Combustion Of Methane-air Mixtures Research Articles

Chemical kinetics modelling for the effect of chimney on diffusion flame in carbon nanotubes synthesis

Hydrocarbon flame syntheses of carbon nanotubes (CNTs) on various metals substrates have been reported in literature, but existing methods are limited to usage of high melting temperature metals substrate due to excessive temperature in conventional diffusion flame burner. To address this limitation, high thermal conductivity chimney is innovatively incorporated into the diffusion burner to control flame temperature. Hence, the aim of this study is to investigate the effect of chimney application on the interaction behaviour between flame temperature profile and concentration of post combustion species distribution of diffusion flame. In this study, Computational Fluids Dynamics (CFD) – Chemical Kinetics (Chemkin) coupling method is used to simulate the flow and transport behaviour of laminar methane-air mixture in combustion environment. Kinetics mechanism is imported into species transport model of Chemkin simulator to further determine the reaction between combustion species and temperature profile of the mixture. Heat transfer models are then integrated into simulation to investigate the cooling effect of chimney during the combustion of methane-air mixture. This numerical simulation study provides insights on effects of chimney application towards cooling, species concentration, flame temperature and paving path for controlling the growth of CNT on low melting temperature metal substrate. Numerical models show improvement in temperature controls, increase in gas species concentration and in surface area that is favourable for growth of CNTs.

Numerical simulation of turbulent combustion of methaneair mixture in moderate Reynolds number conditions

Numerical simulation of turbulent combustion of methaneair mixture in moderate Reynolds number conditions

A comparative and experimental study on performances of Natural gas-Air Combustion, Helium, and Air as Propellants in a Gas Gun

ABSTRACT In this study, the performance of a single-stage gas gun with a natural gas-air mixture as the propellant gas is investigated by measuring the explosion pressure and the projectile velocity. In this work, the combined effect of elevated initial pressure, mixing, equivalence ratio, and ignition energy on the explosion pressure and projectile velocity is studied. To assess the performance of the natural gas-air combustion, a series of tests with helium and air as the propellant gas are carried out to measure the projectile velocity. The fuel used in this study consisted of 84.2% methane, 9.5% ethane, and 3.7% propane. The helium gas was 99.999% pure. The volume fraction of fuel in the natural gas-air mixture varied between 4.98% and 17.3%, and the flammability limits of the natural gas were between 4.41% and 14.47%. The equivalence ratio Φ was used to control the fuel concentration in the mixture. The equivalence ratio in this study was 0.5, 0.75, 1, 1.5, and 2. The experimental results indicated that despite the low percentage of methane and high percentage of higher hydrocarbons in natural gas, the maximum pressure still occurs at the equivalence ratio 1, the finding which had already been proved to be the case for pure methane. However, the maximum combustion pressure is reduced compared to other fuels with higher methane and lower hydrocarbon content. Also, the study showed that the maximum combustion pressure increased with rising the ignition energy also when the mixer was used to blend the natural gas-air mixture. The experimental results showed that at low pressures, the projectile velocity was higher using helium compared to the same pressures produced by the combustion of methane-air mixture. However, at higher pressures, the projectile velocity was much higher for the pressures produced by combustion due to its lower molecular weight and the heat produced in the combustion process. The price analysis also indicated that for the same output pressure, the cost of a test with a methane-air explosion is 78 times lower than that of a test with helium.

The impact of oxidiser humidity on methane combustion in a gas burner

The improvement of thermodynamic and environmental indicators of energy facilities is a vital and topical problem. Numerical simulation methods are used to investigate the processes of combustion and formation of hazardous substances by computation, and to work out recommendations on increasing the effectiveness of fuel combustion. The paper examines the impact of atmospheric air humidity on the process of combustion of a stoichiometric methane-air mixture in a burner. The combustion process is simulated in the 3D unsteady statement in Cartesian coordinates. In the investigation, the combustion of the methane-air mixture is considered in one stage. Air humidity is considered in the 0 to 100% range. The monoxide nitrogen formation process is described using the extended Zeldovich-Fenimore mechanism. A comparative study has found that increasing air humidity up to 100% reduces the mass emission of monoxide nitrogen by a factor of 1.27, as compared to dry air. The study results can be used for designing equipment that uses methane as fuel.

Investigation on the overpressure of methane-air mixture gas explosions in straight large-scale tunnels

To investigate the overpressure of methane-air explosions in straight large-scale tunnels, the computational fluid dynamics (CFD) code of Flame Accelerator Simulator (FLACS) was used and validated against experiments conducted at three different scales, and the effects of the volume concentration of methane in air, the blockage ratio (BR), the tunnel length, and the cross-section were studied. When analysed using the GaussAmp mathematical model, the maximum peak overpressure appears at a volume concentration of 10.30 % of methane in air. Blockage ratios (BR) of 0.15 and 0.3 resulted in the combustion of methane-air mixtures with the volume concentration of 6.5 % and 14.0 % of methane in air, producing a fatal overpressure of 21 kPa. When the BR increases up to 0.75, both the lean and rich mixtures cause a peak overpressure of over 60 kPa. Combustion of the same methane-air mixture produces the same overpressure, which decays approximately linearly at the same slope owing to a smooth wall roughness before travelling near the outlet, independent of the specific tunnel length. A method to characterise the cross-sections was proposed, and the maximum peak overpressure of different lengths of methane-air mixtures in different cross-sectional tunnels was found, presenting various regimes from a hump shape to a wave-like uplift and bowl shape. The cross-section parameters determine the degree of confinement and further control the maximum peak overpressure in the modelled tunnels. An exponential asymptotic model can be used to conveniently obtain the maximum peak overpressure. These phenomena indicate that approximately square-shaped cross-sections should be selected to avoid an extremely high overpressure in large-scale tunnels with the potentially significant accumulation of methane-air mixtures.

An internal combustion alternator with both free piston and cylinder

The free-piston generator has more merits than the traditional reciprocating engines, and has been under extensive investigation. This study focuses on the development two novel perspective ideas unlike already advanced the linear free piston generator. These are: (1) to make both piston and cylinder free that provides ideally balancing of the system; (2) to turn from the linear scheme of a generator to rotary design because of simplicity and compactness. The mathematical simulation of operation of the linear two-stroke cycle alternator with both free piston and cylinder was conducted with using the model of zero-dimensional dynamics. Combustion of methane-air mixture with excess air ratio of 1.2 was considered. Both heat and friction losses were taken into account. Operation particularities of such system were analyzed. It was found that oscillation amplitude of moving parts did not exceed of 70 mm at frequency of ∼50 Hz for alternator with following basic parameters: cylinder bore of 80 mm, length of 400 mm, piston length of 120 mm. Mass of both piston and cylinder was chosen identical and equal of 3 kg. It was shown that the efficiency of the chemical energy conversion reached of 46%, and specific power was high as 40 kW/l at compression ratio of 40. The lab scale pneumatic alternator in rotor variant with both free rotor and cylindrical body was assembled for experimental demonstration of rotary design capability. The alternator was fed with compressed air of low pressure below of 6 atm. The electrical power in the load reached of 10 W at input power with intake compressed air about 100 W. The results indicate that alternators with both free piston and cylinder in linear and rotary designs have the potential to achieve high efficiency and provide the vibration – free operation.

Mathematical Modeling of Processes of Thermal Recovery of Carbon-Containing Waste

The physical scheme and mathematical model of the processes of thermal recovery of carbon-containing waste are presented. As a basis for modeling combustion processes, a kinetic multi-reactor model is used that includes equations for changing composition, enthalpy and mass, equations for calculating temperature and parameters of associated processes. The change in the state of the reacting mixture is due to convective and diffusive heat and mass transfer, chemical interaction between the components of the reacting mixture. The results of numerical studies of the parameters of combustion products during combustion of methane-air mixture, pyrolytic conversion products of litter and sewage sludge are presented.

Modeling of nitrogen oxides emission in the cylinder of tractor gas-diesel engine

The article is devoted to the modeling of process of nitrogen oxides formation in the cylinder of diesel engine operating on compressed natural gas. The combustion of methane-air mixture, ignited by atomized pilot oil that is injected through multijet nozzle, in the cylinder of gas-diesel engine increases the rate of turbulent fluctuations, which leads to intensification of heat and mass exchange in the plume, and significantly increases the rate of formation and decomposition of nitrogen oxides. The calculation of expected emissions of nitrogen oxides in the cylinder of diesel engine and their content in the exhaust gases is carried out. It is done on the basis of the results of kinetic systems modeling for calculation of the kinetics of nitrogen oxides formation during combustion of natural gas and diesel fuel, taking into account the influence of turbocharging, intermediate cooling of charged air and exhaust gas recirculation. In the mathematical modeling of nitrogen oxide formation in the cylinder of diesel engine, the combustion process is divided into two zones, namely the zone of fresh mixture and the zone of combustion products. First zone is a mixture of fresh air with recycled gases (if there is a process of recycling) and residual gases. Before combustion starts, this zone occupies the whole cylinder. During combustion, the volume of zone of combustion products increases. Theoretical researches are carried out on the basis of developed mathematical model for calculation of nitrogen oxides in the cylinders of 4Ch 11.0/12.5 and 4ChN 11.0/12.5 diesel engines. The high precision of theoretical calculations of volumetric content and mass concentration of nitrogen oxides is confirmed, compared to the results of experimental researches (the difference does not exceed 5%).

Numerical Studies of the Combustion of Methane-Air Mixture in a Meso-Scale TPV Combustor with a Heat-Regeneration Reverse Tube and a Porous-Medium Injector

The major objective of this paper is to develop a miniature combustor applied in a portable thermophotovoltaic (TPV) power generation system. The proposed miniature TPV system consists of a swirling combustor with the infrared thermal tube (ZrO2) acting as the emitter, a heat-regeneration reverse tube, and mixing-enhancing porous-medium fuel injection, which improves the low non-uniform illumination or incomplete combustion problems associated with conventional miniature TPV systems. A two-step global reaction mechanism was used to compute the chemical reactions of the reacting flows. Numerical simulations are performed to analyze the details of the flame structure and flame stabilization mechanism inside the meso-scale combustor with and without a reverse tube. In addition, these thermal conditions have strong effects on the combustion especially when the chamber dimension goes small and the ratio of surface area to volume becomes larger. Such as larger heat loss through the chamber wall to the ambient, may lead to the thermal quenching of meso-scale combustion. Therefore, the effect of various heat transfer conditions at chamber wall, e.g. with different wall heat conductivity on the combustion is delineated and discussed by numerical simulation. Results indicate that the proposed swirling combustor with a heat-regeneration reverse tube and porous medium can improve the intensity and uniformity of the combustion chamber (emitter) illumination, and can increase the surface temperature of the chamber wall. Correspondingly, the simulation results also indicated that the stable combustion in a meso-scale TPV combustor may be sustained through to control mass flow rate, heat loss conditions and maintaining proper thermal condition. Consequently, through such systematic numerical analysis, proper operational parameters for the meso-scale TPV combustor are suggested, which may be used as the guideline for meso-scale TPV combustor design.

Effect of olefin + carbon dioxide admixtures on the ignition of methane-air mixtures

Experimental data are reported on the kinetics and mechanism of methane-air combustion in a closed space at atmospheric pressure and on the effects of inhibitor admixtures and inert gases. Ignition arises and develops as a result of a chain avalanche occurring in the mixture, and self-heating is significant only in developing chain combustion. Admixtures containing propene or isobutene and carbon dioxide (5% olefin + 20% CO2) efficiently prevent the ignition and combustion of methane-air mixtures of any composition.

Heat Transfer Investigation During Combustion Of Methane-air Mixture In Reciprocating Engine Cylinder

Heat Transfer Investigation During Combustion Of Methane-air Mixture In Reciprocating Engine Cylinder

Determination of Population Inversion in Microwave-Enhanced Combustion of Methane-Air Mixture in Ceramic Burner.

Microwave enhanced combustion of methane-air mixture in a ceramic burner is studied and the results suggest perturbation of the assumed Boltzmann distribution of molecular energy levels. A small signal gain, which represents the level of amplification of an incident beam in a medium, was detected using a CO2 laser. An increase in the signal ratio of two MCT photodetectors when the combustion zone was irradiated by microwaves (2.45 GHz) indicates amplification of infrared radiation (10.6 μm), hence the possibility of using the system for laser applications. However, statistical analysis reveals that varying levels of microwave power are not significantly different.

Compression Ratio Effect on Methane HCCI Combustion

We have used the HCT (hydrodynamics, chemistry, and transport) chemical kinetics code to simulate HCCI (homogeneous charge compression ignition) combustion of methane-air mixtures. HCT is applied to explore the ignition timing, burn duration, NOx, production, gross indicated efficiency and gross IMEP of a supercharged engine (3 atm. intake pressure) with 14:1, 16:1 and 18:1 compression ratios at 1200 rpm. HCT has been modified to incorporate the effect of heat transfer and to calculate the temperature that results from mixing the recycled exhaust with the fresh mixture. This study uses a single reaction zone that varies as a function of crank angle. The ignition process is controlled by adjusting the intake equivalence ratio and the residual gas trapping (RGT). RGT is internal exhaust gas recirculation, which recycles both thermal energy and combustion product species. Adjustment of equivalence ratio and RGT is accomplished by varying the timing of the exhaust valve closure in either two-stroke or four-stroke engines. Inlet manifold temperature is held constant at 300 K. Results show that, for each compression ratio, there is a range of operational conditions that show promise of achieving the control necessary to vary power output while keeping indicated efficiency above 50 percent and NOx levels below 100 ppm. HCT results are also compared with a set of recent experimental data for natural gas.

Simulation of nitric oxide formation in combustion of methane-air mixtures

Simulation of nitric-oxide (NO) formation in a quasi-one-dimensional constant-pressure chemical reactor has been performed. The oxide yield in the reactor for various initial mixtures was calculated. The mechanisms ofNO formation in chemically reacting systems are described. Four groups of reactions responsible for individual stages of the oxidation and various routes ofNO formation in the oxidation of atmospheric nitrogen are distinguished. A method of integration of the system of kinetic equations that makes it possible to determine the contribution of individual reactions and groups of reactions to the production of the given component is proposed. The reactions providing maximum contributions to the formation and removal ofNO molecules are determined. The role of individual routes of nitric-oxide formation is established.

Influence of the aerosol formed in the fast evaporation of superheated water on the combustion of methane-air mixtures in a closed vessel

The influence of the aerosol formed in the fast evaporation of superheated water at 150°C on the combustion of a methane-air mixture in a near-spherical closed vessel (volume 20 dm3) is investigated experimentally. The concentrational limits of flame propagation and the normal combustion rate are determined as a function of the mass concentration of the water aerosol. It is found that the flegmatizing ability of the aerosol is relatively low; it is not due to the formation of fairly large water drops but practically completely to the presence of saturated water vapor. Exponential reduction in normal combustion rate of the mixture with increase in concentration of the superheated-water aerosol is observed.

Superadiabatic combustion of methane air mixtures under filtration in a packed bed

We have developed a model for the finite deformation thermomechanical response of α-cyclotrimethylene trinitramine (RDX). Our model accounts for nonlinear thermoelastic lattice deformation through a free energy-based equation of state developed by Cawkwell et al. (2016) in combination with temperature and pressure dependent elastic constants, as well as dislocation-mediated plastic slip on a set of slip systems motivated by experimental observation. The kinetics of crystal plasticity are modeled using the Orowan equation relating slip rate to dislocation density and the dislocation velocity developed by Austin and McDowell (2011), which naturally accounts for transition from thermally activated to dislocation drag limited regimes. Evolution of dislocation density is specified in terms of local ordinary differential equations reflecting dislocation–dislocation interactions.This paper presents details of the theory and parameterization of the model, followed by discussion of simulations of flyer plate impact experiments. Impact conditions explored within this combined simulation and experimental effort span shock pressures ranging from 1 to 3 GPa for four crystallographic orientations and multiple specimen thicknesses. Simulation results generated using this model are shown to be in strong agreement with velocimetry measurements from the corresponding plate impact experiments. Finally, simulation results are used to motivate conclusions about the nature of dislocation-mediated plasticity in RDX.

Superadiabatic combustion of methane air mixtures under filtration in a packed bed

Filtration combustion in porous media differs substantially from combustion in a homogeneous media. The difference is the heat transfer between the filtrated gas and the porous medium under conditions of active heat transfer over a highly developed internal solid surface. The reactive transfer is characterized by a thermal wave velocity that determines the velocity of heat accumulation in a porous medium due to the filtrated flow and by a reaction wave velocity. A resonance condition can occur when both velocities nearly coincide and the heat of reaction becomes localized in a moving thermal wave. A temperature can be reached much in excess of the adiabatic reaction temperature and thus self-propagating reactions are possible even in a weakly exothermic medium. The propagation of the localized resonance wave and the superadiabatic reaction effect is confirmed by experiments and the results of a simple mathematical analysis. The model is compared with predictions for the thermal and combustion wave speeds and demonstrates good agreement.

Reduction in nitrogen oxide concentration in homogeneous combustion of methane and its mixtures with nitrogenous substances in air

Experiments were carried out with a jet apparatus with laminar flame at atmosperic pressure in order to study the two-step combustion of pure methane-air mixtures and similar mixtures with nitrogenous substances - methylamine and pyridine - added. The first series of experiments used methane-air mixtures. During combustion, nitrogen oxide was formed from atmospheric nitrogen. In the second series of experiments the methane contained 1.5-2.5% of a nitrogenous additive. In this case, the quantity of nitrogen oxide formed was many times lower. Analogous experiments were performed with addition of 1.5-2.5% pyridine, and similar results were obtained. Under the conditions described, two-step combustion of methane-air mixtures with methylamine or pyridine added also proves quite effective.