Turbulent flame propagation parameters

LETTERS

TO

THE

EDITORS

Letters to the Editors on points of scientific: interest related to combustion and flame are invited. The Editors do not hold themselves responsible for opinions expressed in correspondence. Anonymous contributions cannot be accepted.

Turbulent Flame Propagation Parameters IT iS known that turbulence can increase burning rates appreciably; however, it is not clear which parameters should be considered. It is the purpose of this Note to show that turbulent flame velocities can be expressed as a function of laminar flame velocity, intensity of turbulence, scale of turbulence and equivalence ratio. The recent work of Lefebvre and Reid I offers an excellent review of the progress in turbulent flame propagation studies. Most of the theories reviewed by Lefebvre and Reid include in some manner the laminar flame velocity, intensity of turbulence and free stream velocity. Occasionally the scale of turbulence and the equivalence ratio are included. In addition to their review, new experimental data are presented which tend to support the wrinkled flame concept of turbulent flames. Lefebvre and Reid use laminar flame velocity, intensity of turbulence and free stream velocity to correlate their experimental turbulent flame results. In a previous paper Choudhury and Sanematsu 2 have shown that fo~ low intensity of turbulence 14 A

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and small integral scale of turbulence, turbulent flames can be expressed as a function of laminar flame velocity, intensity of turbulence and integral scale of turbulence. Reworking the dam of other experimenters appeared to support this fmdin& However, with the publication of Lefebvre and Reid's paper extending experimental data to higher intensities of turbulence, there appears to be an equivalence ratio parameter. In addition to the higher intensities of turbulence, the fluctuating velocity component of the turbulent flow is greater in absolute magnitude than the laminar flame velocity. This was not the case studied in Choudhury and Sanematsu's paper. Figure 1 is a graph of Lefebvre and Reid'ss results for a stoichiometric fuel-air mixture for various intensifies of turbulence ranging from 2 to 14 per cent. The symbols Ur and UL refer to turbulent and laminar flame velocities respectively. While u' is the intensity of turbulence, Ly is the integral scale of turbulence in the direction perpendicular to the propagating flame, agd b is a characteristic length scale usually associated with the wire mesh screen or hole diameter causing the turbulent flow. The line drawn in the figure was obtained by the method of least squares.

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FIGURE 2. Turbulent flame velocity for various fuel-ail ratios

Figure 2 is a graph of Lefebvre and Reid's experiments for equivalence ratios of 0.6, 0.8, 1.0, 1.1, 1.2 and 1.4. The effect of the equivalence ratio is apparent from Figure 2. It should be pointed out that in Lefebvre and Reid's experiment, as in nearly all cases studied, the scale of turbulence was not measured. Therefore, the scale of turbulence was estimated by using the correlation data of Baines and Peterson 4. By reworking Lefebvre and Reid's data it can be shown that turbulent flame propagation, Ur, increases with an increase in intensity of turbulence, u', provided the scale of turbulence, Lr, laminar flame propagation, UL, and equivalence ratio are kept constant. On the other hand, turbulent flame propagation, Ur, increases with a decrease in scale of turbulence, Ly, if the intensity of turbulence,

February 1969

LETTERSTO THE EDITORS

93

u', laminar flame propagation, UL, and equivalence ratio are kept constant. The equivalence ratio effect for Lefebvre and Reid's data could be included by changing the abscissa to (u'/U~)(L,,/b)C, where n is 1-3. Thus it is shown that turbulent flame propagation can be ex,3ressed in terms of turbulence as described by the intensity of turbulence and intcgral scale of turbulence, and properties of the fuel such as the laminar flame velocity and equivalence ratio. H. S. SANEMATSU Department of Mechanical Engineering. University of Southern California, Los Anodes , California

(Received February 1968; revised July 1968)

References t LEFEBV~,A. H. and REro, R. Combustion & Flame, I0, 355 (1966) 2 CHOUDHURY,P..R. and SANEMATSU,H. S. "An experimental investigation of turbulent flame propagation'. Paper presented at the Fall 1964 Western States Combustion Meeting, Salt Lake City (26-27 October 1964) a LErL~VRE,A. H. Private communication (28 July 1967) 4 BAINES,W. D. and PETERSON,E. G. Trans. dmer. Mech. Engrs, 73, 467 (1951)

Spectral Characteristics of Hydrocarbon-Air Flames Containing Aluminium, Magnesium and Boron DtraiN6 the last several years the use of hydrocarbon fuels containing several particulate metals ha:~ been evaluated for use in air-breathing engines and rocket motors. Since the addition of metals to the hydrocarbon substantially increases the flame temperature, they have also been suggested for use as working fuels for magnetohydrodynamic systems. In the course of our work it became necessary to measure the ultra-violet, visible, and near infra-red spectra of hydrocarbon-air flames containing particulate aluminium, magnesium and boron. These studies are described below. The hydrocarbon slurries were burned in an annular combustor described previously 1. Though the test mixtures were normally quite rich (equivalence ratio approximately three) the flames were essentially stoiehiometric due to the induction of air into the flameholder from the surroundings. Spectral radiation from the highly turbulen~t flames was measured with a Beckman Universal I R-4 spectrometer. The spectrometer admitted radiant energy from an area on the flame approximatcly 6 mm in height and 0.1 mm in width (depending upon the particular experiment). The entrance slit was focused at the centre of the flame. The instrument response was calibrated on an absolute scale with the aid of a blackbody source and calibrated tungsten filament lamp. Experimental spectral data were not corrected for the effect of atmospheric attenuation. Spectroscopic temperatures were determined for each of the flames examined using the technique discussed by Babrov 2. This involved a measurement of the spectral absorptivity and absolute spectral radiation at a given location in the flame from which the equivalent blackbody radiation and an associated average temperature could be deduced. The aluminium slurry employed in this work was prepared by the Aero Propulsion Laboratory at Wright-Patterson Air Force Base. It contained 50 per cent by weight of the metal, 98 per cent pure, with a particle size car,ging from 2 to 20 tan. Analysis revealed that the hnpurities consisted principally of magnesium, silicon, :,odium and copper. Though the viscosity of the suspension was approximately 100 cP, no particular problems were encountered with transport and a:omization of the fuel. Its combustion was less corqplete than that obtained with the magnesium and boron suspensions, resulting in a considerably larger combustion zone (5 cm diameter and 30 cm length). Some particles appeared to be incompletely oxidized during their average residence time of approximately 200 ms in the flame. Spectral radiation at three different positions in the flame is shown in Figure 1. In addition to considerable continuum radiation from Al and AI203, molecular radiation from the AIO green