Flame Photographs Research Articles

A new approach to the correlation of turbulent burning velocity data

The present investigation highlights a new approach to the correlation of turbulent burning velocity data, which basically stems from the fact that detailed measurements of turbulence, using wire meshes and perforated discs as generators of isotropic turbulence, show a unique trend when the ratio of Kolmogorov microscale η to lateral macroscale L is plotted against the r.m.s. turbulent velocity u ′. For weak turbulence, the data obtained for different generators telescoped into a single curve, whereas for strong turbulence distinct demarcations could be detected for different grids. This cardinal finding was further pursued apropos of turbulent burning velocity data. Systematic measurements were carried out in cold flows using a hot-wire anemometer and a real time wave analyzer from which the requisite intensity and scales of turbulence were deduced. Subsequently burning velocity data for methane-air mixtures burning in an open burner were obtained by analyzing densitometrically the direct photographs of flames. Analysis reveals that two correlations are adequate for interpreting the data obtained. When turbulence intensity is low, the ratio of turbulent to laminar burning velocity S T /S L depends only on η/L in much the same fashion as u ′/S L while for large intensities, u ′/S L turns out to be an additional parameter besides η/L. When this analysis is extended to the data of other investigators, the forms of the correlating equations remain the same as in the present work but require different empirical constants. To sum up, it may be said that for weak turbulence η/L alone is adequate as a correlating parameter, while for strong turbulence both u ′/S L and η/L must be considered.

Electronically produced equidensities from time exposures and instantaneous photographs in the investigation of pool flames

Information contained in the negatives of time exposures and instantaneous photographs of pool flames can be quantitatively evaluated by the use of electronical equidensitometry. In this method, up to 15 colored lines of constant optical density (equidensities) are produced. N-hexane, methanol, n-butane and liquefied natural gas, in pools of varying diameter, as well as a natural gas jet flame, are investigated without influencing the combustion processes. Equidensities obtained from time exposures, instantaneous photographs, and a series of high-speed frames are evaluated. The mathematical relationship between the optical density of the emulsion and the spectral radiance L λ emitted by the flame in the visible region is then derived. Thus the quantitative significance of the equidensities as lines of L λ = const. becomes apparent. The equidensities obtained from time exposures (long-time equidensities) can be related to the time-averaged values of flame properties such as radiance, temperature, flame shape, flame height, and soot concentration. A characteristic dependence on fuel type exists. Furthermore, the minimum sampling time for statistical combustion processes taking place in the flames is easily measured. The equidensities obtained from instantaneous photographs (short-time equidensities) afford a detailed insight into the complicated turbulent flame field. It appears that the eddies are generally elliptical. In hexane and methanol pool flames, the lengths and widths of the largest eddies increase with increasing pool diameter. The dimensions of the smallest eddies, which are almost spherical and in which the transfer of kinetic energy into heat occurs, remain constant. Using equidensities obtained from a series of high-speed frames the dissipation of a single eddy can be visualized in detail. Presently, we are using a high-speed camera to make a film of the equidensities obtained from the pool flames. In this way, the migration velocity, the dynamics of the geometrical dimensions, and the lifetime of the eddies, including their fluctuation quantities, can be determined.

Radiative properties of plastics fires

A variant of the Schmidt Method, in which a narrow-view-angle radiometer looks at a flame against a blackbody background of known temperature, has been used for measuring radiative properties of plastics pool fires or fires spreading over horizontal fuel slabs. Measurements performed at several background temperatures, including cold background (radiance measurement), are interpreted by two alternative models. A gray-emitter homogeneous model yields an effective radiation temperature and emissivity. Alternatively, the assumption of a spectral absorption coefficient inversely proportional to wavelength, ascribing the absorption exclusively to soot, yields radiation temperature and the product of equivalent soot volume fraction and optical path length. The respective gray absorption coefficients and equivalent soot volume fractions were determined from path lengths measured as averages from flame photographs. Data for pool fires of polymethyl methacrylate (PMMA), polypropylene (PP), polystyrene (PS), polyoxymethylene (POM) and fires spreading over polyurethane (PU) foam are presented. At a height of 51 mm above the fuel surface, radiation temperatures ranged from 1190 K (PS) to 1408 K (PU foam), emissivities of 0.3×0.3 m fires from 0.05 (POM) to 0.81 (PS), absorption coefficients (not determined for POM) ranged from 1.3 m −1 (PMMA, PU foam) to 5.3 m −1 (PS) and equivalent soot volume fractions from 0.46×10 −6 (PU foam) to 2.8×10 −6 (PS). Finally, the correlation between emissivities and burning rates, which varied from 6.5 g/m 2 s (POM) to 14.5g/m 2 s (PS), strongly suggested that for sufficiently large fires burning rate is controlled primarily by radiative transfer. Data for a 0.73 m dia PMMA pool fire are also presented.

Decomposition of combustion noise scaling rules by direct flame photography

In order to estimate the scaling rules for combustion noise, a turbulence autocorrelation function of the reaction rate must be estimated. This depends in part upon the active reaction volume in a turbulent flame. This volume may be measured by direct flame photography centering on the radiation of an active radical in the reaction zone. The paper describes experiments on fuel-lean propane, propylene, and ethylene-air turbulent open premixed flames with burners of 0.4″–0.96″ diameter and velocities to 600 fps. The flame volume is measured from photographs of the flame centered on the CH emission. The volume is correlated to the flow velocity, burner diameter, laminar flame speed and fuel mass fraction in the form of a power law by a regression technique. The scaling rule obtained from experiments is shown to follow the law V∝UD 3F 3 S L . The scaling laws generated are compared with estimates previously drawn from Strahle's theory of combustion noise. Implications concerning the turbulence structure in the reaction zone are drawn from the correlations.

Ignition of Aluminum Particle Streams

Abstract The ignition of aluminum particle streams was studied in a steady-flow experiment in which aluminum powder carried in oxygen was ignited by a propane-oxygen pilot flame. Two measured particle size distributions were used, with average diameters of 25 μ and 120μ. Particle-to-gas weight fractions were in the range from 0.1 to 0.5. Ignition times were deduced from flame photographs and high-speed motion pictures were used to follow the ignition event. A simple thermal ignition model was used to predict particle ignition times. The model takes into account the radiation from the burning stream of particles to the unignited particle, as well as to the heating of this particle by a finite volume of hot gas. The measured ignition times were independent of metal concentration. The stream with 35 μ mean size ignited at 0.5 ms and the stream with 120μ mean size at 1.1 ms after injection into the flame, both independent of the metal flow rate. Comparison with the predicted ignition times for the two particl...

Combustion of monosized droplet streams in stationary self-supporting flames

Free-burning flames on single streams of monosized oil droplets of diameters between 500 and 10 μm were produced by a new experimental technique. The effects of initial drop size, fuel characteristics, and the inter-drop distances on the progress and mechanism of combustion of the drops in stationary free-burning flames have been studied photographically using a high-speed flash technique. The plots of (initial diameter) 2 against burning times give straight lines for all the fuels investigated, but the slopes of the lines (termed as the burning constants) were about one-half of those obtained in experiments with single droplets. The explanation for the reduction of the burning constants is considered to be due to droplet interaction which leads to the oxygen depletion around the droplets. For a drop a particular initial diameter, the squares of diameters do not decrease linearly with time in an initial period during which drop diameter is reduced to 90% of its initial value. The results of the flame sizes of the drops show that the ratios of flame to drop diameter d f/d increase markedly during the progress of combustion. These results contradict the quasi-steady-state model of combustion and point to the fact that combustion is an unsteady-state diffusion-controlled process. The Nusselt numbers of the burning drops are reduced by 80%/90% compared with those reported for drops undergoing negligible mass transfer. Photographs of the stationary flames show that the luminous areas of kerosine flames have been found to be inversely proportional to a modified equivalence ratio S Av /D AV (1− N D ), where S Av is the average drop spacing, D Av the average drop diameter, and N D the number of drops within the flame.

Preservation of approach-flow velocity and turbulence in bunsen flames

An attempt is reported to test the validity of (a) the theory of flame-generated turbulence, and (b) the concept of distiguishing between core and mixing-zone flames. A nozzle burner was used to obtain laminar and moderately turbulent open premixed propane-air flames. Four types of experiment have been carried out: o (1) Microdensitometer recordings of time-exposure photographs of turbulent flames show that the width of the turbulent-flame brush increases linearly with height in the flame. (2) Very tall and slender laminar flames (20 or more burner-diameters tall) have been made possible by forcing the flow through tightly packed extremely fine steel wool. (3) Varying amounts of turbulence have been generated outside the flame by using a coaxial air jet as a confinement of the flame. Schlieren photographs show that no turbulence diffuses into the approach flow. (4) Unburned gas velocities have been measured using a laser Doppler anemometer. Direct and spectrum-analyzer records of the beat signal show that the velocity and tue turbulence of the approach flow are preserved and constant throughout the unburned gas. These experiments lead to the conclusion that both the theory of flame-generated turbulence and the concept of distinguishing between core and mixing-zone flames are untenable.

After-Running in a Two-Stroke Gasoline Engine

problems in abnormal combustion have often arisen in high compression engines of recent passanger cars. One of them is after-running which is the continuation of the engine running by autoignitions after electrical ignition cutoff. In this phenomenon the ignition of fuel-air mixture is closely related with the engine speed. From the relation obtained between the firing timing and the engine speed just before firing cycle in various conditions, it is concluded that the ignition in after-running is due to the compression ignition. Moreover, it has been definitely shown by the flame photographs of after-running that the firing originates not from any fixed heat source in the combustion chamber, but from some mixture portions in which the conditions are favourable for the autoignition of the mixture.

Experimental Investigation of Recessed Wall Flameholder

Flameholders are of great importance in the design of ramjet combustors. It was already confirmed by L.H. Huellmantel et al. that recessed wall flameholders had the advantages of smaller pressure drop and wider range of flame stabilization compared with bluff body flameholders. This paper presents experimental investigations of more detailed characteristics of recessed wall flameholders as the preliminary sudy of their application for practical use. First, it is shown through flame photographs that the flame-holding mechanism of the recirculation flow in the groove of a recessed wall flameholder is similar to that of bluff body flameholders. Second, from the experimental results on the influence of the groove width upon the flame stability limits, it is found that the condition l/h=5 is the best for the flame stabilization, where l and h denote the width and the depth of the groove respectively.

Ignition and combustion of aluminium particles in hot ambient gases

Observations have been made of individual aluminium particles entrained in combustion gases under controlled conditions, at atmospheric pressure. Effects of ambient gas temperature and oxygen content and of aluminium particle size on ignition and subsequent combustion phenomena have been determined by flame photography and by microscope examination of collected particles. Ignition occurs only when the ambient gases are above 2210° to 2360° K, this temperature being quite insensitive to free oxygen content and particle size. This result is consistent with an ignition theory which assumes a substantial change in the rate of the surface reaction at the melting temperature of alumina (2300°K). When ignition occurs, the delay time, which varies with the square of particle diameter, is in excellent agreement with simple heat-transfer calculations when the initial lag of the particle velocity relative to the gas is taken into account. Once ignition occurs, combustion phenomena are complex, involving unsymmetrical burning and fragmentation; a qualitative discussion is provided.

Photographic Study of Combustion in a Pre-Combustion Type Diesel Engine : 2nd Report, Combustion in a Main Combustion Chamber

Using a high-speed motion picture camera, the authers made flame photographs thraugh a glasswindow set on the cylinder head of a pre-combustion type Diesel engine. For the cases of various ejecting directions and also for the various forms of cavity on a piston crown, the process of mixing and combustion of the gas mixture blown out from the pre-combustion chamber, swirl creation and flame distribution in the cylinder were investigated. Further, the suitable design of the main combustion chamber for improvement on the combustion was discussed by comparing the results of performance test obtained in a single cylinder engine.

Photographic Study of Combustion in a Pre-Combustion Type Diesel Engine : (2nd Report, Combustion in a Main Combustion Chamber)

Using a high-speed motion picture camera, the authors made flame photographs through a glass window set on the cylinder head of a pre-combustion type Diesel engine. For the cases of various directions of gas outflow and also for various shapes of cavity on a piston crown, the process of mixing and combustion of the gas mixture blown out from the precombustion chamber, swirl creation and flame distribution in the cylinder were investigated. In addition, the suitable design of the main combustion chamber for improvement on the combustion was discussed by comparing the results of performance test obtained in a single cylinder engine.

Flame Photographs of Light Load Combustion Point the Way to Reduction of Hydrocarbons in Exhaust Gas

An investigation to determine why the hydrocarbon content of automobile exhaust gas increases abruptly during deceleration and how this can be prevented is described in this paper. High-speed motion pictures of the flame, taken through a quartz-window cylinder head, show incomplete combustion under these conditions. Additional experiments established that excess residual-gas dilution is the major cause of the incomplete combustion. The concentration of hydrocarbons in the exhaust can be reduced either by admitting extra air and fuel to the intake manifold to limit vacuum attained during deceleration or by shutting off the fuel during deceleration.

Temperature and Latent Energy in Flame Gases

THE following facts relating to the travel of name, initiated by an electric spark, through a homogeneous inflammable gaseous mixture have been established: (1) Flame photographs show that the flame front during the early stages of travel is of low luminosity, but later it builds up a high luminosity indicating a high instantaneous pressure and temperature. (2) Turbulent motion of the inflammable mixture before combustion has no influence upon the rate of travel of the flame front in the early stages, but has a marked influence upon this in the later stages1.

First Direct Photographs of Sun's Hydrogen Flames Made

First Direct Photographs of Sun's Hydrogen Flames Made

Striations in Explosive Flames

IN a recent paper by Egerton and Gates (Proc. Roy. Soc., A, 116, 516; 1927), reference is made to peculiarities exhibited by certain flame photographs (not reproduced) obtained, on a rapidly revolving film, when mixtures 1C2H2: 2·502: 10N2 were ignited at one end of a closed cylinder 19 cm. long and of 10·7 cm. internal diameter, as follows: “Some of the photographs show distinct closely-spaced bands, indicating that the combustion has a vibratory character. The number of vibrations per second in the after-flame is about 2500, and the spacing is consistent with the view that successive sound waves are reflected from the end-plates”.