Laminar Flame Front Research Articles

Some Phenomena During Flame Propagation in a Half‐Open Channel with an Obstacle

A laminar flame front propagating in a closed‐end channel with an obstacle is simulated. Ignition is initiated at the closed end of the channel. The simulations revealed the entrainment of the flame in the vortex behind the obstacle, the expulsion of the vortex from the channel by the flame, and flame fragmentation.

Calculation and Consideration of the Lewis Number for Explosion Studies

When a laminar flame front becomes cellular, the flame front area increases and the rate at which the unburnt gases are consumed is increased. If this is not taken into account when modelling vented or non-vented explosions, the pressures that vessels containing explosions are subject to can be greatly under-predicted. For most explosion models, hydrodynamic instabilities are considered as the only driving force for the formation of cellular flames. This paper shows that the transition to cellular flame propagation can occur earlier due to thermo-diffusive instabilities. To predict when thermo-diffusive instabilities should be considered as the main driving force for cellular flame transition, the Lewis number of the reactant mixture should be compared to a critical value. Calculations of the Lewis number for a range of different fuel/air mixtures that are in common use are presented. The fuels studied are methane, ethane, propane and n-butane. For each of these fuel/air mixtures the critical Lewis number was evaluated using an expression from asymptotic analyses. It is shown that the critical Lewis number based on this type of analysis is under-predicted. This is because the flame thickness is not included. It is proposed that a modified form of the critical Lewis number be used which is greater by an order of 1/Zeldovich number.

Turbulence and scalar transport in premixed bunsen flames of lean hydrogen/air mixtures

A joint particle imaging velocimetry/laser-induced OH fluorescence (PIV/OH-LIF) imaging technique was applied to measure the conditional unburned and burned gas velocities in turbulent premixed Bunsen flames of lean hydrogen/air mixtures. Turbulent scalar flux u ″ c ″ ˜ in a two-dimensional plane was obtained for five flames with u ′/ S L ranging from 1.2 to 3. Flow integral length scales were also derived from PIV measurements based on the two-point correlation functions of the axial velocity fluctuation. Among the investigated flames, countergradient diffusion was observed in the radial component, while gradient diffusion was found in the axial component of u ″ c ″ ˜ . The behavior of countergradient diffusion is consistent with predictions from the Bray number expression. It is, however, contrary to the expectation that turbulent premixed flames will behave in a gradient-diffusion manner for fuel-air mixtures with conditional dilatation peaking near the unburned side in laminar flame fronts. For the statistically two-dimensional turbulent flow field of the jet flame configuration, the axial convective and turbulent diffusive fluxes of the progress variable variance parallel to the turbulent flame brush do not vanish after applying Gauss's theorem. This invalidates the unique relationship between turbulent scalar flux and conditional dilatation found by Swaminathan et al.

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.

Interaction between two premixed laminar V-shaped flame fronts at low Lewis number

In studies of flamelets in premixed turbulent combustion, the interaction between laminar flames is of great importance. To study this, measurements have been made of the interaction between two premixed laminar V-shaped flames (“W” configuration) for a Lewis number less than 1. When the Lewis number differs greatly from unity, differential diffusion effects dominate and lead to considerable complexity. Flow visualization is used in association with laser Doppler velocimetry. Temperatures are measured with a thermocouple and species are sampled through a probe. The results show repulsion between the two oblique flame fronts separated by fresh mixture. This induces a divergent flow in the whole interaction zone and an overall shifting of the two V-shaped flames. The influence of the Lewis number is first noticeable on the unburned gases between the two flames since it induces preferential diffusion of hydrogen in the transverse direction. This leads to an increase of temperature inside the two V-shaped flames. As a consequence, it also prevents the intersection and coalescence of the two inner fronts as they propagate towards each other.

The baroclinic effect in combustion

The baroclinic effect is due to the nonalignment of pressure and density gradients, and its result is to induce vorticity production. Because of the steep density gradients and the almost universal presence of ambient pressure signals in combustion systems, the baroclinic effect is a crucial mechanism for the production of turbulence. In this paper, we provide a review of the results of numerical simulations of baroclinic interactions between planar pressure signals and cylindrical laminar flame fronts. In the first section, the evolution of a flame front affected by a single baroclinic impulse is considered. In the second section, we consider the effect of a double baroclinic impulse on a growing flame ball within a shock tube and examine the effect of viscosity on the evolution of the overall burning rate.

Self-Generated Acceleration of Confined Deflagrative Flame Fronts∗

Abstract This paper deals with the investigation of laminar and turbulent flame front behaviour in the later stage of an confined explosion process where stretch effects can be neglected and for which experimental data of burning velocity and knowledge about the effect of flame-induced turbulence in a spherically propagating flame front are still limited. It could be shown that for laminar explosion experiments there exists a radius range of the propagating flame front in which the influence of stretch due to the overall spherical shape as well as compression of the unburnt gas can be neglected, so that the burning velocity S is mainly influenced by flame-induced acceleration. Also turbulent flame fronts are accelerated, obviously by additional flame-induced turbulence, and in this case the flame front propagation can be based on an effective turbulence intensity (u'/SL, p) eff resulting from the superposition of fan-generated (u'/SL, p) meas and flame-generated (u'/SL, p) ind turbulence intensity. Finall...

A new heterogeneous burning velocity formula for the propagation of a laminar flame front through a polydisperse spray of droplets

The effects of heating programmes, varied charge density and the insertion of partitions parallel to the wall of the oven chamber were investigated as means of obtaining coke of homogeneous quality in the direction of the oven width. The following results were obtained. (1) The difference in the strength after gasification with CO2 at 1100°C (CSR) was reduced by increasing the heating rate in the tail zone to increase the average CSR over the oven width. (2) A higher charge density reduced the CSR difference more markedly than the heating programme did. (3) The insertion of partitions homogenized the CSR of the cokes produced in the head, body and tail zones in spite of different heating rates in the three zones. Development of CSR in the chamber is discussed in terms of dynamic behaviour of the tar produced during coking across the oven width.

Measurement of Scalar Dissipation in Premixed Flames

Abstract The dissipation rate of a scalar appears in the balance equations of many models of turbulent flows, reacting or otherwise. In this study, scalar dissipation in premixed flames is measured three-dimensionally using laser imaging techniques involving two closely spaced parallel beams and intensified CCDcameras. A progress variable, c, is derived from measured Rayleigh scattering intensities and computed results of a laminar flame code. In a lean turbulent premixed flame of Lewis number greater than unity, the conditional mean values of scalar dissipation, defined as $ (where D is diffusivity and ζ is a particular value of c), are found to be much less than the laminar flame values. Fine turbulence scales smaller than the laminar flame front thickness are thought to influence the mixing of radicals and have a broadening effect on the flame front.

Baroclinic distortion of laminar flames

One of the most important divisions in studies of premixed gaseous combustion is that between the theoretically much favoured laminar flames, and the more com­monly observed case of turbulent burning. Laminar premixed flames clearly represent much simpler cases for theoretical and numerical study. Conversely experimental in­vestigations are much simpler in the case of turbulent combustion due to the inherent instability of laminar fluid flows. One mechanism which can effect the transition from laminar to turbulent com­bustion is baroclinicity (i. e. the non-alignment of pressure and density gradients). A laminar deflagration, or slow flame, may be thought of as a reaction front which prop­agates at a low Mach number and whose associated pressure field is therefore close to uniformity. On the other hand, very steep density gradients are associated with the rapid temperature increase due to the exothermic chemical reaction. (Note that a typical deflagration thickness is of the order of 1 mm, and densities may decrease by factors of between five and ten in going from unburnt to burnt gas.) Externally induced pressure disturbances, which are almost universally present in practical com­bustion systems, can introduce a baroclinic effect whenever a steep pressure gradient interacts with a flame front in such a way that the former is misaligned with the den­sity gradient associated with the latter. The differential acceleration of fluid elements can produce significant rotational motion and, if this field of vorticity is sufficiently strong, a laminar flame front may be broken up and the transition to turbulent burning may result. This scenario was clearly demonstrated in an experiment done by Markstein (1963) that involved the double passage of a large amplitude planar pressure signal across an expanding spherical flame bubble in a shock tube. The laminar flame front was com­pletely obliterated, and the evolution to fine grain turbulent combustion was revealed. In the current paper we report on numerical simulations of a number of similar experiments. Although we are here restricted to two space dimensions and cannot therefore investigate fully turbulent behaviour, these simulations do reveal qualita­tively similar behaviour to that found in the early stages of the Markstein experiment. It has been possible to repeat the simulations for a variety of different flames, so that the effects of the various processes (in particular the chemical reaction and the ther-moviscous diffusion) can be assessed. Attention is also given to the question of the grid dependency of the numerical solutions obtained.

A new heterogeneous burning velocity formula for the propagation of a laminar flame front through a polydisperse spray of droplets

A new burning velocity formula for the heterogeneous mode of laminar combustion of a premixed polydisperse spray of droplets is derived, using activation energy asymptotics and the method of multiple scales. For a mass fractional flux of fuel vapor, G F of O(1) at the homogeneous reaction zone the homogeneous burning velocity formula obtained by Silverman et al. [ Combust. Flame 93:97–118 (1993)] is applicable. Here, it is shown that for G F of O( θ −1) (where θ the nondimensional activation energy) heterogeneous combustion is predominant, whence the new formula is developed. The spray of droplets is modelled with the sectional approach, so that an arbitrary initial multisize distribution of droplets may be considered. Calculated results show that the behavior of the heterogeneous burning velocity as a function of the Sauter mean diameter is in qualitative accord with experimental data from the literature. For a polydisperse tetralin spray of low volatility the theory produces evidence suggesting a critical initial SMD at which a switchover from the homogeneous to the heterogeneous mode of flame propagation occurs that is in agreement with independent experimental and theoretical predictions for monodisperse sprays.

Translated from Nensho no Kagaku to Gijutsu, 1, 37 (1992) Fractal Behaviour of Wrinkled Laminar Flame

Abstract The unbumed gas region of wrinkled laminar flame stabilized on a Bunsen burner was visualized by a laser tomography and the fractal analysis was applied to the boundaries of unbumed gas to characterize the geometry of wrinkled laminar flame. The fractal dimension of the wrinkled laminar flame front is independent of any scale of the unbumed mixture turbulence. It increases with the turbulence intensity from 2.20 to 2.32 and approaches at high intensity turbulence to 2.36 which is the fractal dimension of the non-reactive turbulent flow. The existence of inner and outer cut-offs is clearly shown. Both cut-ofls are also independent of any scale of unbumed mixture turbulence. The inner cut-off coincides with the scale of wrinkles caused by the laminar flame instability and the outer cut-off is determined by the flame base diameter.

Microgravity observations of premixed laminar flame dynamics

Microgravity experiments have been performed to investigate the role of buoyancy in the structure and stability of premixed laminar flame fronts. Planar and spontaneously wrinkled (cellular) flames burning in propane-oxygen-nitrogen mixtures and propagating slowly downward through a 6 cm diameter tube against a flow of fresh gas were filmed under normal and low gravity conditions in the NASA Lewis 2.2 Second Drop Tower. Results show that downwardly-propagating flames that are planar in normal gravity become curved toward the unburned mixture in reduced gravity, forming a single cell nearly filling the tube. This behavior is consistent with theoretical predictions that flat flames are possible only when the stabilizing effect of buoyancy in this geometry balances the destabilizing effect of the hydrodynamic instability mechanism. Thus when gravity is reduced, this mechanism is unopposed and produces a curved flame. Behavior of cellular tube flames in microgravity is considerably more complex. Mixtures producing small cells in normal gravity produce flames with larger, irregular cells in low gravity. Often in the microgravity tests, one cell grows at the expense of the others, which move downstream and disappear. The remaining cell propagates through the tube as an oblique curved flame, while smaller cells form and disappear along the front. These observations are interpreted qualitatively in terms of the predicted effects of buoyancy on flame stability and cell sizes. It is concluded that while buoyancy is not the dominant mechanism in the formation of the cells, it contributes significantly to the overall flame stability and structure.

A parametric acoustic instability in premixed flames

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.

Stretching and quenching of flamelets in premixed turbulent combustion

The stretch rate of flamelets in premixed turbulent combustion is computed using (1) detailed numerical simulations of vortex-flame interactions and (2) a model for intermittent turbulence taking into account all possible turbulence scales acting on the flame front. Simulations of interactions between isolated vortices and a laminar flame front are used to obtain a relation between the characteristics of a given vortex and the actual flame stretch generated by this structure. Quenching conditions and quenching times are also given by these simulations. A net rate of stretch is then defined in the case of a complete turbulent flow field as the difference between the total rate of flame stretch and the quenching rate due to scales that have a high enough energy and a long enough lifetime to quench locally the flame front. The net rate of stretch is computed for a variety of parameters of interest in practical applications. It is a function of the large-scale turbulence parameters and the laminar flame speed and flame thickness and may be used as an input in most flamelet models for premixed turbulent combustion. Different criteria for total flame quenching in premixed turbulent combustion are derived and compared (1) to the classical Klimov-Williams theory, (2) to a criterion proposed by Poinsot et al. [8, 9], who studied quenching according to the presence near the flame front of a single eddy able to locally quench combustion, and (3) to the experimental results of Abdel-Gayed and Bradley [6, 7].

Diagrams of premixed turbulent combustion based on direct simulation

The structure and morphology of premixed turbulent flames is a problem of fundamental interest in combustion theory. Diagrams indicating the typical structure of a flame submitted to a given turbulent flow have been constructed on the basis of essentially intuitive and dimensional considerations. Knowing the turbulence integral scale and the turbulent kinetic energy, these diagrams indicate if the flow will feature flamelets, pockets or distributed reaction zones. A new approach to this problem is described in the present paper. The method is based on direct numerical simulations of flame/vortex interactions. The interaction of a laminar flame front with a vortex pair is computed using the full Navier-Stokes equations. The formulation includes non-unity Lewis number, non-constant viscosity and heat losses so that the effects 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. 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 2 or Williams 5 . Results show that flame fronts are 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 to the characteristic scales proposed by Peters. 3 Stretch is not the only important parameter determining flame/vortex interaction. Curvature, viscous dissipation and transient dynamics have large effects, especially for small scales and they strongly influence the boundaries of the combustion regimes. For example, the Klimov-Williams criterion which has been advocated to limit the flamelet region, underestimates the size of this region by more than an order of magnitude.

How to attack complex gas phase combustion systems

Starting from detailed investigations of elementary chemical reactions a strategy for experimental and theoretical analysis of complex combustion systems is described. Experimental results on the effect of selective translational excitation of reactants in the reaction H + O 2 → OH + O are compared with theoretical studies on ab initio potential energy surfaces. For the reaction CN + H 2O → HCN + OH direct measurements of thermal rates in shock tubes are compared with data obtained from a laser-photolysis-laser-induced fluorescence technique. The attack of practical combustion problems with homogeneous models including detailed chemistry is described for the NO x reduction in power plants and the autoignition of hydrocarbonair mixtures in relation to engine knock. Next the interaction of elementary chemical processes with steady and unsteady laminar flows is considered. Laminar counterflow diffusion flames constitute an important basis set for the simulation of more complicated turbulent combustion processes. Having a library of precalculated “flamelets” of different strain rate and fueloxidizer mixture compositions the numerical treatment of turbulent flame structures can be performed on existing computers. Detailed calculations on pure and partially premixed CH 4air counterflow diffusion flames are compared with results of nonintrusive CARS measurements for various strain rates. For the unsteady case as a simple test system the ignition of O 2O 3 mixtures by irradiation with a CO 2 laser along the axis of a cylindrical vessel is considered. Mathematical simulation of the ignition process is done by solving the corresponding system of conservation equations. Spatial retization using definite differences leads to a system of ordinary differential and algebraic equations that can be solved numerically. Experimental data are presented for velocity components of the flame front from IR-UV double resonance experiments. The last part describes two approaches for linking complex chemistry to models of turbulent flow field predictions applied to an enclosed turbulent jet diffusion flame. One is a conserved-scalar approach employing a stretched-laminar-flamelet model, and the other involves a direct closure of the chemical production terms by a pdf method. Numerical results for turbulent COair diffusion flames and experimental results on imaging of laminar and turbulent flame fronts by OH radical and acetaldehyde fluorescence in an internal combustion engine are presented.

Explosions of adiabatically compressed gases in a constant volume bomb

Measurements of the explosion limits for fuel-air mixtures compressed by an expanding laminar flame front have been made in a constant volume bomb. The mixtures studied to date include normal pentane, hexane and heptane with fuel-air equivalence ratios in the range 0.8 to 1.3. An Arrhenius plot of the explosion density as a function of reciprocal temperature in the range 670 to 720 K is consistent with an activation energy of 51 kcal/mole. For fixed initial temperature and pressure the explosion density has a weak minimum at an equivalence ratio near =1.1 and decreases with increasing chain length. This behavior is in good agreement with observations made in rapid compression machines and spark ignition engines. A simple kinetic model based on a branched chain mechanism has been developed to correlate the data. In the first stage region the equations are linear and can be integrated approximately using the WKB method. The resulting expression gives an explicit connection between the explosion limit for gases undergoing adiabatic compression and the first stage induction time at constant density. Quantitative agreement between theory and experiment can be achieved with minor modifications in values of published rate constants.

An evaluation of the Klimov-Williams criterion

Different expressions of the Klimov-Williams criterion are explored and the relations between them are established. Five different calculation methods of the K-W criterion are proposed. The sensitivity of the criterion to its quantitative evaluation based on cold flow or flame turbulence parameters and to the use of various definitions of the laminar flame front thickness is assessed by using experimental data coming from V-shaped turbulent premixed flames. It is shown that when the Zeldovich characteristic length is adopted, the values of the K-W criterion are scattered around 0.2 either when the cold flow or the flame turbulence parameters are used. Its values range around unity when the total visible flame front thickness is used. Other implications for the interpretation of the structure of turbulent premixed flames of these various calculation methods are discussed.

Longwave stability of a laminar flame front in an external electric field

Longwave stability of a laminar flame front in an external electric field