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Atomization and Sprays
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ISSN Print: 1044-5110
ISSN Online: 1936-2684

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Atomization and Sprays

DOI: 10.1615/AtomizSpr.v1.i1.70
pages 113-136

EFFECT OF SHOCK WAVES ON LIQUID ATOMIZATION OF A TWO-DIMENSIONAL AIRBLAST ATOMIZER

Kenneth D. Kihm
Texas A&M University College Station, TX; and Micro/Nano-Scale Fluidics and Energy Transport Laboratory, University of Tennessee, Mechanical, Aerospace and Biomedical Engineering Department, Knoxville TN 37996-2210, USA
Norman Chigier
Department of Mechanical Engineering Carnegie-Mellon University, Pittsburgh, PA 15213-3890

ABSTRACT

Two major questions have been raised by the designers of airblast fuel atomizers: (1) When the atomizing air is choked and underexpanded through a convergent nozzle, do shock waves exist in the presence of liquid? (2) If so, will these shock waves enhance the efficiency and performance of sonic air-blast atomizers? A two-dimensional twin air-blast research atomizer was designed and carefully machined to study these questions and demonstrate the influence of shock waves on atomization processes. Using a shadowgraph technique, shock wave patterns have been visualized with and without liquid injection. The shock wave patterns have also been studied theoretically based on the isentropic flow assumption and the Prandtl-Meyer expansion fan. Droplet size distributions have been measured for both subsonic and sonic conditions by the Malvem Fraunhofer diffraction particle sizing technique for water sprays using air as the atomizing fluid. Measurements of Sauter mean diameter (SMD) were made as a function of the air-to-liquid velocity and mass ratios for several different orifice dimensions of air and liquid flows. The Mach number in the shock cells was varied up to 1.5 as the upstream stagnation pressure was progressively increased. It was found that the velocity of the air relative to the liquid has the most important influence on mean drop size variation. The SMD shows a rapid decrease with increase in the relative velocity as the air jets approach the sonic condition. Near choking and after the flow is choked, SMD decreases very slowly, whereas the density-weighted relative velocity, ρVrel, progressively increases with the stagnation pressure. For the fine atomizer that we employed, a critical Weber number Wecrit was already attained near the choking condition, and a further reduction in SMD was not apparent up to the local Mach number of 1.5. In order to investigate the effect of shock waves on airblast atomization more extensively, supersonic air jets of higher Mach number should be applied so that the Weber number can exceed the critical value after the flow is choked.


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