Begell House
Atomization and Sprays
Atomization and Sprays
1044-5110
5
1
1995
STEADY INJECTION OF IDENTICAL CLUSTERS OF EVAPORATING DROPS EMBEDDED IN JET VORTICES
A model has been developed that describes the evaporation of clusters of drops in a flowing gaseous jet. Each one of these clusters is embedded into a coherent vortex and the drops evaporate as the clusters convect downstream together with the vortex. Because there is a continuous injection of clusters, each cluster represents in fact a statistical average of clusters at that particular location. Thus, the formulation contains a conservation equation for the cluster number density, conservation equations for the gas in the jet, and conservation equations for the drops in the cluster and the vortex containing the cluster. The cluster and vortex models are coupled to the gaseous jet model through boundary conditions. The heat necessary to evaporate the drops comes from the surroundings of the gaseous jet, and this is described through a global, diffusive entrainment model. It is assumed that the turbulent diffusion coefficient is proportional either to the local vortex strength or to the cluster velocity and the multiplier is named the entrainment coefficient.Results are presented here for the stationary case representing the situation when identical clusters are continuously injected and the injection rate is constant. Thus, if a “snapshot” of the calculation is taken at any time, the cluster is observed at that time and the clusters in its wake represent the history of the cluster at previous times. Parametric studies cover the influence of the initial air/fuel mass ratio, the entrainment coefficient, and the initial drop and gas velocities inside the vortices. The results show that quantitative predictions of the evaporation time, the penetration of the clusters into the ambient, and the temperature of the jet depend on details of the entrainment of hot gas into the jet.
Josette
Bellan
Department of Mechanical and Civil Engineering, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
K.
Harstad
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
1-16
UNSTEADY INJECTION OF SEQUENCES OF DROP CLUSTERS IN VORTICES DEPICTING PORTIONS OF A SPRAY
A model of unsteady injection of sequences of drop clusters embedded in jet vortices was applied to describe both vortices in the shear layer of a spray and small-scale vortical structures in the core of a spray. In the first case, the vortices are large compared to the size of the spray, they rotate fast with respect to the injection rate, and their number per area of spray is small. In the second case, the vortices are small compared to the size of the spray, they rotate slowly with respect to the injection rate, and their number per area of spray is large.
Results were obtained for injection sequences where either the drop size or the air/fuel mass ratio varied from cluster to cluster in the injection sequence. The variation was a mono-tonic increase, a monotonic decrease, or a sinusoidal variation. The results thus obtained were compared to baseline results from steady-state calculations. Additionally, both the entrainment from the ambient into the jet and the initial number of clusters per jet area were varied so as to ascertain their influence on cluster penetration and jet properties.
The results show that penetration of a cluster into the ambient is a function of the characteristics of the cluster sequence following the cluster, that the jet temperature is controlled by entrainment from the ambient into the jet in the shear-layer application whereas conduction also becomes important in the spray-core application, and that the fuel mass fraction in the jet is a function of the initial characteristics of the clusters as well as entrainment.
Josette
Bellan
Department of Mechanical and Civil Engineering, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
K.
Harstad
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA
17-44
SPRAY SIZING BY TOMOGRAPHIC IMAGING
A technique for determining the local drop size distribution in liquid sprays is developed. This technique is based on the far-field measurement of the single-wavelength back-scattered light from the spray. A combination of a tomographic imaging technique and the Mie scattering theory is used. The tomographic reconstruction of the spray is realized by using several optical projections, which are obtained by recording the back-scattered light when the spray is illuminated with a coherent and collimated laser beam. Each of these projections corresponds to a specific angular orientation of the spray and a single back-scattering direction. Such an image represents the local time-averaged intensity across the spray for the light scattered in the selected direction. Three tomographic images of the same cross section, corresponding to three different back-scattering directions, are used to determine the local drop size distributions. A log-normal distribution function defined by two variables is assumed, and the scattered light intensity by this distribution is calculated from the Mie theory for the three back-scattering directions. A discrete search technique is then implemented to find the local log-normal distribution that best matches the results provided by the Mie theory and the tomographic images for all three directions. Simulations realized with an axisymmetric spray underline the accuracy of this new method.
C.
Oberle
Department of Mechanical and Aerospace Engineering, State University of New York at Buffalo, Buffalo, New York 14260
Nasser
Ashgriz
University of Toronto
45-73
DENSE SPRAY CORRECTIONS FOR DIFFRACTION-BASED PARTICLE SIZE DISTRIBUTION MEASUREMENTS
The effect of dense sprays on particle size measurement by laser-diffraction-based techniques is examined for log-normal distributions with a wide range of Sauter mean diameters and size distribution widths. A Monte Carlo modeling technique is used to simulate scattering of light by particles, and general scattering phenomena are included by using Mie theory. By regression analysis, corrections for the SMD and size distribution width are obtained and presented in graphical and equation form. The corrections depend on the actual Sauter mean diameter and the actual width parameter of the spray and the obscuration. Simulations are also performed for a limited number of Rosin-Rammler distributions, and the results for the log-normal and Rosin-Rammler distributions are compared. For log-normal distributions and Rosin-Rammler distributions with n greater than 2.0, increasing the Sauter mean diameter of the size distribution increases the measurement error in SMD and decreases the error in width. Increasing the size distribution width also results in an increase in the measurement error in SMD and a decrease in the width. For Rosin-Rammler distributions with n less than 2.0, these trends are the same with one important exception: as the Sauter mean diameter increases, the measurement error in SMD decreases.
Christine M.
Woodall
Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
James E.
Peters
Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, USA
Richard O.
Buckius
Department of Mechanical and Industrial Engineering, University of Illinois at Urbana-Champaign, 158 Mechanical Engineering Building, MC-244, 1206 West Green Street, Urbana-Champaign, IL 61801, USA
75-87
MECHANISM OF ATOMIZATION OF A LIQUID JET
A linear stability analysis is presented for a viscous liquid jet in an inviscid gas medium, with three-dimensional disturbances. Although Squire's theorem is applicable for an inviscid jet, it is not valid for the present case of a cylindrical viscous liquid jet. It is found that three-dimensional waves are relatively unimportant at low Weber numbers, but their growth rate becomes almost identical to that of varicose waves at large Weber numbers. Thus three-dimensional waves become observable at large Weber numbers. In particular, sinuous waves can become the most unstable waves under certain flow conditions within the wind-induced breakup regime. As the Weber number is increased, more and more three-dimensional disturbances become unstable and significant, and hence the jet breakup process becomes more and more complicated. These results agree well with experimental observations. Further, the results suggest the existence of the vanishingly small liquid jet breakup length, thus explaining why in the atomization regime the liquid jet disintegrates almost immediately at the nozzle exit.
Xianguo
Li
University of Waterloo
89-105
NEAR-NOZZLE CHARACTERISTICS OF A TRANSIENT FUEL SPRAY
The near-nozzle characteristics of a transient fuel spray were investigated via the measurement of drop sizes and velocities, and microphotographs of the near-nozzle region for a range of gas-to-liquid density ratios. In addition, a steady-state single-phase hydrodynamic simulation of the internal nozzle flow was performed to observe the effects of needle position on the internal flow.
Measurement of droplet size and velocity near the nozzle on the edge of the spray showed that the average droplet velocity peaked during needle opening and needle closing, and changed throughout the spray event. Drop sizes tended to be small on the spray edge. Microphotographs of the near-nozzle region showed that the spray was most widely dispersed immediately after injection begins, narrowing rapidly to a constant spray angle. The same behavior was observed even for injection into near-vacuum conditions. However, once the spray was established, aerodynamic interactions were necessary for near-nozzle atomization.
The single-phase internal flow calculation showed that the magnitude of the turbulence intensity in the fluid was related to needle position. The position for cavitation, as indicated by regions within the nozzle tip with pressures less than the saturation pressure of the liquid, did not seem to be related to needle position, however.
Jaye
Koo
School of Aerospace and Mechanical Engineering, Korea Aerospace University,
Goyang, Gyeonggi, 412-791, Republic of Korea
Jay K.
Martin
Engine Research Center, University of Wisconsin—Madison, Madison, Wisconsin 53706
107-121