Showing papers on "Diffusion flame published in 1969"

Spread of a laminar diffusion flame

Abstract: A theoretical description is presented for a laminar diffusion flame spreading against an air stream over a solid- or liquid-fuel bed. Both a thin sheet and a semi-infinite fuel bed are considered. The burning process is described as follows: The hot flame heats the unburned fuel bed, which subsequently vaporizes. The resulting fuel vapor reacts with the oxygen supplied by the incoming air, thereby producing the heat that maintains the flame-spread process. The formulated model treats the combustion as a diffusion flame, for which the details of the reaction kinetics can be ignored by assuming infinite reaction rates. The model includes the chemical stoichiometry, heat of combustion, gas-phase conductive heat transfer, radiation, mass transfer, fuel vaporization, and fuel-bed thermal properties. The radiation is mathematically treated as a heat loss at the flame sheet and a heat gain at the fuel-bed surface. The calculated flame-spread formulas are not inconsistent with available experimental data. These results reveal much of the physics involved in a spreading, flame. For instance, the flame-spread rate is strongly influenced by (1) the adiabatic stoichiometric flame temperature, and (2) the fuel-bed thermal properties, except for the fuel-bed conductivity parallel to the propagation direction.

The structure of counterflow diffusion flames in the forward stagnation region of a porous cylinder

Abstract: An experimental study has been made of the structure and the blow-off mechanism of the laminar counterflow diffusion flame established in the forward stagnation region of a porous cylinder. Concentration profiles of the stable species were determined using a microprobe sampling technique and gas-chromatographic analysis for hydrocarbon flames at atmospheric pressure. The over-all structure of the flame was examined by optical interferometry. Experimental results show that there are always various intermediate hydrocarbons and some oxygen on the fuel side of the flame. Hydrogen and carbon monoxide, which are the intermediate products, exist on both the fuel and air sides of the flame. The maximum concentrations of hydrogen and various intermediate hydrocarbons are found near the stagnation point of the counterflow. Although the flame approaches the cylinder surface when the fuel-ejection rate is decreased or the stagnation velocity gradient is increased, the differences in the concentration profiles for the case of a low fuel-ejection rate and for the case of a large stagnation velocity gradient are remarkable. It has been confirmed that critical blow-off of the flame is due to chemical limitations on the combustion rate in the flame zone and is clearly distinguished from blow-off caused by thermal quenching of the flame.

Optical studies of the generation of noise in turbulent flames

Abstract: A study of the mechanism of noise generation in turbulent premixed flames, turbulent diffusion flames and in liquid-spray combustion of hydrocarbon fuels is described. It is shown that all these flames may be represented acoustically as an assembly of monopole sound sources in their combustion zones. The radiated sound pressure is dependent upon the rate of change of the rate of increase of volume of the fuel and oxidant during combustion. The rate of volume increase is proportional to the rate of consumption of the fuel and oxidant in the flame. To measure this quantity, an optical technique has been employed that relies on observations of changes in the intensity of emission from the free radicals C2 generated in the reaction zones of these flames. For the premixed flames, good quantitative agreement is obtained between the radiated sound pressure calculated from these intensity measurements, using simple acoustic theory, and the values recorded simultaneously with a microphone. Similar agreement is obtained for diffusion flames, with the assumption made that the fuel and oxidant burn in stoichiometric proportions. Qualitative agreement is obtained for the liquid-fuel flames. The mean intensities of emission from premixed flames, burning under varied conditions of turbulence, were found to depend linearly upon the total flow rate of the combustible mixture and to be independent of the conditions of turbulence. This observation is compatible with the wrinkled laminar flame model of turbulent flame propagation.

The mechanism of flame spreading over the surface of igniting condensed-phase materials

Abstract: This paper describes an experimental and theoretical investigation of the fundamental mechanism by which a flame spreads over the surface of a condensed-phase materials in a quiescent gaseous environment containing a component with which it can react chemically. It is postulated that the advancing flame vaporizes the surface material lying before it. As these vapors diffuse away from the surface, they undergo an exothermic reaction with the chemically active component in the gaseous environment, and ignite; thus, flame spreading is viewed as continuous diffusive, gas-phase ignition. Flame-spreading velocities have been measured for a variety of solid materials in O2/inert environments between 4 and 415 psia. Well-defined experimental, conditions yielded reproducible results, and thus suggest that flame-spreading, velocity is an intrinsic combustion quantity. All data can be correlated by a power-law relationship between the flame-spreading velocity (V) and two gas-phase parameters-pressure (P) and reactive component mole fraction (Yox)—in the form V ∞ ( P Y m ) Φ . It is concluded that V is controlled by a gas-phase physical process—probably either heat or mass transfer—which supports the mechanism proposed. Temperature distributions ahead of the propagating flame were obtained from surface-mounted, fine-wire thermocouples. The temperature level as the flame passes over the thermo-couple bead is independent of P, Yox, and inert diluent, and about 120°C below that measured previously during steady-state vaporization. Thus, it is concluded that direct surface attack by oxygen is unimportant during flame spreading and that the transient vaporization phenomenon is probably quite different than that of steady pyrolysis. The mathematical statement of the postulated flame-spreading mechanism is sufficiently complex that a complete analytical solution is currently impossible. Postponing numerical solutions, simplistic analyses were conducted that resulted in predicted flame-spreading characteristics that were well supported by the data obtained over the entire range of experimentation. Based on the evidence presented, the authors conclude that the postulated theory is probably valid, and engineering design of systems involving flame-spread control now can be put on a rational basis.

Emission spectra obtained from the combustion of organic compounds in hydrogen flames

Abstract: A study is presented of the emission spectra produced by nebulisation of organic liquids into a nitrogen-hydrogen diffusion flame burning in air, and into a laminar-flow pre-mixed air-hydrogen flame. Both flames exhibit low background emissions and that of the diffusion flame is particularly low even over the OH-band region. Carbon, hydrogen and oxygen-containing compounds exhibit intense bands for CH, CHO, C2 and OH species. Nitrogen-containing organic compounds additionally display NH, NO and CN bands; chlorine-containing compounds give CCl bands in the diffusion flame; sulphur compounds give CS and S2 bands; and phosphorus compounds give HPO and PO bands.The spectral distribution of these bands and their intensities indicate that many types of organic compound can be characterised and identified by direct observation of their emission spectra during combustion in these cool flames.

Spontaneous ignition and combustion of sodium droplets in various oxidizing atmospheres at atmospheric pressure

Abstract: The study of the combustion of a droplet of liquid sodium has been made possible thanks to a novel technique which samples the pure molten metal and keeps it free of contamination until released in a heated furnace in a controlled oxidizing atmosphere. Sixteen mm motion pictures were obtained and frame-by-frame examination made it possible to observe a vaporphase diffusion flame separated from the droplet by a visible dead space. This is adequately described by a concentric zones model. Burning rates and evaporation constants have been computed for different temperatures and oxygen mole fractions ranging from 0.1–0.5. The experimental results are interpreted by considering oxygen diffusion to be the rate-determining step.

Extinguishment of solid propellants by rapid depressurization

Abstract: : A new theory for extinction by depressurization of AP composite propellants is employed to predict the rate of pressure decrease required to achieve flame-out and to rationalize the effects of various composition parameters on the ease of extinguishment The research is concentrated mainly on the 'temporary' extinguishment behavior of solid propellants, but a re- ignition theory is also presented Attention is given in this theory to the proper derivation of the nonsteady heat feedback from the gaseous flame zone to the burning surface Included in the model are the essential physical and chemical rate processes of the granular diffusion flame model, as deduced from the steady state burning characteristics A brief study of double-base propellants which indicates that they are considerably easier to extinguish than composite propellants concludes this report A rough model for the burning of double-base propellants is presented which further indicates the importance of the flame structure in determining the extinguishment characteristics of solid propellants

The extinction problem in an opposed jet diffusion flame with competitive reactions

Abstract: The problem of ignition and extinction for the flow of a compressible fluid with competitive/chain reactions has been considered in the present paper, A typical system of hydrogen-carbon monoxide fuel mixture forming one end of the opposed jet and air the other end has been considered for the numerical study. An efficient numerical procedure has been developed for the solution of two coupled and non-linear second order differential equations describing the chemical kinetics. The present study reveals that the mass flow rates at extinction (Apparent Flame Strength) for the mixture can be obtained by using a simple mixing rule similar to those used in theories of flame propagation. The results of maximum volumetric heal release rates have been obtained and compared with the experimental results of Bittker and Brokaw.

On the process of flame spreading over surface of plastic fuels in an oxidizing atmosphere

Abstract: A general theory of flame spreading over the surface of solid or liquid fuels in an oxidizing atmosphere is developed in the present work. The model of the process considers heating of the fuel ahead of the flame, fuel vaporization and mixing with the gaseous oxidizer and flame propagation through this combustible mixture. The flame spreading velocity results from the balance of all these processes. This theory is of a general type and may be applied to many kinds of flame spreading processes. However, the work has been directed to the case of flame spreading over the surface of plastic fuels in nitrogen-oxygen mixtures. The model of the process is solved by approximate analytical methods and an experimental investigation is also carried out. Theoretical and experimental results are obtained and compared, showing the influence of initial fuel temperature, pressure and mixture composition.

A study of free-jet and enclosed supersonic diffusion flames

Abstract: An experimental study has been made of supersonic diffusion flames produced by the subsonic and supersonic free-jet injection of hydrogen into a high-enthalpy air-stream. The air-stream was flowing at a Mach number=1.98 and had a total temperature of approximately 1900°K and a static pressure of 14.7 psia. An axial, mid-stream mode of fuel injection was adopted. For the case of hydrogen injected subsonically, combustion was found to be complete (i.e. the concentration of unreacted hydrogen was negligible), at a distance of approximately 9 inches from the point of injection. For the supersonic injection of hydrogen this distance was increased by approximately 30%, for the range of fuel velocities used. The tests were repeated with methane as the injected fuel, but ignition did not occur even with the methane preheated to 480°K, or when a bluff-body was inserted into the flow to create shock conditions. The above flames were then enclosed in various combustors of simple geometry, either of constant-area, constant-divergence or some combination of these two. For the conditions specified above, combustion in the diffusional mode was found to be impossible in a constantarea combustor. A diffusion flame was initiated in a combustor which consisted of a short parallel section followed by a section with a divergence of approximately 1°. However no combustion took place within a completely divergent duct even though the divergence was less than 1°. An exponential relationship between pressure, area and length has been proposed, and a one-dimensional analytical treatment for the case of heat addition in a non-constant-area duct is included in this paper. This assumption is shown to be experimentally reasonable and to result in gas dynamic equations which include the effect of process length.

On the process of flame spreading over the surface of plastic fuels in an oxidizing atmosphere

Abstract: A general theory of flame spreading over the surface of solid or liquid fuels in an oxidizing atmosphere is developed in the present work. The model of the process considers heating of the fuel ahead of the flame, fuel vaporization and mixing with the gaseous oxidizer and flame propagation through this combustible mixture. The flame spreading velocity results from the balance of all these processes. This theory is of a general type and may be applied to many kinds of flame spreading processes. However, the work has been directed to the case of flame spreading over the surface of plastic fuels in nitrogen-oxygen mixtures. The model of the process is solved by approximate analytical methods and an experimental investigation is also carried out. Theoretical and experimental results are obtained and compared, showing the influence of initial fuel temperature, pressure and mixture composition.

Turbulent Swirling Jet Diffusion Flames

Abstract: The phenomenological boundary-layer equations describing the flowfield in a turbulent jet diffusion flame with swirl are solved in the von Mises plane. The expressions found are compared with experimental results for three jet flames with different degrees of swirl using semiempirical values of turbulent exchange coefficients in the flame. The combined theoretical and experimental study indicates that an increase of the swirling motion in a flame leads to an increase of the flame width and mass entrainment and to a corresponding decrease of the flame length. These effects are associated with changes of the values of the turbulent eddy viscosity and Prandtl number caused by varying the swirl intensity in the flame. Nomenclature 61/2 = flame width defined in the text, Eq. (33) c = empirical constant defined by Eq. (20) Gx =

Experimental burning rates and combustion mechanisms of single beryllium particles

Abstract: Single spherical beryllium particles from two powder samples having average particle diameters of 32 and 25 μ, respectively, were injected into oxidizing gases, both moist and dry, at atmospheric pressure and at temperatures ranging from 2600°–2960°K. The purpose was accurate measurement of metal burning rates and a study of parameters affecting these rates. Ignition efficiencies, burning rates, and flame diameters of particles were found to vary both with particle diameters and with ambient gas properties. It is concluded that beryllium particles may burn by several distinct modes. One is rapid vapor-phase diffusion flame, favored by high partial pressures of oxygen and high temperatures of the gaseous environment. The burning times corresponding to this mode, ranging from 1.3–4.5 msec, were found to be proportional to the square of the particle diameter and inversly proportional to a power of oxygen pressure slightly less than unity. As the partial pressure of oxygen is decreased below a certain value which is usually between 0.1 and 0.2 atm, the metal flame temperature decreases and vaporphase combustion gradually changes over into slow surface reaction. There is also evidence of a third mode in which combustion is only moderately slower than the rapid vapor-phase burning. This mode, favored by low temperature and by the presence of water vapor in the environment, may be vapor-phase combustion hindered by a heavy coating of beryllia accumulated on the particle during the pre-ignition surface reaction.

Theoretical Studies of Diffusion Flame Structures

Abstract: : Two problems concerning diffusion flames characterized by a coordinate-dependent Damkohler number were studied theoretically, namely the structure of diffusion flames in laminar boundary layers and the one-dimensional unsteady ignition of solid fuels. The model selected for the study of the structure of diffusion flames in laminar boundary layers consists of an oxidant-containing flow over a wedge of condensed fuel. The fuel was assumed to undergo equilibrium vaporization and the chemical reaction was represented by Arrhenius kinetics of arbitrary order. The near-equilibrium regime was analyzed by the method of matched asymptotic expansions. The model for the study of the unsteady ignition of solid fuels consisted of a solid fuel suddenly exposed to hot oxidizing gas. The chemical reaction was represented by second-order Arrhenius kinetics. (Modified author abstract)

Turbulent mass transport and rates of reaction in a confined hydrogen-air diffusion flame

Abstract: Volumetric reaction rates and mass-transport coefficients are reported as a function of position for a ducted, two-dimensional, turbulent, hydrogen-air diffusion flame. Air and hydrogen velocities were 100 and 64 ft/sec, respectively. Basic data consisted of wall static pressures and a complete mapping of total pressure and chemical composition at eight axial locations. These data were used to compute the density, velocity, and mass fraction distributions in the flame. Mass-transport coefficients were determined by numerical solution of the time mean continuity equation for an inert species. Reaction rates were obtained by substituting the known mass-transport coefficient into the continuity equation for a reacting species and solving for the reaction rate, other terms being known. The mass-transport coefficients are at least one order of magnitude greater than the molecular diffusivities and are about one-tenth of previously reported values for confined, rod-stabilized, premixed propane-air flames with the same inlet air velocity. A fivefold increase in the mass-transport coefficient was found for the region of the flame studied. The magnitude and increase of the coefficient is due primarily to the generation of turbulence by the flame. The maximum reaction rates of oxygen are higher than those of a premixed flame with comparable inlet velocity, but the reaction zone is considerably thinner and the integrated reactant consumption is lower. A correlation exists between oxygen and hydrogen concentrations, independent of position and time. It is shown that the existence of the correlation precludes the simultaneous existence of significant amounts of hydrogen and oxygen and implies that local burning occurs at a reactant interface. It follows that turbulent mixing controls the reaction rate. An equation is developed which shows that volumetric reaction rate is directly proportional to the mass-transport coefficient.

A precise test of the flame-stretch theory of blow-off

Abstract: The postulate that blow-off is caused by excessive flame stretch in the stabilization zone has been tested for methane-air flames inhibited with methyl bromide. This involved the determination of a series of burning velocity vs composition curves corresponding to constant methyl bromide/methane volume ratios of 0, 0.01, 0.02, and 0.03. The maximum burning velocity for methane-air flames was found to be 38.3 cm/sec at 10.25% methane in air. The addition of methyl bromide reduced the maximum burning velocity to 35.1, 32.7, and 30.7 cm/sec for ratios of 0.01, 0.02, and 0.03, respectively, with a shift in the mixture composition at the maximum towards stoichiometric. Blow-off flow rates of inhibited methane-air flames were determined under laminar flow conditions on long cylindrical burners ranging from 0.62 to 1.30 cm diam. For any particular methane-air mixture, blow-off was found to occur at a fixed value of the Karlovitz flame-stretch factor, irrespective of the inhibitor content of the mixture. It is concluded that blow-off is almost certainly caused by excessive flame stretch in the stabilization zone. The critical value of the flame-stretch factor at which blow-off occurs, appears to increase rapidly as the methane content of rich primary mixtures is increased. It is suggested that this apparent increase is due to the error involved in the assumption that the burning velocity in the flame-stabilization zone is the normal burning velocity of the primary mixture. The burning velocity in the stabilization zone at the base of the flame will be significantly affected by (a) intermixing of the primary mixture with the surrounding atmosphere prior to combustion, and (b) heat transfer to the primary flame front from the outer diffusion flame.

Laboratory Studies of Carbon Formation in Fuel-Rich Flames at High Pressures

Abstract: Experimental studies are described in which continuous flow model flames of uniform composition are used to examine the effects of the individual variation of the main combustion parameters on flame composition and radiation. A detailed description is given of the chemical and physical behaviour of rich, high-pressure hydrocarbon–air flames and comparison is made with the results of previous work at N.G.T.E. on small premixed flames of C5 and C6 hydrocarbons and on the constant volume combustion of rich methane–oxygen mixtures. It is shown that there is considerable similarity between the combustion processes in the modern gas turbine and the diesel engine and that conclusions which are important in diesel engine combustion can be drawn from continuous flow experiments of this kind.

The turbulent diffusion combustion of a gas in a cylindrical chamber

Abstract: The turbulent diffusion combustion of a gas jet in a surrounding air flow bounded by the walls of a cylindrical chamber is studied theoretically and experimentally. A generalized universal equation is obtained for the coefficient of completeness of burning in the chamber as a function of the parameters of the geometry and modes of operation

Flame radiation studies using a model gas turbine primary zone

Abstract: A model primary zone is described, based on the simple toroidal vortex generated by a confined, circumferentially uniform annular jet of air. Fuel is either premixed in the vapor state with the air, or is fed radially to the air jet as a flat sheet of droplets of substantially uniform diameter from a spinning disk atomizer. The air is dry and can be preheated to 500°C and the fuel to 400°C. The flame can be operated at pressures up to 28 atm. Radiation measurements are made using the Schmidt technique. By using a flame of uniform composition, the model avoids difficulties inherent in measurements of this kind in developed gas turbine engine combustion systems, that are due to the highly stratified nature of the flame. The effects of the more important combustion parameters on flame emissivity can be studied independently through ranges of values spanning those occurring in present-day gas turbine engines. In common with most studies of this kind, there is some loss of accuracy with flames of very low emissivity due to absorption of radiation at the surface of the flame.

A study of premixed turbulent flames by scattered light

Abstract: A nonflow-disturbing, light-scattering technique for continuous measurements of both mean and fluctuating values of temperature has been developed that is capable of high-frequency response. The technique was used to study turbulent atmospheric premixed flames of natural gas and air. Thermally stable, submicron, magnesium oxide particles were introduced steadily into a premixed unburned gas-air stream. Since the combustion occurred under conditions of turbulent convective transport at constant pressure with no molal change and no dilution with external fluid, the particle concentration at any point in the flame was inversely proportional to the absolute temperature. The concentration was determined from photomultiplier tube measurements of mercury-arc light scattered at 90 degrees from within a volume approximately 0.1-inch diameter by 0.1-inch long. The technique did measure temperature and temperature fluctuations and can be a powerful tool for the study of premixed turbulent flames. Analysis of the fluctuations gave information directly relatable to the speeds, amplitudes, and wavelengths associated with turbulent combustion. Flame shapes determined from high-speed photographs were consistent with interpretations based on the light-scattering measurements. Under the conditions studied, the influence of upstream turbulence on flame structure was negligible except for its action as a triggering mechanism for downstream flame turbulence. Demonstrations are given of the use of the technique both for measurements of local average volumetric burning rates within the flame brush and for measurements of gross volumetric burning rates.