Begell House
Atomization and Sprays
Atomization and Sprays
1044-5110
3
3
1993
THE EFFECT OF VAPORIZATION AND GAS COMPRESSIBILITY ON LIQUID JET ATOMIZATION
A linear stability analysis is presented for an evaporating jet. The development of the surface hydrodynamic instability is assumed to be much faster than the surface evolution due to evaporation. This allows the process to be considered as quasi-steady, and the normal mode method for the steady basic solution is applicable as an approximation. It is found that for low-speed jets undergoing Rayleigh breakup, jet surface evaporation is a destabilizing factor, while for high-speed atomizing jets, surface evaporation becomes stabilizing. This is due to the fact that the evaporation flux distributions at the troughs and crests of the waves on the surface of the liquid jet are different for these two eases. The effect of gas compressibility is also analyzed. For subsonic jets, the maximum growth rate and the corresponding wavenumber is found to be underestimated when the effects of the gas compressibility are neglected, since the gas pressure and gas density at the interface are higher than predicted by the conventional incompressible gas theory.
Rolf D.
Reitz
Engine Research Center, University of Wisconsin-Madison, Rm 1018A, 1500 Engineering Drive, Madison, Wisconsin 53706, USA
X. W.
Lian
Engine Research Center, Department of Mechanical Engineering, University of Wisconsin, Madison, Wisconsin 53706
249-264
AERODYNAMIC EFFECTS ON PRIMARY BREAKUP OF TURBULENT LIQUIDS
An experimental study of primary breakup of turbulent liquids is described, emphasizing liquid/gas density ratios less than 500 where aerodynamics effects are important. The experiments involved multiphase mixing layers along round water jets (3.6 and 6.2 mm in diameter) injected at various velocities into still helium, air, and Freon 12 at pressures of 1 and 2 atm with fully developed turbulent pipe flow at the jet exit. Puked shadowgraph photography and holography were used to find conditions at the onset of breakup as well as drop properties as a function of distance from the jet exit. Two main aerodynamic effects were observed, as follows: (1) enhanced primary breakup near the onset of breakup; and (2) merged primary and secondary breakup when the Rayleigh breakup times of ligaments formed by turbulent fluctuations were longer than the secondary breakup times of similar sized drops. The predictions of phenomenological theories based on these ideas were in good agreement with the measurements.
P.-K.
Wu
Department of Aerospace Engineering, The University of Michigan, Ann Arbor, Michigan, USA
G. M.
Faeth
Department of Aerospace Engineering, the University of Michigan, Ann Arbor, Michigan 48109-2140, USA
265-289
A SIMPLIFIED MAXIMUM-ENTROPY-BASED DROP SIZE DISTRIBUTION
A simplified approach is presented for description of the drop size distribution in a spray, based on the application of the maximum entropy formalism (MEF). This approach succeeds in separating the constraints on drop she from those on drop velocity, providing results for the size distribution that are independent of the constraints on velocity. The solution is insensitive to the limits of integration, deriving all physical information directly from the constraint equations.
Results are presented that compare the present work with phase Doppler measurements by the authors, as well as with data from the literature. These results show that the MEF drop size distribution compares favorably with other methods of predicting and describing drop size distributions.
Mehran
Ahmadi
Amirkabir University of Technology
R. W.
Sellens
Department of Mechanical Engineering, Queen's University at Kingston, Kingston, Ontario, Canada K7L 3N6
291-310
MULTIPLE SCATTERING AND SIZE DISTRIBUTION EFFECTS ON THE PERFORMANCE OF A LASER DIFFRACTION PARTICLE SIZER
The effect of multiple scattering on the performance of the Malvern particle sizer was studied experimentally. Two particle samples with the same volume median diameter but with different size distributions were used in the experiments. One sample could be described by a log-normal distribution function, the other by a Rosin-Rammler distribution function. The particle samples were specifically designed to simulate drop size distributions in liquid sprays.
The model-independent, log-normal, and Rosin-Rammler models were used in the data analysis, and the results were compared. It was found that even when the sample concentration was low and multiple scattering did not occur, the results obtained with the log-normal and Rosin-Rammler models could be misleading, particularly in the high end of the distribution (large particles). When the log-normal test sample was used, the data analysis with the model-independent and log-normal models gave good results, but the data analysis with the Rosin-Rammler model seriously underestimated the fraction of large particles. When the Rosin-Rammler test sample was used, the data analysis with the model-independent and Rosin-Rammler models gave good results, but the data analysis with the log-normal model seriously overestimated the fraction of large particles.
When the sample concentration is high and multiple scattering occurs, data analysis with the model-independent model is not feasible. Correction equations are available in literature for data analysis with the log-normal and Rosin-Rammler models. It was found that when the correction equations were used, both the log-normal and Rosin-Rammler models provided the mean particle size reasonably well. However, there were considerable difficulties in the high end of the distribution. This was not surprising, since the experiments with a low particle concentration had already shown that the data analysis with the log-normal and Rosin-Rammler models can produce misleading results.
Tuomas
Paloposki
A.
Kankkunen
Helsinki University of Technology, 02150 Espoo, Finland
311-320
STRUCTURE OF VAPORIZING PRESSURE ATOMIZED SPRAYS
A detailed characterization of a vaporizing spray produced by a pressure atomizer is presented. The study is part of an ongoing effort to understand the behavior of sprays under a variety of conditions. The atomizer utilized is a standardized research atomizer that can be operated (without modifying the geometry of the atomizer) as either a simplex atomizer or an air-assist atomizer, with both swirling and nonswirling atomizing air, and with or without reaction. Phase Doppler interferometry, laser diffraction, and infrared extinction/ scatter are utilized in the characterization. The measurements include: (1) drop size, velocity, cross-correlations, volume flux, and concentration; (2) gas-phase velocities with and without the spray; and (3) the vapor concentration within the spray. The present data are acquired in the absence of reaction, and include temporally resolved measurements of the droplets. In the present spray, strong dependency of velocity on drop size is observed. Entrainment of surrounding air and gravitational forces lead to segregation of drops based on inertia. The result is that the modulation of the gas-phase velocities (both mean and rms) is correlated in a nontrivial way to the concentration, size, and slip velocity of the drops. The vapor concentration is saturated along the centerline of the spray, and decays in the radial direction due to the entrainment of air. The vapor concentration diffuses similarly to the gas-phase momentum. Finally, comparison of different techniques for measurement of drop size is reasonable, as is comparison of different techniques for measurement of the spray vaporization. The data presented provide a detailed set of measurements for vaporizing, pressure atomized sprays suitable for modeling challenges.
G. Scott
Samuelsen
Department of Mechanical and Aerospace Engineering, University of California, Irvine, California 92697-3550, USA
Vincent
McDonell
Department of Mechanical and Aerospace Engineering, University of California at Irvine, USA
321-364