Premixed Flame Front Research Articles

Magnifying and Tracking Observation of Micro PET Particles Passing through a Plane Premixed Flame Front

In order to observe detailed behavior of micro plastic particles under rapid heating, a fundamental investigation is made by introducing two ingenious devices; one is the construction of a plane laminar premixed burner exhibiting extremely excellent two-dimensionality, the other is the construction of a magnifying particle-tracking system composed of a pair of rotating plane mirrors and a fixed high-speed video camera. Taking account of the flow patterns obtained using a PIV/PTV system, a series of heating processes from melting to burning of micro PET particles passing through a laminar flame sheet is optically observed. It is found that the proposed high-speed and magnifying tracking system can realize a wide straight range of particle tracking up to 50 mm and clarify many interesting facts concerning the ignition and burning processes of micro PET particles.

Contribution of small scale turbulence to burning velocity of flamelets in the thin reaction zone regime

There is a growing body of experimental evidence that passive surface character of the premixed flamelets may not be preserved beyond medium turbulence intensities, and their thermal structure deviate from that of a laminar flamelet. Further, the experimental measurements of flame surface characteristics indicate that the flame surface area is not the dominant factor in increasing the turbulent burning velocity under the conditions corresponding to the thin reaction zones regime. Approaches to estimating the turbulent burning rates based on the area increase of the premixed flame front surfaces may not be the right models and may require additional mechanisms for proper representation of the burning rate. This paper proposes a simple scheme to estimate the contribution of the flame front alteration by small scale turbulence on flamelet burning velocity. An expression was derived to estimate the contribution of flame front alteration as a consequence of the small scale turbulent eddies that may penetrate into the preheat layer of the premixed flame front. The derivation was based on the experimental evidence of flame front alteration by active eddies penetrating into the preheat layer and enhancing the transport. As a first approximation it was assumed that these active eddies have a characteristic size about the Taylor microscale. Further, the formalism that demonstrates that within the turbulence cascade the volume occupied by a certain size eddy and its characteristic velocity obey power-law relationships (i.e. structure functions) that are dictated by the intermittency of the turbulent field, was used. The predictions of the proposed expression were compared to the available experimental data.

Flame oscillations in tubes with nonslip at the walls

A laminar premixed flame front propagating in a two-dimensional tube is considered with nonslip at the walls and with both ends open. The problem of flame propagation is solved using direct numerical simulations of the complete set of hydrodynamic equations including thermal conduction, diffusion, viscosity, and chemical kinetics. As a result, it is shown that flame interaction with the walls leads to the oscillating regime of burning. The oscillations involve variations of the curved flame shape and the velocity of flame propagation. The oscillation parameters depend on the characteristic tube width, which controls the Reynolds number of the flow. In narrow tubes the oscillations are rather weak, while in wider tubes they become stronger with well-pronounced nonlinear effects. The period of oscillations increases for wider tubes, while the average flame length scaled by the tube diameter decreases only slightly with increasing tube width. The average flame length calculated in the present work is in agreement with that obtained in the experiments. Numerical results reduce the gap between the theory of turbulent flames and the experiments on turbulent combustion in tubes.

Pulsating Mode of Flame Propagation in Two-Dimensional Channels

Flame propagation in channels and cracks is a problem of considerable interest with applications in many practical combustion devices, in fire hazard scenarios, and in the emerging micropropulsion technologies. Understanding the dynamics and stability characteristics of flame propagation in channels is, therefore, important both for fundamental research as well as for practical applications. In this work, we examine the propagation of a premixed flame front in a two-dimensional channel in the presence of a Poiseuille flow. Our primary objective is to determine within the flammable regions the structure of the flame front and the conditions that result in steady propagation and those leading to a pulsating mode of propagation. Special attention is given to the difference between propagation in narrow and wide channels, heat losses to the channel’s walls, and an imposed flow that either supports or opposes the propagation. In general, flame oscillations are found to occur in mixtures for which the effective Lewis number is sufficiently large. They are more likely to occur in narrow or wide channels and particularly at near-extinction conditions where the critical Lewis number is reduced to physically accessible values.

Behavior of partially premixed flame fronts excited with a fuel tube resonance frequency

Behavior of partially premixed flame fronts excited with a fuel tube resonance frequency

The Temporal Rate of Fractal Dimension Growth in Flame Front Wrinkles during the Early Phase of Flame Propagation in an SI Engine.

Turbulent premixed flame fronts during the early phase of flame propagation were visualized by a laser-light sheet technique in an optically accessible spark ignition engine. Time-resolved continuous images of wrinkling flame fronts were captured by a high-speed video camera. The test engine was operated at various engine speeds and compression ratios. The image data were processed to measure the fractal dimension in a time series for each cycle and to investigate the nature of fractal dimension growth in the early phase of flame development. The results show that the temporal rate of fractal dimension growth increases as the engine speed and the compression ratio increase. The empirical correlation was derived for the temporal rate of fractal dimension growth as a function of turbulence intensity and pressure.

A similarity solution describing the collision of twoplanar premixed flames

The problem of the head-on collision of two planar premixed flame fronts is considered using the idealized model of a single step reaction controlled by the deficient species and an Arrhenius reaction law. Density changes are neglected. It is shown that a similarity solution exists if the deficient species and temperature are equidiffusive (Lewis number unity). The similarity solution, derived using activation energy asymptotics, is valid in the intermediate region when the flames are close enough that their pre-heat zones overlap but their reaction zones may be considered to be well separated. A one–dimensional numerical simulation shows good agreement with the analytical solution.

Vented gaseous deflagrations: modelling of translating inertial vent covers

Abstract A model of explosion pressure build up in enclosures with translating inertial vent covers is presented. The previous approach, valid for inertia-free vents, is advanced by appending to it a new model of translating inertial vent cover displacement. The model and CINDY code are validated against experiments by Hochst and Leuckel (J. Loss Prev. Process Ind. 11 (1998) 89) in a 50-m 3 vessel with vertically translating covers with surface densities of 42 and 89 kg/m 2 at conditions of initially quiescent and turbulent mixtures. It is demonstrated for the first time that modelling of the vent cover jet effect is crucial for prediction of interdependent pressure-time and cover displacement-time transients, whereas air drag force and cushioning effects are negligible. The model was used further to investigate the influence of vent cover surface density on venting generated turbulence, via comparisons with experimental data of Cooper et al. (Combust. Flame 65 (1986) 1) in a 1-m 3 enclosure with vertically translating covers of various surface densities up to 200 kg/m 2 . The increase of the turbulence factor, i.e. total premixed flame front wrinkling factor, with cover inertia is obtained and explained.

Nonlinear response of premixed-flame fronts to localized random forcing in the presence of a strong tangential blowing

Using a modified Michelson-Sivashinsky evolution equation (EE) as the starting point, we study the nonlinear dynamics of a premixed gaseous flame when a significant gas flow velocity u ∥ exists parallel to the front. The chosen u ∥ noticeably exceed the critical value u c corresponding to the transition between absolute and convective instability, whereby an external forcing is needed to trigger some wrinkling in the long-time limit. The selected excitation is spatially localized and evolves randomly in time according to an Ornstein–Uhlenbeck process with adjustable intensity and correlation time. Once suitably periodized spatially, the EE is solved for the front shape by a Fourier pseudo-spectral numerical method. Integrations over wide spatial domains and long times reveal the following. (1) The instantaneous spatial development of the Landau–Darrieus (LD) instability ultimately comprises three successive regions: (I) a linear zone, adjacent to and downstream of the exciting source, where the instability ultimately amplifies preferentially some wavelengths from the random forcing; (II) a transition zone, downstream of zone I, where nonlinearity acquires full importance; and (III) a fully nonlinear zone, where crest mergers make the cell amplitudes and wavelengths increase with downstream distance, till the end of the integration domain is reached. (2) On time average, only zone (I) depends significantly on the noise characteristics whereas zone (III) widens self-similarity: flame-brush thickness, wrinkle wavelengths, PDF of front fluctuations about the mean are all governed by a single length scale Λ(x) that increases linearly with downstream distance (x). The mean degree of wrinkling, flame slope and cell aspect ratio are uniform there. Changing u ∥ yields dΛ∼u c/u ∥. This self-similarity may be attributed to the scale invariance of the LD instability and of the Huygens nonlinearity. Strongly resembling the temporal (u ∥=0) development of wrinkles despite the spatial nonlocality of the LD instability and the absence of any infrared cut-off frequency in the exciting spectrum, the above findings raise open questions as to the consequences of a spatially distributed forcing when u ∥≫u c.

Computing the Effective Hamiltonian in the Majda--Souganidis Model of Turbulent Premixed Flames

Turbulence enhances the speed of propagation of a premixed flame front. According to the Majda--Souganidis model, the procedure to predict this enhancement involves computing the effective Hamiltonian in a small-scale nonlinear cell-problem. We first discuss how to transform this problem into computing the steady-state solution of a system of conservation laws whose vector solution represents the gradient of the eigenfunction associated with the effective Hamiltonian. Theoretical arguments as well as numerical evidence are presented to emphasize the importance of enforcing the constraint that the vector solution must effectively be the gradient of a scalar function. We introduce a scheme that satisfies this constraint exactly by relying on staggered grids for the gradient components. Also discussed is the issue of selecting a time integrator to achieve fast convergence to a steady state. Validation is performed by examining convergence under grid refinement and by comparison with analytical results when available.

Premixed flame front structure in intense turbulence

Six methane/air flames of equivalence ratio 0.7, Ka =1–17 stabilized on a intense turbulence low-swirlburner of were investigated by Rayleigh laser sheet measurements of the gas density within the preheat zone (approximately 400–1000 K). Lean premixed turbulent flames with Karlovitz numbers Ka , greater than unity are becoming of increasing practical importance but only limited detailed experimental data are available. This regime is often called the distributed reaction zone and a new approach to analyzing flame front broadening was developed to address issues of flame three dimensionality. Instantaneous progress variable, c , contour sets constitute the basic data and a statistical analysis of three aspects of this data are presented: (1) c contour spacing (2) flame front curvature, and (3) flame surface density. A detailed investigation revealed little change of these parameters with turbulence, even at the highest Ka numbers: the contour spacings were similar to those derived from laminar flame calculations and neither the flame front curvature nor the flame surface density varied significantly with c . Geometric effects of turbulence on flame front structure still predominate at these turbulence levels. The primary effect of increasing turbulence is increasingly to convolute the flame front and so increase the flame surface density, reduce the scalar length scales, and increase the burning rate. This finding is also reflected in the significant broadening of the flame front curvature distributions with increasing u′/S L . The data presented here indicate that the diagrams often used to delimit the various regimes of premixed turbulent combustion and hence guide modeling are based on parameters which need to be carefully investigated to determine their appropriateness. The results are consistent with an interpretation of combustion/turbulence interactions which sees the reaction zone thickness as the significant scalar length scale.

Adaptive Spectral Element Simulations of Thin Premixed Flame Sheet Deformations

The high order Spectral Element Method is used to solve the reaction-diffusion-advection problem as described by the premixed flame case. An h–p adaptive refinement-coarsening algorithm is developed based on a posteriori error estimators. Simulations of the wrinkling of a premixed flame front are used to illustrate how the mesh is adaptively refined. A similar, idealized heat transfer problem is used to show the adaptive refinement and coarsening of the mesh. Adaptivity efficiently provides high resolution in areas of the domain where large or rapidly varying physical changes exist, while saving unnecessary computation where the solution is smooth or physical phenomena have passed by.

Theoretical investigation of unsteady flow interactions with a premixed planar flame

This paper presents the results of a theoretical study of the interactions between a laminar, premixed flame front and a plane acoustic wave. Its objective is to elucidate the processes that damp or drive acoustic waves as they interact with flames. Using linear analysis, the characteristics of the acoustic field, the flame's movement and wrinkling in response to acoustic perturbations, and the acoustic energy that is produced or dissipated at the flame are calculated. These calculations show that the net acoustic energy flux out of the flame is controlled by competing acoustic energy production and dissipation processes. Energy is added to the acoustic field by unsteady heat release processes resulting from the unsteady flux of unburned reactants through the flame by fluctuations in the flame speed or density of the unburned reactants. Energy is dissipated by the transfer of acoustic energy into fluctuations in vorticity that are generated at the flame front because of the misaligned fluctuating pressure and mean density gradients (i.e. the baroclinic vorticity production mechanism). The paper concludes by showing how these results can be generalized to determine the response of planar flames to an arbitrarily complex acoustic field. The principal contribution of this work is its demonstration that the excitation of vorticity and fluctuations in the flame speed have significant qualitative and quantitative affects on the interactions between flames and acoustic waves.

Thermal Interaction of Two Flame Fronts Propagating in Channels with Opposing Gas Flows

A one‐dimensional nonstationary model is presented for propagation of two premixed flame fronts in narrow plane channels with regard for thermal interaction of flames through a dividing wall. The gas flows in channels considered are oppositely directed and equal in magnitude. It is shown that heat transfer through the heat‐conducting wall separating the plane channels leads to the appearance of a number of special features. The scheme of filtrational combustion of gases considered may be called the scheme of combustion with counter filtration; it is a new modification of systems with enthalpy excess. It is shown that the temperature at the combustion‐wave front may be greater than the adiabatic temperature of free plane flame with the same composition of the combustible mixture even if there are heat losses through external walls of the system. By solving the problem, dependences of the velocity of combustion waves on the distance between them are obtained, and the dynamic behavior of combustion waves is studied. The region of problem parameters, where self‐stabilization of combustion waves is possible, is found.

Flamelet modeling of lifted turbulent methane/air and propane/air jet diffusion flames

The stabilization mechanism of lifted turbulent jet diffusion flames is a test problem for models of partially premixed turbulent combustion. In these flames, combustion processes occur in both the nonpremixed and the premixed mode. For the flame stabilization process, however, flame propagation of the premixed branches seems to play a crucial role. In this paper, a flamelet model for partially premixed turbulent combustion is presented that combines flamelet models for non-premixed and premixed combustion. A new model for the turbulent burnign velocity in partially premixed flows is proposed. It is based on a formulation for a conditional turbulent burning velocity which depends on mixture fraction. The effect of partially premixing is taken into account by using the presumed probability density function (pdf) approach in terms of the mixture fraction. Mean scalar quantities on both sides of the premixed flame front are calculated in the same way. From a computational point of view, the model has the advantage that the calculation of the chemical processes can be decoupled from the flow calculation, allowing for simulations of realistic configurations, yet retaining detailed chemistry. The model is used to simulate the stabilization process of turbulent methane/air and propane/air jet diffusion flames. The calculated lift-off heights compare favorably with experimental data from various authors.

A comparison of the structures of lean and rich axisymmetric laminar Bunsen flames: application of local rectangular refinement solution-adaptive gridding

Axisymmetric laminar methane–air Bunsen flames are computed for two equivalence ratios: lean (Φ=0.776), in which the traditional Bunsen cone forms above the burner; and rich (Φ=1.243), in which the premixed Bunsen cone is accompanied by a diffusion flame halo located further downstream. Because the extremely large gradients at premixed flame fronts greatly exceed those in diffusion flames, their resolution requires a more sophisticated adaptive numerical method than those ordinarily applied to diffusion flames. The local rectangular refinement (LRR) solution-adaptive gridding method produces robust unstructured rectangular grids, utilizes multiple-scale finite-difference discretizations, and incorporates Newton's method to solve elliptic partial differential equation systems simultaneously. The LRR method is applied to the vorticity–velocity formulation of the fully elliptic governing equations, in conjunction with detailed chemistry, multicomponent transport and an optically-thin radiation model. The computed lean flame is lifted above the burner, and this liftoff is verified experimentally. For both lean and rich flames, grid spacing greatly influences the Bunsen cone's position, which only stabilizes with adequate refinement. In the rich configuration, the oxygen-free region above the Bunsen cone inhibits the complete decay of CH4, thus indirectly initiating the diffusion flame halo where CO oxidizes to CO2. In general, the results computed by the LRR method agree quite well with those obtained on equivalently refined conventional grids, yet the former require less than half the computational resources.

Experimental study of nonfuel hydrocarbon concentrations in coflowing partially premixed methane/air flames

Centerline measurements have been made of temperature, CH 4, O 2, CO 2, and C2 to C12 nonfuel hydrocarbons in a CH 4/air nonpremixed coflowing flame and in five partially premixed coflowing flames with primary equivalence ratios varying from 12.3 to 2.5. Partial premixing decreases the flame height and thereby compresses all of the profiles towards the burner surface, so a nondimensional vertical coordinate has been developed to account for this and make other effects more apparent. The temperature and major species results show that partial premixing reduces radial heat and mass transfer in the lower part of the flames, and causes an inner rich premixed flame front to form at one-half the height of the outer flame front. Partial premixing increases the mole fractions of the oxygenated hydrocarbons CH 2O and C 2H 2O to hundreds of parts per million, and of C 2H 4O and C 3H 4O to parts per million. The mole fractions of regular hydrocarbons are decreased by partial premixing, in roughly the same proportion as they are reduced by dilution with nitrogen, which suggests that fuel dilution is the primary cause. The decrease in concentrations is progressively greater for larger hydrocarbons. In the flames that exhibit a double flame structure, nonfuel hydrocarbons are formed inside the inner rich premixed flame front, peak at this front, and are completely consumed in the region between the flame fronts.

Topology of turbulent premixed flame fronts resolved by simultaneous planar imaging of LIPF of OH radical and rayleigh scattering

Planar images of Rayleigh scattering and laser-induced predissociative OH-fluorescence (OH-LIPF) have been obtained simultaneously in turbulent premixed jet flames on a single-shot basis. The geometric structure of temperature and OH isocontours were extracted for fractal analysis. A power-law fractal behavior can be identified in the ensemble-averaged flame length measure. It was found that the inner and outer cut-off scales of OH contours are larger than those of the iso-temperature contours; while the OH images show comparatively smaller fractal dimensions. The joint-pdf ’s between flame temperature and OH LIPF signals at different heights are also derived to evaluate the flame stretch effect on local flame structure. Comparison of image pairs near the extinction limit suggests that Rayleigh thermometry is more adequate to characterize the fine-scale flame front wrinkling in highly stretched turbulent premixed flames.

First Experimental Study of the Darrieus-Landau Instability

This paper presents the first direct experimental measurements of the growth rate of the Darrieus-Landau instability on a planar laminar premixed flame front. Prior to measurements, the intrinsically unstable flame is maintained stable by a novel technique based on the response of the flame to an acoustic parametric forcing. The growth rates of the instability, measured when the forcing is removed, are in agreement with the theoretically predicted values.

Turbulent flame propagation in partially premixed combustion

Stratified spark-ignition engines that use direct fuel injection into the combustion chamber feature both small- and large-scale spatial variations in unburned mixture composition. In these configurations, the spark-ignited turbulent flame propagates into a mixture with variable equivalence ratio. Under lean-rich conditions, the reaction zone can be described as a staged combustion system with a first stage corresponding to a propagating premixed flame front followed by a second stage corresponding to multiple post-diffusion flames that burn the remaining excess fuel and excess oxidizer. Direct numerical simulations (DNS) are used in the present study to bring basic information onto the effects of partial premixing both on the turbulent flame topology and the overall mean reaction rate and to determine if and how flamelet combustion models should be modified to account for these effects. The numerical configuration corresponds to three-dimensional partially premixed flames propagating into a temporally decaying turbulent flow. The simulations use different degrees of scalar inhomogeneity around mean stoichiometric conditions. It is found that while partial premixing has a negligible contribution to the premixed flame surface wrinkling, it has a net negative impact on the overall mean premixed reaction rate. This effect is related to the reduced values of mean flamelet mass burning rate per unit flame surface area observed in partially premixed configurations compared to perfectly premixed flames.