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Molecular dynamics simulations using the Lennard-Jones energy potential are compared with continuum solutions of reaction and diffusion in a dilute gas. The reaction model is a passive one in which high-energy bath atoms create a species, at dilute concentrations, which may have a very fast consumption reaction. This construction is designed based on typical fast reaction pathways involved in the fuel breakup in a hydrocarbon flame. Using reaction rates and diffusivities obtained from the molecular simulations allows the continuum solution to describe the reactive atom density spatial distribution with good accuracy. Based on this agreement, it is possible to estimate which reaction rates will produce negligible diffusive spreading, and hence, which species might be assumed to be in chemical equilibrium in continuum reacting flow calculations.

A premixed flame within a turbulent flow exhibits a decreasing enhancement of fuel consumption rate with increasing turbulence intensity, an effect known as the bending effect. Denet has shown that flow time correlations may be one cause of the bending effect. Using a Damkohler-Huygens front propagation model, we illustrate that the removal of flow components with reduced frequencies greater than unity (ω >kS L) causes a small reduction in front area but a large reduction in the flow intensity, which is the bending effect (ω is the frequency and k is the wavenumber). To be effective in producing front area, a flow mode must have a phase velocity, ω/k, smaller than the laminar burning velocity, S L.

A premixed flame, propagating away from a point ignition source into an unlimited domain displays an increasing flame speed after the flame size has grown beyond a transition radius. Experiments by Gostintsev et al are described by the relation R = R1 + At3/2, where t is the time from ignition and, where SL is the flame burning velocity and is the thermal diffusivity. The non-dimensional function a() is determined from the experimental results to be equal to 0.0022, where is the density ratio across the flame. In the present work, two-dimensional Lagrangian simulations of flame propagation also display a radial growth with a 3/2 power-law behaviour. This is a potential flow model - no vorticity is included. Hence, the Darrieus - Landau hydrodynamic instability by itself can generate flame acceleration. The numerical results are summarized by the relation R = R1+(2/40)L(SLt/L)3/2, where L is a reference length and is the volume production ratio, = - 1. Equating the zone of velocity jump in the numerical sc...

Three-dimensional turbulence simulations which include three different mean directions of passive flame propagation are analyzed for the isotropic nature of the flame shape, as given by the fluctuations in the G scalars which determine the passive Huygens propagation. When the turbulence intensity is greater than the flame burning velocity, the fluctuations appear to be isotropic as assumed in the analysis of Peters (1992)

Flame propagation through an array of vortices was studied with a model which incorporated the variation of the local burning velocity with stretch. Assuming incompressible flow, the mean burning velocities were calculated and compared to those of the Huygens limit. It was found that stretch causes a decrease in the mean burning velocity, and a mechanism which explains this trend was identified. The study also demonstrated that, as expected, stretch only has a significant effect on the mean burning velocity for vortices whose size is of the same order as that of the flame thickness.

Flame propagation into a swirling flow allows the creation of burnt gas vorticity which may enhance the forward motion of the flame. This baroclinic push on the flame differs from the axial pressure model suggested by Chomiak (1976). In particular, the baroclinic enhancement will depend upon the location of the burnt gas with respect to the flame, as shown by two vortex configurations: straight and curved. Additionally, the baroclinic effect, depending upon the flame density gradient and the swirling flow radial pressure gradient, will provide an enhancement that is proportional to the ambient pressure.

A flame ball growing into a turbulent mixture exhibits a speed which is proportional to its size - this effect is a consequence of the distorted flame surface: the inner flame regions “push” the outer regions, the magnitude of this velocity depends upon the flame surface density distribution. A simple two-dimensional, zero-thickness flame model interacting with defined eddies illustrates how rapidly this flame distortion occurs after a spark ignition. This distributed volume source effect is also the functional form of Groffs modified entrainment model of flame propagation. A result of this size dependent growth is exponential growth of flame area which is an essential feature of spark-ignition internal combustion engines.

The structure of a nonpremixed methane-air flame interacting with avortex is described by a two-step reduced mechanism. The mixture-fraction field Z for the flamelet is approximated by a stretched parabola. This synthetic mixture-fraction field is used to relate the vortex swirl velocity to the tangential velocity along the flamelet and to identify a rate of scalar dissipation χ appropriate for parabolically shaped flame tongue. The flame temperature and χ along the tongue are calculated in the context of rate-ratio asymptotics. It is found that the peak temperatures, and hence the smallest χ, occur at the flame tip. In addition, analytical estimates are made to evaluate the importance of the transverse convective effects in the reaction zone relative to the usually assumed diffusive-reactive balance. Away from the tip of the flame tongue but near it, the influences of transverse convection are important, but the relative magnitude of these effects is dependent on the curvature κ of the flamelet as well as on the spacing l between the mixture-fraction contour of the oxidizer stream (Z = 0) and that of the fuel stream (Z = 1); the convective influence becomes more pronounced as κ and/or l is increased. However, the estimates show that the importance of transverse convection is not as large as would be indicated using activation-energy asymptotics.

Computed flame motion through and between swirling eddies exhibits a maximum advancement rate which is related to the time duration of flame motion between eddies. This eddy spatial structure effect upon the apparent turbulent flame speed appears to be similar to the square-root dependence observed in wrinkled flamelet data. The rate-limiting behavior at one eddy length-scale can be removed by inclusion of smaller eddies which reside between the larger eddies. This large-eddy, small-eddy concept yields a recursion relation and repeated functional iteration can be done to approximate a desired flame speed relation. As an example, an iteration to produce ST In ST = u' is given for the range of u' observed in liquid flames. Currently, the iteration process is a post-diction of flame speed, but if a universality can be developed, then a predictive theory of turbulent flame propagation might be achieved.

Computed vorticity-flame interactions in two-dimensions suggest that a wrinkled flame is a common occurrence in reacting turbulent flow and that these wrinkles are formed in a small fraction of an eddy turn-over time. An experiment in which the Tsuji porous-cylinder burner is spinning may be a way to achieve a steady, wrinkled diffusion flame under laboratory conditions. The wrinkled flame differs from the previously studied stagnation, counterflow flame in that the former has a large spatial variation in diffusive gradients near the wrinkle. These gradient variations may explain the extinction observed in unsteady, pulsed jets by Lewis et al

A discrete vortex dynamics time-dependent simulation of a premixed wrinkled laminar flame shows that the computed turbulent shear stress in the flame brush region based on unconditioned velocities is substantial. By comparison, the computed shear stresses in the burnt and in the unburnt flow are negligible. Thus, it is the intermittent flame motion that gives the appearance of counter-gradient fluxes in a turbulence model based on unconditioned velocities. The flame is considered to be of zero thickness and the flame location is described in terms of arc length from the flame holder. Two-dimensional V-shaped and bunsen burner flames have been simulated in planar geometry. A flame speed relation is used which depends on flame curvature and in some calculations on flame stretch. Discrete volume sources are distributed along the flame location to represent the volume expansion at constant pressure combustion in this open system. The estimated vorticity production due to fluctuating pressure gradients combine...

A numerical simulation of the response of a premixed flame to various randomly defined two-dimensional flow fields is performed in order to obtain an insight into Lewis number effects in turbulent premixed combustion. A one-step reaction with a large, but finite, activation energy in the limit of zero heat release is assumed.The numerical simulation uses vortex dynamics to describe the velocity field, while the scalar field is computed on a mesh.The flow field integral length scale is typically ten times the flame thickness, and the r.m.s. velocity fluctuation is of the order of the flame velocity. Ensemble averages are taken over a total number of 30 realization for Le of 1/2 and 37 realizations for Le of 2, where the Lewis number is Le = λ/(ρcpD). Since the density is kept constant, there is no feedback from the reacting scalar field to the velocity field. For the Le of 1/2 case, there is a clear manifestation of the thermo-diffusive instability mechanism leading to a cellular flame front struc...

We show asymptotically and numerically that a constant-density flame will exhibit different geometrical shapes which depend on the flow time-scales. In particular, a frozen flow may yield a flame surface composed of bulges connected by cusps, whereas a pulsating flow may dampen the flame motion to the extent that only a flat, laminar flame surface will be possible. The cusp nature disappears when the eddy frequency becomes comparable to uLk, where uL is the laminar flame speed and k is a characteristic wave number of the flow. Flame shapes obtained with a sheet of laser light within a spark-ignited engine show a cusp nature only at low engine RPM, in agreement with the frequency criteria given above.

Numerical simulation of premixed flame propagation through a two-dimensional vorticity distribution exhibits the coupling between combustion heat release and fluid dynamics. Effects of both thermal expansion and vorticity generation via baroclinic torques are considered. The rotational part of the velocity field is described by discrete vorticity. The irrotational velocity. given by a velocity potential. is determined by a Poisson equation where the chemical reaction determines the spatial distribution of volume expansion. The transport of a single reacting scalar is computed on an Eulerian mesh with a small-Mach-number flow assumption so that the density gradient is nonzero only within the reaction zone. The vector cross product of density gradient and pressure gradient defines the baroclinic generation of vorticity. With a single initial vortical region-the monopole interacting with a premixed flame-the baroclinic effect produces opposite circulation, and thus. creates a dipole configuration. W...