Showing papers on "Diffusion flame published in 2000"

Large-eddy simulation of a turbulent piloted methane/air diffusion flame (Sandia flame D)

Abstract: The Lagrangian Flamelet Model is formulated as a combustion model for large-eddy simulations of turbulent jet diffusion flames. The model is applied in a large-eddy simulation of a piloted partially premixed methane/air diffusion flame (Sandia flame D). The results of the simulation are compared to experimental data of the mean and RMS of the axial velocity and the mixture fraction and the unconditional and conditional averages of temperature and various species mass fractions, including CO and NO. All quantities are in good agreement with the experiments. The results indicate in accordance with experimental findings that regions of high strain appear in layer like structures, which are directed inwards and tend to align with the reaction zone, where the turbulence is fully developed. The analysis of the conditional temperature and mass fractions reveals a strong influence of the partial premixing of the fuel.

Morphology and burning rates of expanding spherical flames in H2/O2/inert mixtures up to 60 atmospheres

Abstract: Recognizing that previous experimental studies on constant-pressure, outwardly propagating, Spherical flames with imaging capability were limited to pressures less than about 5 atm, and that pressures within internal combustion engines are substantially higher, a novel experimental apparatus was designed, to extend the environmental pressure to 60 atm. Results substantiate previous observations of the propensity of cell formation over the flame surface due to hydrodynamic and diffusive-thermal instabilities and provide convincing evidence that wrinkled flame is the preferred mode of propagation in hydrogen/air mixtures in environments with pressures above only a few atmospheres. It is further shown that, by using helium as the diluent, and by reducing the oxygen concentration of the combustible, diffusional-thermal instability can be mostly suppressed and the hydrodynamic instability delayed. Stretch-free laminar flame speeds were subsequently determined for such smooth flames up to 20 atm and were compared with the calculated values, allowing for detailed chemistry and transport.

Optical Properties in the Visible of Overfire Soot in Large Buoyant Turbulent Diffusion Flames

Abstract: Nonintrusive measurements of the optical properties of soot at visible wavelengths (351.2-800.0 nm) were completed for soot in the overfire region of large (2-7 kW) buoyant turbulent diffusion flames burning in still air at standard temperature and pressure, where soot properties are independent of position and characteristic flame residence time for a particular fuel. Soot from flames fueled with gaseous (acetylene, ethylene, propylene, and butadiene) and liquid (benzene, cyclohexane, toluene, and n-heptane) hydrocarbon fuels were studied. Scattering and extinction measurements were interpreted to find soot optical properties using the Rayleigh-Debye-Gans/polydisperse-fractal-aggregate theory after establishing that this theory provided good predictions of scatter-ing patterns over the present test range. Effects of fuel type on soot optical properties were comparable to experimental uncertainties. Dimensionless extinction coefficients were relatively independent of wavelength for wavelengths of 400-800 nm and yielded a mean value of 8.4 in good agreement with earlier measurements. Present measurements of the refractive index function for absorption. E(m), were in good agreement with earlier independent measurements of Dalzell and Sarofim and Stagg and Charalampopoulos. Present values of the refractive index function for scattering, F(m), however, only agreed with these earlier measurements for wavelengths of 400-550 nm but otherwise increased with increasing wavelength more rapidly than the rest. The comparison between present and earlier measurements of the real and imaginary parts of the complex refractive index was similar to E(m) and F(m).

An experimental estimation of mean reaction rate and flame structure during combustion instability in a lean premixed gas turbine combustor

Abstract: The interactions between the local flame structure produced by a premixed swirl-stabilized injector with combustion instabilities were experimentally studied for a model gas turbine combustor operating at high pressure and temperature. The model gas turbine combustor studied utilizes a sudden-expansion dump combustor with a single swirler and bluff body for enhancing mixing rate and flame stabilization, respectively. Laser-based measurements were made for both stable and unstable operating conditions. The local flame front structure was visualized using planar laser-induced fluorescence (PLIF) from the OH⊙, and the global heat release zone was interpreted from flame emission measurements. For stable combustion conditions, the mean reaction rate estimated independently from both OH-PLIF and OH * chemiluminescence measurements showed good agreement, thereby indicating confidence in the use of OH-PLIF measurements for extracting the local mean reaction rate. For unstable combustion conditions, the flamefront characteristics, including flame surface density and mean reaction rate, were evaluated together with the information from the OH * chemiluminescence measurements to identify the boundary of the heat release region at discrete phases of the unstable flame. Analysis of the flame structures during combustion instability indicated significant variations during different phases of the instability. The heat release flow field, particularly in the recirculation regions appearing at the corner and inner face of the dump plane, varied substantially. Rayleigh index information indicated that the recirculation zones play an important part in driving the instability. In contrast, the high shear layer formed along the interface between reactants and hot products produced a region where the instability was damped due to a lowering of the heat release.

Scalar mixing and dissipation rate in large-eddy simulations of non-premixed turbulent combustion

Abstract: Predictions of scalar mixing and the scalar dissipation rate from large-eddy simulations of a piloted nonpremixed methane/air diffusion flame (Sandia flame D) using the Lagrangian-type flamelet model are presented. The results obtained for the unconditionally filtered scalar dissipation rate are qualitatively compared with general observations of scalar mixing from experiments in non-reactive and reactive jets. In agreement with experimental data, provided the reaction zone has an inward direction, regions of high scalar dissipation rate are organized in layerlike structures, inwardly inclined to the mean flow and aligned with the instantaneous reaction zone. The analysis of single-point time records of the mixture fraction reveals ramplike structures, which have also been observed experimentally and are believed to indicate large-scale turbulent structures. The probability density function (pdf) of the instantaneous resolved scalar dissipation rate at stoichiometric mixture evaluated at cross sections normal to the the nozzle axis is shown to be described accurately by a lognormal pdf with σ=1. A new model for the conditionally averaged scalar dissipation rate has been proposed and is shown to account for local deviations from the simple mixing layer structure. The stabilizing effect of the pilot flame in the present configuration is also discussed. Finally, the influence of the resolved fluctuations of the scalar dissipation rate on the flame structure is investigated, revealing only a weak influence on temperature and nitric oxide predictions. However, the model requires further refinement for situations in which local extinction events become important.

Unsteady Flamelet Modeling of Soot Formation in Turbulent Diffusion Flames

Abstract: The unsteady flamelet model is applied in a numerical simulation of soot formation in a turbulent C2H4 jet diffusion flame. A kinetically based soot model is used, which relies on a detailed kinetic mechanism to describe the formation of small polycyclic aromatic hydrocarbons. To describe the formation, growth, and oxidation of soot particles, flamelet equations for the statistical moments of the panicle size distribution are derived. Since the effective Lewis number of large panicles tends to infinity, a formulation is given, which allows the investigation of the effect of different diffusion coefficients of the particles on the soot formation process. The results of the calculation are compared to experimental data, showing very good agreement for the temperature, which is shown to depend strongly on soot and gas radiation. The predicted soot volume fraction compares reasonably well with the measured data, if differential diffusion of the panicles is considered, Calculations with unity particle...

A numerical study of the laminar flame speed of stratified methane/air flames

Abstract: Freely propagating laminar methane/air flames were modeled for spatially stratified equivalence ratio conditions at atmospheric pressure. The GRI 2.11 mechanism was used with a modified version of the Lawrence Livermore National Laboratory HCT code to describe the reaction kinetics. Results showed that the laminar flame speed was strongly affected by the equivalence ratio gradient and by the burned gas composition and temperature. Production of molecular hydrogen from the original fuel and its transport to the reaction zone as well as heat transfer from the burned to the fresh gases are key factors in understanding the influence of stratification on laminar flame speed. Combinations of different mixture stratification conditions can either enhance or reduce the flame velocity when compared with homogeneous mixtures. Mixture stratification is also responsible for higher flame resistance to extinction both on the lean and rich sides of the equivalence ratio. The importance of heat and mass transfer on the observed results implies that their extrapolation to high pressure and to turbulent systems must be made with care.

Raman-Rayleigh-LIF measurements of temperature and species concentrations in the Delft piloted turbulent jet diffusion flame

Abstract: We report a series of Raman-Rayleigh-LIF measurements in two turbulent natural-gas jet diffusion flames produced by the Delft piloted jet diffusion flame burner. The main objective of the Raman-Rayleigh-LIF measurements was to obtain detailed information on the major species concentrations in the flames. The measurements provide simultaneous data on temperature, the concentrations of the major species and the radicals OH and NO and mixture fraction. The application of the Raman technique in the undiluted natural-gas flames proves to be very challenging because of the high fluorescence interference levels. The interference contributions to the recorded Raman signals are identified and subtracted using empirical correlations between the Raman signals and the signals on fluorescence interference monitor channels. The calibration and data reduction of the Raman-Rayleigh and LIF signals are discussed in detail. The resulting dataset compares excellently with data from previous experiments. Because the Raman-Rayleigh-LIF data provide quantitative concentrations and accordingly quantitative mixture fractions, they form a valuable and useful extension of the existing database for the Delft piloted jet diffusion flame burner.

Experiments on the instability modes of buoyant diffusion flames and effects of ambient atmosphere on the instabilities

Abstract: Large scale dynamic behavior of buoyant diffusion flames were studied experimentally. It was found that buoyant diffusion flames originating from circular nozzles exhibit two different modes of flame instabilities. The first mode results in a sinuous meandering of the diffusion flame, characteristic of flames originating from small diameter nozzles. This instability originates at some distance downstream of the nozzle exit in the contraction region of the buoyant flame envelope and develops into a sinuous motion of the flame. The second mode is the varicose mode which develops very close to the nozzle exit as axisymmetric perturbations of a contracting flame surface. In this mode, flame oscillations result in the formation of toroidal vortical structures that convect through the flame and cause periodic burn out at the flame top resulting in the observed flame height fluctuations. The average flame heights are found to be typically shorter for these flames. The oscillation frequencies and their scaling for the two modes are also different with the sinuous mode having higher frequencies than the varicose mode. It was also observed that the instability can switch from one mode to the other and the probability of observing the varicose mode appears to increase with increasing Richardson number. Additionally, the feasibility of altering the behavior of buoyant diffusion flames was explored through variation of the oxidizer medium density. It was found that the flame oscillations can be completely suppressed for flames burning in helium rich helium–oxygen mixtures. At lower helium concentrations, the oscillation frequency can be significantly reduced. In order to enhance the buoyancy effect, CO2–O2 mixtures were also studied. However, the density increase and its effects on flame oscillation frequency were found to be small compared to those flames burning in air. These experiments point towards the feasibility of altering buoyant flame behavior under earth gravity and studying the large scale dynamical aspects of buoyant flames without the need of variable gravity environment.

Numerical modeling of turbulent premixed flames in the thin-reaction-zones regime

Abstract: A new flamelet model has been developed for large-eddy simulation of turbulent premixed cornbustion in the thin-reaction-zones regime. In this regime of turbulent combustion, turbulent eddies smaller than the flame thickness exist and this small-scale turbulence disturbs the flame structure. Recent experimental observations show that turbulence interacts mostly with the chemically inert preheat zone while the thin reaction zone of the flame where all chemical processes occur remains unchanged. Therefore, flamelet models can be extended to this regime. The present model accounts for the different type of interactions between the flame and turbulence of different scales. The large-scale (larger than the flame thickness) turbulence increases the turbulent burning velocity by wrinkling the flame. On the other hand, the small-scale (smaller than the flame thickness) turbulence modifies the laminar flame propagation by enhancing the intensity of transport processes within the preheat zone. The develope...

Dynamic Response of Strained Premixed Flames to Equivalence Ratio Gradients

Abstract: Premixed flames encounter gradients of mixture equivalence ratio in stratified charge engines, lean premixed gas-turbine engines, and a variety of other applications. In cases for which the scales—spatial or temporal—of fuel concentration gradients in the reactants are comparable to flame scales, changes in burning rate, flammability limits, and flame structure have been observed. This paper uses an unsteady strained flame in the stagnation point configuration to examine the effect of temporal gradients on combustion in a premixed methane/air mixture. An inexact Newton backtracking method, coupled with a preconditioned Krylov subspace iterative solver, was used to improve the efficiency of the numerical solution and expand its domain of convergence in the presence of detailed chemistry. Results indicate that equivalence ratio variations with timescales lower than 10 ms have significant effects on the burning process, including reaction zone broadening, burning rate enhancement, and extension of the flammability limit toward learner mixtures. While the temperature of a flame processing a stoichiometric-to-lean equivalence ratio gradient decreased slightly within the front side of the reaction zone, radical concentrations remained elevated over the entire flame structure. These characteristics are linked to a feature reminiscent of “back-supported” flames—flames in which a stream of products resulting from burning at higher equivalence ratio is continuously supplied to lower equivalence ratio reactants. The relevant feature is the establishment of a positive temperature gradient on the products side of the flame which maintains the temperature high enough and the radical concentration sufficient to sustain combustion there. Unsteadiness in equivalence ratio produces similar gradients within the flame structure, thus compensating for the change in temperature at the leading edge of the reaction zone and accounting for an observed “flame inertia”. For sufficiently large equivalence ratio gradients, a flame starting in a stoichiometric mixture can burn through a very lean one by taking advantage of this mechanism.

Flamelet modeling of lifted turbulent methane/air and propane/air jet diffusion flames

Abstract: The stabilization mechanism of lifted turbulent jet diffusion flames is a test problem for models of partially premixed turbulent combustion. In these flames, combustion processes occur in both the nonpremixed and the premixed mode. For the flame stabilization process, however, flame propagation of the premixed branches seems to play a crucial role. In this paper, a flamelet model for partially premixed turbulent combustion is presented that combines flamelet models for non-premixed and premixed combustion. A new model for the turbulent burnign velocity in partially premixed flows is proposed. It is based on a formulation for a conditional turbulent burning velocity which depends on mixture fraction. The effect of partially premixing is taken into account by using the presumed probability density function (pdf) approach in terms of the mixture fraction. Mean scalar quantities on both sides of the premixed flame front are calculated in the same way. From a computational point of view, the model has the advantage that the calculation of the chemical processes can be decoupled from the flow calculation, allowing for simulations of realistic configurations, yet retaining detailed chemistry. The model is used to simulate the stabilization process of turbulent methane/air and propane/air jet diffusion flames. The calculated lift-off heights compare favorably with experimental data from various authors.

Effects of flow transients on the burning velocity of laminar hydrogen/air premixed flames

Abstract: The effects of unsteady strain rate on the burning velocity of hydrogen-air premixed flames are studied in an opposed nozzle configuration. The numerical method employs adaptive time integration of a system of differential-algebraic equations. Detailed hydrogen-air kinetic mechanism and transport properties are considered. The equivalence ratio is varied from lean to rich premixtures in order to change the effective Lewis number. Steady Markstein numbers for small strain rate are computed and compared with experiment. Different definitions of flame burning velocity are examined under steady and unsteady flow conditions. It is found that, as the unsteady frequency increases, large deviations between different flame speeds are noted depending on the location of the flame speed evaluation. Unsteady flame response is investigated in terms of the Markstein transfer function which depends on the frequency of oscillation. In most cases, the flame speed variation attenuates at higher frequencies, as the unsteady frequency becomes comparable to the inverse of the characteristic flame time. Furthermore, unique resonance-like behavior is observed for a range of rich mixture conditions, consistent with previous studies with linearized theory.