Showing papers on "Premixed flame published in 2001"

The effects of obstructions on overpressure resulting from premixed flame deflagration

Abstract: This paper introduces a new experimental set-up for investigating the effects of obstruction geometry, blockage ratio and venting pressure on overpressures resulting from premixed flame deflagration. Obstructions shaped as cylinders, triangles, squares, diamonds and plates or walls are studied here covering blockage ratios ranging from about 10% to more than 75%. It is found that the deflagration overpressure increases with increasing venting pressure. Also, the maximum overpressure increases, generally with increasing blockage ratio but the rate of increase depends on the obstruction geometry. The wall/plate type obstruction leads to the highest overpressures and the cylindrical obstruction yields the lowest overpressure. The time taken to reach the maximum overpressure decreases with increasing blockage ratio and changes with obstruction geometry implying that the flame accelerates faster due to changed local turbulence levels and length scales.

Constructive and Destructive Interference of Acoustic and Entropy Waves in a Premixed Combustor with a Choked Exit

Abstract: Thermoacoustic instabilities are a cause for concern in combustion applications as diverse as small household burners, gas turbines or rocket engines. In this work, a feedback mechanism is analyzed, which couples combustion chamber acoustics with convectively transported fluctuations of entropy (entropy waves) generated within a premixed flame. Essential elements of this thermo-acoustic feedback loop are fluctuations in fuel concentration, induced by acoustic disturbances at the location of fuel injection, convective transport of fuel inhomogeneities through the premixing section of the burner, modulations in heat release rate and hot gas entropy resulting from the consumption of fuel/air mixture with varying fuel concentration by the flame, and the generation of sound through entropy non-uniformities at the turbine inlet. From a qualitative analysis based on relative phases, it is concluded that depending on the various convective and acoustic time lags involved, entropy waves may couple constructively as well as destructively with combustor acoustics. However, such qualitative analysis does not indicate whether the coupling between entropy and acoustic waves is strong enough to significantly influence thermo-acoustic stability. Therefore, a linear model has been constructed to estimate the effect of entropy waves on the thermo-acoustic response and stability of a combustor with a choked exit nozzle, as it might be found in a gas turbine. Note that phenomena like dispersion of convective waves, distributed heat release, vortical velocities, etc., have not been taken into account, as they would burden the presentation with unnecessary complexity. Results obtained indicate that the interaction between combustor acoustics and entropy waves can be significant, especially for the lowest non-axisymmetric modes, and even at frequencies higher than those usually associated with convective waves. As expected, it was observed that the coupling between pressure and entropy waves at the exit nozzle can enhance as well as reduce the thermo-acoustic stability of a combustor, or the responsiveness to an external or internal fluid-mechanic excitation mechanism. It is concluded that comprehensive thermo-acoustic analysis of a premixed combustor with a choked exit must in general include generation and propagation of entropy waves and coupling with combustor acoustics.

Large-eddy simulation of turbulent premixed flames in the flamelet regime

Abstract: Large Eddy Simulations (LES) of turbulent premixed flames in the flamelet regime are carried out using a new subgrid combustion model. This model explicitly incorporates the effects of large-scale advection through a Lagrangian volume transport scheme and the effects of small-scale turbulent stirring and flame propagation (subgrid effects) using a one-dimensional linear eddy model. Three-dimensional turbulent stagnation point premixed flames are simulated with the new model and a conventional flame speed model. It is seen that the conventional model implemented using a finite difference discretization of the scalar equations on a typical three-dimensional LES grid is unable to capture the flamelet nature of the stagnation premixed flames. On the other hand, the subgrid model in the new formulation provides enough fine-scale resolution to accurately capture the flamelet nature of undisturbed laminar flame propagation and small scale wrinkling of the flame surface. As a result, the intermittency associated ...

Measurement of adiabatic burning velocity in methane-oxygen-nitrogen mixtures

Abstract: Experimental measurements of the adiabatic burning velocity in methane-oxygen-nitrogen mixtures are presented. Non-stretched flames were stabilized on a perforated plate burner at 1 atm. The oxygen content in the artificial air was varied from 16 percent to 21 percent. The Heat Flux method was used to determine burning velocities under conditions when the net heat loss of the flame is zero. Major attention in this work has been paid to the identification of possible uncertainties and errors of the measurements. The overall error of the burning velocities is estimated to be smaller than ± 0.8 cm/s. Experimental results are in very good agreement with recent literature data for methane-air mixtures. They also agree well with detailed chemical model predictions.

A study of flame propagation mechanisms in lycopodium dust clouds based on dust particles' behavior

Abstract: Following our earlier study on the behavior of lycopodium dust flames, further experiments using a particle image velocimetry system with a high-resolution video camera have been conducted to clarify the mechanisms of laminar dust flame propagation in a vertical duct. Lycopodium, a nearly equal-sized particle, has been recognized as being monodispersed, but it was found that an actual lycopodium dust cloud consisted of individual and agglomerated particles. Corresponding to the particle forms, the reaction zone showed a double flame structure, consisting of enveloped diffusion flames (spot flame) of individual particles and diffusion flames (independent flame) surrounding some particles. Due to the convective flow caused by a flame, part of the gravitational settling particles was shifted to the surrounding sides and the rest of the particles changed their movements to upwards in front of the flame. Such particle movement causes a dynamic variation in dust concentration ahead of the flame, which propagates at lower dust concentration rather than the mean concentration. Although the flame moved discontinuously on a micro scale, an overall constant flame velocity was found, presumably due to the dynamic variation in dust concentration and induced flow ahead of the flame. Judging from the above-mentioned movement of single particles in front of the flame, a residence time of the unburnt particle in the preheating zone is needed to form combustible gases close to the particle. This residence time depends on the preheating zone thickness, the particle velocity and the flame propagation velocity. The observation of the movement of a single particle suggested a flame propagation mechanism where an enveloped and diffusion lycopodium dust flame discontinuously propagates from one particle to those adjacent in a laminar suspension.

Modelling turbulent premixed combustion in the intermediate steady propagation regime

Abstract: Numerical simulation and comparison with standard experimental data of turbulent premixed combustion occurring at large Reynolds and moderately large Damkohler numbers (a situation which is typical in industrial burners) have been presented. The simulation has been performed in the framework of the Turbulent Flame-speed Closure (TFC) combustion model, developed in [1-4], which makes use of a theoretical expression for the turbulent combustion velocity for the closure of the progress variable transport equation. This model is based on the concept of the Intermediate Steady Propagation (ISP) regime of combustion in real combustors, i.e. when the turbulent flame propagates with equilibrium turbulent flame speed but has flame brush thickness growing according to the turbulent dispersion law. These ISP flames precede usually analysed 1D stationary flames, and from the theoretical point of view they are in fact intermediate asymptotic of the combustion process between the period of formation of developed turbulent flames and 1D stationary flames. Numerical results of turbulent premixed combustion in a two-dimensional planar channel at parameters that correspond to real industrial combustors have been compared with corresponding standard experimental data on a high speed turbulent premixed flame [9]. Finally, it has been explained in the framework of the TFC combustion model that "countergradient transport", i.e. the necessity to use a negative effective diffusion coefficient to describe experimental heat and progress variable fluxes inside the flame, is an inherent feature of turbulent premixed flames, and is connected with direct dependence of the second order velocity-scalars correlation on combustion. It has been shown that the existence of the countergradient transport phenomenon is not in contradiction with the actual increasing of the flame brush width.

Probe sampling measurements and modeling of nitric oxide formation in methane-air flames

Abstract: Probe sampling measurements of the concentrations of O2, CO2, CO, and NO in the postflame zone of the methane-air flames are reported. A heat flux method was used for stabilization of nonstretched flames on a perforated plate burner at 1 atm. Major attention in this work has been paid to the identification of possible uncertainties and errors of the measurements. Radial and axial profiles of the concentrations of stable species and NO in the postflame zone were used to evaluate the influence of the ambient air en-trainment. flame expansion, and downstream heat losses. In the core region of the flames, the radial profiles of the major species and NO are flat from moderately lean to moderately rich mixtures. In the very lean mixtures an ambient air dilutes the burnt gases, while in the very rich mixtures it causes oxidation of the combustion products. The buoyancy and radial flame expansion can significantly modify observed concentration gradients in the postflame region. In the methane-air mixtures, the NO...

Theoretical investigation of unsteady flow interactions with a premixed planar flame

Abstract: This paper presents the results of a theoretical study of the interactions between a laminar, premixed flame front and a plane acoustic wave. Its objective is to elucidate the processes that damp or drive acoustic waves as they interact with flames. Using linear analysis, the characteristics of the acoustic field, the flame's movement and wrinkling in response to acoustic perturbations, and the acoustic energy that is produced or dissipated at the flame are calculated. These calculations show that the net acoustic energy flux out of the flame is controlled by competing acoustic energy production and dissipation processes. Energy is added to the acoustic field by unsteady heat release processes resulting from the unsteady flux of unburned reactants through the flame by fluctuations in the flame speed or density of the unburned reactants. Energy is dissipated by the transfer of acoustic energy into fluctuations in vorticity that are generated at the flame front because of the misaligned fluctuating pressure and mean density gradients (i.e. the baroclinic vorticity production mechanism). The paper concludes by showing how these results can be generalized to determine the response of planar flames to an arbitrarily complex acoustic field. The principal contribution of this work is its demonstration that the excitation of vorticity and fluctuations in the flame speed have significant qualitative and quantitative affects on the interactions between flames and acoustic waves.

The role of secondary instabilities in the stabilization of a nonpremixed lifted jet flame

Abstract: A nonpremixed lifted jet flame is studied dynamically in the hysteresis zone. High-speed laser tomography images show clearly that, in the case of an organized jet, the flame is located on streamwise counter-rotating vortex filaments generated to secondary instabilities and ejected towards ambient air. Particle image velocimetry is used to evaluate the amplitude of the translational and rotational velocity of these filaments. The use of an acoustic field to force jet instabilities shows that the flame, following large filament ejections, moves back upstream very close to the nozzle without anchoring at it. The role of streamwise vortices in the stabilization mechanism of the lifted flame is confirmed by measurements obtained with a disordered jet, from a straight tube burner. From these results, it is proposed that secondary vortices at the flame base are sufficiently strong to create a premixed zone and to oppose flame propagation.

Benzene Oxidation: Experimental Results in a JDR and Comprehensive Kinetic Modeling in JSR, Shock-Tube and Flame

Abstract: New experimental results have been obtained for the oxidation of benzene in a jet-stirred reactor at high temperature (950–1350 K) at atmospheric pressure and variable equivalence ratio (0·3≤ 0 ≤1·5). Molecular species concentration profiles of reactants, stable intermediates and final products were obtained by probe sampling followed by on-line and off-line GC analyses. The oxidation of benzene in these conditions was modeled using a detailed kinetic reaction mechanism (120 species and 921 reactions, most of them reversible). The proposed mechanism was also used to simulate the oxidation of benzene at low pressure (0·46 atm) and high pressure in stirred reactor conditions. The burning velocities of benzene-air mixtures were well-predicted by the proposed kinetic scheme that was also used to simulate the MBMS results of Bittner and Howard obtained for a fuel-rich benzene-oxygen-argon premixed flame. The ignition delays of benzene-oxygen-argon mixtures measured by Burcat over the range of equivalence ratio...