandated that, in order to thin the slurry sheet, the thickness of the lip which separates the inner gas and slurry had to be increased accordingly. Increased lip thickness reduced the interfacial shear and increased the thickness of the gas boundary layer immediately adjacent to the slurry sheet. This suppressed the sheet instability and reduced the resulting liquid breakup. Lastly, velocity point correlations revealed that an inertial subrange was difficult to find in any of the model permutations and that droplet length scales correlate with radial velocities. As anticipated, a higher viscosity resulted in larger droplets. Both the incremental impact of viscosity and the computed slurry length scale matched open literature values. Additionally, the employment of a full 360° computational domain produced a qualitatively different spray pattern. Partial azimuthal models exhibited a neatly circumferentially repeating outer sheath of pulsing spray ligaments, while full domain models showed a highly randomized and broken outer band of ligaments. The resulting quantitate results were similar especially farther from the injector; therefore, wedge models can be used for screening exercises. Lastly, droplet size and turbulence scale predictions for two external literature cases are presented. Copyright © 2015 by ASME Country-Specific Mortality and Growth Failure in Infancy and Yound Children and Association With Material Stature Use interactive graphics and maps to view and sort country-specific infant and early dhildhood mortality and growth failure data and their association with maternal

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... The basis for the current work is the experimental and computational three-stream studies of Strasser [20], and Strasser and Battaglia [21][22][23]. Therein, self-generating and self-sustaining pulsatile flow for air-water systems were considered, and this produced a ''Christmas tree" type of spray as noted by Zhao et al. [24]. The violently bulk pulsatile behavior creates a vigorous atomization process that would not otherwise be present in a non-pulsing system, essentially, improving atomization without additional energy input. ...
... Special attention was given to developing correlations which related retraction to important metrics. Additionally, video analysis methods were developed for air-water systems [20], where the radially expanding Nomenclature AS axisymmetric AW air-water feed stocks AWE air-water equivalent (conjugate pair for a slurry-high pressure gas cases) COV coefficient of variation = standard deviation/ mean * 100% D 32 Sauter [20,21] and non-Newtonian slurry systems [21,23]. There is apparently nothing in the open literature by other research groups regarding issues of geometry, feed conditions, force stream modulation, and numerics on the NN flow field of a similar injector. ...
... Lip thickness and gas boundary layer influence low Reynolds number liquid feeds in that their primary instability mechanism is a competition between the gas phase turbulence and liquid surface tension to produce Kelvin-Helmholtz instabilities [14]. However, one of the dominant liquid film disruptive forces in the geometry of this work is the temporal pressure gradient moving throughout the pre-filming section [23]. ...
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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.
... It is expected that the thicker plates will have a negative effect on primary atomization by creating a thicker gas boundary layer (velocity deficit near the liquid interface) and a reduced instability driving force. This should be of lesser importance for these high liquid Reynolds number simulations, where the primary instability mechanism is liquid phase turbulence (Xiao et al. [12]), but is important for studies involving slurry as is shown in [29]. In addition to liquid phase turbulence, one of the dominant liquid film buffeting forces is the temporal pressure gradients moving throughout the pre-filming section [29]. ...
... This should be of lesser importance for these high liquid Reynolds number simulations, where the primary instability mechanism is liquid phase turbulence (Xiao et al. [12]), but is important for studies involving slurry as is shown in [29]. In addition to liquid phase turbulence, one of the dominant liquid film buffeting forces is the temporal pressure gradients moving throughout the pre-filming section [29]. ...
... Apparently increasing grid refinement allows the liquid jet to spread faster than in the base mesh, and is consistent with Chesnel et al. [3] who showed breakup was delayed when using a coarse mesh. Mesh effects on atomization might be non-intuitive as is discussed in [29]. ...
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Although coaxial airblast primary atomization has been studied for decades, relatively little attention has been given to three-stream designs; this is especially true for transonic self-pulsating injectors. Herein, the effects of nozzle geometry, grid resolution, modulation, and gas flow rate on the acoustics and spray character within an industrial scale system were investigated computationally using axisymmetric (AS) and three-dimensional (3D) models. Metrics included stream pressure pulsations, spray lift-off, spray angle, and primary droplet length scale, along with the spectral alignment among these parameters. Strong interactions existed between geometry and inner gas (IG) feed rate. Additionally, inner nozzle retraction and outer stream meeting angle were intimately coupled. Particular attention was given to develop correlations for various metrics versus retraction; one such example is that injector flow capacity was found to be linearly proportional to retraction. Higher IG flows were found to widen sprays, bringing the spray in closer to the nozzle face, and reducing droplet length scales. Substantial forced modulation of the IG at its dominant tone did not strongly affect many metrics. Incompressible 3D results were similar to some of the AS results, which affirmed the predictive power by running AS simulations as surrogates. Lastly, normalized droplet size versus normalized distance from the injector followed a strikingly similar trend as that found from prior two-fluid air-slurry calibration work.
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