Showing papers on "Diffusion flame published in 1983"
TL;DR: In this paper, the formation and growth of soot particles in a coannular diffusion flame have been studied using a laser extinction/scattering technique for particle size measurement, and the results reveal that the flame can be broadly divided into two regions.
632 citations
TL;DR: In this article, a theoretical analysis of turbulent jet diffusion flames is developed in which the flame is regarded as an ensemble of laminar diffusion flamelets that are highly distorted, and the condition for flamelet extinction is derived in terms of the instantaneous scalar dissipation rate, which is ascribed a log-normal distribution.
Abstract: A theoretical analysis of turbulent jet diffusion flames is developed in which the flame is regarded as an ensemble of laminar diffusion flamelets that are highly distorted. The flow inhomogeneities are considered to be sufficiently strong to produce local quenching events for flamelets as a consequence of excessive flame stretch. The condition for flamelet extinction is derived in terms of the instantaneous scalar dissipation rate, which is ascribed a log-normal distribution. Percolation theory for a random network of stoichiometri c sheets is used to predict quenching thresholds that define liftoff heights. Predictions are shown to be in reasonably satisfactory agreement with experimentally measured liftoff heights of methane jet diffusion flames, within experimental uncertainties. UEL issuing from a tube or duct into an oxidizing atmosphere forms a jet in which combustion may occur. The associated combustion process is the most classical example of a diffusion flame. At sufficiently high velocities of fuel flow (fundamentally, at sufficiently large Reynolds numbers) the entire diffusion flame is turbulent. The turbulent jet diffusion flame begins at the mouth of the duct for a range of values of the exit velocity. When a critical exit velocity is exceeded, the flame abruptly is detached from the duct and acquires a new configuration of stabilization in which combustion begins a number of duct diameters downstream. Flames in this state, stabilized in the mixing region, are termed lifted diffusion flames, and the critical exit velocity at which they appear is called the liftoff velocity. The liftoff height is the centerline distance from the duct exit to the plane of flame stabilization. A further increase in the exit velocity increases the liftoff height without significantly modifying the turbulent flame height (the centerline distance from the duct exit to the plane at which, on the average, combustion ceases). There is a second critical value of the exit velocity, called the blowoff velocity, beyond which the flame cannot be stabilized in the mixing region. The present study addresses questions of the structure of lifted turbulent diffusion flames at exit velocities between liftoff and blowoff values. Attention is focused especially on the calculation of liftoff heights. Liftoff characteristics for turbulent jet diffusion flames are of practical importance in connection with flame stabilization. Conditions for liftoff and blowoff must be known in developing rational designs of burners, e.g., in diffusion-flame combustors for power production or in flaring applications for the petroleum industry. They are also of interest in connection with extinguishment of certain fires that may occur in oil or gas rigs. The present work is directed toward developing an improved fundamental understanding of liftoff phenomena that may later prove useful for these applications.
430 citations
TL;DR: In this paper, heat transfer and gas phase chemical kinetic aspects of the flame spread process are addressed separately for the spread of flames in oxidizing flows that oppose or concur with the direction of propagation.
Abstract: Recent advances in the experimental study of the mechanisms controlling the spread of flames over the surface of combustible solids are summarized in this work. The heat transfer and gas phase chemical kinetic aspects of the flame spread process are addressed separately for the spread of flames in oxidizing flows that oppose or concur with the direction of propagation. The realization that, in most practical situations, the spread of fire in opposed gas flows occurs at near extinction or non-propagating conditions is particularly significant. Under these circumstances, gas phase chemical kinetics plays a critical role and it must be considered if realistic descriptions of the flame spread process are attempted. In the concurrent mode of flame spread, heat transfer from the flame to the unburnt fuel appears to be the primary controlling mechanism. Although gas phase chemcial kinetics is unimportant in the flame spreading process, it is important in the establishment and extension of the diffusion ...
266 citations
TL;DR: In this article, the convective effects associated with the gas expansion are fully taken into account in the study of the flame structure and only one scalar "flame stretch" is shown to control the shape and the motion of the front.
Abstract: The recent theory of Clavin and Williams [1] concerning the premixed flame fronts in non uniform as well as unsteady flows is extended to the case of large amplitudes of the front corrugation. The convective effects associated with the gas expansion are fully taken into account in the study of the flame structure. Only one scalar « the flame stretch » is shown to control the shape and the motion of the front This stretch can be split in two parts, one accounts for the geometry of the front (mean curvature) and the other for the non uniformity of the flow (rate of strain tensor).
166 citations
TL;DR: In this paper, the linear stability of a premixed flame attached to a porous plug burner is examined using activation energy asymptotics, and a dispersion relation is obtained which defines the stability boundaries in the wave-, Lewis-number plane, and the movement of these boundaries is followed as the mass flux is reduced below the adiabatic value.
Abstract: We examine the linear stability of a premixed flame attached to a porous plug burner, using activation energy asymptotics. Limit function-expansions are not an appropriate mathematical framework for this problem, and are avoided. A dispersion relation is obtained which defines the stability boundaries in the wave-, Lewis-number plane, and the movement of these boundaries is followed as the mass flux is reduced below the adiabatic value and the flame moves towards the burner from infinity. Cellular instability is suppressed by the burner, but the pulsating instability usually associated with Lewis numbers greater than 1 is, at first, enhanced. For some parameter values the flame is never stable for all wavenumbers; the Lewis number stability band that exists for the unbounded flame disappears. For sufficiently small values of the stand-off distance the pulsating instability is suppressed.
85 citations
TL;DR: In this paper, the results of an extensive wind-tunnel study into the shapes and sizes of hydrocarbon jet diffusion flames in a horizontal cross-wind are presented, where the shape of a turbulent diffusion flame in a cross wind can be described by the frustum of a cone, which can be defined by five different parameters of shape.
80 citations
01 Sep 1983-Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science
TL;DR: In this paper, the axial temperature profiles of the flame and wall and axial profiles of heat flux to the solids bed and refractory wall were modeled mathematically.
Abstract: Heat flow in the flame zone of a direct-fired rotary kiln has been modeled mathematically. The flame has been assumed to be cylindrical in shape, backmixed radially, and moving axially in plug flow. The length of the flame and the rate of entrainment of secondary air have been characterized by empirical equations reported in the literature. It has been shown that the axial component of radiation can be reasonably neglected since it is relatively small compared to the radial component. The resulting one-dimensional model is capable of predicting the axial temperature profiles of the flame and wall and the axial profiles of heat flux to the solids bed and refractory wall. The model has been employed to study the influence on heat flow to the bed of the following variables: fuel type (fuel oil, natural gas, producer gas), firing rate, temperature of secondary air, pct primary air, and oxygen enrichment. Of the three fuels, combustion of fuel oil gives the longest flame and the greatest heat input to the solids in the flame zone. Raising the secondary-air temperature increases the flame length significantly but has a small effect on the maximum flame temperature and heat flux to the solids. Increasing percent primary air decreases the flame length and increases the peak values of flame temperature and solids heat flux but reduces the quantity of heat received by the solids in the flame zone. Oxygen enrichment results in a shorter flame, higher maximum flame temperature, and increase in the heat transferred to the solids in the flame zone.
62 citations
31 Oct 1983
TL;DR: In this paper, the influence of flame temperature on NOx, particulate and hydrocarbon emissions from a single-cylinder light-duty direct-injection diesel engine was examined by varying the composition of the intake air with the engine operating at different speeds and loads.
Abstract: The influence of flame temperature on NOx, particulate and hydrocarbon emissions from a single-cylinder light-duty direct-injection diesel engine was examined by varying the composition of the intake air with the engine operating at different speeds and loads. At a fixed engine speed, load, and start-of-combustion timing, the effects of intake-gas composition on emissions were found to correlate with variations in the characteristic diffusion flame temperature. Furthermore, this flame temperature dependence was not significantly affected by the engine operating conditions. These results indicate that the flame temperature correlations originally developed for divided-chamber diesel engines can be applied to direct-injection diesel engines.
58 citations
TL;DR: In this paper, the relative roles of premixed and diffusion burning in diesel combustion have been examined in terms of characteristic times for ignition delay, combustion, and fuel-air mixing.
Abstract: Relative roles of premixed and diffusion burning in diesel combustion have been examined in terms of characteristic times for ignition delay (tau sub ig), combustion (tau sub c) and fuel-air mixing (tau sub m) Results indicate that the majority of the diesel combustion process is diffusion controlled, as in gas turbine combustion, since tau sub c is much less than tau sub m over the entire range of operating conditions During the ignition phase, some premixed burning can occur in the fringe of the fuel spray where tau sub m is less than tau sub ig; however, most of the fuel injected prior to ignition also burns in the diffusion mode, since tau sub m is greater than tau sub ig in the majority of the fuel spray The fraction of premixed burning which occurs during the ignition phase is increased by longer tau sub ig, high rates of fuel injection, high air swirl and the use of multi-hole injectors, which increase the surface area of the spray
48 citations
TL;DR: In this paper, the size, number density, and volume fraction of soot present in a laminar diffusion flame with and without metallic additives were evaluated from scattered light intensities at 45 and 135°.
41 citations
TL;DR: In this article, the authors compared the predicted and measured steady gasification rates of JP-10 drops supported at various locations in an open turbulent diffusion flame and made predictions considering both the presence and absence of envelope flames.
TL;DR: In this article, the structural features of a plane steady flame fall into several distinct classes, as follows when the Mach number is very small the flame is of the well-known thermal type, with a convection-diffusion-dominated preheat zone followed by a flame sheet within which diffusion and reaction balance one another.
TL;DR: In this article, simultaneous laser Doppler and Mie scattering measurements have been made in a horizontal turbulent hydrogen diffusion flame in a co-flowing stream, and the resulting light scattering signal is used to obtain scalar time traces which are processed together with the record of radial velocity to yield turbulent fluxes of the main species, temperature and mixture fraction.
Abstract: Simultaneous laser Doppler and Mie scattering measurements have been made in a horizontal turbulent hydrogen diffusion flame in a co-flowing stream. The nozzle fluid is seeded with aluminium oxide particles. The resulting light scattering signal is used to obtain scalar time traces which are processed together with the record of radial velocity to yield turbulent fluxes of the main species, temperature and mixture fraction. The measurement technique has been tested by experiments in a known isothermal flow. The profiles and values of normalized Favre mixture fraction fluxes are close to those of non-reacting jets only after burnout, far downstream, and it appears that chemical reaction reduces the upstream flux. The mixture fraction radial gradients and the corresponding radial fluxes show similar profiles throughout the measured region, x/D = 40 to 160, with no significant instance of counter-gradient flux. Radial pressure gradients modify the scalar fluxes significantly; estimates point to augm...
TL;DR: In this article, the variation of the flame propagation rate across a thermally-thick fuel as a function of opposed flow velocity is described as consisting of three regimes: low opposed flow velocities, dominated by the naturally induced flow and shows relatively little variation of propagation rate.
Abstract: The variation of the flame propagation rate across a thermally-thick fuel as a function of opposed flow velocity is described as consisting of three regimes. The first at low opposed flow velocities is dominated by the naturally induced flow and shows relatively little variation of the propagation rate. The second regime shows a near linear increase of propagation rate with opposed flow and is dominated by thermal processes alone. The last regime shows a decline in rate with opposed flow and is an indication of the dominance of chemical rate processes. Surface temperature and flow field measurements ahead of the flame indicate that the mechanism by which the flame propagates is not by thermal conduction through the solid ahead of the flame, but rather by fuel diffusing from behind the flame through the quench layer to create a lean flammable mixture ahead of the flame.
TL;DR: In this article, the authors consider the hypothesis that the anomalously high rates of propagation of unconfined vapour cloud explosions are caused by radiation-induced multi-point ignition due to small particles ahead of the main flame front.
Abstract: We consider the hypothesis that the anomalously high rates of propagation of unconfined vapour cloud explosions are caused, or aggravated, by radiation-induced multi-point ignition due to small particles ahead of the main flame front. The mechanism by which loose agglomerates of fine fibres ignite at very low radiation fluxes is examined and the measured dependence of ignition lag on the level of radiation provides the basis for calculating maximum flame speed enhancement. It is shown that, for compact particles, there is an absolute minimum size for radiative ignition due to vaporization below the minimum critical energy and that fibrous aggregates are more likely to be raised and dispersed by convection than any solid particle capable of inducing ignition. Modelling the growth of such flames for various densities of igniting particles displays the development in time of a convoluted flame front with greatly increased surface area and rate of burning, resembling in appearance the consequences of large-scale turbulence. The effective flame speeds are of the order required to account for observed levels of damage, even when no other enhancement is considered; in fact the mechanism suggests that interaction with other proposed causes of flame speed enhancement is highly probable
TL;DR: In this article, theoretical results on heterogeneous flame suppression, the thermal and chemical inhibition effects on premixed flames were investigated, and the chemical effectiveness of CF3Br was experimentally measured and obtained in an Arrhenius form for the overall inhibition reaction.
TL;DR: In this article, a gas-phase transient diffusion is used to suppress the relaxation process of fuel vapor accumulation through the use of d 2 -law results as the initial conditions, and the authors demonstrate that transient diffusion enhances the vaporization and burning rates, reduces the flame front standoff ratio, and elevates the flame temperature.
TL;DR: In this article, an exact representation of the irrotational flow field upstream of the flame is obtained by utilizing a suitable distribution of sources on the duct axis. But the authors do not consider the flame front as a surface of discontinuity separating the cold fuel oxidizer mixture and hot combustion products.
Abstract: Nonsteady behavior of a flame stabilized at the center of a two-dimensional duct is studied using an integral technique. An exact representation of the irrotational flow field upstream of the flame is obtained by utilizing a suitable distribution of sources on the duct axis. Time-dependent solutions exhibit traveling wave patterns with significant amplification along the flame region. These solutions enable the calculation of the acoustic transmission and reflection properties of the flame region. HE control and elimination of combustion instability in systems of practical interest and the rational interpretation of subscale experiments require an understanding of the fundamental mechanism. In many instances response of the combustion processes to local pressure and velocity fluctuations is an important factor that feeds considerable energy into the system to sustain the oscillations. When the frequency is less than a few hundred hertz, the chemical reaction time delay is relatively unimportant, rather, the fluid mechanical adjustments of the flame region play the dominant role governing the detailed response of the system. As a result of fast chemical kinetics, flame fronts are usually very thin compared to length scales associated with combustion devices of practical interest. Therefore, in problems where detailed calculations of flame structure are not important, one often considers the flame front as a surface of discontinuity separating the cold fuel oxidizer mixture and hot combustion products. The flowfields on either side of the flame front are matched by relations analogous to the jump conditions across the shock discontinuities. Nonsteady behavior of a flame stabilized at the center of a two-dimensional duct is studied as a specific but typical example of the low-frequency behavior. Steady-state flame spreading of the confined premixed flames in two
TL;DR: In this paper, the authors discuss the stability of a large class of diffusion flames when the activation energy characterizing the reaction rate is large and examine the evolution from the stationary solution on a time scale so short that changes are confined to the thin flame sheet where all the reaction occurs.
TL;DR: In this article, a new and efficient numerical method has been developed to calculate time dependent, multidimensional flames, which adapts the grid in an intelligent or solution dependent fashion so that the physics and chemistry of the flow are properly resolved.
TL;DR: In this paper, it was shown that a travel distance of more than 5 cm is required for a flame to establish steady propagation in a horizontally layered mixture of Methane-Diluted with Ar or N2, or CO2.
Abstract: Combustion of horizontally layered methane/air mixtures have been studied. Methane-diluted with Ar, or N2, or CO2—and air, initially separated by a thin plate, form a flat combustible layer only by interdiffusional mixing upon withdrawal of the plate. Simultaneous line ignition was made at one side of the layer for the whole range of the combustible mixture ratio. The ignition took place at only a stoichiometric position and failed elsewhere. Addition of the inert gases caused serious ignition delay. The induction distance depends primarily on the amount of inert gases in the fuel. The flame speed first accelerated and, as the flame traveled way off the igniter wall it drastically decreased down to 1.5 times the burning velocity of the corresponding stoichiometric premixed flame. It has been found that a travel distance more than 5 cm is required for the flame to establish steady propagation.
TL;DR: In this article, an intense ultraviolet chemiluminescence of SO(A 3 Π) attributed to the reaction C + SO 2 → CO + SO, was observed when SO 2 was added.
TL;DR: The results of an investigation of the dynamic behavior of a bluff-body stabilized diffusion flame are reported in this article, where the dynamic characteristics of flames established by different air and fuel flow rate conditions are investigated using 500 frames/s cine pictures and two spectrophotpmeters tuned to the CH emission at 431.5 nm.
Abstract: The results of an investigation of the dynamic behavior of a bluff-body stabilized diffusion flame are reported. The dynamic characteristics of flames established by different air and fuel flow rate conditions are investigated using 500 frames/s cine pictures and two spectrophotpmeters tuned to the CH emission at 431.5 nm. Each spectrophotometer viewed a 1.8x64-mm area of the flame with the long dimension symmetrically located along the diameter. Downstream of the recirculation zone the flame consists of large, discrete "fireballs" or flame turbules separated by axial regions where the flame is not visible. The flame turbules are quasiperiodic with a frequency that decreases as they move downstream. Flame turbule time widths are typically between 1.5 and 12 ms depending on fuel and air flow rates and axial location and their average velocity is approximately equal to the annulus air injection velocity.
TL;DR: In this paper, the extinction limits of the counterflow diffusion flame, which depend on fuel and oxygen concentration, flow velocity, and which side (fuel or air) the inhibitor is added, are measured by increasing the amount of inhibitor.
TL;DR: In this article, laser-light scattering and attenuation techniques are applied simultaneously in a complementary fashion to measure the properties of the soot particles in a laminar diffusion flame.
Abstract: Abstract—Laser-light scattering and attenuation techniques are applied simultaneously in a complementary fashion to measure the properties of the soot particles in a laminar diffusion flame. The measurements of local soot particle size and number density in the laminar diffusion flame are made together with the measurements of soot mass concentration, gas species concentration and temperature in the same flame. The results are as follows. (I) The average diameters (D;v) of soot panicles in the flame lie between 50 and 120 nm. The largest particles probably are formed in the fuel-rich region just inside the position of the temperature peak. The number density of soot particles is in the range between 108 and 1010 particles/cm3 and the volume fraction occupied by the particles in the flame is estimated to be 10-7 to 3 X 10-6 cm 3/cm3 and is a maximum near the flame axis, The soot density is about 2 g/cm3cm. (2) Most likely soot is produced in the fuel-rich region just inside the flame front where u...
TL;DR: In this paper, a series of measurements of flame speed in which ignition and velocity enhancement processes are separate and independent is described, and it is shown that an enhancement in flame speed is achieved in the vicinity of the jet and that this enhancement is not necessarily related to some jet-related perturbation of the ignition process.
10 Jan 1983
TL;DR: Combustion noise was discussed as early as 1802 in a technical note on singing flames as mentioned in this paper, and it has been a continual broadening of the areas of interest: Now the phenomena related to combustion noise are considered in a host of specific uses of combustion.
Abstract: Combustion noise was discussed as early as 1802 in a technical note on singing flames. Occasional papers followed this early work up to about 1950, when jet propulsion spurred interest. Since then there has been a continual broadening of the areas of interest: Now the phenomena related to combustion noise are considered in a host of specific uses of combustion. This review is based on the authors' observations of nonpropulsive combustion in the residential, commercial, and industrial fields, although it is realized that there are many parallels to specific propulsive applications. Three specific areas of combustion noise are discussed in some detail, namely, combustion roar, combustion-driven oscillations, and pulse combustion. Combustion roar, in the absence of acoustic distortion effects, is characterized by a smooth noise spectrum related to the reacting chemistry of the flame and the turbulence level of the combustion region. Combustion-driven oscillations are characterized by a discrete frequency and a feedback cycle to maintain the oscillation. Pulse combustion is the positive application of combustiondriven oscillations. In addition, some other combustion noise phenomena are discussed, such as the interaction with vortex shedding and the combustion amplification of flow phenomena.
01 Jan 1983
TL;DR: Much evidence has accumulated in the literature indicating the importance of ions in sooting flames and a number of workers have previously suggested ionic mechanisms, so that there is no difficulty in accounting for the observed polycylcic structures.
Abstract: In a recent review1 of soot nucleation mechanisms it was demonstrated that mechanisms based upon neutral free radical species are inadequate to explain soot formation in flames. Either rates are too slow to account for the rapid rate of soot formation or there are difficulties in accounting for the large numbers of polycyclic rings observed in soot particles. These problems can be overcome by assuming an ionic mechanism. Ion molecule reactions are extremely fast compared to free radical reactions and ions have a propensity to quickly rearrange to the most stable structure so that there is no difficulty in accounting for the observed polycylcic structures. Much evidence has accumulated in the literature indicating the importance of ions in sooting flames and a number of workers have previously suggested ionic mechanisms. For this, the reader is referred to a recent review.1
TL;DR: In this article, a numerical model of flame spread on a thermally thin fuel in opposed flow was used to study the effect of the initial fuel bulk temperature on the extinction limit, and the distribution of temperature, fuel fraction, reactivity, and local equivalence ratio were presented and analyzed for the near-limit case.
Abstract: A numerical model of flame spread on a thermally thin fuel in opposed flow (Frey and T'ien, 1979) has been used to study the effect of the initial fuel bulk temperature on the extinction limit. The distribution of temperature, fuel fraction, reactivity, and local equivalence ratio are presented and analyzed for the near-limit case. When the fuel temperature is decreased, the fuel pyrolysis rate near the pyrolysis front decreases, the flame spread rate drops, the pyrolysis length diminishes and the flame size is reduced. When the flame reaches a certain critical size, depending on a number of parameters, extinction occurs.