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Abstract Ignition process in a premixed methane-air swirl configuration is studied using a large eddy simulation method with Smagorinsky sub-grid scale model. A developed thickened flame combustion approach with two-step methane-air mechanism is used. Non-reacting mean and RMS axial, tangential and radial velocity profiles are validated against the experimental results. It is shown that the flow field consists of four zones: Inner Recirculation Zone, Inner Shear Layer, Outer Shear Layer and Corner Recirculation Zone. The mean and RMS of velocities and temperature in reacting flow are then validated against the experimental data. Large eddy simulation is used to investigate the ignition sequence by sparking in the four zones in the flow field. Flame growth, propagation and stabilization are studied for these cases. Results show that sparking in IRZ has the fastest flame growth and takes the minimum time to reach flame stabilization. Propagating flame surface in all cases has sharp flame edges, without any hysteresis for flame position. Finally, flame structures are analyzed by flame curvature and the effect of flow field velocity on the flame surface. Highlights • Ignition process is investigated by large eddy simulation and thickened flame model. • Sparking in IRZ has the fastest flame growth and minimum time to flame stabilization. • Flame attachment position doesn't show any hysteresis with respect to spark position. • The major controlling mechanism is the velocity direction in the propagation phase. [ABSTRACT FROM AUTHOR]

Abstract In the current study, the effect of external thermal pattern on the dynamics and characteristics of methane-oxygen and methane-oxygen-carbon dioxide premixed flames in non-adiabatic meso-scale cylindrical reactors is investigated experimentally. In this regard, two different external thermal patterns were imposed on the outer surface of the reactors. The results showed that imposing method/direction and also temperature level of the external thermal pattern have impressive effect on flame dynamics and chemiluminescence in the non-adiabatic meso-scale reactors. Also, it was shown that increasing the temperature level of the external thermal pattern could significantly extend the flame stability and its presence range in the meso-scale reactors, especially for the vitiated mixtures (methane-oxygen-carbon dioxide). Moreover, the results demonstrated that decreasing the inner diameter of a meso-scale reactor, which was subjected to an external thermal pattern, could increase the flame controllability and its presence range in the non-adiabatic meso-scale reactors. Highlights • Imposing external heat source affects the flame dynamics in small scale reactors. • External heat source and its imposing direction affect asymmetric flame shape. • Flame dynamics may be accompanied by variation in the flame chemiluminescence. • Effect of external heat source on stability of the vitiated flames is more obvious. • Decreasing the diameter of a heated meso reactor extends flame controllability. [ABSTRACT FROM AUTHOR]

Large eddy simulation (LES) is applied to investigate blended CH 4 –H 2 swirling inverse diffusion flames. In this regard, two different compositions of fuel stream including 30% and 80% molar fraction of hydrogen are utilized. Two distinct swirl numbers of 0.667 and 1.33 are applied to the central air stream. The considered configuration for studying swirling inverse flames is extracted from the well-known DLR (Deutsches Zentrum Luft- und Raumfahrt) burner with modifications to allow investigating inverse flames. The DLR-A benchmark is regarded as a reference to contrast Large Eddy Simulation (LES) and Unsteady Reynolds Averaged Simulation (URAS) predictions. It is found that LES results outperform URAS computations in a sufficiently fine grid. Investigating blended CH 4 /H 2 inverse flames revealed that the jet penetration length is reduced by an augmentation in hydrogen concentration. This is ascribed to the increased mixing with hydrogen enrichment which leads to decreased flame length as well. It is found that with curtailed concentration of hydrogen, the major axis of vortical structures is aligned with the flow direction. In the other words, the eddies are flattened and the lateral size of mixing region is reduced. The consequence is an enhancement in flame length. A noticeable decrease in the mass fraction of carbon monoxide pollutant is observed with hydrogen enrichment. Reduction in inflow carbon atoms and rapid CO oxidation owing to excessive air in the mid- and far-field regions are authorized as the contributing factors. The lowered peak temperature in presence of enhanced hydrogen concentration is attributed to the related reduction in thermal power. [ABSTRACT FROM AUTHOR]

In the present study, we experimentally evaluated the effects of geometrical parameters, Reynolds number, dilution percent, and equivalence ratio on methane–oxygen–carbon dioxide flame dynamics in non-adiabatic cylindrical meso-scale reactors with the backward facing step. Five flame regimes of blow-out, marginal, stationary (asymmetric and spinning), RERI, and flashback were observed during the tests. The results showed that adding carbon dioxide to methane–oxygen mixture could extend RERI flame regimes in the meso-scale reactors. Moreover, it was shown that the presence of RERI flame regime could lead to more uniform temperature distribution on the outer surface of the lengthy meso-scale reactors as compared to the stationary flame regimes. Also, it was demonstrated that increasing the average flow speed in the meso-scale reactors decreased the average frequency of RERI flame regimes. [ABSTRACT FROM AUTHOR]

In this study, we experimentally investigated the effects of geometrical parameters (such as the reactor length ( L ) and the inner diameter ( D R )), the step height ( r in − r R ) , the Reynolds number (Re), and the equivalence ratio ( φ )) of the inlet mixture on the behavior of the rich fuel methane–oxygen flames in non-adiabatic cylindrical meso-scale reactors with the backward facing step. During the experiments, seven different flame regimes were observed. These flame regimes were the blow-out, marginal, stationary (stable), RERI, stationary (stable)-flashback, RERI-flashback, and flashback, respectively. Also, the results showed that the length and diameter of reactors could strongly affect flame dynamics, especially the borders among the observed flame regimes in the meso-scale reactors. As a result, it was demonstrated that the reactor length and diameter obviously influenced the traveling speed and frequency of RERI flame regime. Also, it was shown that decreasing the step height from 1.4 mm to 0.4 mm suppressed RERI flame regime in the meso-scale reactors. Moreover, in the certain ranges of the equivalence ratio, increasing the Reynolds number decreased the flame presence range in the meso-scale reactors. Finally, it was concluded that RERI flame regime is a prevalent operational regime for applications in which an almost uniform temperature pattern on the outer surface of lengthy meso-scale reactors is required. However, it was shown that this privilege was not valid as compared to the temperature distribution patterns caused by the stationary flame regimes in the small length reactors. [ABSTRACT FROM AUTHOR]

Highlights: [•] Flame regimes of stable asymmetric and RERI have been observed in meso scale reactor. [•] By increasing the inlet velocity, the equivalence ratio is decreased to near unity. [•] The increase of velocity more than a level reduces the flame presence in the reactor. [•] Increasing the dilution percent restricts the asymmetric stable flame region. [•] In such a meso scale reactor just asymmetric flame as a stable flame can be formed. [ABSTRACT FROM AUTHOR]

Abstract: Reduction of air pollutants formation from hydrocarbon combustion process requires improvements in combustion systems. The moderate and intense low oxygen dilution (MILD) combustion technique is an opportunity to achieve such a goal. MILD combustion is a combustion regime which can be attained by high temperature preheating and high level dilution. In this paper, the mechanism of CO and CO2 formation for a CH4/H2 fuel mixture is studied under MILD combustion condition of a jet in hot coflow (JHC) burner. This investigation is done using the computational fluid dynamics (CFD) and also zero dimensional well-stirred reactor (WSR) analysis. The RANS equations with modified k–ε equations are solved in an axisymmetric 2D computational domain. The DRM-22 reduced mechanism is considered to represent the chemical reactions. The effects of oxidizer oxygen concentration and fuel hydrogen content are studied on methane oxidation pathways. Results show that the higher hydrocarbon oxidation pathways are effective on CO and CO2 formation under MILD condition. In the methane oxidation mechanism, the ratio between the main route and ethane route is the main reason of CO increment at higher O2 level under MILD condition in JHC laboratory burner. The WSR analysis illustrates that a decrease of O2 concentration in oxidizer does not necessarily lead to lower production of CO and CO2. [Copyright &y& Elsevier]

Abstract: Large eddy simulation (LES) method is employed to investigate the effect of the hydrogen content of fuel on the H2/CH4 flame structure under the moderate or intense low-oxygen dilution (MILD) condition. The turbulence–chemistry interaction of the numerically unresolved scales is modelled using the PaSR method, where the full mechanism of GRI-2.11 represents the chemical reactions. The influence of hydrogen concentration on the flame structure is studied using the profiles of temperature, CH2O and OH mass fractions and the diffusion profiles of un-burnt fuel through the flame front. Furthermore, more details are investigated by contours of OH, HCO and CH2O radicals in an area near the nozzle exit zone. Results show that increasing the hydrogen content of fuel reinforces the MILD combustion zone and increases the peak value of the flame temperature and OH mass fraction. This increment also increases the flame thickness and reduces the OH oscillations and diffusion of the un-burnt fuel through the flame front. [Copyright &y& Elsevier]

Abstract: In this study, an approach to modification and improvement of CH4/air pre-mixed flame in a micro-stepped tube is numerically studied. The effects of added hydrogen to methane as an additive, entrance mixture velocity and some physical properties such as the micro-stepped tube wall thermal conductivity and the outer wall convective and radiative heat transfer coefficients on temperature distribution and combustion progress in a micro-stepped tube are calculated using a high order, high accuracy 2D numerical laminar steady state code. The results show that adding hydrogen to methane in a micro-stepped tube can play a pivotal role in modification processes of combustion phenomena in a micro combustor. Also, it is found that adding hydrogen to CH4 can assure the flame presence in some certain conditions in comparison to the simple backward facing step method. Moreover, adding hydrogen can improve the concentration of the combustion vital radicals and also temperature distribution along the micro combustor, impressively. In this regard, adding hydrogen to methane as an additive sustains the combustion vital radicals against the variation of the heat transfer conditions in or around a micro-combustor. [Copyright &y& Elsevier]

In this paper, the importance of fluctuations in flow field parameters is studied under MILD combustion conditions. In this way, a turbulent non-premixed CH4+H2 jet flame issuing into a hot and deficient co-flow air is modeled using the RANS Axisymmetric equations. The modeling is carried out using the EDC model to describe the turbulence-chemistry interaction. The DRM-22 reduced mechanism and the GRI2.11 full mechanism are used to represent the chemical reactions of H2/methane jet flame. Results illustrate that although the fluctuations in temperature field are small and the reaction zone volume are large in the MILD regime, the fluctuations in temperature and species concentrations are still effective on the flow field. Also, inappropriate dealing with the turbulence effect on chemistry leads to errors in prediction of temperature up to 15% in the present flame. By decreasing of O2 concentration of hot co-flow air, the effect of fluctuations in flow field parameters on flame characteristics are still significant and its effect on species reaction rates does not decrease. On the other hand, although decreasing of jet inlet Reynolds number at constant inlet turbulence intensity addresses to smaller fluctuations in flow filed, it does not lead to lower the effect of turbulence on species distribution and temperature field under MILD combustion conditions. [ABSTRACT FROM PUBLISHER]

Abstract: Energy crises and the preservation of the global environment are placed man in a dilemma. To deal with these problems, finding new sources of fuel and developing efficient and environmentally friendly energy utilization technologies are essential. Hydrogen containing fuels and combustion under condition of the moderate or intense low-oxygen dilution (MILD) are good choices to replace the traditional ones. In this numerical study, the turbulent non-premixed CH4+H2 jet flame issuing into a hot and diluted co-flow air is considered to emulate the combustion of hydrogen containing fuels under MILD conditions. This flame is related to the experimental condition of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147–1154]. In general, the modelling is carried out using the EDC model, to describe turbulence–chemistry interaction, and the DRM-22 reduced mechanism and the GRI2.11 full mechanism to represent the chemical reactions of H2/methane jet flame. The effect of hydrogen content of fuel on flame structure for two co-flow oxygen levels is studied by considering three fuel mixtures, 5%H2+95%CH4, 10%H2+90%CH4 and 20% H2+80%CH4(by mass). In this study, distribution of species concentrations, mixture fraction, strain rate, flame entrainment, turbulent kinetic energy decay and temperature are investigated. Results show that the hydrogen addition to methane leads to improve mixing, increase in turbulent kinetic energy decay along the flame axis, increase in flame entrainment, higher reaction intensities and increase in mixture ignitability and rate of heat release. [Copyright &y& Elsevier]

The effects of the diameter and location of an inserted wire on methane–air flame characteristics in a micro-burner, with a backward-facing step, were investigated numerically. Our goal was to shed light on the parameters that the authors had not already considered in the previous study. To do so, the effects of the studied parameters on the flame location and distribution of temperature, H, and OH species, were scrutinized. It was shown that increasing the inserted wire's diameter and relocating the inserted wire towards the outlet had polynomial and linear effects on the flame location in the burner, respectively. Although changing these two parameters did not have any obvious effects on the maximum temperature of the auxiliary axis in the burner or the external wall, effects on the peak values of the hot-flame critical chemical species of OH and H were recognized. Furthermore, it was shown that the temperature distribution on the outer surface of the burner was more influenced by the wire's axial location in the burner, rather than the wire's diameter. This effect may be of interest for designing micro-TPVs or micro-TEGs. [ABSTRACT FROM AUTHOR]

• Variation in Reynolds number shifted the location of maximum flame speed. • Fuel-rich flames were more stable than fuel-lean ones in the small-scale reactors. • Heat loss and instabilities controlled the flame propagation speed in the reactors. • Instabilities increased the flame propagation speed in the reactor. • Most of flame fronts in the reactors with 1 mm in inner diameter were symmetric. In this paper, the effects of Reynolds number, geometrical parameters, and equivalence ratio on methane-oxygen premixed flame dynamics in non-adiabatic micro-reactors were investigated experimentally. The results showed that decreasing the inner diameter of the reactor could significantly influence flame operational regimes and propagation speed in the reactors in terms of suppressing hydrodynamic instabilities and compatibility with theory. In addition, the results showed that increasing the Reynolds number could shift the location of the maximum flame propagation speed in the reactors towards more fuel-lean mixtures. Therefore, it was considered that fuel-rich flames were more stable than fuel-lean ones. Moreover, two mechanisms of heat loss and flame instabilities were found to control flame propagation speed in small-scale reactors. In this regard, unlike the effect of heat loss mechanism on flame propagation speed, the instabilities on the flame surface and the consequent flame surface stretching could lead to a significantly higher flame propagation speed in the reactor than the free propagation flame speed. Meanwhile, it was shown that most established flames in reactors with 1 mm inner diameter were symmetric and more controllable. However, the flame shape in reactors with 2 mm inner diameter could be asymmetric under certain conditions. [ABSTRACT FROM AUTHOR]

Abstract: In this study, an approach to modify and improve of CH4(methane)/air pre-mixed flame in a micro-stepped tube combustor is numerically studied. The effects of wire on flame in micro combustor are calculated using a high order, high accuracy 2D steady state code. The results show that the wire insertion within a micro combustor has significant effects on stabilizing of the flame in a micro combustor. Also, the wire insertion method is strongly able to modify the flame location within a micro combustor. In such a manner, applying this method can guarantee the flame presence in some certain conditions in comparison to the simple backward facing step method. The wire insertion method improves the combustion process by activating the chemical reactions which have main effect on the combustion initiation. Moreover, it is shown that the wire insertion method can effectively improve and also increase temperature distribution and its level along the wall and the axis of the micro combustor. Meanwhile, it is shown that the wall thermal conductivity and its outer surface emissivity coefficient with the convective heat transfer coefficient on the outer wall of the micro combustor have great influences on combustion characteristics. [Copyright &y& Elsevier]