Showing papers on "Diffusion flame published in 1976"

An Asymptotic Analysis of Unsteady Diffusion Flames for Large Activation Energies

Abstract: The limit of large activation energy is studied for the process of simultaneous mixing and chemical reaction of two reactants undergoing a one-step irreversible Arrhenius reaction. Consideration is restricted to problems of the evolution type—like unsteady mixing and boundary-layer combustion—for which the solution is uniquely determined in terms of the initial conditions. The continuous transition from the nearly-frozen to the near-equilibrium regimes is described. The analysis uncovers the existence of: (i) An ignition regime, in which a mixing layer develops with only minor effects of the chemical reaction, until a thermal runaway occurs somewhere within the mixing region; at this location chemical equilibrium then is established rapidly, (ii) A deflagration regime, in which premixed flames originate from the ignition point and move through the mixing region to burn completely the reactant not in excess. And (iii) a diffusion-flame regime, in which a thin diffusion flame, that is established w...

Mathematical Models of Turbulent Flames; A Review

Abstract: The problem of predicting turbulent flame phenomena is reviewed, with special reference to the turbulent diffusion flame and to the confined pre-mixed flame downstream of a flame-holder. Attention is given to means of describing instantaneous and time-average states, and to the distinctions between the micro-scale aspects of the two flame types which these permit. A model for micro-scale reduction is proposed; and this is employed to derive the influence of the Schmidt number on burning rates in diffusion flames, and the influence of laminar flame speed on burning rates in confined pre-mixed flames. Both influences are weak. It is shown that the mixing-length version of the eddy-break-up model is likely to be closer to the truth than the model employing the local dissipation rate of turbulence energy.

Quasi-steady diffusion flame theory with variable specific heats and transport coefficients

Abstract: A theory is developed to account for the variable nature of the specific heats C and the transport coefficients of thermal and mass diffusion, λ and D, in quasi-steady one-dimensional diffusion flames, using bipropellant droplet combustion as an illustration. In the theory C, λ and D are all temperature dependent whereas C and λ are further concentration-weighted; consequently all the existing variable-property droplet combustion models are special cases of the present generalized model. By allowing these coefficients to assume realistic functional forms, explicit expressions are derived for the concentration-weighted coefficients, the temperature and species profiles, the mass burning rate, and the flame-front standoff ratio. Predicted results yield improved, yet much less ambiguous, agreement with experimental observations. Finally it is shown that such good agreement can also be achieved by utilizing a simplified, temperature-independent, model and by evaluating the concentration-weighted coef...

The Transition to Instability in a Steady Hydrogen-Oxygen Diffusion Flame

Abstract: —Blow-off of a streamwise diffusion flame does not usually show features intermediate between those characteristic of the burner-attached flame and a quasi-stable lifted flame. The present work has, however, characterised an unusual intermediate stage in the blow-off of a lean hydrogen-oxygen diffusion flame, where the flame base takes on a stable cellular structure. From previous work, it is known that, near extinction, there is transfer of fuel through the base of the flame into the oxidant stream. The partial premixing resulting from such inter-stream transfer gives rise in the particular case of this lean flame to a composite premixed/diffusion flame structure at the flame base which is most probably responsible for the cellularity.

Temperature measurement in conical flames by laser interferometry

Abstract: Reasons are developed for choosing a laser interferometer, and in particular the design due to Murty, to undertake flame temperature measurements in small-scale conical flames. The difficulties of obtaining a simple polynomial to represent the fringe shift distribution for an axisymmetric diffusion flame are discussed and a method of solution using the extended spline fit technique is described. The organisation of computer programs to plot fringe shift profiles and to compute radial distributions of refractivity and final temperature is given. The technique is applied to a methane diffusion flame and a comparison made with results obtained using a fine-wire thermocouple. The paper concludes that the technique is especially suitable where lateral fringe shift distributions are complex functions of lateral distance and in particular gives flame temperature profiles completely compatible with the more conventional probe methods.

Investigation of Liquid Fueled Turbulent Diffusion Flames

Abstract: —Pollutant emissions data gathered from within a simulated gas turbine combustor primary zone (flame stabilized in the wake of a disc with liquid fuel injected into the wake region from the center of the disc) for three widely varied operating conditions are reported and discussed. The basic flame structure was found to consist of a hollow reaction region (shear layer) along the boundary between the recirculation zone and the free stream. As the heterogeneous properties of the combustion process increased in importance, a free stream flame zone following the fuel spray was also detectable. In addition, CO and NOx emissions were found to originate in separate regions of the flame structure.

Models for metal particle combustion with extended flame zones

Abstract: A class of models is proposed to describe the vapor-phase combustion of metal particles. Optional features of the models include surface condensation of the oxide vapor and the existence of an extended flame zone, formed by condensing part or all of the outwardly-diffusing oxide vapor. Two types of extended flames, an isothermal flame and a non-isothermal flame with the inner edge hotter than the outer edge, are found to exist. It is shown that by including the additional gas-phase condensation process, the mass burning rate increases slightly whereas the outer edge of the flame can attain substantial size. Sample solutions are obtained for magnesium reacting with oxygen. Fair agreements exist between theoretical predictions and experimental results.

A kinetic-theory approach to turbulent chemically reacting flows

Abstract: The paper examines the mathematical and physical foundations for the kinetic theory of reactive turbulent flows, discussing the differences and relation between the kinetic and averaged equations, and comparing some solutions of the kinetic equations obtained by the Green's function method with those obtained by the approximate bimodal method. The kinetic method described consists essentially in constructing the probability density functions of the chemical species on the basis of solutions of the Langevin stochastic equation for the influence of eddies on the behavior of fluid elements. When the kinetic equations are solved for the structure of the diffusion flame established in a shear layer by the bimodal method, discontinuities in gradients of the mean concentrations at the two flame edges appear. This is a consequence of the bimodal approximation of all distribution functions by two dissimilar half-Maxwellian functions, which is a very crude approximation. These discontinuities do not appear when the solutions are constructed by the Green's function method described here.

Heat‐pipe‐oven reactor (HPOR): I. A new device for flame studies; photon yields in the reaction of Na with CCl4 and N2O

Abstract: A new device based on the heat‐pipe oven has been demonstrated to have a unique capability for studying reactions of metal atoms with oxidizers. This device allows spherical diffusion flame studies under known, uniform, and easily adjustable, metal atom concentration. low gas pumping rate, clean window operation, and minimized problems of reactive solid disposal. The reactor was used for the study of chemiluminescence of the reactions of sodium vapors with CCl4, with N2O, and with both CCl4 and N2O. The emission from these reactions in the range 2000–9000 A was identified to be from various excited atomic levels of sodium and from excited C2, mostly in the Swan bands. Even though the emission was very intense, especially in the case of Na+N2O+CCl4 (photon yields of about 10%), it was found that the Na excited‐state populations had a Boltzmann distribution corresponding to an electron temperature of 2260 °K. This suggests that the emission of sodium radiation from both the Na+CCl4 and Na+CCl4+N2O flames re...

Combustion control method for gas fuel in industrial furnace

Abstract: PURPOSE:To avoid local overheat, by making gas combustion flame in an industrial furnace to be hollow thin flame to reduce NOx while allowing the adjustment of the length of flame from stable flame to short flame shortened to not over than the half.

Formation and Control of Nitric Oxide from Fuel Nitrogen : 1st Report, Experimental and Modeling Studies of Fuel NO in Premixed Flat Flames

Abstract: The formation of nitric oxide from small additions of ammonia in premixed methane/air and hydrogen/air flat burner flames was studied and the conversion ratio of ammonia to nitric oxide was compared with the ;model calculation proposed. Experimental results show that fuel-NO is formed promptly above the equilibrium concentration in and near the flame zone and that the conversion ratio becomes larger (i) at higher air ratio, (ii) for less fuel-N, and (iii) at higher flame temperature. These characteristics can be explained by the kinetic scheme based primarily on extended Zeldovich mechanism if an assumption is made that fuel-N decays finally into N atom.

Studies on the Flame Structure of a Spray Burner : 2nd Report, Experimental Comparison with the Gas Diffusion Flame and Discussion on the Behavior of Droplets

Abstract: It was conjectured in the first report that most of the droplets within the flame do not burn individually, but fuel vapor evaporated from them collects and burns like a gas diffusion flame. The main object of this study is to confirm this conjecture. First, various measurements were made on a spray combustion flame and a gas diffusion flame under the same conditions. Comparing the two sets of data, it was found that the flames are similar in structure. Experiments on five spray combustion flames having different sizes, produced by changing the flow rate of combustion air, were secondly conducted. From the results, the conjecture of the first report was confirmed and it was found that the turbulent diffusion process has much greater influence upon the shape of the flames used in this study than the evaporation process of droplets. Finally, assuming that the droplets simply evaporate within the flame, their behavior was analyzed by making use of the knowledge which has been obtained for a single droplet. The calculated results showed fairly close agreement with the measured results.