Methane-air Combustion Research Articles

Modeling of premixed gas combustion in porous media composed of Coarse-Sized particles: 1-D description with discrete solid phase

A one-dimensional non-steady-state model of gas combustion in a porous medium, treated as a discrete structure, is proposed. Each element of this regular periodic structure consists of the three elements: a solid particle, a gas flow zone, and a gas stagnation zone. Combustion occurs in the flow zones; the solid particles and stagnation zones are chemically inert. Numerical simulation of the opposing waves of premixed methane-air combustion in the discrete-modeled porous medium reveals the pulsating nature of the process. The maximal temperature and burning rate in the combustion front as well as the velocity of its propagation are low in the regions of the gas flow zone, which are adjacent to the particles, and increase in the neighborhood of the stagnation zone. At the size of elementary cells greater than some critical value d cr (1) (approximately 3 mm), the flame breaks into the high velocity regime of propagation in the neighborhood of the stagnation zone; the amplitude of pulsation of the combustion characteristics is increased considerably in this period. After crossing the threshold value d cr (1) , the high frequency oscillation (∼100 Hz) of the position of combustion front and its temperature were found in a region of the flow zone adjacent to the stagnation zone. When the diameter of particles becomes greater than the second critical value d cr (2) , the flame propagates in the high velocity regime over all the regions of the flow zone: the amplitude of pulsation of combustion characteristics is reduced. The discrete model of gas combustion in a porous medium, composed of coarse-sized particles, leads to a qualitatively new result when the process under consideration is sensitive to high-temperature reactions in the gas phase, and the final composition of the gas mixture is of main practical interest.

Direct simulation vs volume-averaged treatment of adiabatic, premixed flame in a porous medium

For the case of flame thickness being of the order of the pore linear dimension, the flame structure and speed in adiabatic, premixed methane-air combustion in porous media are examined. The local, volume-averaged conservation equations that assume a local thermal equilibrium between the solid and the gas phases (i.e. the single-medium treatment) or allow for a thermal nonequilibrium (i.e. the two-medium treatment) are used along with the direct application of the pointwise conservation equation to a two-dimensional porous medium model (ordered arrangement of discrete or connected square cylinders). The effective properties of the porous medium in the volume-averaged treatments, including the interfacial Nusselt number, are found by applying the local volume-averaging principles. The results show that, although significant variations of the temperature and species concentrations occur over a pore, the flame structure, thickness, speed, and excess temperature (i.e. local gas temperature in excess of the adiabatic temperature) are fairly well predicted by the two-medium model (the single-medium treatment is unable to predict the local excess temperature). However, the volume-averaged treatments are unable to predict the pore-level, local high temperature region in the gas phase (which can be up to 40% above the adiabatic temperature), and the pore-level variation in the flame speed with respect to the flame location in the pore (which can vary by up to 20%). Other shortcomings of the volume-averaged treatments are also revealed through a parametric examination involving the pore-geometry variables, solid to gas conductivity ratio, equivalence ratio, porosity, and flame location within the pore.

Measurements of Emissions and Radiation for Methane Combustion within a Porous Medium Burner

Abstract In this paper, we report the results of an experimental investigation of methane-air combustion within a porous medium burner for various equivalence ratios and flow rates. Measurements of emissions and radiant output are presented. The results indicate that CO and NOx, emissions increase with flame speed. For a given equivalence ratio, however, NOx, emissions are fairly constant over the range of flame speed studied. The radiant output increases but the radiant thermal efficiency decreases with flame speed.

Ignition and extinction of flames near surfaces: Combustion of CH4 in air

AbstractIgnition and extinction characteristics of homogeneous combustion of methane in air near inert surfaces are studied by numerical bifurcation theory for premixed methane/air gases impinging on planar surfaces with detailed chemistry involving 46 reversible reactions and 16 species. One‐parameter bifuraction diagrams as functions of surface temperature and two‐parameter bifurcation diagrams as functions of equivalence ratio and strain rate are constructed for both isothermal and adiabatic walls. Lean and rich composition limits for ignition and extinction, and energy production are determined from two parameter bifurcation diagrams. For a strain rate of 500 s−1, CH4/air mixtures exhibit hysteresis from ∼ 0.5% up to ∼ 12.5% and from ∼ 5.5% up to ∼ 13.5% near isothermal surfaces and adiabatic walls, respectively. Ignition temperature rises with composition from 1,700 to 1,950 K, without a maximum around the stoichiometric ratio. Under some conditions multiple ignitions and extinctions can occur with up to five multiple solutions, and wall quenching, kinetic limitations, and transport can strongly affect flame stability. Flames near the stoichiometric ratio cannot be extinguished by room temperature surfaces for sufficiently low strain rates. The role of intermediates in enhancing or retarding ignition and extinction is studied, and implications of the effect of catalytic surfaces on homogeneous ignition and extinction are discussed. Removal of H atoms and CH3 radicals by wall adsorption can increase extinction and ignition temperature of 6% CH4 in air by up to 300 K for a strain rate of 500 s−1.

Pt-catalyzed combustion of CH 4C 3H 8 mixtures

Experiments on a Pt foil, combined with simulations using a stagnation point flow model, were used to study the influence of propane on the combustion of methane in air over Pt catalysts. The steady-state multiplicity of the system was studied in terms of the surface temperature of the catalyst as a function of the input power. The system exhibited up to three stable steady states for some fuel ratios. Propane ignites at a much lower temperature and power input, and we observed that the whole mixture ignited at these conditions for sufficiently high propane contents. The model, which uses one-step global reaction rates for the kinetics, reproduced these results, predicting the ignition, extinction and autothermal temperatures fairly accurately. The nature of each stable state could be determined from the model. The low temperature stte is the extinguished state (with negligible reaction of either fuel), the intermediate state corresponds to only propane oxidation, and the third shows oxidation of both methane and propane. The model also predicts that the system exhibits two distinct autotherms for lean methane—propane mixtures.

A lagrangian model for predicting turbulent diffusion flames with chemical kinetic effects

The present study is a new step in the development of a lagrangian turbulent combustion model devotedto the prediction of non-premixed flames in which kinetic effects occur. Taking into account, in a refined manner, the interaction between chemical reaction and turbulence, the model includes a spectrum of turbulent timescales instead of a single mean timescale. The more recent improvement of the model consists in including a library of chemical delay times derived from a six-step reduced reaction mechanism for methane-air combustion, which is valid over a wide range of equivalence ratio. The predictions of the combustion model in which the library was implemented were compared with experimental data on turbulent jet diffusion flames near extinction. These calculations were performed with a finite volume technique devoted to variable density Navier-Stokes equations. A standard two-equations turbulence model was used and provided reliable results for dynamic variables (mean velocity and kinetic energy of turbulence). Comparisons between computed and measured mean mixture fraction and mixture fraction root mean square (rms) are quite satisfactory, proving that scalar transport is correctly modeled. The more salient point is the prediction of partial extinction and reignition phenomena, in agreement with experiments. This result clearly demonstrates the ability of the turbulent combustion model to simulate the interaction between turbulence and chemical reactions, which occurs in flames displaying kinetic effects.

Analysis of a systematical reduction technique

The systematical reduction of chemical reaction mechanisms is studied by using a first-order sensitivity analysis. Two important aspects in the reduction strategy are considered here: the identification of the steady-state species and the truncation of the resulting algebraic equations for these species. The analysis is applied to the four-step model of Peters et al. based on the skeletal mechanism for lean methane-air combustion. The identification of the steady-state species with the sensitity analysis confirms the choices made bySeshdri and Peters. Even more, with the analysis it is predicted that the differences in burning velocity between the four-step mechanism and the skeletal mechanism can be reduced, by taking CH 3 not in steady state, effectively yielding a five-step model. The effect of truncation is also investigated, and the differences of several approximations in the fourstep model are studied systematically. With the sensitivity analysis introduced here, we are able to predict the qualitative effect of truncation. Applied to the truncation of the expression for OH mole fraction, it shows that the performance of the four-step model can be improved considerably if another reaction is included in this expression. The computations of the burning velocity with this “extended” OH expression confirm the prediction. It can be concluded that the presented sensitivity analyses can be used effectively to analyze and determine systematically reduced mechanisms.

A computational study of methane-air combustion over heated catalytic and non-catalytic surfaces

A boundary layer model for the combustion of methane-air mixtures over a heated catalytic and noncatalytic surface maintained at constant temperature has been developed. Finite differences and a modified damped Newton's method were implemented to solve the coupled nonlinear parabolic partial differential equations for the conservation of total mass, momentum, energy, and chemical species. Adaptive gridding was used to obtain high resolution into the boundary layer and at the flame front. Detailed gas-phase chemistry was included in the model. The effect of different surface reaction boundary conditions on flame propagation and on the development of unstable species profiles in the gas-phase has been studied. An inert surface and a surface promoting gas-phase oxidation of the fuel to stable products have been simulated. In addition, the effect of desorption of hydroxyl radicals from the wall during the oxidation of the fuel on the unstable species profiles and on the propagation of the flame into the boundary layer was studied. The results illustrate the sensitivity of the predicted methane flame propagation on the surface boundary conditions used, which is more pronounced than for a hydrogen flame studied earlier.

Mechanism of methane-air combustion on the surface of a porous ceramic plate.

The mechanism of mathane-air combustion on the surface of a porous ceramic plate was studied by experimental testing and analysis of a simplified theoretical model based on one-dimensional low of methane-air mixture and the overall chemical reaction rate. The effects of such parameters as thickness of porous ceramic plates, equivalence ratio of mixed-gas and heat load on the combustion characteristics were examined. A thicker plate achieves higher surface temperature as premixed gas is preheated on the porous ceramic plate. The combustion zone is closest to the porous ceramic plate with equivalence ratio Φ=1.2. The surface temperature has peak value at a certain heat load

A Perfectly-Stirred-Reaction Description of Chemistry in Turbulent Nonpremixed Combustion of Methane in Air

Explanation of the observed peak CO concentration in turbulent nonpremixed jet combustion of methane in air

Characterizing The Unconfined Ceiling Jet Under Steady-state Conditions: A Reassessment

Detailed velocity and temperature measurements using cross-correlation velocimetry were obtained for unconfined ceiling jets under ceiling transient and steady-state conditions. Small fires of 0.5 to 2.0 kW were produced with a premixed methane-air burner. These measurements represent one of the most detailed studies of unconfined ceiling jets to date and seem to be in general agreement with large-scale data. Empirical correlations for the steady-state ceiling jet temperature and velocity profi1es are described herein. The correlations compare we1 1 with, and have improved, results from previous works. The maximum temperature may not be scaled as a function of fire-to-ceiling heights for large radial distances. Furthermore, the Gaussian and boundary 1ayer thermal and momentum thicknesses of the ceiling jet are not equal, contrary to the assumptions made in other works.

On reduced mechanisms for methaneair combustion in nonpremixed flames

A four-step mechanism for the combustion of methane in air in nonpremixed flames is obtained by making steady-state and partial equilibrium approximations for minor species. The model gives good predictions of laminar flame structure, including steady-state minor species, for a wide range of flame stretch and across the whole breadth of the reaction zone, including rich mixtures. A good prediction of extinction is obtained. Further reduction to a three-step or two-step mechanism is discussed.

The ESR Detection of N Atoms in Methane–Air Combustion Flames Doped with Nitrogen Compounds

Abstract The ESR spectra of nitrogen atoms were detected for the first time in the combustion flames of methane doped with ammonia. The dependence of the N-atom concentration on the flame conditions was examined. The results were then discussed with reference to the models of the reduction of NH3, leading to the NO formation.

Temperature fluctuations in a jet-stirred reactor andmodelling implications

Temperature pdfs, obtained from compensated thermocouples, and mean species concentrations have been measured, under conditions of intese turbulence, at different points in a conical jet-stirred reactor. The effects of the measured turbulent fluctuations on chemical source terms, for premixed methane-air combustion in an overall mathematical model, are evaluated on the basis of a laminar flamelet sub-model. Allowance for turbulence in this way improves the overall model, but suggests some limitations in the sub-model. Possibly these arise from an extended and fragmented reaction zone. The observed r.m.s. temperature fluctuations through the flame are compared with those of other workers and generalised in terms of the ratio of chemical to eddy lifetimes, expressed by u′(u l R L 0.5 ) −1 , where u ′ is the r.m.s. turbulent velocity of the unburnt premixture, R L the turbulent Reynolds number and u l the laminar burning velocity. The fluctuations decrease with an increase in this ratio over the measured range. Laminar flamelet assumptions demonstrate how an increase in the ratio, can cause volumetric chemical reaction and heat release rates to approach laminar flame values. This approach suggests possible simplifications to mathematical models.

The Effect of Exhaust Gas Recirculation and Turbulence on the Burning Velocity, Dead Space Thickness, and Minimum Ignition Energy in Premixed Methane-Air Combustion

Abstract The dilution of a homogeneous combustible mixture with products of combustion has proven to be a successful method of reducing the oxides of nitrogen produced by a combustion process. The Exhaust Gas Recirculation (EGR) affects the flame velocity, ignition energy, and quenching behavior. An experimental program has been conducted to determine the effects of EGR on these fundamental aspects of a turbulent methane/air flame in a constant volume bomb. Data are presented showing that the decrease in flame velocity caused by the EGR can to some extent be compensated for by an increase in the turbulence intensity at a price of increased ignition difficulty. The effect of the EGR, as determined from both heat transfer and ionization measurements, is to increase the single-wall quenching distance.

Methane-Air Combustion Gases as an Aerodynamic Test Medium

Covers advancements in spacecraft and tactical and strategic missile systems, including subsystem design and application, mission design and analysis, materials and structures, developments in space sciences, space processing and manufacturing, space operations, and applications of space technologies to other fields.

The effect of oscillatory combustion on the formation of atmospheric pollutants

The difference between the relaxation times of the mechanical and thermal transport processes which occur in flow combustors often leads to a coupling phenomenor, which may result in severe acoustical oscillations. Previously reported experiments have indicated that the concentrations of certain chemical species may be significantly affected by these pressure-temperature oscillations. Some chemical species, such as the oxides of nitrogen, which are apparently enhanced, are directly involved in the formation of atmospheric pollutants and chemical smog. Because oscillatory combustion occurs in many industrial applications, an understanding of the complex chemical thermodynamics of combustion systems is very important. In this work approximate integration techniques have been employed to solve the set of simultaneous kinetic rate equations which describe the oscillatory combustion of certain hydrocarbons. The dynamic concentrations for each chemical species have been calculated as a function of time and position in a model flow combustor. Three chemical systems were investigated: the combustion of carbon monoxide in oxygen, the combustion of carbon monoxide in air, and the products of the combustion of methane in air. In addition, calculations of the point-to-point equilibrium concentrations for the chemical systems investigated were made for comparison. Some major conclusions reached as a result of this analytical study were as follows: 1. Pressure-temperature oscillations significantly affect the concentrations of several “trace” components which contribute to air pollution. 2. The amplitude of the temperature oscillations seems to be of more importance than the frequency of the oscillations for the particular combustor model chosen for this study. The effects due to amplitude become more significant with increased temperature. 3. Deviation from equilibrium in flow combustors is very significant for important trace components such as monatomic oxygen and nitric oxide.

Recent developments and notes

Temperature pdfs, obtained from compensated thermocouples, and mean species concentrations have been measured, under conditions of intese turbulence, at different points in a conical jet-stirred reactor. The effects of the measured turbulent fluctuations on chemical source terms, for premixed methane-air combustion in an overall mathematical model, are evaluated on the basis of a laminar flamelet sub-model. Allowance for turbulence in this way improves the overall model, but suggests some limitations in the sub-model. Possibly these arise from an extended and fragmented reaction zone.The observed r.m.s. temperature fluctuations through the flame are compared with those of other workers and generalised in terms of the ratio of chemical to eddy lifetimes, expressed by u′(ulRL0.5)−1, where u′ is the r.m.s. turbulent velocity of the unburnt premixture, RL the turbulent Reynolds number and ul the laminar burning velocity. The fluctuations decrease with an increase in this ratio over the measured range. Laminar flamelet assumptions demonstrate how an increase in the ratio, can cause volumetric chemical reaction and heat release rates to approach laminar flame values. This approach suggests possible simplifications to mathematical models.