Showing papers on "Diffusion flame published in 1993"
TL;DR: In this article, an experimental study of buoyant propane diffusion flames was undertaken to identify the mechanism responsible for the periodic oscillations near the source of these flames, referred to as "puffing", which exhibits itself as quasi-periodic oscillations of the diffusion flame front near the axisymmetric source of a fire with formation of large scale flaming vortical structures.
299 citations
TL;DR: In this paper, a coannular ethene diffusion flame was analyzed using cross sections for polydisperse aggregates and the fractal dimension of the aggregates in the flame was determined.
261 citations
TL;DR: In this paper, an analysis for the transition characteristics from momentum-to buoyancy-controlled turbulent jet diffusion flames is presented, based on dimensional arguments for the physics in turbulent diffusion flames.
234 citations
TL;DR: In this article, a planar laser-induced fluorescence imaging of superequilibrium OH molecules in the counterflow flamefront region near the vortex leading edge was used to infer the thin flame limit.
216 citations
TL;DR: In this paper, a simple analysis and measurements are presented, which show that adiabatic premixed laminar flames can be stabilized on a flat flame burner, especially designed for this purpose.
Abstract: A simple analysis and measurements are presented, which show that adiabatic premixed laminar flames can be stabilized on a flat flame burner, especially designed for this purpose. The physical properties of these flames are identical to those of flat freely propagating flames. The adiabatic state can be accomplished in practice when the burner plate temperature is well above the temperature of the unburnt mixture. The net heat loss of the flame to the burner is zero (i.e. the flame is adiabatic) when the measured radial temperature profile of the burner plate is uniform. These flames are particularly suitable for comparison with theoretical or numerical flat flame studies.
195 citations
TL;DR: In this paper, a seeded laminar counterflow diffusion flame (CDF) operated at low strain-rate as a well-defined combustion system, offering at the same time a low velocity and high temperature gradient environment.
Abstract: Thermophoresis, meaning particle drift down a local gas temperature gradient, is now known to be important to many combustion-related technologies. Until now, however, no direct experimental determinations of primary and aggregated particle thermophoretic diffusivities, αT D, in high temperature combustion environments have been reported. To perform such measurements, we selected a seeded laminar counterflow diffusion flame (CDF) operated at low strain-rate as a well-defined combustion system, offering at the same time a low velocity and high temperature gradient environment. We established a CH4/ O2Inert opposed jet diffusion flame in which the gaseous fuel/oxygen ratio, and the diluent flow rates were adjusted to obtain a flat, stable flame, approximately coincident with the gas stagnation plane (GSP). Particles fed to or formed on either or both sides of the GSP move toward this plane until the local axial velocity is exactly counterbalanced by the thermophoretic velocity. As a result of this ...
129 citations
TL;DR: In this article, laser-induced fluorescence has been observed form the formaldehyde A 1 A 2 -X 1 A 1 electronic transition in a well characterized, laminar methane/air diffusion flame burning at atmospheric pressure.
124 citations
TL;DR: In this article, a time-dependent, axisymmetric, third-order-accurate computational-fluid-dynamics-based model was studied using a jet diffusion flame, and the outer-vortex structures were developed as part of the solution, while a weak shear-layer perturbation was required to generate the inner structures.
112 citations
TL;DR: In this paper, the authors found that when a fuel gas flowed in the direction of a decreasing field strength, the burning velocity was found to increase and the supply of air to the flame front increased by applying inhomogeneous field.
111 citations
TL;DR: In this paper, balance equations for continuity, momentum, energy and mass fractions of the chemical species involved are described. But the balance equations are highly non-linear differential equations and require specific numerical solution techniques.
Abstract: The exciting phenomenon called a flame results from the interaction of convection and molecular diffusion with many chemical reactions on small length scales. This interaction can be described on the basis of balance equations for continuity, momentum, energy and mass fractions of the chemical species involved. The balance equations are highly non-linear differential equations and require specific numerical solution techniques. Such techniques have been developed for one-dimensional flame problems by different groups since the early 1970’s [1.1]–[1.19] and are quite efficient and reliable today [1.20]–[1.22]. During the same time and stimulated by this working tool, the detailed kinetic mechanisms for hydrocarbon flames have continuously been refined and improved, resulting, however, in a continuously growing number of reactions and species.
101 citations
TL;DR: In this paper, a premixed flat flame flows over a flat deposition substrate that lies perpendicular to the flow and parallel to the burner face, and the optimal growth conditions occur when the flame is lifted from the burner surface and stabilized at the deposition surface.
TL;DR: In this article, the role of flame radiation in NOx production in jet flames was explored further, and the effects of dilution and premixing were strongly dependent on fuel type.
TL;DR: In this paper, the authors compared experimental concentration measurements of the major stable species and five radical species (OH, H atom, O atom, CH, and CH3) obtained on a rectangular Wolfhard-Parker slot burner with a detailed computation of the chemical structure of an axisymmctric laminar, CH4/air diffusion flame burning at atmospheric pressure.
Abstract: Experimental concentration measurements of the major stable species and five radical species (OH., H atom, O atom, CH., and CH3) obtained on a rectangular Wolfhard-Parker slot burner are compared with a detailed computation of the chemical structure of an axisymmctric laminar, CH4/air diffusion flame burning at atmospheric pressure. In order to examine these CH4/air flames with different geometries and different sizes, the species profiles are plotted as functions of the local mixture fraction, and the scalar dissipation rate has been matched in a region around the stoichiometric surface. The overall agreement in the absolute concentrations, the shape of the profiles, and their location in terms of the local mixture fraction is good to excellent for the stable species (except for O2) and for the most abundant radicals OH, H atom, and O atom. For example, the calculated OH- maximum concentration is in much better agreement with the experimental results than are full equilibrium and partial equilib...
TL;DR: In this article, a comparison of the influence of nitrogen dilution and flame temperature on soot formation in diffusion flames of ethylene was made, and the relative importance of the two effects was quantified.
TL;DR: In this article, the authors derived the species conservation relation in the counterflow flames and the tubular flames described by similarity solutions and made it possible to evaluate NO x emission indices.
TL;DR: In this paper, a method for estimating total radiant output of turbulent jet flames based on the measurement of radiative heat flux at a single location is reported, and the radiative flux from a variety of jet flames was measured and plotted in normalized coordinates to establish the feasibility of this approach.
TL;DR: The application of tunable diode lasers for in situ diagnostics in laminar hydrocarbon diffusion flames is demonstrated and carbon monoxide concentrations and the local flame temperature are determined by both direct-absorption and wavelength-modulation techniques.
Abstract: The application of tunable diode lasers for in situ diagnostics in laminar hydrocarbon diffusion flames is demonstrated. By the use of both direct-absorption and wavelength-modulation (second-derivative) techniques, carbon monoxide concentrations and the local flame temperature are determined for a laminar methane–air diffusion flame supported on a Wolfhard–Parker slot burner. In both cases the results are found to be in excellent agreement with prior measurements of these quantities using both probe and optical techniques.
TL;DR: In this article, the authors presented a planar image of laser-induced flurescence from OH · radicals and elastic scattering from soot particles in time-varying, laminar CH4/air diffusion flames burning in a co-flowing, axisymmetric configuration at atmospheric pressure.
TL;DR: In this paper, the influence of the Lewis number on turbulent flame front geometry is investigated in a premixed turbulent stagnation point flame, where a laser tomography technique is used to obtain the flame shape, a fractal analysis of the multiscale flame edges is performed and the distribution of local flame front curvature is determined.
Abstract: The influence of the Lewis number on turbulent flame front geometry is investigated in a premixed turbulent stagnation point flame. A laser tomography technique is used to obtain the flame shape, a fractal analysis of the multiscale flame edges is performed and the distribution of local flame front curvature is determined. Lean H2/Air and C3H8/Air mixtures with similar laminar burning rates were investigated with Lewis numbers of 0·33 and 1·85 respectively. At the conditions studied the laminar H2/Air mixture is unstable and a cellular structure is observed. Turbulence in the reactant stream is generated by a perforated plate and the turbulent length scale (3 mm) and intensity (7%) at the nozzle exit are fixed. The equivalence ratio is set so that the laminar burning velocity is the same for all the cases. The results show clearly that the turbulent flame surface area is dependent on the Lewis number. For a Lewis number less than unity surface area production is observed. The shape of the flame f...
TL;DR: In this paper, an experimental and numerical study of laminar flames formed by the counterflow of a monodisperse fuel spray with an air stream is reported, where the influence of the phase transfer terms on the flame structure is analyzed.
TL;DR: Asymptotic methods are used to evaluate a characteristic diffusion time (the recipro-cal of the so-called “scalar dissipation rate”) for counterflow diffusion flames using three distinct layers inside the viscous layer.
Abstract: Asymptotic methods are used to evaluate a characteristic diffusion time (the recipro-cal of the so-called “scalar dissipation rate”) for counterflow diffusion flames. Attention is restricted to the practically important limit of small values of the stoichiometric mixture fraction, which results in large values of the stream function at the reaction sheet, a quantity that is employed here as a large expansion parameter. Three distinct layers are identified inside the viscous layer: (i) a convective-diffusive layer on the oxidizer side of the reaction sheet, (ii) a rotational inviscid layer on the fuel side of the reaction sheet, and (iii) a fuel-product mixing layer in the vicinity of the stagnation plane; all three of these layers are analyzed and matched. The matching produces analytic expressions for the stream function and mixture fraction at the reaction sheet and corrects formulas for the characteristic diffusion time previously derived on the basis of constant-density-flow or nonreactive-mixing-laye...
TL;DR: In this article, the divergence of the net radiative flux is formulated in terms of wide-band absorptance model parameters for combustion products, and is valid for all degrees of optical thickness.
Abstract: An expression is derived for the radiative source term governing the interaction of molecular gas band radiation and flow in nonhomogeneous, plane-parallel reacting flow problems. The divergence of the net radiative flux is formulated in terms of wide-band absorptance model parameters for combustion products, and is valid for all degrees of optical thickness. When optical thickness is finite, the net absorption is obtained by integrating the radiation field solution over the band lineshapes and taking hemispherical averages. Illustrative calculations for counterflow diffusion flames will be discussed.
28 Jun 1993
TL;DR: In this paper, the authors considered polyethylene, a reference material for laboratory scale investigation, hydroxyl terminated polybutadiene, and glycidyl azide polymer.
Abstract: The regression rate of a fuel for hybrid propulsion applications results from the coupling between the heat flux density issuing from the reacting cross {Jow and the energy required by the condensed phase process. Three fuels are considered polyethylene, a reference material for laboratory scale investigation, hydroxyl terminated polybutadiene, a candidate material for booster applications, and glycidyl azide polymer. which although being a fuel has an autonomous burning rate. The condensed phase process is shown to be an irreversible thennal pyrolysis not directly affecred by pressure. The characteristics of this process are presented for the three fuels. The ablation process, measured here or in the literature on laboratory devices or small motors, can be described quite accurately by the diffusion flame approach for the pure fuels. The ablation of GAP is dominated by its self burning with probably some added effect of the cross flow difTusion flame, with, as a consequence, high regression rates.
TL;DR: In this paper, an accurate numerical algorithm was employed to simulate two model flames, an unconfined lifted and a confined, co-flowing, methane-air jet diffusion flame using detailed chemistry and complex transport models.
Abstract: We employ an accurate numerical algorithm to simulate two model flames—an unconfined lifted and a confined, coflowing, methane-air jet diffusion flame using detailed chemistry and complex transport models. The algorithm employs Newton's method to obtain the primitive variable solution of the large system of strongly coupled elliptic governing equations. The Newton equations are solved by a block-line tridiagonal method. We employ a global grid refinement technique which equidistributes meshes according to the gradients and curvatures of the solution obtained on the previous mesh and bounds the ratio of adjacent grid step size. The algorithm can be applied to problems ranging from non-reacting flows to reacting flows in two- or three-dimensional configurations. In the unconfined case, the lifted flame and the “triple flame” are both predicted in the numerical solution. The computed solutions agree well with the experimental results. The comparison of the present solutions with the previously repor...
TL;DR: Strong fluorescence backgrounds from OH radicals found in the high-temperature regions of the flame are discussed, and a data reduction scheme based on the analysis of the entire spectral shape are reported.
Abstract: Spectra with simultaneous peaks of CO(2), O(2), N(2), CH(4), and H(2)O are taken by the use of spontaneous Raman scattering from free jets and a turbulent CH4 diffusion flame. A narrow-band XeCl excimer laser working at 308 nm and an intensified multichannel camera are used to give full information about all major species and the spectral background. Knowledge of the structure of the background is important for data analysis. For O(2), an enhancement that is due to near-resonant Raman scattering is found. Assuming no influence of this enhancement on the temperature dependence of the O(2) Raman-scattering cross section, temperature and concentrations of all major molecules are determined simultaneously from the intensity of the Raman bands. Experimental details and a data reduction scheme based on the analysis of the entire spectral shape are reported. Strong fluorescence backgrounds from OH radicals found in the high-temperature regions of the flame are discussed.
TL;DR: In this article, a flame-capturing technique is used to simulate a flame front advected by a hydrodynamical flow in a manner independent of zoning, and the flame is resolved down to a spatial scale λ c, where it becomes stable with respect to the Rayleigh-Taylor instability.
Abstract: We study thermonuclear, carbon-oxygen deflagration flames in supernovae. A flame-capturing technique is used to simulate a flame front advected by a hydrodynamical flow in a manner independent of zoning. The flame is resolved down to a spatial scale λ c , where it becomes stable with respect to the Rayleigh-Taylor instability. At scales >λ c , the flame shows a complex behavior. Large bubbles of hot, burned matter rise, kill smaller bubbles, and tend to dominate the flow. The flame folds, thin layers of cold fuel penetrate deep into products, and the flame becomes multiply connected. The geometrical complexity of the flame front and the instantaneous turbulent flame speed vary with time. A substantial acceleration of burning is found
TL;DR: In this paper, an experimental investigation has been made with the objective of studying the mixing mechanism near the nozzle exit in a tone excited non-premixed jet flame, where the fuel jet was pulsed by means of a loudspeaker-driven cavity.
Abstract: An experimental investigation has been made with the objective of studying the mixing mechanism near the nozzle exit in a tone excited non-premixed jet flame. The fuel jet was pulsed by means of a loudspeaker-driven cavity. The excitation frequencies were chosen for the two cases of the non-resonant and resonant frequency identified as a pipe resonance due to acoustic excitation. The effect of different sinusoidal excitation conditions on mixing pattern near the nozzle exit and flame was visualized using various techniques, including schlieren photograph and laser light scattering photograph from TiO2 seed particles. In order to clarify the details of the flame feature observed by visualization methods, hotwire measurements have been made. Excitation at the resonant frequency makes strong mixing near the nozzle. In this case, the fuel jet flow in the vicinity of nozzle exit breaks up into disturbed fluid parcels. This phenomenon affects greatly the combustion characteristics of the tone excited j...
TL;DR: In this article, a coal combustion model was developed to represent the limiting scenario of infinitely fast secondary pyrolysis followed by combustion of gaseous fuels and soot in flame sheets, either on or around individual coal particles.
TL;DR: In this article, the authors studied the general structure and extinction characteristics of diffusion and premixed flames in the presence of heat loss, and their dependence on the relative rates of reaction, transport, and loss.
TL;DR: In this paper, the authors consider a stationary uniform, laminar supersonic mixing layer of hydrogen and air and investigate some conditions under which ignition occurs, specifically, the influences of viscous dissipation effects, initial temperature and/or velocity gradients.
Abstract: Development of Scramjet engines requires a detailed knowledge of the coupling between supersonic flow and chemical kinetics of combustion. In this paper we consider a stationary uniform, laminar supersonic mixing layer of hydrogen and air and investigate some conditions under which ignition occurs. More specifically, the influences of viscous dissipation effects, initial temperature and/or velocity gradients are investigated. This problem, where detailed chemistry for H2-air is used, is solved numerically. The structure of the reactive mixing layer is shown to consist of an induction region, a thermal runawayregion, a region where both premixed and diffusion flames coexist, and a pure diffusion flame region. The exact length of the induction zone is found to depend sharply on the upstream boundary conditions and more precisely on the intensity of viscous dissipation together with amplitude and direction of transverse temperature and velocity gradients.