Showing papers on "Premixed 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.

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.

Flame quality and presence monitor for multiburner furnaces

Abstract: Flame-monitoring equipment for indicating the state of a burner flame in the presence of other burner flames is arranged to respond to a range of frequencies higher than those normally utilized in flame-monitoring equipment and which are confined to the root portion of the burner flame. The equipment provides a measure of the flame quality and has a light sensitive element which is arranged to view the root portion of the burner flame and to feed signals to the associated electrical circuitry which determines the flame quality from the number, rather than the amplitude, of the high frequencies present in the electrical output of the element and lying within said range.

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.

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 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 Study on Radiation from a Luminous Flame

Abstract: From such a standpoint that a radiation from a luminous flame is due to the small solid particles (so-called"soot"), a fundamental research for the radiation from the luminous flames of the propane-air and city gas-air was carried out on the basis of Mie theory concerning radiation. The purpose of this paper is to measure the scattered parts and attenuated parts of incident lights about a flame and study the radiation of a luminous flame systematically. A mean diameter D32 of soot particles, defined as [numerical formula], where N (D) dD is the probability that the soot diameter is in the range of D and D+dD, is suggested to discuss the relation between the scattered light from the flame and the attenuated light by the luminous flame. It is showed that the values of D32 obtained by scattering and attenuation are 1 400A regardless of diffusion or premixed flame. The density of the soot particles, Cv, is experimentally obtained from the attenuation of the incident light in our research, and general consideration of radiation from luminous flames is given.

Effect of sound on a turbulent flame

Abstract: 1. A theoretical analysis shows that the nature of the stability of a gas jet depends significantly on the location of the flame front and on the ratio of the densities of the combustion products and the fresh mixture. 2. It has been experimentally established that there is an analogy between the sensitivity of a turbulent flame and a jet of unignited gas to sound. Eddy formation in flames takes place in the presence of weak acoustic oscillations, whose energy density is a millionth of the kinetic energy of the gas flow.

Method and apparatus for flame working mineral bodies

Abstract: Flame cutting mineral bodies with an improved form of flame jet to produce spalling is employed to carry out a channelling operation wherein a vertical channel is cut in an exposed vertical face of granite in a quarry. Emission of a stream of products of combustion from a burner at superatmospheric pressure is controlled by means of a specially formed exit orifice in a flame jet nozzle to provide a jet flame which is distorted out of the normally conical shaped configuration so as to provide a fan shaped flame operative over a relatively extended area of impingement on a vertical face of stone with more efficient utilization of flame energy being realized in spalling.

Endothermic gas reactions in flame without - catalyst

Abstract: (Part of) charge is reacted in flame produced from combustible gas mixture, gas stream (secondary gas) being intermixed in flame front. Secondary gas may consist of inert gas or one or more gaseous reactants. It can be introduced in same stream as combustible gas mixture or in transverse stream. Process is suitable for synthesis of unsaturated 2C hydrocarbons from light hydrocarbons and O2 and HCN from light hydrocarbons, NH3 and O2. Satisfactory yields are attained.

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.

Some recent experimental observations on flame spreading over solid fuel surfaces

Abstract: Flame spreading velocity characteristics over solid fuel surfaces, discussing environmental pressure, chemical reactivity, pyrolysis, forced convection, etc