Lean Methane-air Mixtures Research Articles

Effects of Gravity on Plane-Symmetric Rod-Stabilized Flame Stabilization

The current research is focused on the study of methane-air flame stabilization under the flow geometry variation for normal and upside-down (reverse) flame orientation. The experimental studies of plane-symmetrical rod-stabilized flames under the normal and reverse-oriented gravity conditions were carried out. The results of numerical simulation are presented. The blow-off is shown to be the function of stabilization-body position. We consider both V-shaped and M-shaped plane-symmetrical open flames for different Reynolds numbers, fuel-air ratio and flame orientation relative to the gravity direction. Blow-off limits appear to be independent on the gravity for the lean methane-air mixtures while the quenching processes are different for the normal and reverse-oriented gravity conditions. Blow-off is also accompanied by the chemiluminescence intensity decrease under reverse-oriented gravity conditions, which is exhibited in localized plume extinction and gradual quenching. While under the normal gravity, it appears via stepwise vortex separation from the lateral plume parts. Under such conditions chemiluminescence intensity remains almost constant. The blow-off time scale under the normal gravity conditions is bigger as compared to reversed-oriented ones by several times.

Numerical investigation of catalytic combustion in a honeycomb monolith with lean methane and air premixtures over the platinum catalyst

The stringent regulation requires low hydrocarbon emissions in combustion engines with high priority to environmental protection. For low emission technologies, catalytic combustor is an attracting option for the stable combustion process, which operates at lower temperatures than conventional engines. In this work, numerical analysis of the catalytic combustion with lean premixed methane and air is performed employing a cylindrically shaped honeycomb monolith combustor coated with the platinum catalyst. Detailed diagnostic investigations of complete catalytic combustor have been carried out using honeycomb monolith reactor in order to examine the comprehensive gaseous and catalytic surface reactions. The surface chemistry introduced by Chou on the monolith reactor consisting of numerous channels engaged along the cross-section is employed to examine the monolith temperatures and methane conversions during surface ignition. The species and surface site fractions are predicted with adsorbed species as the function of the monolith. The parameters such as the inlet temperature, fuel/air ratio, and velocity serve to govern the behavior of the catalytic combustion monolith. The reasonable requirement of catalytic combustor length for the complete combustion of the lean methane-air mixture is also determined by the numerical simulation over the platinum-coated honeycomb monolith.

Effect of coal dust on the air-methane mixture combustion in the Swiss-roll burner

The 2-D numerical model has been developed to simulate the combustion of lean coal-methane-air mixture in ?Swiss-roll? burner and investigate the effect of coal particles content and mixture feed rate on the stable burner operation. Five homogeneous and three heterogeneous chemical reactions are considered. The Eulerian model for gas phase and the discrete particle model (Lagrangian) for particle phase taking into account the radiation are used in this study. Simulations are performed for the two turns ?Swiss roll? burner with heat insulated outer wall, channel width of 6 mm and inner wall thickness of 2 mm. It is shown that the presence of coal particles in a lean methane-air mixture expands the range of stable operation of the "Swiss-roll" burner. It is found that an increase in the content of coal particles reduces the combustion time of the particles by in-creasing the combustion temperature, changes the shape of the reaction zone, and shifts it to the burner inlet.

Large-eddy simulation of dual-fuel ignition: Diesel spray injection into a lean methane-air mixture

In the present study, large-eddy simulation (LES) together with a finite-rate chemistry model is utilized for the investigation of a dual-fuel (DF) ignition process where a diesel surrogate (n-dodecane) spray ignites a lean methane-air mixture in engine relevant conditions. The spray setup corresponds to the Engine Combustion Network (ECN) Spray A configuration enabling an extensive validation of the present numerical models in terms of liquid and vapor penetration, mixture distribution, ignition delay time (IDT) and spatial formaldehyde concentration. The suitability of two n-dodecane mechanisms (54 and 96 species) to cover dual-fuel chemical kinetics is investigated by comparing the predicted homogeneous IDTs and laminar flame speeds to reference values in single-fuel methane-air mixtures. LES of an n-dodecane spray in DF conditions is carried out and compared against the baseline ECN Spray A results. The main results of the study are: (1) ambient methane impacts the ignition chemistry throughout the oxidation process. In particular, the activation of the low-temperature chemistry is delayed by a factor of 2.6 with both mechanisms, whereas the high-temperature chemistry is delayed by a factor of 1.6–2.4, depending on the mechanism. (2) The ignition process starts from the spray tip. (3) There exists a characteristic induction time in the order of 0.1 ms between the start of the first high-temperature reactions and the time when maximum methane consumption rate is achieved. (4) The high-temperature ignition process begins near the most reactive mixture fraction conditions. (5) The role of low-temperature reactions is of particular importance for initiation of the production of intermediate species and heat, required in methane oxidation and (6) both applied mechanisms yield qualitatively the same features (1)–(5) in the DF configuration.

Dual-pulse laser ignition model

A two-dimensional mathematical model of dual-pulse laser ignition that self-consistently integrates Navier-Stokes, translational and vibrational energy, and neutral and charged species equations has been presented. The results showed that the ignition kernel dynamics depends on the shape and initial energy distribution in the energy spot created by the first ultraviolet laser pulse. The results also suggest that the ignition delay time and the flame kernel development depend on the laser intensity, vibrational-nonequilibrium, and initial electron number density. For the high initial degree of ionization, we have obtained ignition of the lean methane–air mixture with the equivalence ratio of 0.6. Vibrational-nonequilibrium taken into account by the Landau-Teller model leads to the slower ignition kernel growth and the significant increase in the ignition delay time. For the case modeled, a change in the overlap of the second laser with the focal point of the first laser pulse leads to the early split up of the kernel and the flame extinguishment.

Combustion peculiarities of coal-methane-air mixtures in a recuperative burner

Numerical modeling of the combustion of a lean methane-air mixture containing fine coal particles entering the “Swiss-roll” type recuperative burner is considered. The mathematical model is constructed under the following assumptions: the flow field is two-dimensional; the gas mixture is an ideal incompressible gas consisting of oxygen, methane, coal volatile substances, carbon monoxide, carbon dioxide, water vapor, hydrogen and nitrogen. In the gas phase four oxidation reactions, in which methane, volatile matter of coal, carbon monoxide, hydrogen participate and the reaction of carbon dioxide decomposition take place. On the surface of the coal particle, there are three oxidation reactions involving oxygen, carbon dioxide and water vapor, resulting in the formation of carbon monoxide. It is assumed that coal contains 8% of ash, 12.9% of volatile substances and 79.1% of carbon. It is shown that for a two percent methane-air mixture the reaction zone shifts toward the center of the burner as the feed rate of the mixture increases. An increase in the content of coal particles leads to a shift of the reaction zone into the inlet part of the burner, and the heat release in the burner increases.

Analytical solutions of superadiabatic filtration combustion

Analytical solutions for superadiabatic filtration combustion of lean methane-air mixtures in a monolithic porous burner are sought. The one-dimensional, local volume-averaged equations of energy and species conservation that assume a non-thermal equilibrium (i.e., the two-medium treatment), are converted by a coordinate transformation using a combustion wave speed and solved to obtain close-form solutions. A parametric examination varying inlet gas velocity, fuel equivalence ratio, porosity, and thermal conductivity and diffusivity of the solid phase of the porous burner proves the validity of the analytical solutions which are in an excellent agreement with the numerical benchmark. The analytical solutions depict the key features of the filtration combustion such as non-thermal equilibrium between the solid and gas phases, superadiabatic flame temperature, and internal heat recirculation between solid and gas phases of the porous burner.

Burning stability of the coal-dust methane-air mixture in a recuperative burner

The physical mathematical model of combustion of the coal-dust methane-air mixture in a recuperative burner has been developed. The influence of the particles (inert and reacting) on the burning stability of coal-dust methane-air mixture with low concentration of methane has been determined. The regimes of stable combustion for lean methane-air mixture with coal suspension depending on the fuel flow rate at the inlet of the burner and the fuel content have been defined.

Stabilization of premixed lean methane-air combustion using dielectric barrier discharge with low pollutant emissions

Catalytic combustion is a promising technology to stabilize lean combustion with low pollutant emissions. Catalytic combustion has been applied to gas turbine combustors; however, some drawbacks of this technology remain to be addressed. In this work, a new concept is demonstrated to overcome the problems of catalytic combustion by using dielectric barrier discharge (DBD) instead of a catalyst. A premixed lean methane-air mixture preheated to 400 °C with an equivalence ratio of 0.45 is flowed through the DBD reactor under atmospheric pressure. Almost complete combustion is achieved with a DBD power of 0.7% of the net calorific value of the mixture. The exhaust emissions are ppm, ppm, ppm, and ppm. This work demonstrates that DBD-assisted combustion is a potential alternative to catalytic combustion.

The Role of Hydrodynamic Enhancement on Ignition of Lean Methane-Air Mixtures by Pulsed Nanosecond Discharges for Automotive Engine Applications

ABSTRACTThe downsizing and boosting of automotive engines for increased fuel economy poses challenges in both obtaining stable ignition at boosted intake pressures and high dilution conditions. Pulsed nanosecond discharge ignition technologies have shown promise in more reliably igniting dilute charge mixtures in internal combustion engine experiments. However, reasons for this combustion enhancement remain unclear. In this study, we ignited lean methane-air mixtures in a constant volume chamber at 2 bar absolute pressure to evaluate pulsed discharge ignition using a novel electrode geometry. The in-chamber pressure history indicates faster flame development times than those produced by traditional inductive spark. High-speed schlieren imaging reveals a significant hydrodynamic component to the observed enhancement: a more wrinkled flame kernel structure and increased burning rates from increased flame surface area. Increasing the number of pulses increased expulsion of the flame kernel. Our results clarify the enhancement observed by other researchers in internal combustion engine experiments.

Turbulent Flame Speed Dependencies in Lean Methane-Air Mixtures under Engine Relevant Conditions

Direct numerical simulations are carried out to study the behavior of turbulent flames propagating in lean methane-air mixtures of equivalence ratio 0.5 and for non-dimensional turbulence intensities (urms/SL) of 2-25. The turbulent Reynolds number (ReT) varies from 44-549, the Damköhler (Da) number from 0.26-3.2, and the Karlovitz number (Ka) from 1.1-49.4. The mixture pressure is 20 bar and temperature is 810 K to simulate approximate conditions in lean-burn natural gas engines. A 13-species reduced mechanism and a global mechanism are employed in the study. It is shown that the normalized turbulent flame speeds (ST/SL) can be related to the flame area enhancement (AT/AL) resulting from turbulence interactions with the laminar flame and the efficiency factor (Io) which is close to unity (∼ 1.07 ± 0.04) when evaluated at the temperature of peak heat release rate. This area enhancement increases with increasing ReT and decreasing Ka. The effect of integral length scale on flame area enhancement is related to the effect induced by the spectrum of eddies of the turbulent cascade. A correlation is proposed for the turbulent flame speed. While the global mechanism is adequate for predicting flame speed and area enhancement, some differences in the structure of the flame are observed between predictions with the global and reduced mechanisms.

A 3-D DNS and experimental study of the effect of the recirculating flow pattern inside a reactive kernel produced by nanosecond plasma discharges in a methane-air mixture

This paper presents 3-D DNS and experimental schlieren results used to study a non-equilibrium plasma discharge in a lean methane-air mixture. A detailed combustion mechanism and a plasma model, developed in our previous work, are used in the 3-D computations in order to study the impact of gas flow recirculation on the temporal evolution of species and gas temperature in the vicinity of the discharge zone. The results show that the formation of a fresh gas counterflow, with stagnation plane at the centre of the discharge and parallel to the electrodes, changes the topology of the hot kernel from an initial cylindrical shape to a toroidal one. This phenomenon leads to an increase of the area/volume ratio of the reactive kernel, that may result under certain conditions in kernel extinction. The results also show the importance of considering this 3-D gas flow recirculation to correctly predict the temporal evolution of the hot kernel. In particular, we show that the temperature and species concentrations in the central region of the discharge return to fresh gas conditions shortly after the end of the pulse. In the experimental case investigated here, this time is of the order of 150 µs. This result is particularly important for ignition by Nanosecond Repetitively Pulsed (NRP) discharges, because the gas conditions at the beginning of each successive pulse depend strongly on the time interval between pulses, thus on the pulse frequency

Influence of the nanoaerosol fraction of industrial coal dust on the combustion of methane–air mixtures

The mechanism of formation of nanosized aerosol particles during mechanical grinding of coal from Kuzbass mines is studied. The concentration and size spectrum of aerosol particles in a mine tunnel during cutter operation were measured using an aerosol spectrometer. It is found that 90% of the particles are less than 200 nm in size. In the nanometer range, there are two peaks corresponding to average diameters of 20 and 150 nm, the first of which is due to single particles, and the second to aggregates consisting of single particles. The formation of aerosol during mechanical coal grinding in a continuous flow mill was studied. The spectrum and morphology of the particles produced in the laboratory mill are in qualitative agreement with those for the nanoaerosol formed in the mine. The influence of the coal aerosol on the combustion of gas mixtures was studied. Laboratory experiments showed that the presence of the nanoaerosol in a lean methane–air mixture significantly increased its explosibility. This was manifested in an increase in the maximum pressure and a significant increase in the pressure rise rate during explosion. The study leads to the conclusion that the nanoaerosol is formed from the organic coal components released into the gas phase during local heating of coal on the cutter teeth.

Experimental and numerical investigation of hetero-/homogeneous combustion-based HCCI of methane–air mixtures in free-piston micro-engines

The hetero-/homogenous combustion-based HCCI (homogeneous charge compression ignition) of fuel–lean methane–air mixtures over alumina-supported platinum catalysts was investigated experimentally and numerically in free-piston micro-engines without ignition sources. Single-shot experiments were carried out in the purely homogeneous and coupled hetero-/homogeneous combustion modes, involved temperature measurements, capturing the visible combustion image sequences, exhaust gas analysis, and the physicochemical characterization of catalysts. Simulations were performed with a two-dimensional transient model that includes detailed hetero-/homogeneous chemistry and transport, leakage, and free-piston motion to gain physical insight and to explore the hetero-/homogeneous combustion characteristics. The micro-engine performance concerning combustion efficiency, mass loss, energy density, and free-piston dynamics was investigated. The results reveal that both purely homogeneous and coupled hetero-/homogeneous combustion of methane–air mixtures in a narrow cylinder with a diameter of 3mm and a height of approximately 0.3mm are possible. The coupled hetero-/homogeneous mode can not only significantly improve the combustion efficiency, in-cylinder temperature and pressure, output power and energy density, but also reduce the mass loss because of its lower compression ratio and less time spent around TDC (top dead center) and during the expansion stroke, indicating that this coupled mode is a promising combustion scheme for micro-engine. Heat losses result in higher mass losses. Heterogeneous reactions cause earlier ignition, which is very favorable for the micro-device.

Combustion of lean methane–air mixtures in a slot burner with adiabatic outer walls

The problem of the combustion of a methane–air mixture in a slot burner with adiabatic outer walls is solved. The problem is investigated numerically in a one-dimensional formulation and dimensional variables. The range of existence of stable high-temperature combustion of the methane–air mixture is determined as a function of gas flow rate and methane content in the mixture. The mechanism of failure of stable combustion is determined. The effect of heat transfer of the gas insert with an inner insert on combustion stability is shown.

Accurate Measurements and Analysis of the Thermal Structure of Turbulent Methane/Air Premixed Flame

This paper provides further experimental and numerical results concerning the premix turbulent combustion of lean methane-air mixture. For V-shaped flame, the experimental data were acquired by two-dimensional Rayleigh scattering technique. The main purpose of this investigation is to obtain quantitative information on the instantaneous thermal structure of the flame front for both laminar and turbulent conditions. Four values for turbulence intensity have been considered. The flame surface density is closely related to the two-dimensional temperature gradient. For turbulent combustion, a general decreasing trend of averaged temperature gradient was observed. However, this tendency is inverted for very high turbulence intensity when the instantaneous temperature gradient presents high fluctuations. The flame front thickness PDF and the curvature PDF decrease with the turbulence intensity. The joint PDF of curvature and the maximum of the progress variable's gradient have the tendency to rotate counterclockwise with the increase of turbulence intensity. Negative curvature brings more energy in preheat zone of flame and enhances combustion; consequently the temperature gradients increase.

ГОРЕНИЕ БЕДНЫХ МЕТАНОВОЗДУШНЫХ СМЕСЕЙ В ЩЕЛЕВОЙ ГОРЕЛКЕ С АДИАБАТИЧЕСКИМИ ВНЕШНИМИ СТЕНКАМИ

ГОРЕНИЕ БЕДНЫХ МЕТАНОВОЗДУШНЫХ СМЕСЕЙ В ЩЕЛЕВОЙ ГОРЕЛКЕ С АДИАБАТИЧЕСКИМИ ВНЕШНИМИ СТЕНКАМИ

Comparison of simulated results for CO fields at the flame front by the RANS and LES methods

Turbulent combustion of a lean methane–air mixture in a model combustion chamber behind a step is analyzed by numerical methods. Comparison of the RANS and LES methods shows that the neglect of pulsations in the RANS method may result in significant errors in the calculation of CO fields.

Effects of Electric Fields on the Combustion Characteristics of Lean Burn Methane-Air Mixtures

In this work, the effects of the electric fields on the flame propagation and combustion characteristics of lean premixed methane–air mixtures were experimentally investigated in a constant volume chamber. Results show that the flame front is remarkably stretched by the applied electric field, the stretched flame propagation velocity and the average flame propagation velocity are all accelerated significantly as the input voltage increases. This indicates that the applied electric field can augment the stretch in flame, and the result is more obvious for leaner mixture. According to the analyses of the combustion pressure variation and the heat release rate, the peak combustion pressure Pmax increases and its appearance time tp is advanced with the increase of the input voltage. For the mixture of λ = 1.6 at the input voltage of −12 kV, Pmax increases by almost 12.3%, and tp is advanced by almost 31.4%, compared to the case of without electric fields. In addition, the normalized mass burning rate and the accumulated mass fraction burned are all enhanced substantially, and the flame development duration and the rapid burning duration are remarkably reduced with the increase of the input voltage, and again, the influence of electric field is more profound for leaner mixtures. The results can be explained by the electric field-induced stretch effects on lean burn methane-air mixture.

Combustion of lean methane–air mixtures in monolith beds: Kinetic studies in low and high temperatures

The paper summarizes results of Gosiewski et al. (2009), Pawlaczyk and Gosiewski (2013) and Pawlaczyk (2013) kinetic studies of non-catalytic (thermal) combustion of lean methane–air mixtures, carried out in a low and high temperature, in monolith bed and compared with similar results for a free space. The study reveals an influence of size, type of monolith’s surface and temperature in combustion zone on the reaction mechanism and its kinetics. It was formulated a hypothesis that the share of combustion type: heterogeneous with surface effect (on the monolith’s wall) and homogeneous (in the free space) depends on the temperature in the combustion zone. The appropriate kinetic equations were estimated. Moreover the paper presents corrected values of kinetic parameters which in contrast to Gosiewski et al. (2009) and Pawlaczyk and Gosiewski (2013) were estimated using concentrations of components related to actual gas volume in the combustion zone.