Showing papers on "Diffusion flame published in 1997"

OPPDIF: A Fortran program for computing opposed-flow diffusion flames

Abstract: OPPDIF is a Fortran program that computes the diffusion flame between two opposing nozzles. A similarity transformation reduces the two-dimensional axisymmetric flow field to a one-dimensional problem. Assuming that the radial component of velocity is linear in radius, the dependent variables become functions of the axial direction only. OPPDIF solves for the temperature, species mass fractions, axial and radial velocity components, and radial pressure gradient, which is an eigenvalue in the problem. The TWOPNT software solves the two-point boundary value problem for the steady-state form of the discretized equations. The CHEMKIN package evaluates chemical reaction rates and thermodynamic and transport properties.

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.

Effects of pressure gradients on turbulent premixed flames

Abstract: In most practical situations, turbulent premixed flames are ducted and, accordingly, subjected to externally imposed pressure gradients. These pressure gradients may induce strong modifications of the turbulent flame structure because of buoyancy effects between heavy cold fresh and light hot burnt gases. In the present work, the influence of a constant acceleration, inducing large pressure gradients, on a premixed turbulent flame is studied using direct numerical simulations. A favourable pressure gradient, i.e. a pressure decrease from unburnt to burnt gases, is found to decrease the flame wrinkling, the flame brush thickness, and the turbulent flame speed. It also promotes counter-gradient turbulent transport. On the other hand, adverse pressure gradients tend to increase the flame brush thickness and turbulent flame speed, and promote classical gradient turbulent transport. As proposed by Libby (1989), the turbulent flame speed is modified by a buoyancy term linearly dependent on both the imposed pressure gradient and the integral length scale l t . A simple model for the turbulent flux uc is also proposed, validated from simulation data and compared to existing models. It is shown that turbulent premixed flames can exhibit both gradient and counter-gradient transport and a criterion integrating the effects of pressure gradients is derived to differentiate between these regimes. In fact, counter-gradient diffusion may occur in most practical ducted flames.

Dual-pump coherent anti-Stokes Raman scattering measurements of nitrogen and oxygen in a laminar jet diffusion flame

Abstract: Dual-pump coherent anti-Stokes Raman scattering (CARS) has been demonstrated for the simultaneous measurement of gas-phase temperature and concentrations of molecular nitrogen and oxygen. A polarization technique was used to vary the relative intensities of the two CARS signals and expand the dynamic range of the relative concentration measurements. Detailed temperature and oxygen mole fraction measurements were performed in the stabilization region of a hydrogen-nitrogen jet diffusion flame. These results indicate that there is a region below the nozzle exit where significant amounts of oxygen are found on the fuel side of the peak flame temperature profile.

Refractive Indices at Visible Wavelengths of Soot Emitted From Buoyant Turbulent Diffusion Flames

Abstract: Measurements of the optical properties of soot, emphasizing refractive indices, are reported for visible wavelengths (350-800 nm). The experiments considered soot in the fuel-lean (overfire) region of buoyant turbulent diffusion flames in the long residence time regime where soot properties are relatively independent of position in the overfire region and residence time. Flames fueled with acetylene, propylene, ethylene, and propane that were burning in still air provided a range of soot physical and structure properties. Measurements included soot composition, density, structure, gravimetric volume fraction, and scattering and absorption properties. These data were analyzed to find soot fractal dimensions, refractive indices, refractive index functions, and dimensionless extinction coefficients assuming Rayleigh-Debye-Gans scattering for polydisperse mass fractal aggregates (RDG-PFA theory). RDG-PFA theory was successfully evaluated using measured scattering properties. Soot fractal dimensions were independent of both fuel type and wavelength, yielding a mean value of 1.77 with a standard deviation of 0.04. Refractive indices were independent of fuel type within experimental uncertainties and were in reasonably good agreement with earlier measurements for soot in the fuel-lean region of diffusion flames due to Dalzell and Sarofim (1969). Dimensionless extinction coefficients were independent of both fuel type and wavelength, yielding a mean value of 5.1 with a standard deviation of 0.5, which is lower than earlier measurements for reasons that still must be explained.

Real-Time Quantitative Analysis of Combustion-Generated Polycyclic Aromatic Hydrocarbons by Resonance-Enhanced Multiphoton Ionization Time-of-Flight Mass Spectrometry.

Abstract: We have combined resonance-enhanced multiphoton ionization (REMPI) time-of-flight mass spectrometry with on-line flame sampling to determine the centerline concentrations of naphthalene, fluorene, and anthracene in a pure methane + oxygen/argon (1:5) diffusion flame. Naphthalene concentrations between 100 parts per billion by volume (ppbV) and 6 parts per million by volume (ppmV) and fluorene concentrations below 50 ppbV are determined using one-color REMPI on jet-cooled samples extracted from the flame; anthracene concentrations in the 5−40 ppbV range are determined using two-color REMPI. The REMPI ion signals are converted to absolute concentrations in real time by performing gas-phase standard additions to the flame sample. Isomer-selective detection of larger polycyclic aromatic hydrocarbons, such as perylene and benzo[a]pyrene, is possible using the two-color REMPI approach.

Extinction of diffusion flames with nonunity Lewis numbers

Abstract: Quasisteady extinction of diffusion flames with nonunity Lewis numbers is analyzed for counterflow diffusion flames in the diffusion-flame regime of activation-energy asymptotics. Particular attention is placed on an excess or deficiency of the total energy in the reaction region, associated with leakage of the reactants. If the Lewis number is less than unity, there is diminished diffusive loss of thermal energy that leads to an increase of the total energy in the reaction zone as reactants penetrate. The resulting excess total energy strengthens the chemical reaction, so that the flame becomes more robust and resistant to extinction. On the other hand, flames with Lewis numbers greater than unity are found to extinguish more easily. An extinction criterion is provided that is valid for nonunity Lewis numbers.

Comparison of different radiation treatments for a one-dimensional diffusion flame

Abstract: A comparison of several radiative heat transfer models is made for a stagnation-point diffusion flame at low stretch rate, with CO2 and H2O as the participating media. Computed results of the radiative source distribution for wideband, narrowband and SLWSGG show reasonable agreement with each other. Results from the optically thin and grey gas models with Planck mean absorption coefficient are shown to underestimate the self-absorption and overestimate the emission substantially for the low stretch flame. The relative computation times of using the various radiation models are also given.

The Effect of Buoyancy on Flickering in Diffusion Flames

Abstract: Flame flickering of jet diffusion flames reflects the instability of the hot gases ascending column, This phenomenon, which is related to buoyancy, has been characterized in terms of pressure and gravitational force by experiments conducted on the ground and on board of an aircraft. The measurements show that the relationship between the frequency F and the gravity g is very close to F∼ & g2/3 when the burners have a sufficiently small outlet diameter. These measurements are entirely in agreement with a theoretical model based on the hydrodynamic instability of the layer of burnt gas under the influence of buoyancy. It is also confirmed, by varying the ambient pressure, that a model of the form F ∼ P1/3 closely fits the observed phenomena with gas jet diffusion flames.

Counter-gradient diffusion in a confined turbulent premixed flame

Abstract: In the present study, we develop a full second order model and a new flamelet closure relation for the mean chemical source term to describe turbulent reactive flows with premixed reactants. This description ensures that non-gradient and counter-gradient diffusion which can appear in such flames, are taken into account in 2-D or 3-D geometries. 2-D calculations are performed in the case of a confined turbulent premixed flame stabilized by a backward facing step, a configuration which reveals experimentally counter-gradient diffusion and combustion induced production of turbulence. Numerical results are found to be in excellent agreement with measurements of Shepherd et al. [Nineteenth Symposium (International) on Combustion (The Combustion Institute, Pittsburgh, 1982)]. The important influence of counter-gradient diffusion on first order quantities, i.e., mean velocity and length of the recirculation zone is shown.

Scalar Dissipation Measurements in Turbulent Jet Diffusion Flames of Air Diluted Methane and Hydrogen

Abstract: Simultaneous two-dimensional Rayleigh and fuel Raman images have been collected in air-diluted methane and hydrogen jet diffusion flames. Temperature, fuel mass fraction and mixture fraction images are derived by a two-scalar approach based on one-step chemistry and equal species diffusivities. This enables calculation of two components of the scalar dissipation rate x-The inherently weak Raman signal has been maximised by intra-cavity measurements, using a flashlamp-pumped dye laser. In addition, the Raman signal-to-noise ratio is drastically improved by a novel contour-aligned smoothing technique which exploits the high correlation between the Rayleigh and Raman signals. Quantitative measurements of scalar dissipation are presented, including probability density functions for components of x- Profiles of mean and rms mixture fraction show the usual features already documented in other published results for this type of flame. Probability density functions of ξ are close to Gaussian on the axis,...

Synthesis of SiO2 and SnO2 particles in diffusion flame reactors

Abstract: Silica and stannic (tin) oxide powders were synthesized by oxidation of their respective chlorides in single and double (inverse) diffusion flame reactors. The effect of reactant gas mixing on the characteristics of these powders (size and morphology) was investigated by altering the position of the fuel (CH 4 ) and oxidant (air or O 2 ) streams in the burner. Reactant gas mixing plays a key role in controlling particle size since it affects the temperature history, residence time, and initial particle concentration in the flame, thus, yielding a simple technique for particle-size control over a wide size range in flame reactors. The different material properties (such as sintering) of silica and stannic oxide result in particles of different size and morphology, although they were made at nearly identical flame conditions. Moreover the oxidant composition affects significantly the properties of silica particles.

Experimental Assessment of Naphthalene Formation Mechanisms in Non-Premixed Flames

Abstract: Concentration profiles of stable hydrocarbons were measured along the centerline of several antisymmetric co-flowing methane/air non-prcmixed flames whose fuels were doped with 2500 to 1900 ppm of benzene, toluene, elhylbenzene. styrene. and phenylacctylene. The results indicate that the H-abstraction/C2H2,-addition mechanism proposed by Frenklach and co-workers was responsible for the naphthalene formed in the undoped, benzene-, styrene-, and phenylacetylene-doped flames. However, a second source of naphthalene, possibly a reaction between benzyl and propargyl radicals, was important in the toluene- and ethylbenzene-doped flames. Soot volume fractions were also measured in each flame; the maximum values correlated well with the maximum naphthalene concentrations, indicating that naphthalene formation is the critical soot formation step for these fuel mixtures containing one-ring compounds.