Showing papers on "Laminar flame speed published in 1997"

Gradient and counter-gradient scalar transport in turbulent premixed flames

Abstract: In premixed turbulent combustion, the modelling of the turbulent flux of the mean reaction progress variable c, pul i c, remains somewhat controversial. Classical gradient transport assumptions based on the eddy viscosity concept are often used while both experimental data and theoretical analysis have pointed out the existence of counter-gradient turbulent diffusion. Direct numerical simulation (DNS) is used in this paper to provide basic information on the turbulent flux of c and study the occurrence of counter-gradient transport. The numerical configuration corresponds to two-or three-dimensional premixed flames in isotropic turbulent flow. The simulations correspond to various flame and flow conditions that are representative of flamelet combustion. They reveal that different flames will feature different turbulent transport properties and that these differences can be related to basic dynamical differences in the flame-flow interactions: counter-gradient diffusion occurs when the flow field near the flame is dominated by thermal dilatation due to chemical reaction, whereas gradient diffusion occurs when the flow field near the flame is dominated by the turbulent motions. The DNS-based analysis leads to a simple expression to describe the turbulent flux of c, which in turn leads to a simple criterion to delineate between the gradient and counter-gradient turbulent diffusion regimes. This criterion suggests that the occurrence of one regime or the other is determined primarily by the ratio of turbulence intensity divided by the laminar flame speed, u'/s L , and by the flame heat release factor, τ? (T b - T u )/T u , where T u and T b are respectively the temperature within unburnt and burnt gas. Consistent with the Bray-Moss-Libby theory, counter-gradient (gradient) diffusion is promoted by low (high) values of u'/s L and high (low) values of τ. DNS also shows that these results are not restricted to the turbulent transport of c. Similar results are found for the turbulent transport of flame surface density, Σ. The turbulent fluxes of c and Σ are strongly correlated in the simulated flames and counter-gradient (gradient) diffusion of c always coincides with counter-gradient (gradient) diffusion of Σ.

Correlation of Flame Speed with Stretch in Turbulent Premixed Methane/Air Flames

Abstract: In the flamelet approach of turbulent premixed combustion, the flames are modeled as a wrinkled surface whose propagation speed, termed the {open_quotes}displacement speed,{close_quotes} is prescribed in terms of the local flow field and flame geometry. Theoretical studies suggest a linear relation between the flame speed and stretch for small values of stretch, S{sub L}/S{sub L}{sup 0} = 1 - MaKa, where S{sub L}{sup 0} is the laminar flame speed, Ka = {kappa}{delta}{sub F}/S{sub L}{sup 0} is the nondimensional stretch or the Karlovitz number, and Ma = L/{delta}{sub F} is the Markstein number. The nominal flame thickness, {delta}{sub F}, is determined as the ratio of the mass diffusivity of the unburnt mixture to the laminar flame speed. Thus, the turbulent flame model relies on an accurate estimate of the Markstein number in specific flame configurations. Experimental measurement of flame speed and stretch in turbulent flames, however, is extremely difficult. As a result, measurement of flame speeds under strained flow fields has been made in simpler geometries, in which the effect of flame curvature is often omitted. In this study we present results of direct numerical simulations of unsteady turbulent flames with detailed methane/air chemistry, thereby providing an alternative method of obtaining flame structure and propagation statistics. The objective is to determine the correlation between the displacement speed and stretch over a broad range of Karlovitz numbers. The observed response of the displacement speed is then interpreted in terms of local tangential strain rate and curvature effects. 13 refs., 3 figs.

Characteristics of the liquid flame spray process

Abstract: Liquid flame spraying (LFS) is a new thermal spray process. Liquid feedstock is injected and atomized in an oxygen-hydrogen flame where the liquid phase is evaporated and thermochemical reactions are completed to produce fine particles. Production of nanoparticles requires a thorough understanding of the process. Therefore, various process stages were studied; i.e., the atomization of liquid feedstock, and characterization of the flame and flame-droplet interactions. Experimental techniques included laser diffraction anemometry for droplet size distribution, laser doppler velocimetry for particle velocity, pulsed laser Rayleigh back scattering for flame temperature and Schlieren photography for flame structure. Atomization is optimized with an organic solvent, such as isopropanol, nebulized with hydrogen gas at a high flow rate. Liquid droplets injected into the flame are subjected to a maximum temperature of 2600°C and are accelerated to about 160 m s−1. The flame length can be controlled by flame velocity and the solvent type. Water produces a shorter flame whereas ispropanol extends the flame. Injection of the aerosol produces a “pencil-like” region which does not experience turbulence for most of the flame length. Experimentation with manganese nitrate and aluminium isopropoxide or aluminium nitrate showed conversion to a

The propagation of premixed flames in closed tubes

Abstract: A nonlinear evolution equation that describes the propagation of a premixed flame in a closed tube has been derived from the general conservation equations. What distinguishes it from other similar equations is a memory term whose origin is in the vorticity production at the flame front. The two important parameters in this equation are the tube's aspect ratio and the Markstein parameter. A linear stability analysis indicates that when the Markstein parameter α is above a critical value αc the planar flame is the stable equilibrium solution. For α below αc the planar flame is no longer stable and there is a band of growing modes. Numerical solutions of the full nonlinear equation confirm this conclusion. Starting with random initial conditions the results indicate that, after a short transient, a at flame develops when α>αc and it remains flat until it reaches the end of the tube. When α<αc, on the other hand, stable curved flames may develop down the tube. Depending on the initial conditions the flame assumes either a cellular structure, characterized by a finite number of cells convex towards the unburned gas, or a tulip shape characterized by a sharp indentation at the centre of the tube pointing toward the burned gases. In particular, if the initial conditions are chosen so as to simulate the elongated finger-like flame that evolves from an ignition source, a tulip flame evolves downstream. In accord with experimental observations the tulip shape forms only after the flame has travelled a certain distance down the tube, it does not form in short tubes and its formation depends on the mixture composition. While the initial deformation of the flame front is a direct result of the hydrodynamic instability, the actual formation of the tulip flame results from the vortical motion created in the burned gas which is a consequence of the vorticity produced at the flame front.

Premixed flame-wall interaction in a turbulent channel flow : budget for the flame surface density evolution equation and modelling

Abstract: Turbulent premixed flame propagation in the vicinity of a wall is studied using a three-dimensional constant-density simulation of flames propagating in a channel. The influence of the walls is investigated in terms of the flamelet approach, where flamelet speed and flame surface density transport are used to describe the flame. The walls have constant temperature and lead to flamelet quenching for sufficiently small wall-flame distances. Starting from the exact evolution equation for the surface density of propagating interfaces (Trouve & Poinsot 1994; Candel & Poinsot 1990; Pope 1988), a budget for the flame surface density equation is presented before, during, and after the interaction with the wall. Before the flame interacts with the wall, flame propagation is controlled by a balance between surface production and annihilation. During the interaction, high flame surface density gradients near the wall are responsible for the predominance of the transport terms. Closures of all terms of the flame surface density equation are proposed. These models are based on flamelet ideas and take into account wall effects. Enthalpy loss through the wall affects flamelet speed, flamelet annihilation and flame propagation. Decrease of turbulent scales near the wall affects turbulent diffusion and flame strain. This model is compared to DNS results using two types of tests: (i) a priori tests, where individual terms of the modelled flame surface density equation are compared to the terms of the exact interface density propagation equation, calculated with the DNS; (ii) a posteriori tests, where the final model is used to obtain total reaction rate, mean fuel mass fraction, heat flux at the wall and fuel mass fraction at the wall in the configuration used in the DNS. For both types of tests the model compares well with the DNS results.

A Mass-Based Definition of Flame Stretch for Flames with Finite Thickness

Abstract: The flame stretch concept is extended for the case of 3D instationary flames with finite flame front thickness. It is shown that additional contributions to the stretch rate appear apart from the terms which are usually used in flame studies. These extra terms are associated with variations in the mass density along the flame iso-contours and with variations in flame front thickness in time and space. It is finally shown that the following definition for the stretch rate is applicable: K = \/m (dm/dt), denoting the fractional change of mass in an infinitesimally small flame volume.

On the shape of flames under strong acoustic forcing : a mean flow controlled by an oscillating flow

Abstract: A conical flame, in the presence of high-frequency (≈1000 Hz) and high-amplitude acoustic modulation of the cold gases, deforms to a shape which is approximately hemispherical. It is shown that the acoustic level required to produce a hemispherical flame is such that the ratio of acoustic velocity to laminar combustion velocity is about 3. This flame flattening is equivalent to the phenomenon of acoustic restabilization observed for cellular flames propagating in tubes. The transition between the conical flame and a hemispherical flame is described. The surface area of the reaction zone of the flame is found to be unmodified when the flame flattens. The velocity field at the burner outlet is examined with and without a flame. The mean flow lines are strongly deflected when the hemispherical flame is present. We show that the presence of the flame creates an unusual situation where the oscillating flow controls the geometry of the mean flow.

Darrieus - Landau instability, growing cycloids and expanding flame acceleration

Abstract: 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...

Stirred Reactor Analysis of Cavity Flame Holders for Scramjets

Abstract: Scramjet combustor technology is currently under development by the Air Force and a key component of scramjet combustors is the flame holder. This study investigated the flame holding properties of recessed cavities in supersonic flow using numerical analysis techniques. The numerical models developed for this analysis included several perfectly stirred reactor models. A simplified analytical model indicated that an important property for flame holding was the lower limit residence time. This model also showed that under certain conditions, the solution for combustion systems was not unique. It was found that ignition delay times and lower limit residence times varied by orders of magnitude with reaction mechanism. The perfectly stirred reactor model also indicated that trace species diffusion should increase flame spreading rate, and that heat loss reduces flame holding limits. Reduced mechanisms for hydrocarbons were also shown to have orders of magnitude variation in lower residence times. The methodology developed in this research provides a design guide for the size of cavity required to provide flame holding for a scramjet combustor.

Effect of sprays of water and NaCl-water solution on the extinction of laminar premixed methane-air counterflow flames

Abstract: The response of laminar premixed fuel-lean methane-air counterflow flames to the combined effects of stretch and cooling by a fine spray of water and NaCl-water as well as surfactant-water solution is studied experimentally. Results show that flame extinction is promoted both by increased flame stretch and by a higher concentration of liquid. An unexpected finding is that at a given concentration an NaCI-water solution is significantly more effective than pure water in causing flame extinction. On the other hand, at the concentration studied, results from a spray of water containing a surfactant (Synperonic PE/L62) are indistinguishable from those with pure water.

Flame Front Curvature Distributions in a Turbulent Premixed Flame Zone

Abstract: Distributions of flame front curvature obtained by laser sheet tomography agree with those derived from numerical simulations of passive flame propagation within three-dimensional Navier-Stokes turbulence. The experimental configuration is that of grid turbulence impinging upon a plate which stabilizes a premixed methane/air flame, planar images of the flame allow construction of flame curvature as a function of flame location within the spatial zone that contains products and reactants. In the simulations the flame burning velocity is twice the turbulence intensity and the Reynolds number based on the computed Taylor length scale is approximately 55. The computed flame geometry and flame strain rate are obtained as a function of location based on the mean progress variable (defined by the passive surface displacement or by the scalar fluctuations defined over transverse planes). The shape of the mean progress variable profile compares well with experiment and with two reaction-diffusion models of propaga...

Large–Scale Dynamics of an Unconfined Precessing Jet Flame

Abstract: Visual observations, high speed movie sequences and image processing techniques have been used to examine unconfined vertical lifted turbulent diffusion flames issuing from precessing jeta (PJ) nozzles. These techniques provide qualitative information about the dynamic motions in the flame and quantitative data on the size and number of flame structures, the celerity of those structures, flame dimensions, residence times and characteristic strain rates. The information is used to provide new insight into the flame stabilisation mechanism of, and combustion processes occurring in, a PJ flame and to enable a comparison with similar studies in free turbulent jet diffusion flames and pool fires. Large-scale structures are seen to form near to the base of the PJ flame and move downstream with a slow, nearly constant speed in a manner reminiscent of the puffing motions in a pool fire. The entrainment and mixing of these structures is such that the flame tip oscillates as the residual unburned mixture i...

Experimental flame correlations and dimensional relations in turbulent ceiling fires

Abstract: Flame size and heat flux correlations were obtained by experiments for circular turbulent flame sheets developing from a downward injection source beneath an unconfined inert ceiling, and are compared against those for one-dimensional ceiling flames. These correlations show proportionality of the flame sheet area to heat release rate and the representation of flame heat transfer as a function of the distance from the source normalised by flame length. Heat release rate per unit flame sheet area in circular flames is found to be significantly smaller than that in one-dimensional flames. It suggests a weaker entrainment of ambient air to circular flames than to one-dimensional flames. Total heat flux to the ceiling surface from the flame sheet is less than 30 kW/m 2 and is not enough to accelerate flame spread. This suggests the importance of preheating of a combustible ceiling by a hot gas layer for the fast fire spread generally observed in real and experimental room fires. Dimensional analysis based on the experimental results suggests the proportionality of horizontal velocity to the distance from the upstream end of the burning surface, and faster velocity in one-dimensional flames than in circular flames.