Showing papers on "Diffusion flame published in 1996"

A numerical study on flame stability at the transition point of jet diffusion flames

Abstract: Flame stability of a fuel jet diffusion flame was studied numerically by using finite rate chemistry. The flow is time dependent and plane two-dimensional, and the chemical reaction is described by simplified, overall one-step kinetics. The variable parameters are the jet Reynolds number, P Rc o and Damkohler number, Da o : and three types of flame stability behavior were observed depending on values of these parameters. The first one is the local and occasional extinction at the transition point from a laminar to a turbulent flame. When Re 0 is kept at a rather high value and Da 0 is decreased, local extinction at the transition point begins to occur at a certain critical value. The occasional extinction is caused at the instant when the local scalar dissipation rate in the reaction zone becomes too large, producing a rupture in the reaction zone layer. The rupture is quickly connected again to recover the continuous reaction zone layer. As Da 0 is decreased further, however, the frequency of rupture increases, and at another critical value, complete extinction is produced at the transition point, leaving a short, residual rim flame immediately downtream of the injector. This is the second type of flame stability. As Da 0 is decreased further, the third and final stability characteristic is observed: the blow-off of the whole flame from the injector rim. When the flme is extinguished completely at the transition point, most of the injected fuel flows downstream as a fuel jet entraining the surounding air to produce a lifted, turbulent diffusion flame in the downstream flow. This study of the structure of the flame has shown that it is actually an ensemble of instantaneous local premixed, diffusion., and partially premixed flames.

A Numerical Study of a Bluff-Body Stabilized Diffusion Flame. Part 2. Influence of Combustion Modeling And Finite-Rate Chemistry

Abstract: This is the second and last part of a paper on numerical prediction of a bluff-body stabilized turbulent diffusion flame of syngas and air. Part 1 investigates the effect of the turbulence model an...

Flame liftoff in diesel sprays

Abstract: Flame liftoff and stabilization in nonstationary n-heptane sprays is studied for diesel engine-like conditions using a numerical simulation involving complex chemistry and a novel subgrid stirred reactor model of turbulence-chemistry interaction. By following ignition and flame formation processes, it is shown that the flame stabilizes at a certain point due to the upstream propagation of a triple flame where the leading edge is at the stoichiometric surface and the combustion occurs both on the lean and on the rich side. Due to the evaporative cooling, large injection velocities, and the strong flame stretch, the stabilization distance is very large. This allows a considerable amount of air to enter the central part of the flame to feed the internal rich flame, and a large amount of fuel to escape combustion in the stoichiometric flame and support the external lean flame. Both soot and NOx emissions reduction are expected to follow for large liftoff distances (i.e., high pressure and small orifice injection). The flame stabilization mechanism is a result of complex physical and chemical interactions and cannot be described by a simplified theory but to the leading order is determined by the chemical reaction time at the leading edge, the turbulent diffusivity, and the flow velocity so that there exists a balance between the local convection velocity and the triple flame propagation speed. The unburned/burned gas density ratio determing the shape of the leading edge of the flame is important in this process. Due to fast evaporation, the spray properties have little effect on flame stabilization except for the heat of evaporation affecting the temperature at the stoichiometric surface and the combustion kinetics of the fuel. Liftoff trends are studied, and available data on liftoff distances are compared with the predictions. The accuracy achieved is good.

Influence of hydrogen addition to fuel on temperature field and soot formation in diffusion flames

Abstract: Overventilated, coflowing axisymmetric laminar diffusion flames of ethylene, propane, and butane were used to study the influence on soot of hydrogen addition to the fuel. The flame temperatures were measured by CARS along the flame axis as well as at off-axis radial locations. CARS spectra taken in heavily sooting regions exhibited poor fits due to C 2 absorbtion of part of the fundamental band of the nitrogen spectrum. It was found that C 2 absorbtion was confined to frequencies greater than 2313 cm −1 . We, therefore, implemented a strategy that fitted only the CARS spectra in the frequency range smaller than 2313 cm −1 . Measured soot concentrations and the flame temperature data with and without hydrogen and helium dilution were evaluated and the relative influences of dilution and direct chemical interaction on soot formation, as a result of hydrogen addition, are presented. It is shown that when hydrogen or helium is added to the fuel as a diluent in moderate quantities, the changes in the temperature field of the coflow diffusion flames are negligible. When allowance is made for the influence of dilution, addition of hydrogen to the fuel side of an ethylene diffusion flame reduces the soot formation. For propane and butane flames, hydrogen addition does not show any influence on soot formation apart from the dilution effect.

Experimental analysis of flame surface density models for premixed turbulent combustion

Abstract: Flame surface density (FSD) balance equations are now used widely to model turbulent combustion. The FSD transport equation may be derived from first principles but needs closure assumptions. These closure rules may be deduced from detailed analysis, direct numerical simulations, and experiments. The last approach is exploited in this work by performing detailed measurements on a two-dimensional turbulent premixed propane-air V flame. Velocity profiles are obtained from laser Doppler velocimetry. The flame front is visualized by high-speed laser sheet imaging. Results are then processed to extract flame front characteristics (flame surface density, flame front normal vector, curvature). Emission from CH radicals provides an estimate of the reaction rate. First, the flame front dynamics, modified by the thermal expansion, are analyzed in terms of turbulent transport showing the occurrence of counter-gradient turbulent diffusion. Important terms of the transport equation for flame surface density are then examined. The strain rate acting on the flame surface is controlled mainly by thermal expansion. The flame orientation factors may be closed from Reynolds shear stresses as proposed by Mantel and Borghi. A new model is devised for the curvature and propagation terms that act as source terms on the fresh gas side and sink terms in the burned stream. The stabilization region behind the flame holder needs a specific description. In this region, the surface density takes large values but the reaction rate remains low because of the chemical induction time.

OH radical imaging in a DI diesel engine and the structure of the early diffusion flame

Abstract: Laser-sheet imaging studies have considerably advanced our understanding of diesel combustion; however, the location and nature of the flame zones within the combusting fuel jet have been largely unstudied. To address this issue, planar laser-induced fluorescence (PLIF) imaging of the OH radical has been applied to the reacting fuel jet of a direct-injection diesel engine of the ``heavy-duty`` size class, modified for optical access. An Nd:YAG-based laser system was used to pump the overlapping Q{sub 1}9 and Q{sub 2}8 lines of the (1,0) band of the A{yields}X transition at 284.01 nm, while the fluorescent emission from both the (0,O) and (1, I) bands (308 to 320 nm) was imaged with an intensified video camera. This scheme allowed rejection of elastically scattered laser light, PAH fluorescence, and laser-induced incandescence. OH PLIF is shown to be an excellent diagnostic for diesel diffusion flames. The signal is strong, and it is confined to a narrow region about the flame front because the threebody recombination reactions that reduce high flame-front OH concentrations to equilibrium levels occur rapidly at diesel pressures. No signal was evident in the fuel-rich premixed flame regions where calculations and burner experiments indicate that OH concentrations will be below detectable limits. Temporal sequences of OH PLIF images are presented showing the onset and development of the early diffusion flame up to the time that soot obscures the images. These images show that the diffusion flame develops around the periphery of the-downstream portion of the reacting fuel jet about half way through the premixed burn spike. Although affected by turbulence, the diffusion flame remains at the jet periphery for the rest of the imaged sequence.

Spectral extinction coefficients of soot aggregates from turbulent diffusion flames

Abstract: The spectral extinction coefficients of soot aggregates were studied in the fuel-lean (overfire) region of buoyant turbulent diffusion flames. Extinction measurements were carried out in the wavelength region of 0.2-5.2 μm for flames fueled with acetylene, propylene, ethylene, and propane, burning in air. The present measurements were combined with earlier measurements of soot morphology and light scattering at 0.514 μm in order to evaluate the spectral soot refractive indices reported by Dalzell and Sarofim (1969), Let and Tien (1981), and Chang and Charalampopoulos (1990). The specific extinction coefficients and emissivities were predicted based on Rayleigh-Debye-Gans theory for polydisperse fractal aggregates, which has been recently found to be the best approximation to treat optical cross sections of soot aggregates. The results indicated that available refractive indices of soot do not predict the spectral trends of present measurements in the ultraviolet and infrared regions. Soot complex refractive index was inferred to be m = 1.54 + 0.48i at 0.514 μm, which is surprisingly in best agreement with the values reported by Dalzell and Sarofim (1969). Additionally, specific extinction coefficients of soot aggregates varied with wavelength as λ -0.83 from the visible to the infrared. Finally, soot refractive indices were found to be relatively independent of fuel type for the visible and infrared spectral regions over the H/C ratio range of 0.08-0.22.

Flame surface density and burning rate in premixed turbulent flames

Abstract: The flame surface density has been measured in hydrocarbon/air stagnation point and v-shaped premixed turbulent flames. A method is proposed to determine the flame surface density from the data obtained by laser sheet tomography. The average flame length and flame zone area as a function of the progress variable are calculated from a map of progress variable and a set of flame edges obtained from the tomographs. From these results a surface density estimate in two dimensions is determined. By this technique it is possible to avoid the difficulties which arise when using an algebraic model based on the measurement of the flame front geometry and a scalar length scale. From these results the burning rate can be obtained which compares well with estimates calculated using the fractal technique. The present method, however, is not constrained by a minimum window size as is the case for the fractal determinations.

The Morphology of Macroscopic Soot

Abstract: The morphology of soot collected from a laminar acetylene/air diffusion flame was studied. Collection methods included both thermophoretic and impaction sampling from both the luminous and nonluminous portions of the flow. The soot was viewed with both electron and optical microscopy. Cluster sizes ranged over four orders of magnitude from 50 nm to 400 yam to include some clusters visible to the naked eye. A new method of micrograph analysis, necessary when the clusters were large, was developed to account for the unresolved primary particles. Over this entire size range, the same fractal morphology was found with a fractal dimension of D = 1.8 and, within a rather large uncertainty, the same prefactor k0=1.7. Thus, the fractal morphology of soot remains constant from clusters of about 10 primary particles per aggregate to macroscopic clusters of over 108 primary particles.

Detailed Kinetic Modelling of Toluene Combustion

Abstract: A detailed chemical kinetic mechanism for the combustion of toluene has been assembled and evaluated for a wide range of combustion regimes. The latter include counterflow diffusion flames, plug flow reactors, shock tubes and premixed flames. The reaction mechanism features 743 elementary reactions and 141 species and represents an attempt to develop a chemical kinetic mechanism applicable to intermediate and high temperature oxidation. Toluene thermal decomposition and radical attack reactions leading to oxygenated species are given particular attention. The benzyl radical sub-mechanism is expanded to include izomerization and thermal decomposition reactions, which are important at flame temperatures, and a molecular oxygen attack path to form the benzylperoxy radical, which is found to be relevant at lower temperatures, The final toluene kinetic model results in excellent fuel consumption profiles in both flames and plug flow reactors and sensible predictions of the temporal evolution of the hydrogen ra...

Comprehensive two-dimensional soot diagnostics based on laser-induced incandescence (LII)

Abstract: A strategy for a comprehensive description of soot sizes and concentrations is described, which relies on various two-dimensional techniques based on laser-induced incandescence (LII) and elastic scattering. Time-resolved LII (TIRE-LII) is an important tool in this process, it allows a direct measurement of primary particle size dp as a central quantity for the description of soot. The paper discusses the essential features of the method which is based on the acquisition of the LII signals at two delay times after an initial laser pulse, where monomer size can be deduced from the local signal ratio. Delays of about 100 and 800 ns should be chosen as moments of observation in order to obtain precise information for a wide range of primary particle sizes. Local soot volume fractions f, may be directly obtained from the prompt LII signal, where a necessary calibration factor can be deduced from a simple line-of-sight extinction measurement. In combination with elastic scattering, this technique can also be used to give relative values for the volume-equivalent diameter D of soot clusters. Additionally, the local mean number n of monomers within a cluster and number concentrations Np and Na of primary particles and aggregates, respectively. can be determined. Although some uncertainties persist regarding the absolute monomer sizes measured, and some data can be given only on a relative basis, experimental results obtained for the upper region of a laminar ethene diffusion flame are in good agreement with the established models of soot oxidation and aggregation.

Premixed flame response to unsteady strain-rate and curvature

Abstract: The interaction of a premixed stoichiometric methane-air flame with a counter-rotating vortex-pair is studied using a skeletal C{sub 1} chemical description of the reaction process. The focus is on the modification to flame structure and dynamics due to unsteady strain-rate and curvature. The detailed description of flame structure and dynamics in response to unsteady flow is necessary to establish relevant extinction criteria in unsteady multi-dimensional flow, which, based on recent experimental evidence, may be significantly different from those of steady one-dimensional counterflow stagnation flames. Present results suggest that the increasing unsteady tangential strain-rate causes modification of flame structure that leads to reduced reaction rates of key chain-branching reactions which are active on the products side of the flame. This causes a reduction in the concentrations of active radicals, such as H, OH, and O, which are necessary for the breakdown of hydrocarbons on the reactants side of the flame.

Influence of gravity and pressure on pool fire-type diffusion flames

Abstract: Experiments are conducted to study the effects of gravity and pressure on the characteristics of diffusion flames of the pool fire type, that is, a diffusion flame stabilized on a burning horizontal fuel surface. In the experiments, the pool fire is simulated by injecting at very low velocity, a gaseous fuel (ethane) through a small-scale, porous, flat, horizontal surface burner and generating a diffusion flame over the burner by the reaction of the gaseous fuel with air. The resulting diffusion flame is characterized by a low Froude number. The diffusion flame characteristics (visual appearance, height, radiant output and temperature, and velocity distribution) are investigated at gravity levels ranging from microgravity (parabolic trajectory of an aircraft) to 12 times normal gravity (centrifuge facility—atmospheric pressure), and at ambient pressures ranging from 0.03 to 0.3 MPa (normal gravity). The results provide information about the effects of these variables on the flame characteristics and data for validation of numerical models of diffusion flames. Furthermore, they also help in understanding some of the limitations of Froude, or pressure, modeling of fires. The experiments indicate that the effect of gravity and pressure on the flame characteristics appears primarily through their effect on the buoyantly induced entrainment of air by the flame plume. Although at elevated pressures the effects are similar on the flame size and shape, important differences are observed on their effect on soot formation. It is found that for pressures above atmospheric, pressure has a major influence in soot formation and, consequently, on the radiant characteristics of the flames, increasing as pressure is increased. It is also found that at pressures below atmospheric pressure and gravity have opposite effects on flame size and soot formation and that consequently their effects on flame radiation also differ.

A Numerical Study of a Bluff-body Stabilized Diffusion Flame. Part 1. Influence of Turbulence Modeling and Boundary Conditions

Abstract: This is the first part of a paper on numerical prediction of a bluff-body stabilized turbulent diffusion flame of syngas and air. This part considers the influence of turbulence modeling and boundary conditions on the predictions. Part 2 investigates the effect of the turbulence-chemistry interaction model and the effect of finite-rate chemistry. Results based on the “standard” k-s model and a Reynolds-stress-equation (RSE) model are compared. Measurements are taken from the literature. The RSE model predicts results in better agreement with the measurements than the k-e model. The two models predict significantly different composition and temperature levels in the recirculation bubble created by the bluff body. The specification of the turbulence level in the fuel-jet has a substantial influence on the axial decay of mixture fraction. Grid-resolution studies show that a relatively coarse grid is capable of representing the present flow with sufficient accuracy to evaluate the various sub-models.

Characterization of Turbulent hytVAir Jet Diffusion Flames by Single-Pulse Spontaneous Raman Scattering

Abstract: A spontaneous Raman scattering apparatus based on a flashlamp-pumped dye laser has been developed for single-pulse temperature and concentration measurements of very high accuracy. Comprehensive data sets from measurements in two turbulent H2/ N2/ air jet diffusion flames are presented comprising mean values and fluctuations of the temperature and major species concentrations. Differences in the behaviour of an attached and lifted flame, operated under identical flow conditions and fuel composition, are shown. The effect of differential diffusion and its dependence on flow velocity are investigated in detail. One of the main aims of this work is the establishment of data sets which are suitable for testing mathematical flame models. As an example, the experimental results from one flame are compared to theoretical predictions from a Reynolds-Stress model.

Experimental and numerical studies of two-stage methanol flames

Abstract: A laminar counterflow configuration is investigated in which a fuel-rich methanol spray is transported by air against an apposing air stream. The fuel-stream equivalence ratio ranges from 1.6 to 3.0, and the fuel-side strain rate from 50 s −1 to 100 s −1 . Under these conditions, there is a vaporization plane in the fuel stream at which the spray disappears, a pale green fuel-rich premixed flame in the fuel stream, and a brighter blue diffusion flame in the vicinity of the stagnation plane. Temperature profiles are measured by thermocouples, and concentration profiles of stable species are measured by gas chromatography of samples withdrawn by a fine probe. Computational methods are employed to calculate the flame structure, with detailed chemistry and transport included. Chemical-kinetic descriptions available in the literature predict the premixed flame to be more than 0.5 mm closer to the diffusion flame than observed experimentally and give nearly twice the measured peak CH 4 concentration and more than twice the measured concentration of C 2 H 2 in the premixed flame. Modification of the rate data by introducing a temperature-dependent branching ratio to the isomers CH 3 O and CH 2 OH, in the H attack on CH 3 OH, patterned after the known variation in the OH attack and by including the step CH 3 O+M→CH 2 OH+M, produces good agreement between all experimental and computational results. A reaction path for methanol in these flames is suggested, including routes to CH, important for prompt NO formation.