Lean Methane Air Research Articles

Numerical simulation of nanosecond pulsed DBD in lean methane–air mixture for typical conditions in internal engines

Detailed two-dimensional numerical simulations of a high energy loading nanosecond dc pulse DBD in a lean methane–air mixture were conducted for plasma-assisted combustion by integrating individual models of plasma chemistry, photoionization and energy loading. The DBD streamer propagation process with radical productions was clarified at 10 atm and 600 K as under the condition of actual internal engines at ignition. Energy is loaded to the streamer first by the formation of plasma channel and then ceased due to the self-shielding effect. Because of the inversed electric field in a discharge space during decrease in applied voltage, energy is loaded to the discharge again. It was found that higher energy is loaded to the DBD streamer for larger dielectric constant even at lower applied voltage, and higher number density of oxygen radical is produced at almost the same radical production efficiency.

Phase Resolved Analysis of Flame Structure in Lean Premixed Swirl Flames of a Fuel Staged Gas Turbine Model Combustor

A scaled model of a gas turbine (GT) burner with coaxially mounted swirlers has been used to study the effects of fuel staging on the behavior of lean premixed methane air flames. Lean flames are known to be susceptible to instabilities that can lead to unsteady operation, flame extinction, and thermo-acoustic oscillations. High speed (10 kHz) laser and optical diagnostic techniques have been used to investigate the fuel staging effect on the mechanisms involved in such instabilities. Methane air flames at atmospheric pressure have been investigated at a constant thermal power of 58 kW. The global equivalence ratio was kept constant, while the fuel staging was varied. The bulk flow velocity at the exit plane was kept constant at 20 m/s. Simultaneous high speed OH PLIF, OH* CL, and acoustic measurements were performed at kHz repetition rate to characterize the flames and determine the operability limits of the combustor. The characterization measurements reveal significant changes in flame shape for various staging ratios as well as onset of self-excited thermo-acoustics in flames with more than 55% fuel injection in the outer swirler. The phase resolved analysis of the OH* CL revealed pulsation in the heat release due to acoustics in flames with higher percentage of fuel in the outer swirler. Comparison of the pressure oscillation in the combustion chamber with the heat release yielded a clear picture regarding the feedback mechanism that sustains the self-excited thermo-acoustic pulsations. The variation of local equivalence ratio of the mixture seems to be the driving force that initiates the onset of acoustics pulsations.

Large-eddy simulation of lean hydrogen–methane turbulent premixed flames in the methane-dominated regime

The application of large-eddy simulation (LES) to the prediction of H2-enriched lean methane–air turbulent premixed combustion is considered. A presumed conditional moment (PCM) subfilter-scale combustion model is coupled with the flame prolongation of intrinsic low-dimensional manifold (FPI) chemistry tabulation technique. The LES and PCM-FPI modelling procedures are then applied to the prediction of laboratory-scale axisymmetric Bunsen-type turbulent premixed flames. Both premixed methane–air and H2-enriched methane–air flames are considered and the predicted solutions are examined and compared to available experimental data. The enriched flame has 20% H2 in terms of mole fraction and lies in the methane-dominated regime of hydrogen–methane mixtures. The LES simulations predict similar qualitative trends to those found in the experiments for flame height and curvature. The addition of H2 decreases the flame height and broadens the curvature probability density functions, which show a Gaussian-type shape centred around zero. Moreover, the enriched flame displays a higher degree of wrinkling with sharper ridges of negative curvature and larger pockets of positive curvature. Overall, the proposed treatment for the PCM-FPI combustion model, in terms of progress variable and tabulated data, seems to perform well for the H2-enriched methane flame in the methane-dominated regime.

Numerical simulation of a turbulent partially premixed flame with inhomogeneous equivalence ratio

Reynolds averaged numerical simulation (RANS) of a turbulent partially premixed flame is performed using a presumed probability density function (Pdf) approach. The mixture fraction and the fuel mass fraction are respectively chosen to describe the local composition and the chemical reaction progress. The so called LW–P model was employed for the calculation of a lean methane–air flame stabilized by a stoichiometric pilot-flame. Numerical predictions of the jet-core as well as the mean quantities of the flame, namely the modified progress variable and the static temperature are compared successfully with the experiments.

Simplified Kinetic Model for Thermal Combustion of Lean Methane–Air Mixtures in a Wide Range of Temperatures

Abstract The paper presents results of kinetic studies on thermal methane combustion over honeycomb monoliths. An analysis of the previous experiments [1] has shown that equations proposed there and their kinetic parameters satisfactorily describe kinetics only for relatively low temperatures up to approx. 700°C. The present study corresponds to that related and supplements the previous data with new kinetic parameters obtained in higher temperatures in the reaction zone up to 900°C. A method of the reaction rate calculation for further simulation studies combining the kinetic parameters obtained in both ranges of low (LT) according to Gosiewski et al. [1] and high temperatures (HT) are also presented in the paper.

Preliminary investigation of a swirl stabilized premixed combustor

Experiments were conducted in a swirl stabilized premixed combustor at atmospheric pressure in order to study the stability and structure of lean premixed methane air flames. Swirl number of the combustor was 0.74. The lab scale combustor was operated up to 30kW of thermal power during measurements. Experiments focused on two main aspects of the combustion system. First the relation between the cold flowfield and the flame stabilization is examined. It is observed that, the structure of the flame near the dump plane corresponds closely to the features of the non-reacting air flowfield. The flame is seen to be stabilized on the boundary of the recirculation region. Lean blowout limits of the combustor that operates within the flamelet regime were also recovered. A correlation linking the blowout equivalence ratio to laminar flame speed is presented.

Thermal combustion of lean methane–air mixtures: Flow reversal research and demonstration reactor model and its validation

Problem of the coal mine shafts ventilation air methane (VAM) utilization is briefly presented. A research and demonstration plant experiments were carried out as the first step to a future industrial application. The plant contained a thermal flow reversal reactor (TFRR) as the main unit where lean methane–air mixture is combusted. The study consisted of the plant experiments and mathematical model simulations as well. The paper presents the mathematical model description and its experimental validation based on comparison of the model results with measured plant data. The aim of the paper is to assess whether dynamic, one dimensional (in space) model may be useful for a fairly large scale reactor simulations. The model was developed and tested in the 2 versions: without and with heat accumulation in the reactor wall.

Study of interaction of entrained coal dust particles in lean methane–air premixed flames

This study investigates the interaction of micron-sized coal particles entrained into lean methane–air premixed flames. In a typical axisymmetric burner, coal particles are made to naturally entrain into a stream of the premixed reactants using an orifice plate and a conical feeder setup. Pittsburgh seam coal dust, with particle sizes in the ranges of 0–25μm, 53–63μm, and 75–90μm, is used. The effects of different coal dust concentrations (10–300g/m3) entrained into the mixture of methane–air at three lean equivalence ratios, ϕ, of 0.75, 0.80 and 0.85, on the laminar burning velocity are studied experimentally. The laminar burning velocity of the coal dust–methane–air mixture is determined by taking high quality shadowgraph images of the resulting flames and processing them using the cone-angle method. The results show that the laminar burning velocity reduces with the addition of coal dust having particle sizes in the ranges of 53–63μm and 75–90μm, irrespective of the equivalence ratio values. However, burning velocity promotion is observed for one case with particle size in the range of 0–25μm at an equivalence ratio of 0.75. Two competing effects are considered to explain these trends. The first effect is due to volatile release, which increases the overall equivalence ratio and thus, the flame temperature and burning velocity. The second is the heat sink effect that the coal particles take up to release the volatiles. This process reduces the flame temperature and accordingly the burning velocity also. A mathematical model is developed considering these effects and it is seen to successfully predict the change of laminar burning velocity for various cases with different dust concentrations and equivalence ratios of the gas mixture. Furthermore, the implication of this study to coal mine safety is discussed.

A Simulation of the Effects of Varying Repetition Rate and Pulse Width of Nanosecond Discharges on Premixed Lean Methane-Air Combustion

Two-dimensional kinetic simulation has been carried out to investigate the effects of repetition rate and pulse width of nanosecond repetitively pulsed discharges on stabilizing premixed lean methane-air combustion. The repetition rate and pulse width are varied from 10 kHz to 50 kHz and from 9 ns to 2 ns while the total power is kept constant. The lower repetition rates provide larger amounts of radicals such as O, H, and OH. However, the effect on stabilization is found to be the same for all of the tested repetition rates. The shorter pulse width is found to favor the production of species in higher electronic states, but the varying effects on stabilization are also found to be small. Our results indicate that the total deposited power is the critical element that determines the extent of stabilization over this range of discharge properties studied.

A combined computational and experimental characterization of lean premixed turbulent low swirl laboratory flames: I. Methane flames

This paper is the first in a series that presents a combined computational and experimental study to investigate and characterize the structure of premixed turbulent low swirl laboratory flames. The simulations discussed here are based on an adaptive solution of the low Mach number equations for turbulent reacting flow, and incorporate detailed models for transport and thermo-chemistry. Experimental diagnostics of the laboratory flame include PIV and OH-PLIF imaging, and are used to quantify the flow field, mean flame location, and local flame wrinkling characteristics. We present a framework for relating the simulation results to the flame measurements, and then use the simulation data to further probe the time-dependent, 3D structure of the flames as they interact with the turbulent flow. The present study is limited to lean methane–air flames over a range of flow conditions, and demonstrates that in the regime studied, local flame profiles are structurally very similar to the flat, unstrained steady (“laminar”) flame. The analysis here will serve as a framework for discussing a broader set of premixed flames in this same configuration. Papers II and III will discuss corresponding analysis for pure hydrogen–air and hydrogen–methane mixed fuels, respectively.

From Large-Eddy Simulation to Direct Numerical Simulation of a lean premixed swirl flame: Filtered laminar flame-PDF modeling

Large-Eddy Simulations (LES) and Direct Numerical Simulation (DNS) are applied to the analysis of a swirl burner operated with a lean methane–air mixture and experimentally studied by Meier et al. [ 19]. LES is performed for various mesh refinements, to study unsteady and coherent large-scale behavior and to validate the simulation tool from measurements, while DNS enables to gain insight into the flame structure and dynamics. The DNS features a 2.6 billion cells unstructured-mesh and a resolution of less than 100 microns, which is sufficient to capture all the turbulent scales and the major species of the flame brush; the unresolved species are taken into account thanks to a tabulated chemistry approach. In a second part of the paper, the DNS is filtered at several filter widths to estimate the prediction capabilities of modeling based on premixed flamelet and presumed probability density functions. The similarities and differences between spatially-filtered laminar and turbulent flames are discussed and a new sub-grid scale closure for premixed turbulent combustion is proposed, which preserves spectral properties of sub-filter flame length scales. All these simulations are performed with a solver specifically tailored for large-scale computations on massively parallel machines.

Effects of equivalence ratio variation on lean, stratified methane–air laminar counterflow flames

The effects of equivalence ratio variations on flame structure and propagation have been studied computationally. Equivalence ratio stratification is a key technology for advanced low emission combustors. Laminar counterflow simulations of lean methane–air combustion have been presented which show the effect of strain variations on flames stabilized in an equivalence ratio gradient, and the response of flames propagating into a mixture with a time-varying equivalence ratio. ‘Back supported’ lean flames, whose products are closer to stoichiometry than their reactants, display increased propagation velocities and reduced thickness compared with flames where the reactants are richer than the products. The radical concentrations in the vicinity of the flame are modified by the effect of an equivalence ratio gradient on the temperature profile and thermal dissociation. Analysis of steady flames stabilized in an equivalence ratio gradient demonstrates that the radical flux through the flame, and the modified radical concentrations in the reaction zone, contribute to the modified propagation speed and thickness of stratified flames. The modified concentrations of radical species in stratified flames mean that, in general, the reaction rate is not accurately parametrized by progress variable and equivalence ratio alone. A definition of stratified flame propagation based upon the displacement speed of a mixture fraction dependent progress variable was seen to be suitable for stratified combustion. The response times of the reaction, diffusion, and cross-dissipation components which contribute to this displacement speed have been used to explain flame response to stratification and unsteady fluid dynamic strain.

LES of a laboratory-scale turbulent premixed Bunsen flame using FSD, PCM-FPI and thickened flame models

Large-eddy simulations (LES) of a turbulent premixed Bunsen flame were carried out with three subfilter-scale (SFS) modelling approaches for turbulent premixed combustion. One approach is based on the artificially thickened flame and power-law flame wrinkling models, the second approach is based on the presumed conditional moment (PCM) with flame prolongation of intrinsic low-dimensional manifolds (FPI) tabulated chemistry, and the third approach is based on a transport equation for the flame surface density (FSD). A lean methane–air flame at equivalence ratio ϕ=0.7, which was studied experimentally by Yuen and Gülder, was considered. The predicted LES solutions were compared to the experimental data. The resolved instantaneous three-dimensional structure of the predicted flames compares well with that of the experiment. Flame heights and resolvable flame surface density and curvature were also examined. In general, the average flame height was well predicted. Furthermore, the flame surface data extracted from the simulations showed remarkably good qualitative agreement with the experimental results. The probability density functions of predicted flame curvature displayed a Gaussian-like shape centred around zero as also observed in the experimental flame, although the experimental data showed a slightly wider profile. The results of the comparisons highlight the weaknesses and the strengths of SFS modelling approaches commonly used in LES of turbulent premixed flames.

Transition metal-exchanged LTA zeolites as novel catalysts for methane combustion

The performance of different transition metal-exchanged zeolites for catalytic combustion of lean methane–air mixtures was studied in this work. Transition metals (Fe, Co, Mn) were incorporated to the parent LTA zeolite by ion exchange, using metal nitrates as precursors. Catalytic performance of these materials was compared to the corresponding to the bulk metal oxides in terms of activity and stability. In general terms, Co-exchanged zeolites present very similar behavior to the parent metal oxide, whereas the Mn-exchanged present very poor performance, and the Fe-exchanged zeolite present markedly better performance than the parent bulk oxide. Characterization of the fresh and used catalysts suggests that the superior behavior of the iron-exchanged zeolite is caused by a change in the reducibility properties of the metal species caused by the interaction with the zeolite framework (observed in TPR and O 2-TPD experiments).

Ignition strategies for fuel mixtures in catalytic microburners

Ignition of methane–air and propane–air mixtures over platinum catalyst in a parallel-plate microburner is studied numerically and a comparison of their ignition characteristics is presented. The ignition behaviour of the two fuels is compared for the case of heated feed and the strategy of using propane–methane mixed fuel is analysed. We show that adding small quantities of propane reduces the ignition temperature of lean methane–air mixture. Transient response of the mixed methane–propane fuel reveals sequential ignition of propane followed by methane. Sensitivity analysis on physical properties of methane and propane shows that the higher apparent activation energy of methane combustion accounts for most of the observed differences in their ignition behaviour. Ignition by resistive preheating, specifically the effect of locally preheating initial section of the burner is investigated. The amount of electric power required for ignition decreases with decrease in the electrical preheating length. This reduction in ignition power is especially significant for low conductivity walls, compared to highly conducting walls. Finally, the gap size of the channel has a relatively small effect on ignition in catalytic microburners.

Propagation and Extinction of a Cylindrical Premixed Flame Undergoing Equivalence Ratio Fluctuation Near the Lean Limit

Experimental study was made to investigate the propagation and extinction characteristics of a stretched cylindrical flame undergoing periodic fluctuation of equivalence ratio near the lean limit. With a lean methane-air and a lean propane-air mixture, burning velocity, flame luminosity and flame stretch rate were measured or evaluated for the fluctuation frequencies of 5Hz and 20Hz. The results were summarized as follows: (1) In some part of a period, burning velocity and flame luminosity of the dynamic flame near the lean limit were possible to become lower than those at the lean flammability limit of the static flame. (2) At the high frequency of 20Hz, the burning velocity took a negative value in a certain time range. In spite of this loss of propagation ability, the flame was not extinguished but sustained, indicating the recovery of the flame intensity due to the dynamic effect of fluctuating flame. (3) Flame recovery phenomenon could occur more easily for the methane flame which was strengthened by the Lewis number effect than the propane flame which was weakened by that effect.

Enhanced model predictive control of a catalytic flow reversal reactor

AbstractThe combustion of lean methane air mixtures in a catalytic flow reversal reactor (CFRR) is studied using a two dimensional heterogeneous continuum model, based on mole and energy balance equations for the solid (the inert and catalytic sections of the reactor) and the fluid phases. Following a design of experiments (DOE), many simulations were carried out to investigate the reactor performance. The results show the impact on the methane conversion and the maximum temperature in the reactor of key process parameters such as the methane inlet concentration, the superficial gas velocity, the switching time, and the mass extraction rate. A simple empirical model is deduced to predict the maximum temperature and conversion of methane in the reactor at stationary state. This model is combined with a model predictive control (MPC) strategy in the form of a terminal constraint to improve the controller performance. Results show that the control of the reactor is improved.

Flame stabilization between two beds of alumina balls in a porous burner

Conditions for flame stabilization in a porous media combustor formed by two beds of different sizes of alumina balls were studied. Premixed combustion of lean methane–air mixtures were used as variables. Measurements performed included temperature profiles and chemical products compositions. Stabilized flames were observed in the range of volumetric flow rate from 7.01 l/min to 19.00 l/min at equivalence ratio of ϕ = 0.6 and ϕ = 0.7. Low pollutants emissions were found in the entire operation range.

A study on thermal combustion of lean methane–air mixtures: Simplified reaction mechanism and kinetic equations

Homogeneous combustion of lean methane–air mixtures occurs in both thermal and catalytic reverse-flow reactors. The modeling and design of such reactor requires a reliable and relatively simple kinetic model of the combustion. Although a number of detailed descriptions of the kinetics of homogeneous combustion are available in the literature (including those which take into account complex free-radical reactions), their practical usefulness is doubtful. In the present study a simple one- or two-stage model is proposed which directly describes global kinetics and neglects intermediates and radicals. Special attention is paid to the mechanism of the possible formation of carbon monoxide. The studies were done both for the combustion in the free space and for the process taking place over a monolith packing.

Procedures for heat recovery in the catalytic combustion of lean methane–air mixtures in a reverse flow reactor

The performance of different heat recovery strategies for reverse flow catalytic reactors, considering both the gas extracting point (centre or end) and the possibility of returning the cooled gases, has been studied in this work. The study was carried out by means of simulations of the combustion of methane over a monolithic Pd-catalyst using a heterogeneous 1D dynamic model. The effect of methane feed concentration (3000–9000 ppmV), has also been considered. The withdrawal of the hot gases from the end of the catalytic bed without returning the cooled gases has been shown to provide the most stable performance, whereas the extraction of the gases from the central part of the bed severely affects reactor performance. Finally, the amount of high-pressure steam that can be generated using the best heat recovery method has been also estimated.