ACERC
Abstracts - 1989 |
ACERC
Abstracts - 1989 |
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Thrust Area 4: Turbulent, Reacting Fluid Mechanics and Heat Transfer |
Queiroz, M. and Yao, S.C.
Combustion and Flame, 76, 351-368, 1989. Funded by ACERC (National Science
Foundation and Associates and Affiliates), the State of Utah, and US Department
of Energy.
An exploration of the dynamic nature of flame propagation in planar sprays at ambient conditions has been performed. The parametric influence of three different levels of fuel-vapor concentrations (corresponding to three liquid fuel initial temperatures of 43ºC, 25ºC, and 15ºC), two droplet sizes (305 and 407 mm), three fuel compositions (hexane, iso-octane, and a 50-50 by weight mixture of these fuels), and two levels of gas-phase turbulence intensity (17% and 2.5%) on the average speed of flame propagation and its fluctuations have been investigated using sequential photographic information. It was found that higher fuel-vapor concentrations, smaller droplet sizes, and more volatile fuels caused the average flame speed to increase while reducing its fluctuations. The effect of higher gas-phase turbulence intensity also caused the average flame speed to increase, but increased its fluctuations. Some of the observed phenomena can be qualitatively explained through the ignition process in a spray. When the presence of fuel vapor in the gas-phase becomes significant due to the evaporation of droplets before the flame front, a premixed-gas type of ignition process prevails. At this condition the variations of average flame speed and its fluctuations may be explained through the effect of gas-phase equivalence ratio on the premixed-gas average flame speed. When the presence of fuel vapor in the flame front is reduced at high turbulence, a relay type of ignition process occurs. In this latter case, the increase of average flame speed and of flame speed fluctuations may be interpreted from the enhanced turbulent diffusion of thermal energy and from the turbulent fluctuations caused by the eddy transport phenomenon in the flow field, respectively.
Ashurst, W.T. and McMurtry,
P.A.
Comb. Sci, and Tech., 66, 17-37, 1989. Funded by US Department of Energy.
Numerical simulation of premixed flame propagation through a two-dimensional vorticity distribution exhibits the coupling between combustion heat release and fluid dynamics. Effects of both thermal expansion and vorticity generation via baroclinic torques are considered. The rotational part of the velocity field is described by discrete vorticity. The irrotational velocity, given by a velocity potential, is determined by a Poisson equation where the chemical reaction determines the spatial distribution of volume expansion. The transport of a single reacting scalar is computed on an Eulerian mesh with a small-Mach number flow assumption so that the density gradient is nonzero only within the reaction zone. The vector cross product of density gradient and pressure gradient defines the baroclinic generation of vorticity. With a single initial vortical region - the monopole interacting with a premixed flame - the baroclinic effects produce opposite circulation, and, thus, creates a dipole configuration. We use the dipole description in a loose sense, because in some solutions the two signs of vorticity are spatially intermingled. However, in comparison to the monopole, the "dipole" solution has stronger local velocity fluctuations but weaker long-range velocities. Thus, the turbulence in the burnt gas is more intense with smaller length scales than the configuration before the flame-vortex interaction.
McMurtry, P.A.
1st International Congress. on Tox. Comb., Los Angeles, California, 1989.
Funded by ACERC (National Science Foundation and Associates and Affiliates).
Results from numerical simulations of chemically reacting, turbulent mixing layers are presented. Effects of turbulent mixing, chemical heat release, and chemical nonequilibrium are discussed. Effects of chemical heat release in turbulent mixing layers are shown to result in lower chemical formation rates than similar isothermal reactions. A study of the flow in terms of vorticity dynamics and linear stability indicates that the large-scale characteristic vortex structures of mixing layers are inhibited by heat release. Phenomena resulting from chemical nonequilibrium are also presented. In agreement with previous analytic studies, incomplete combustion and flame quenching is shown to result when local diffusion time scales are lowered to those of characteristic chemical time scales. The statistical description of the scalar dissipation is the most important parameter describing the turbulence-chemical nonequilibrium interactions. An understanding of these phenomena is crucial for efficient and complete combustion in turbulent shear flows. On going research activities and suggestions for future work are presented.
Montgomery, C.J.; Son, S.F.
and Queiroz, M.
Western States Section, The Combustion Institute, Pullman, Washington,
1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).
Measurements of average gas-phase temperature and concentration of major stable gaseous species, as well as rms, power spectral densities, probability density functions, autocorrelations and other statistical data for temperature are presented for a simplified turbulent spray flame. The flame consists of an array of six vertical streams of nearly-monosized hexane droplets anchored at one edge by a small hydrogen pilot flame. Composition profiles were obtained by microprobe sampling and gas chromatography. Temperatures were measured by a fine wire thermocouple and compensated for thermal inertia using a digital deconvolution technique. The above measurements are presented for initial fuel temperatures of 28ºC and 45ºC. The measurements show that very rapid chemical reaction and heat release take place in the flame's blue partially premixed zone. In the yellow diffusion-flame zone following the blue region, temperatures and species concentrations change more slowly because fuel droplets exist well upstream into the flame and continue supplying fuel vapor that reacts quickly with oxygen entering the flame zone through turbulent mixing. These results demonstrate that the flame studied here is quite different from a gaseous flame because of the significant effect of the liquid phase on the combustion process. Since this may also be the case in many practical systems, it is important that reliable experimental data on spray combustion be obtained, both to aid the development of numerical models and to enhance our understanding of the phenomena involved.
Bonin, M.P. and Queiroz,
M.
Western States Section, The Combustion Institute, Pullman, Washington,
1989. Funded by ACERC (National Science Foundation and Associates and Affiliates).
A newly-developed, laser based instrument which is capable of non-intrusively measuring the size and velocity distribution of particles in a two-phase reacting flow has been applied to a monodispersed stream of liquid fuel droplets burning in a turbulent, co-flowing air stream. This instrument determines the size distribution of particles having diameters ranging from 0.5 to 200 mm with corresponding velocities as high as 400 m/s. Therefore, the instrument is a valuable diagnostic tool for the investigation of both simplified and more complex spray flames. Measurement uncertainty is typically ten percent of the indicated droplet size.
Previous applications of the instrument were primarily concerned with size measurement in light absorbing environments consisting of solid particles such as coal, coal slurries or powdered metals. The present study describes the first documented application of this sizing technique to liquid fuel droplets. Before actual measurements were made, an appropriate instrument response function specific to non-absorbing (liquid) particles was created. With the corrected response function, a parametric examination of a simplified spray flame was undertaken to demonstrate the sizing capability of the instrument under non-absorbing conditions. The parametric study was designed to track the influence of variable turbulence intensity, fuel type and initial droplet size on the droplet vaporization rate. Temperature measurements made with digitally compensated thermocouples further quantified the mechanisms affecting the droplet size history. Hot-wire measurements were also performed to characterize the co-flowing air stream. Comparisons between measured and predicted droplet sizes using single droplet evaporation theories indicate a lower experimental value resulting from group combustion effects. Under certain experimental conditions, droplet agglomeration was observed downstream of the ignition point. The cause of the agglomeration is not clear, however it is thought to result from droplet collision and subsequent coalescence in the turbulent flow.
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