Begell House
Atomization and Sprays
Atomization and Sprays
1044-5110
9
1
1999
SIMPLE DESCRIPTION OF THE COMBUSTION STRUCTURES IN THE STABILIZATION STAGE OF A SPRAY JET FLAME
The structure of a two-phase flame has been investigated within Us stabilization region in the near field of a spray jet. The spray is produced by a coaxial air-blast injector fed with liquid methanol. We focus on a specific structure of the two-phase flame, observed experimentally where the flame presents a double structure with a predominant diffusion character for each reaction zone. The analysis is justified from experimental results of phase Doppler anemometry and planar laser-induced fluorescence of OH. The dynamics of the spray obtained from phase Doppler velocimetry are studied in terms of size classes defined from the Stokes number. The size classification shows that, where the flame stabilizes, the spray is composed of two fluids, one with high inertia (high Stokes number), the other characterized by a low Stokes number. The structure of the two-phase flame is analyzed in the low-inertia part of the spray. The emphasis is put on a regime where τch < τvap < τmix; a double structure may develop in what we called the "vaporization regime," since droplets can cross reaction zones. Such a double structure has been predicted by Continillo and Sirignano [25] and by Greenberg and Sarig [26, 27] through numerical modeling of a two-phase counterflow flame. The present article gives an experimental confirmation of a real occurrence of such aflame structure in turbulent spray jets and proposes a simplified description in the low-inertia part of the spray and for the flame sheet approximation.
D.
Stepowski
UMR 6614—Coria, CNRS et Universite de Rouen, Site universitaire du Madrillet, Avenue de l’universite, BP 12, 76 801 Saint Etienne du Rouvray Cedex, France
P.
Goix
CORIA—UMR 6614, CNRS—Universite & INSA de Rouen, 76821 Mont Saint Aignan, France
A.
Cessou
CORIA—UMR 6614, CNRS—Universite & INSA de Rouen, 76821 Mont Saint Aignan, France
1-27
A PREDICTIVE MODEL FOR DROPLET SIZE DISTRIBUTION IN SPRAYS
Spray combustion remains the dominant mode of energy conversion, providing the majority of the world's energy requirements. A good understanding of spray formation processes and spray droplet size distributions is essential for the design and operation of spray combustion systems with high energy efficiency and low pollutant emissions. The early stage of the spray formation process is clearly deterministic, with distinct unstable wave motion, whereas the final stage of spray formation process is more or less random, chaotic, and stochastic due to nonlinear effects of the unstable wave development. The number of droplets produced in a spray is enormous, and the description of each individual droplet becomes highly improbable, thus requiring a statistical treatment. The present model incorporates the deterministic aspect through the linear and nonlinear stability theory, and the stochastic aspect through the maximum entropy principle. It can predict, from a given flow condition at the nozzle exit, the spray formation process and the probability distribution of subsequently formed droplets in sprays. The effect of flow conditions at the nozzle exit on the droplet size distributions has been investigated. The present predictive model gives the initial distribution of droplet diameters and velocities in sprays, and hence will be useful as a submodel for overall spray combustion modeling.
Sushanta K.
Mitra
University of Alberta
Xianguo
Li
University of Waterloo
29-50
THE EFFECT OF MANIFOLD CROSS-FLOW ON THE DISCHARGE COEFFICIENT OF SHARP-EDGED ORIFICES
The objective of this study is to determine the effect of manifold cross-flow on the discharge coefficient and cavitation characteristics of sharp-edged orifices over a wide range of flow rates, back pressures, and cross-flow velocities. The geometries studied cover a range of orifice diameters, length-to-diameter ratios, and orifice angles characteristic of impinging-element liquid rocket injectors. Experimental results for an orifice angle of 90° with respect to the manifold are presented. Along with the experimental effort, an analytical model has been developed. The model predicts the discharge coefficient for a sharp-edged orifice over a wide range of flow regimes including cavitating and noncavitating flow, and for a wide range of orifice geometries. The analytical model generally shows good agreement with the experimental data over the range of conditions studied here. The model also closely follows the experimental data for cavitating flow except when the orifice length-to-diameter ratio is small, in which case the model overpredicts the discharge coefficient.
Douglas
Talley
USAF Research Lab
P. A.
Strakey
Air Force Research Laboratory, Edwards Air Force Base, California, USA
51-68
EVAPORATION OF A SALT WATER DROP WITH CRYSTALLIZATION
Simultaneous measurements for weight loss and temperature history of an evaporating pendant salt-solution drop into dry air were made for a wide range of air flow rates, ambient temperatures, and initial salt concentrations. The effect of crystallization of salt on the rates of evaporation of an aqueous sodium chloride and potassium chloride solution drop into dry air was examined. The presence of salt crystals was first observed near the front stagnation point, which took a major role in mass transfer of a single drop, and the formation of crystals occurred before the average salt concentration reached the equilibrium value. The dimensionless gas-phase diffusion fluxes of water vapor were affected by the vapor-pressure depression and the decrease in effective surface area caused by crystal growth of inorganic salt. A new simulation method for evaporation of an aqueous salt-solution drop into dry air during crystallization of salt was proposed, and the results were in better agreement with the observed data.
Izumi
Taniguchi
Department of Chemical Engineering, Tokyo Institute of Technology, Tokyo, Japan
Tsuguo
Inoue
Department of Chemical Engineering, Tokyo Institute of Technology, Tokyo, Japan
Koichi
Asano
Department of Chemical Engineering, Tokyo Institute of Technology, Tokyo, Japan
69-85
UNSTEADINESS IN EFFERVESCENT SPRAYS
The ideal spray theory of Edwards and Marx was used to investigate the dependence of effervescent spray unsteadiness on operating conditions, spatial location, and fluid physical properties. Droplet size, velocity, and arrival time at a particular spray location were measured using a Phase/Doppler Particle Analyzer. The droplet arrival times were used in calculations of interparticle arrival time gaps and interparticle time distribution functions. The spray was determined to be steady (interparticle time distribution function obeying inhomogeneous Poisson statistics) or unsteady (interparticle time distribution function not obeying inhomogeneous Poisson statistics) by comparing experimental and theoretical (steady) interparticle time distribution functions with results reported in terms of the number of deviations between the two. Since the spray was assumed to be a Poisson process, the expected deviation is the inverse of the square root of the number of interparticle events. A chi-square analysis was performed on the discrepancy.
Results demonstrate that all droplet size classes, which range from diameters of 3.2 to 60.4 μm, exhibit unsteady behavior. Stokes number calculations show that the largest droplets are incapable of following the turbulent flow field motions. Gas-phase turbulence can therefore be eliminated as a cause of unsteadiness for those drops. Chi-square calculations demonstrate that the probability for obtaining such results from random fluctuations is less than 0.001. Hence, it is concluded that effervescent atomization is an inherently unsteady process.
Results also indicate that spray unsteadiness is influenced by the air-to-liquid ratio by mass (ALR) and the liquid mass flow rate, depending on the properties of the liquid used in the spray, and that fluid viscosity and surface tension can affect the level of spray unsteadiness only when the spray is operating in the bubbly or intermittent slug regime. For such conditions, the spray is more unsteady when a lower-viscosity or higher-surface-tension fluid is utilized. When using a liquid that has a low viscosity (0.03 kg/m-s) and high surface tension (0.065 kg/s2), a decrease in ALR or liquid mass flow rate causes the spray to be more unsteady. The use of a high-viscosity (0.124 kg/m-s) liquid lessens the effect of operating conditions on spray unsteadiness. Finally, it was found that the spray is more unsteady at its edge, as well as at greater downstream distances.
John T. K.
Luong
Thermal Sciences and Propulsion Center, School of Mechanical Engineering, Purdue University, West Lafayette, Indiana, USA
87-109