Showing papers on "Premixed flame published in 1991"

Quenching processes and premixed turbulent combustion diagrams

Abstract: The structure of premixed turbulent flames is a problem of fundamental interest in combustion theory. Possible flame geometries have been imagined and diagrams indicating the corresponding regimes of combustion have been constructed on the basis of essentially intuitive and dimensional considerations. A new approach to this problem is described in the present paper. An extended definition of flamelet regimes based on the existence of a continuous active (not quenched) flame front separating fresh gases and burnt products is first introduced. Direct numerical simulations of flame/vortex interactions using the full Navier–Stokes equations and a simplified chemistry model are then performed to predict flame quenching by isolated vortices. The formulation includes non-unity Lewis number, non-constant viscosity and heat losses so that the effect of stretch, curvature, transient dynamics and viscous dissipation can be accounted for. As a result, flame quenching by vortices (which is one of the key processes in premixed turbulent combustion) may be computed accurately. The effects of curvature and viscous dissipation on flame/vortex interactions may also be characterized by the same simulations. The influence of non-unity Lewis number and of thermo-diffusive processes in turbulent premixed combustion is discussed by comparing flame responses for two values of the Lewis number (Le = 0.8 and 1.2). An elementary (‘spectral’) diagram giving the response of one flame to a vortex pair is constructed. This spectral diagram is then used, along with certain assumptions, to establish a turbulent combustion diagram similar to those proposed by Borghi (1985) or Williams (1985). Results show that flame fronts are much more resistant to quenching by vortices than expected from the classical theories. A cut-off scale and a quenching scale are also obtained and compared with the characteristic scales proposed by Peters (1986). Results show that strain is not the only important parameters determining flame/vortex interaction. Heat losses, curvature, viscous dissipation and transient dynamics have significant effects, especially for small scales and they strongly influence the boundaries of the combustion regimes. It is found, for example, that the Klimov–Williams criterion which is generally advocated to limit the flamelet region, underestimates the size of this region by more than an order of magnitude.

A parametric acoustic instability in premixed flames

Abstract: We present an experimental and theoretical investigation of some aspects of the coupling between a premixed laminar quasi-planar flame front and acoustic standing waves in tubes. A multidimensional instability of the front arises from its interaction with the oscillating field of acceleration. This instability can be described by the Clavin–Williams laminar wrinkled flame theory in which the periodic acceleration created by the acoustic field is added to the acceleration due to gravity. As first suggested by Markstein, the resulting equation can be reduced to the Mathieu equation for a parametric oscillator. A cellular instability appears with a finite excitation threshold. This instability is responsible for the spontaneous generation of intense acoustic oscillations observed elsewhere. The value of the acoustic field at the threshold of instability and the wavelength of the cellular structures are measured experimentally for propane flames and are found to be in good agreement with the calculated values. It is also seen, both experimentally and theoretically, that for certain amplitudes of pumping, the parametric mechanism can also stabilize an initially unstable system.

Turbulent premixed flame propagation models for different combustion regimes

Abstract: Conceptual models of turbulent premixed flame propagation speeds for the three combustion regimes have been proposed and tested by comparing to the available well-defined data obtained on a variety of experimental rigs. The model for the wrinkled flame regime has been based on the Tennekes and Kuo-Corrsin concept of isotropic turbulence which proposes that dissipative eddies are most probably consist of vortex tubes with a diameter of the order of Kolmogorow scale and a spacing of the order of the Taylor length scale. Using this concept of turbulence structure, it was shown that the normalized turbulent flame propagation speed, ut/ul, is proportional to the square root of the normalized turbulence intensity, u′/ul and the turbulent Reynolds number based on Kolmogorov length scale. Rn. In the intermediate regime, assuming that the effective flame front surface area grows exponentially with the turbulence intensity, ut/ul. is linearly dependent on Rν and exponentially dependent on the square root of u′/ul. In the distributed reaction zones regime, starting with the Damkohler's formulation, dimensionless numbers contributing to the flame propagation have been selected, and it was found that ut/u′ is proportional to the 3/4th power of ul/u′. Comparison of the predictions of the models and the experimental data from a variety of rigs shows excellent agreement despite the uncertainties involved in the measurement of the turbulence parameters and propagation velocities.

Modelling of flamelet surface-to-volume ratio in turbulent premixed combustion

Abstract: A model is proposed, valid in the laminar flamelet regime, for the surface-to-volume ratio of a turbulent premixed flame. The new model is in a form suitable for incorporation into an existing model of turbulent premixed combustion. Exact equations are derived which describe the dynamics of the constant-property surface representing the flame interface. Unknown terms in the exact equations are modelled for the simplified case of constant-density combustion in a specified turbulence field. Numerical solutions of the modelled equations are carried out for a one-dimensional test case. Preliminary results indicate that the model is capable of predicting effects present in turbulent flame propagation, and a parametric study shows that correct trends are observed.

Large-Eddy Simulations of Combustion Instability in an Axisymmetric Ramjet Combustor

Abstract: The large-eddy simulation (LES) model previously developed for cold flow in an axisymmetric ramjet combustor is extended to include a model for premixed fuel combustion. The combustion model explicitly uses a subgrid-scale local turbulent flame speed in the governing equation. This model not only reduces the computational effort, when compared to a model with detailed finite-rate chemical kinetics, but also avoids the potential error in the amount of heat release causd by numerical diffusion. Both stable and unstable combustion are simulated with evidence of decaying and growing pressure oscillations, respectively. The observed flow features on unstable combustion. such as a propagating hooked flame and its phase relation with the pressure oscillation, agree with experimental observations. These simulations clearly establish that the LES model contains the essential physics of combustion instability in a ramjet. Although the simulations show general qualitative features of combustion instability, further ...

Flame Propagation in a Nonuniform Mixture: Analysis of a Propagating Triple-Flame

Abstract: A situation in which a dilffusion flame reaches an end at some position in a medium of non-premixed reactants is studied. The mixing of reactants that takes place ahead of the diffusion flame leads to the formation of a “triple-flame”, a structure which consists of a fuel-rich premixed flame, a fuel-lean premixed flame, and a diffusion flame that starts where the two premixed flames meet. An important property of such an end-point is its ability to propagate. The limits of low heat release, unit Lewis number and large Zeldovich number are considered. The structure of the triple-flame and the unique relationship between propagation speed and transverse mixture Traction gradient are computed numerically. For the range of values considered here, the end of the diffusion flame is shown to extend itself at a rate that can be substantially reduced, but that remains positive as the gradient of the mixture fraction is increased.

Mechanisms of microgravity flame spread over a thin solid fuel - Oxygen and opposed flow effects

Abstract: Microgravity tests varying oxygen concentration and forced flow velocity have examined the importance of transport processes on flame spread over very thin solid fuels. Flame spread rates, solid phase temperature profiles and flame appearance for these tests are measured. A flame spread map is presented which indicates three distinct regions where different mechanisms control the flame spread process. In the near-quenching region (very low characteristic relative velocities) a new controlling mechanism for flame spread - oxidizer transport-limited chemical reaction - is proposed. In the near-limit, blowoff region, high opposed flow velocities impose residence time limitations on the flame spread process. A critical characteristic relative velocity line between the two near-limit regions defines conditions which result in maximum flammability both in terms of a peak flame spread rate and minimum oxygen concentration for steady burning. In the third region, away from both near-limit regions, the flame spread behavior, which can accurately be described by a thermal theory, is controlled by gas-phase conduction.

Turbulent premixed combustion modelling using fractal geometry

Abstract: The fractal geometry concept has been applied to the problem of turbulent premixed flame propagation This approach has been based on the fact that turbulent surfaces are wrinkled over a wide range of scales, and experimental evidence shows that the scales are self-similar Self-similarity of the scales over a wide range is the distinctive characteristics of fractal objects A new model, derived using the fractal geometry, for the wrinkled flames (or flames-sheets) regime has been proposed The distinct feature of the present formulation is the adoption of a variable inner cutoff scale as a function of turbulent and molecular diffusivities With a fractal dimension of 7/3 and an outer cutoff approximating the integral length scale of turbulence, our analysis has yielded u t /u 1 ∞(u′/u 1 ) 1/2 Re L 1/4 This model suggests a wrinkled flame structure influenced by a turbulent length scale as well as the turbulence intensity Comparison of the predictions of the model and the experimental data from a variety of rigs shows excellent agreement despite the uncertainties involved in the measurement of the turbulence parameters, propagation velocities, and the intuitive assumptions made in the theories

Turbulent premixed flames and sound generation

Abstract: The intensity and frequency dependence of sound emitted by a premixed flame are related to the dynamics of the flame surface within the corrugated Hamlet approximation For the turbulent flame brush, its velocity, thickness, and surface area are reviewed If the turbulence has a Kolmogorov spectrum the acoustic power spectrum varies as ω−5/2

Calculations of NOx Formation Pathways in Propagating Laminar, High Pressure Premixed CH4/Air Flames

Abstract: Nitric oxide formation in laminar, adiabatic, stoichiometric. premixed CH4-air flames wascalculated over a range of pressures from 0.1 to 20 atmospheres. The detailed kinetics model (46 speciesand 212 reversible reactions) included four chemical pathways to NO formation, and the relative importanceof these pathways was found to vary with pressure and with position in the flame. In the flamefront,the rates of the Zeldovich mechanism and the N20 intermediate mechanism were increased substantiallyby the presence of superequilibrium °atoms. The Fenimore mechanism also contributed substantialamounts of NO. In the postflame gases, the calculations suggested that most of the NO was formed fromthe Zeldovich mechanism, but the N20 intermediate mechanism became important at high pressures whereit contributed as much as 10% of the postflame NO. Experimental measurements of superequilibriumradical concentrations and ffamefront NO formation are clearly essential to confirm the suggested importanceof superequilibrium (...

The Visible Structure of Turbulent Jet Diffusion Flames: Large-Scale Organization and Flame Tip Oscillation

Abstract: High speed visual observations of free turbulent jet diffusion flames are presented. Computer graphic volume rendering, whereby many frames from a movie sequence are displayed simultaneously, is used to provide a unique view of the flow evolution. Two fuels were used, acetylene and ethylene, to span Ihe range from the momentum-driven to the buoyancy-driven regime. The data show that, for all flow regimes, the flames exhibit a large-scale organized motion in the far-field of the jet as manifested by a progression of organized structures up through the length of the visible flame. Occasional pairings of structures are also observed. These structures eventually become the flame tip, which burns out in a quasi-periodic manner. Under no conditions was a random, small-scale breakup and disappearance of the flame tip ever observed. In all cases the speed of the organized structure is seen to be constant and equal to 12 ± 2° of the jet exit velocity. The constancy of speed occurs in spite of the axial de...

Structure and extinction of diffusion flames with flame radiation

Abstract: Using droplet combustion as a model problem, and capitalizing on the temperature-sensitive nature of radiative heat transfer, the structure and extinction of diffusion flames with flame radiation is studied via multi-scale activation energy asymptotics. The flame structure analyzed consists of an O(∈) reaction zone embedded within an O(δ) radiation zone which in turn is situated in the O(1) diffusive-convective flow field, where ∈≪δ≪1. The analysis yields the structure equation for the reaction zone, which can be cast in the same form as that of Linan's adiabatic diffusion flame problem such that his extinction results can be readily used. Present results show that radiative heat loss promotes flame extinction in general, as expected. Furthermore, it can also lead to the phenomenon of dual extinction turning points in which flame extinction due to reactant leakage and thereby kinetic limitation occurs not only for sufficiently small droplets, as is well established, but also for sufficiently large droplets as a result of excessive heat loss from the correspondingly large flame. Consequently there exist diffusive-reactive-radiative systems for which steady combustion is not possible for all droplet sizes. An estimation of the dimensional radiative extinction droplet size is also given for the sample system studied.

Preferential Diffusion Effects on the Surface Structure of Turbulent Premixed Hydrogen/Air Flames

Abstract: An experimental study of the surface structure of high Reynolds number turbulent premixed hydrogen/air jet flames is reported. Test conditions involved various values of turbulence intensities relative to the laminar flame speed, and stable/neutral/unstable conditions for preferential diffusion, within the wrinkled and mixing-limited thin flamelet regimes. Measurements included laser light sheet imaging to characterize flame surface properties and condilional laser velocimetry to characterize the turbulence properties of the unburned gas. It was found that flame surface area (and thus the local turbulent burning velocity), flame brush thickness and the fractal dimension of the flame surface progressively increased with distance from the flameholder, with maximum values eventually limited by approach to the flame tip. Additionally, the rate of development of these properties with distance from the flameholder increased as turbulence intensities relative to the laminar flame speed increased. Finall...

Nonlinear dynamics of laboratory combustor pressure oscillations

Abstract: The mechanisms responsible for driving and damping of pressure oscillations in a laboratory combustor have been investigated. The chamber contains a turbulent methane/hydrogen/air premixed flame stabilized behind a rearward-facing step. Shadowgraph cinematography reveals the shedding of large vortices from the step at frequencies of the system acoustic modes. Variations in the fuel equivalence ratio and the mean flow speed result in a wide variety of nonlinear dynamical behavior. Typically, large cycle-to-cycle variations are observed such that energy may be added or subtracted over one cycle of oscillation but zero net energy change occurs many cycles of oscillation. A quantitative version of Rayleigh's Criterion is constructed by using the cross-spectral-density of the measured flame radiation and pressure. The results under one set of operating conditions show that large driving near the flameholder is balanced by equally large damping further downstream. A second set of conditions results in large energy addition to an acoustic mode balanced by attenuation at the mode subharmonic.

Influence of wrinkled premixed-flame dynamics on large-scale, low- intensity turbulent flow

Abstract: Premixed turbulent flame propagation is analysed under the assumptions of stationarity and transverse homogeneity by expansions for small values of the ratio of the turbulence intensity to the laminar burning velocity. For large Zel’dovich numbers, the effects of diffusive-thermal phenomena within the flame, gas expansion, buoyancy and Lewis and Prandtl numbers different from unity are taken into account under the constraint that turbulence scales are large compared with the laminar flame thickness. A general formulation is given, involving solutions through Fourier decompositions. Parametric results for turbulent burning velocities are obtained, and the evolution of components of turbulent kinetic energies through the flame is calculated. It is shown how buoyancy counteracts the tendency for gas expansion to increase transverse components of the turbulent kinetic energy, pressure fluctuations and vorticity generation across the wrinkled flame. Strong readjustments in components of the turbulent kinetic energy are shown to occur in the downstream hydrodynamic zone. It is established that, with the effects of the hydrodynamic zones fully taken into account, the flame can induce anisotropy in initially isotropic turbulence such that the final velocity fluctuations exhibit higher intensities in the longitudinal mode than in transverse modes, while the enhanced vorticity fluctuations are entirely transverse.

An experimental and theoretical study of porous radiant burner performance

Abstract: We have studied experimentally the stability and heat transfer characteristics of lean premixed, methane-air flames embedded in a porous layer. The work is directly relevant to understanding the performance and operating behavior of porous radiant burners (PRB). Flame speed and radiant output data were obtained for different stoichiometries and flame locations in porous ceramic foam. The results indicate that stable combustion at elevated flame speeds can be maintained in two different spatial domains: one spanning the upstream half of the porous region and the other in a narrow region near the exit plane. The heat release and radiant output are also found to increase as the flame is shifted toward the middle of the porous layer. A one-dimensional laminar premixed flame model incorporating a radiatively participating inert porous medium was used to describe the test conditions. Calculations using a one-step reaction confirmed the observation of two stable flame regions. The predicted flame speeds and radiant output agree favorably with the experimental trends.