Showing papers on "Premixed flame published in 1983"

Liftoff characteristics of turbulent jet diffusion flames

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.

Premixed flames in large scale and high intensity turbulent flow

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).

Stability of the Porous Plug Burner Flame

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.

Heat transfer from flames in a rotary kiln

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.

Stochastic Calculation of Laminar Wrinkled Flame Propagation via Vortex Dynamics

Abstract: This paper explores the interaction between time dependent turbulent flow structure and an irreversible one-step decomposition reaction. A Lagrangian-calculated velocity field is combined with Eulerian scalar transport to describe premixed flame propagation in two dimensions. The time dependent flow structure is calculated by the discrete vortex dynamics method. The flow is two-dimensional and is confined to a unit square through the use of either periodic or non-equilibrium boundary conditions. The velocity field from the known vortex locations is used to describe the convection of a reacting scalar on a fixed Eulerian mesh. By variation of the numerical vortex parameters, the flow field length scale and root-mean-square (RMS) velocity can be changed. For this study, unit Lewis number with temperature-independent transport coefficients has been assumed. Some of the work also assumed a constant density flame model. In addition to calculating reacting scalar transport, calculations have also been ...

The Influence of Stretch on a Premixed Flame With Two-Step Kinetics

Abstract: The structure of a steady, planar. premixed flame in a slowly expanding gas flow is analyzed. The reaction kinetics are represented by a mechanism consisting of a chain branching reaction and a chain breaking reaction. An asymptotic analysis is performed in the limit of a large value for the activation energy of the chain branching reaction, with the activation energy for the chain breaking reaction presumed to be zero. The analysis is valid for cases where the mass fraction of the intermediate species is of order unity, An analytical expression is obtained for the change of the mass burning rate with the Karlovitz number, the parameter characterizing flame stretch. This expression shows the role of differential diffusion of heat and the reactant, of differential diffusion of reactant and intermediate species and of enhanced diffusion of intermediates on flame propagation. It is seen that these three different effects may cancel each other. Depending on the parameter range the mass burning rate m...

Extinction of Premixed Flames in a Stagnation Flow Considering General Lewis Number

Abstract: The effects of the velocity gradient on the premixed flame in the stagnation flow field have been studied theoretically considering general Lewis number The steady two-dimensional stagnation flow and the uni-molecular reaction were assumed, and the governing equations were solved numerically The influence of the velocity gradient and the Lewis number on the flame temperature were discussed There are two effects of the velocity gradient on the flame One effect is that the velocity gradient causes the flame temperature to increase or to decrease due to the imbalance between the excess heat flow from the reaction zone to the unburned gas and the excess diffusion flow of the reactant from the unburned gas to the reaction zone, and this effect largely depends on the Lewis number By the velocity gradient, the flame temperature increases when Le>1 and decreases when Le< 1 When Le= 1, the velocity gradient has no effect on the flame temperature Another effect is tha velocity gradient causes the fl

Combustion state diagnostic method

Abstract: A method of diagnosing a combustion state in a furnace of a boiler wherein the shape of the root part of a flame (namely, the part closer to the fuel jet port of a burner) is picked up by an image sensor or the like, and the combustion state is diagnosed from the shape. Concretely, the relationships between the shapes of the root parts of flames and the combustion states of the furnace, e.g., the amount of CO or the amount of NOx are experimentally or empirically ensured and are stored in advance, one of the patterns of the stored flame shapes is selected on the basis of the shape of the root part of the flame actually observed, and the combustion state of the furnace is judged the combustion state of the selected flame shape.

Calculation of Burning Velocity and Flame Structure in Methanol – Air Mixtures

Abstract: An a priori calculation has been made of laminar burning velocities of methanol-air mixtures over the composition range 8% to 20% CH3OH at 298 K and 1 bar, and for stoichiometric mixtures over the pressure range 0.0625 to 16 bar and over the temperature range 298 to 1200 K. The time dependent conservation equations were solved repeatedly until a time undependent solution was obtained, giving the required flame profiles. The kinetic data set was based on a full survey of high temperature literature data, and comprised 40 elementary reactions with 15 chemical species. The results show generally very good agreement with experimentally measured burning velocities. A kinetic sensitivity analysis shows which elementary reactions are important in determining the flame properties, and enables us to state for which of these reactions it is particularly important to obtain improved rate data.

Correlating Downward Flame Spread Rates for Thick Fuel Beds

Abstract: Flame spread rates are presented for polymethylmethacrylate fuel beds as a function of gravitational acceleration and ambient pressure and oxygen concentration. The data are correlated by plotting a dimensionless spread rate that is a measure of the heat transferred forward of the flame, required to sustain the flame, compared to the maximum possible forward heat transfer against a Damkoehler number. The latter parameter indicates the approach of the flame to its maximum temperature, where the maximum possible forward heat transfer occurs, such that the dimensionless spread rate approaches unity at large Damkoehler number. Surface regression effects on the flame spread process are incorporated into the dimensionless spread rate, and the final correlation is independent of bed thickness.

Characteristic Scales for Density Fluctuations in a Turbulent Premixed Flame

Abstract: Light scattering from particles introduced into premixed flames, both open and confined, is used to distinguish between the predominant scalar states of unburnt mixture and fully burnt gas. Single-point time histories have been analysed to yield information on “packet” size distribution in the turbulent flame zone. Time duration for the respective scalar states is shown to be log normally distributed with substantial differences apparent between the most probable and average values. Characteristic features of open and confined burning emerge which suggest significant differences in turbulent flame structure.

The Detailed Processes Involved in Flame Spread over Solid Fuels

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.

Regulation of blue flame combustion emissions

Abstract: A flame insert for the reduction of emissions of oxides of nitrogen in atmospheric type burners is disclosed. The insert provides heat radiating surfaces which are contacted by the inner cones of the flames. The insert surfaces are sized and shaped to accommodate variations in flame position and to guide flames into contact therewith. A combined flame insert and secondary air baffling system is also disclosed.

Nonsteady flame spreading in two-dimensional ducts

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

Flame signal enhancer for post-mixed burner

Abstract: An apparatus for reliably monitoring a flame of a post-mixed burner without significantly altering the flame characteristics by forming a small but intense signal within the burner which is unaffected by furnace conditions and which corresponds to the actual flame.

Combustion of Stratified Methane/Air Layers

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.

Dynamic behavior of a bluff-body diffusion flame

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.

Hydrodynamical Coupling between the Motion of a Flame Front and the Upstream Gas Flow

Abstract: Etude du mouvement fluctuant d'un front de flamme et de ses effets sur l'ecoulement amont dans le cas d'un front de propagation intrinsequement stable dans un ecoulement faiblement turbulent

Diameters and number densities of soot particles in premixed laminar flat flame propane/oxygen.

Abstract: Submicronic particle sizing by means of diffusion broad­ ening spectroscopy (DBS) was first extensively used in liquid suspensions, often with biological interest. Then, the method was extended to the study of particles in moving and (or) hostile environments, thanks mainly to technical improve­ ments in spectrum analyzer and photocorrelator designs. In particular, Penner et al. 1,2 pioneered in 1976 the study of soot growth in flames by means of DBS. Following their work, other similar experiments have been carried outshowing that DBS is able to provide reliable results in this field. The aim of this Letter is to present soot diameters and number density measurements in a premixed propane/oxygen flat flame. These data are validated by comparison with re­ sults obtained by more classical method. In the case of suspensions at rest, the isothermal diffusion coefficient D is measured from the halfwidth at half-height (HW) of the Lorentzian power spectrum of the photomultiplier anode current through the simple relation