Numerical Simulation of the Pulse-Periodic Nanosecond Discharge in the Methane–Air Mixture

Lean-burn combustion using higher spark energy can enhance the combustion characteristics and emission performance. A laser is electrodeless with a high energy ignition source and can achieve multi-point ignition easily. This paper presents the computational fluid dynamics (CFD) modeling of combustion of methane-air and hydrogen-air mixture in a constant volume combustion chamber (CVCC) by using multi-point laser-induced spark ignition (LISI). The numerical simulation is carried out using ANSYS Fluent software, and a standard turbulence model (k – ε) is used. The pressure-time history and reaction progress was plotted for various equivalence ratios (ϕ). Numerical result shows that multi-point ignition can completely burn methane-air and hydrogen-air mixture in the range of 0.77 to 1.68 and 0.36 to 4.1, respectively. The maximum peak pressure (Pmax) and shortest combustion duration were observed at ϕ of 1.29 for the methane-air mixture and ϕ of 1.0 for the hydrogen-air mixture. The relative error between the numerical and the experimental results from the literature are within 10%. The faster reaction progress was observed for the hydrogen-air mixture compared to the methane-air mixture for all equivalence ratios. Finally, the validated numerical model was used to predict the CO, CO2, and NOx emission trends. For the methane-air mixture, CO emission increases as the equivalence ratio increases. However, the maximum CO2 emissions were observed at stoichiometric conditions. The highest NOx emissions were observed for the hydrogen-air mixture compared to the methane-air mixture at a stoichiometric equivalence ratio.

Experimental investigation of flame/solid interactions in turbulent premixed combustion

Comparative investigations of flame kernel development in a laser ignited hydrogen–air mixture and methane–air mixture

The spontaneous combustion of the coal microparticles of fractions 1–20 μm and 20–32 μm in an air atmosphere and the inflammation of the coal microparticles of fraction 20–32 μm in a methane–air mixture at temperatures of 700–1100 K were investigated with the use of a rapid compression machine. A contactless measurement of the temperature of the coal particles ignited spontaneously in a gas has shown that this temperature can reach 2500 ± 200 K and substantially exceeds the temperature of the gas at the end of its compression stroke. It was established that the coal microparticles burning in a stoichiometric methane–air mixture are local hotbeds of fire in this mixture at a temperature as high as 1400 K and that the gas is ignited in the neighborhood of these hotbeds. The limiting temperatures of ignition of the coal microparticles in an air atmosphere free of methane and in a methane–air mixture were determined. The measured times of delay in the ignition of the methane by the coal microparticles in a hybrid methane–air mixture agree with the delay times of ignition of a pure methane–air mixture under the same conditions to within the experimental error.

In order to clarify the effects of different kinds of fuel and fuel equivalence ratio on flame structure, a numerical simulation of triple flame developed in a co-flowing methane-air or hydrogen-air mixture and air stream was made taking into account the elementary chemical reaction mechanism. The following conclusions were reached: (1)The relation between the apparent burning velocity of the triple flame and the fuel equivalence ratio shows a similar tendency to that of the one-dimensional premixed flame of the corresponding fuel. However, the fuel equivalence ratio at which the apparent burning velocity is the largest is a little larger than that of the one-dimensional premixed flame. The apparent burning velocities are two and three times higher than that of the one-dimensional premixed flame for the methane air or hydrogen-air mixture. (2)The flame thrusts out forward in the downstream of the boundary between mixture and air stream, and a part of the flow is bent and forks out in this protruding flame so that a triple flame is originated; this triple flame is composed of fuel rich and lean premixed flame branches and a diffusion flame branch. The change in shape of the convex part, caused by the effect of the one-dimensional premixed flame, is further promoted by the effect of hydrodynamic instability originated in the expansion brought about by heat release. A considerably strong diffusion flame branch exists almost in the center of the two premixed flame branches for the methane air mixture, while a considerably weak diffusion flame branch approaches the fuel lean premixed flame branch for the hydrogen air mixture. (3)Near the fuel equivalence ratio at which the burning velocity of the one-dimensional premixed flame is the largest, the effect of the one-dimensional premixed flame becomes large and the fuel rich premixed flame advances and becomes vertical to the flow direction. As a result, the effect of hydrodynamic instability is weakened. Thus, both of these effects demonstrate that the fuel equivalence ratio at which the apparent burning velocity is the largest is a little larger than that of the one dimensional premixed flame.

Kinetic effects of hydrogen addition on the catalytic self-ignition of methane over platinum in micro-channels

Effects of hydrogen addition on oblique detonations in methane–air mixtures

A detailed numerical study of laminar burning speed for fuel–air mixture is conducted using laminarReactingLMFoam solver which is a modified version of reactingFoam solver based on openfoam code. It accounts for detailed mixture-averaged transport property calculation for reacting flow using low-Mach number governing equations. The effect of various equivalence ratio gradients is studied on stratified hydrogen–air and methane–air mixture with mixture-averaged transport model and unity Lewis number for all species, and corresponding laminar burning speed is compared with homogeneous mixture. For both the fuel–air mixture, rich to lean stratified mixture resulted in a higher laminar burning speed and no significant difference was noticed for lean to rich stratified mixture when compared with homogeneous mixture at same local equivalence ratio. Increased burning speed is explained based on higher burnt gas temperature and molecular diffusion of lighter species from burnt gas referred to “Chemical Effect” in this study. The effect of thermal and molecular diffusion from the burnt gas on laminar burning speed is studied for stratified and homogeneous mixture using mixture-averaged transport model and unity Lewis number for all species. It is shown that the molecular diffusion effect from burnt gas (“Chemical Effect”) is more prominent as compared with the thermal diffusion effect. Extension in lean flammability limit for stratified mixture of both the fuel is shown based on higher heat release rate as compared with homogeneous mixture and extension in flammability limit for stratified mixture is explained based on higher Chemical Effect from burnt gas.

<div class="htmlview paragraph">The combustion processes of of lean mixtures of methane in air is examined by employing a detailed chemical kinetic scheme consisting of 178 elementary reaction steps with 41 species. The changes with time in the concentrations of the major relevant reactive species are determined from the preignition reactions to the time near equilibrium conditions. The results of such an approach to the combustion process are considered over a wide range of initial temperatures (1000 K - 1600 K) and equivalence ratios (0.2 - 1.2) while the pressure was kept at atmospheric. Calculated results obtained while using this model tend to be in good agreement with the corresponding experimental values of ignition delay. The ignition delay of methane-air mixture correlated by the following empirical expression</div> <div class="htmlview paragraph"> <div id="FD1" class="formula"> <div class="graphic-wrapper"><img class="article-image equation block" src="1999-01-3483_fig0001.jpg" alt="No Caption Available"/></div> </div> </div> <div class="htmlview paragraph">in which constants <b>A</b> and <b>B</b> are function of the equivalence ratio while <b>T<sub>i</sub></b> is the initial mixture temperature in °K.</div>

On the influence of singlet oxygen molecules on the speed of flame propagation in methane–air mixture

The subject of this study is the comparative analysis of the kinetic mechanisms that proceed in a methane–air mixture when O2 molecules are excited to the electronic state by laser photons with wavelength λI = 762.346 nm and when O2 molecules dissociate due to the absorption of laser radiation with λI = 193.3 nm. The efficiencies of both methods of combustion initiation are compared with each other and against the method of laser-induced thermal ignition. Numerical simulation shows that for methane–air mixture the excitation of O2 molecules to the state is more effective in reducing the induction time and in lowering the ignition temperature than the method of photodissociation of O2 molecules by laser radiation at 193.3 nm wavelength. In order to ignite a stoichiometric CH4–air mixture at identical temperature it is needed to supply twofold greater energy upon photodissociation of O2 molecules than in the case of O2 molecule excitation. However, both the laser-induced excitation of O2 molecules to the state and O2 molecule dissociation by laser photons with λI = 193.3 nm are much more effective in combustion initiation than the method based on heating the mixture by laser radiation.

Explosion characteristics of methane–air mixtures in a spherical vessel connected with a duct

A review on understanding explosions from methane–air mixture

Effect of concentration and ignition position on vented methane–air explosions

Laminar burning velocity and Markstein lengths of methane–air mixtures