CH4-air Mixtures Research Articles

Experimental and reaction mechanism study on laminar burning velocity and characteristics of OH/NH generation in ammonia co-combustion

Developing fuels without carbon, such as ammonia (NH3), is crucial for achieving carbon reduction. Laminar burning velocity (LBV) is investigated, and distribution characteristics of OH and NH radicals are identified through planar laser-induced fluorescence. Rate of production of main free radicals and sensitivity analysis are also studied. The addition of NH3 to CH4-air mixtures results in a decrease in the LBV, a rise in preheating zone height, a reduction in OH radical fluorescence intensity, and an enhancement in NH radical fluorescence intensity. Simulation reveals that the high NH3 ratio enhance the negative impact of nitrogen-containing elementary reactions on LBV. The addition of NH3 enhances the initial consumption of more OH via reaction R248:NH2+OH=NH+H2O, leading to increased NH production, and indicating a negative effect on LBV. The decrease in OH concentration is the main factor leading to a decrease in LBV. The reactions affecting the OH generation and consumption are R39:H+O2=O+OH and R85:OH+H2=H+H2O, respectively. The addition of NH3 leads to a more significant reduction in R39.

Spatiotemporal Surface Temperature Measurements Resolving Flame-Wall Interactions of Lean H2-Air and CH4-Air Flames Using Phosphor Thermometry

AbstractSpatiotemporal wall temperature (Twall) distributions resulting from flame-wall interactions of lean H2-air and CH4-air flames are measured using phosphor thermometry. Such measurements are important to understand transient heat transfer and wall heat flux associated with various flame features. This is particularly true for hydrogen, which can exhibit a range of unique flame features associated with combustion instabilities. Experiments are performed within a two-wall passage, in an optically accessible chamber. The phosphor ScVO4:Bi3+ is used to measure Twall in a 22 × 22 mm2 region with 180 µm/pixel resolution and repetition rate of 1 kHz. Chemiluminescence imaging is combined with phosphor thermometry to correlate the spatiotemporal dynamics of the flame with the heat signatures imposed on the wall. Measurements are performed for lean H2-air flames with equivalence ratio Φ = 0.56 and compared to CH4-air flames with Φ = 1. Twall signatures for H2-air Φ = 0.56 exhibit alternating high and low-temperature vertical streaks associated with finger-like flame structures, while CH4-air flames exhibit larger scale wrinkling with identifiable crest/cusp regions that exhibit higher/lower wall temperatures, respectively. The underlying differences in flame morphology and Twall distributions observed between the CH4-air and lean H2-air mixtures are attributed to the differences in their Lewis number (CH4-air Φ = 1: Le = 0.94; H2-air Φ = 0.56: Le = 0.39). Findings are presented at two different passage spacings to study the increased wall heat loss with larger surface-area-to-volume ratios. Additional experiments are conducted for H2-air mixtures with Φ = 0.45, where flame propagation was slower and was more suitable to resolve the wall heat signatures associated with thermodiffusive instabilities. These unstable flame features impose similar wall heat fluxes as flames with 2–3 times greater flame power. In this study, these flame instabilities occur within a small space/time domain, but demonstrate the capability to impose appreciable heat fluxes on surfaces.

Development of a multiple laser-sheet imaging technique for the analysis of three-dimensional turbulent explosion flame structures

The present work describes the development of a multiple laser-sheet imaging technique for the study of turbulent, premixed flame surface structures at high Karlovitz stretch factor values. Experiments were conducted using CH4 air mixtures at 365 K and 0.5 MPa and up to a root mean square turbulence velocity of 1.5 m/s. A high-speed Nd:YAG laser capable of pulsing up to 60 kHz in conjunction with a high-speed camera and a rotating mirror was used to reconstruct time-resolved three-dimensional turbulent flames. This has, for the first time, enabled the direct measurement of wrinkled flame surface areas, along with reaction progress variable and flame brush thickness. These are important parameters for the characterization of turbulent burning rates and provide more insight into the dynamic nature of the flames and their structures. In addition, the current data aid toward direct comparison with results from combustion simulation studies.

Numerical investigation of NO generation reaction pathway with novel embedded flue-gas internal recirculation combustion loop consisting of flue-gas and CH4-air mixture

The novel embedded flue-gas internal recirculation (FIR) combustion technology has garnered increasing attention in gas burners due to its notable advantages in combustion efficiency and ultra-low NOx emissions. In this study, we design FIR channels for a gas burner and investigate the impact of recirculation ratio (R) on temperature, combustion efficiency, concentrations of free radicals (H, O, CH), and NO concentration via Computational Fluid Dynamics (CFD) method. In addition, this paper explores and elucidates the dynamics of four NO production pathways, namely, thermal NO, prompt NO, N2O, and NNH, as well as the contributions of the four pathways to total NO. Observations indicate a constrained range of FIR recirculation ratios, specifically between 23.2 % and 35 %. The combustion efficiency of the FIR burner is 96 % and 99.8 % at R = 23.8 % and R = 35.0 %, which is superior to conventional burners. Moreover, as R increases, both pathways for NO generation from N2O and NNH are intensified as R exceeds 23.8 %. In the embedded FIR combustion system, the contribution of N2O to total NO increases from 0.23 % to 0.78 %, while the percentage of prompt NO decreases to 0.19 %. These findings provide a development direction and technical guidance for the practical implementation of the embedded FIR technology.

Numerical Investigation of Hydrogen Role on Detonation of CH4/H2/air Mixtures

ABSTRACT Steady one-dimensional Zeldovich-von Neumann-Dӧring (ZND) calculation and thermo-chemical analysis are performed to investigate the hydrogen role on ZND detonation of CH4/H2/air mixtures. Different hydrogen blend ratios (i.e. Hbr) ranging from 0% to 100% are selected in this paper. The results show that hydrogen blend ratio has a significant impact on the ZND detonation reaction pathway of stoichiometric CH4-air mixtures. When Hbr ≤ 20%, CH4 is converted to CO2 by route 1: CH4 => CH3 => C2H6 => … => CO2 and route 2: CH4 => CH3 => CH3O => CH2O => HCO => CO => CO2. When Hbr ≥ 50%, CH3 in route 2 is directly converted to CH2O through R97 (CH3 + O <=> CH2O + H). This causes different ZND detonation structures. The induction length decreases with hydrogen addition. This is because R0 (H + O2 <=> O + OH) governs the heat release during the induction stage, enhancing the energy absorption in induction stage and the molecular thermal decomposition after leading shock wave. Finally, the effect of hydrogen blend ratio on the detonation cell size is explored, and a prediction model of detonation cell size incorporated with detonation stability parameter and induction length is developed for CH4/H2/air mixtures.

Experimental and chemical kinetic modeling study on the inhibition performance of pentauoroethane blended with carbon dioxide in methane-air mixture explosions

To explore the inhibition effectiveness and mechanisms of pentafluoroethane (C2HF5), carbon dioxide (CO2), and their blends, 9.5% methane (CH4)-air mixture explosions under different inhibitors were tested by employing a 20 L spherical explosion system. A quadratic regression model was developed to predict impacts of factors on maximum explosion pressure (pm) through response surface method. The reactions and mechanisms under blended inhibitors were numerically investigated. Results depicted that increasing volume fractions of inhibitor led to decrease in pm, maximum rate of pressure rise, and delay in explosion induction time. Blended inhibitors demonstrated superior inhibition effectiveness, optimal inhibition-environmental was achieved at 4.5% C2HF5 blended with 5.5% CO2. A quadratic polynomial relation existed among pm, volume fractions of C2HF5, CO2, and CH4 during the explosion. Key free radicals were weakened and the promoted explosive elementary reactions were primarily inhibited. CH4+OHCH3+H2O and CH4+HCH3+H2 were the most diminished reactions by blended inhibitors compared to CO2 alone. Carbon-containing reactions R82 and R84, and fluorinated reactions R1025 and R1099 reduced the key free radicals, interrupted the chain reactions and promoted the production of HF. Findings will provide new ideas for prevention of CH4-air mixture explosions and reduce hazardous effect.

A study of the effect of hydrogen on methane deflagration characteristics in a square confined space

Macroscopic and microscopic explosion kinetic properties of H2/CH4/air mixtures are investigated. The explosion characteristics of CH4/air mixtures with different hydrogen blending ratios(0–25%) are investigated experimentally. It is found that the increase of hydrogen blending ratios has almost no effect on the peak overpressure of the methane gas explosion, but the rate of maximum pressure rise, the explosion index and the flame propagation velocity increase significantly. The effect of hydrogen blending ratios on the chemical kinetics of CH4-air mixtures is analyzed based on the CHEMKIN software. The concentration of key radicals, which play a major role in the reaction, does not change much as the hydrogen blending ratio increases. A prediction model of explosion overpressure in a square confined space is established using the segmentation method, and it is verified that the relative errors between the model calculation results and experimental results are less than 10%.

LES of pilot n-heptane ignition and its interaction with the lean premixed methane–air mixture in a dual-fuel combustion engine

Large eddy simulations of pilot fuel ignited, lean premixed, natural gas engines are performed to study the pilot-ignition process and its subsequent interaction with the premixed charge. The injection pressure (Pinj) and injection duration (Δinj) are varied (i.e. 800 bar/760μs, 800 bar/500μs, and 400bar/760μs) to study the impact of the injection process on the subsequent combustion evolution. Open-cycle simulations considering the full engine geometry are used to predict the in-cylinder flows, while combustion is modeled using a finite-rate chemistry model. In-cylinder methane (CH4) is shown to delay the low-temperature ignition of the pilot fuel, regardless of the pilot injection setting, which subsequently prolongs the overall pilot fuel ignition delay. Moreover, all simulated cases show the occurrence of back-supported combustion (BSC), where the burning of CH4-air mixture is “back-supported” by pilot fuel radicals. Despite both the 800 bar/500μs and 400 bar/760μs cases having the same injected pilot fuel mass, the peak in-cylinder pressure and burning rate of the premixed CH4-air mixture in the former case are higher. Higher Pinj and shorter Δinj lead to better mixing between the pilot fuel and the premixed CH4-air charge. Subsequently, this forms a larger volume of regions with elevated equivalence ratio due to the presence of pilot fuel (ϕ>ϕCH4) which, consequently promotes the formation of BSC. The impact of in-cylinder flow fields on the dual-fuel combustion process is investigated by performing two closed-cycle 800 bar/500μs cases with one assuming solid-body rotation and another without solid-body rotation (i.e. zero velocity field). In-cylinder flow field is shown to have a visible impact on the transition stage between the pilot ignition stage and the premixed flame propagation stage, but have an insignificant effect on the pilot fuel ignition process. In the transition stage, slower flame propagation is observed in the zero-velocity case. The results show that this is not only due to the turbulence effect on premixed flame but also due to differences in the volume and distribution of pilot fuels that impacts BSC.

Effect of Pressure and Turbulence Intensity on the Heat Flux During Flame Wall Interaction (FWI)

Combustion applications such as internal combustion engines are a major source of power generation. Renewable alternative fuels like hydrogen and ammonia promise the potential of combustion in future power applications. Most power applications encounter flame wall interaction (FWI) during which high heat losses occur. Investigating heat loss during FWI has the potential to identify parameters that could lead to decreasing heat losses and possibly increasing the efficiency of combustion applications. In this work, a study of FWI (CH4-air mixture) in a constant volume chamber, with a head-on quenching configuration, at high pressure in both laminar and turbulent conditions is presented. High-speed surface temperature measurement using thin junction thermocouples coupled with high-speed flow field characterization using particle image velocimetry (PIV) are used simultaneously to investigate the effect of pressure during FWI (Pint) and turbulence intensity (q) on the heat flux peak (QP). In laminar combustion regimes, it is found that QP is proportional to Pint0.35. The increase in q is shown to affect both Pint and QP. Finally, comparing QP versus Pint for both laminar and turbulent combustion regimes, it is found that an increase in q leads to an increase in QP (b = 0.76).

Effect of ammonia addition on laminar burning velocity of CH4/H2 premixed flames at high pressure and temperature conditions

Global emissions of nearly 30% stem from fossil fuel combustion for long-distance transportation vehicles. Therefore, investigations on newer alternatives for clean and green fuels is desired in pursuance of net-zero carbon emissions by 2050. Given this contemplation, the current study aims at studying the effect of ammonia addition, a carbon-free fuel, on laminar burning velocity of CH4/H2 fuel blends at high-pressure and high-temperature conditions utilizing the high-pressure diverging-channel method. The NH3 varies from 0% to 30% in the CH4/H2 fuel blend by volume. With regard to adiabatic flame temperature, this NH3/CH4/H2 mixture is comparable to pure CH4-air mixtures, but with a reduction in carbon emissions of about 26.6%. The research involves conducting measurements at high temperatures between 300 K and 750 K, pressures between 1 atm and 5 atm, and equivalence ratios between ϕ = 0.7 and 1.4. When the pressure is 5 atm, the obtained burning velocities of NH3/CH4/H2 blends are analogous to that of CH4-air mixtures. The temperature exponent, α, does not change when the NH3 fraction or pressure changes, but at high temperatures there is a slight shift in the pressure exponent, β. The experimental results suggest that the Okafor and Li kinetic models align well with the measurements made in the current work up to 5 atm, while the Reduced GRI Mech 3.0 mechanism overestimates the burning velocity values. Additionally, an increase in NH3 fraction and pressure results in a decrease in burning velocity and NO formation due to the presence of NHi (i = 0–2) radicals. Overall, this study convincingly demonstrates the viability of NH3/CH4/H2-air mixtures as a practical alternative to reduce carbon emissions.

Minimum ignition energy theoretical model for flammable gas based on flame propagation layer by layer

To achieve the rapid prediction of minimum ignition energy (MIE) for premixed gases with wide-span equivalence ratios, a theoretical model is developed based on the proposed idea of flame propagation layer by layer. The validity and high accuracy of this model in predicting MIE have been corroborated against experimental data (from literature) and traditional models. In comparison, this model is mainly applicable to uniform premixed flammable mixtures, and the ignition source needs to be regarded as a punctiform energy source. Nevertheless, this model can exhibit higher accuracy (up to 90%) than traditional models when applied to premixed gases with wide-span equivalence ratios, such as C3H8-air mixtures with 0.7–1.5 equivalence ratios, CH4-air mixtures with 0.7–1.25 equivalence ratios, H2-air mixtures with 0.6–3.15 equivalence ratios et al. Further, the model parameters have been pre-determined using a 20 L spherical closed explosion setup with a high-speed camera, and then the MIE of common flammable gases (CH4, C2H6, C3H8, C4H10, C2H4, C3H6, C2H2, C3H4, C2H6O, CO and H2) under stoichiometric or wide-span equivalence ratios has been calculated. Eventually, the influences of model parameters on MIE have been discussed. Results show that MIE is the sum of the energy required for flame propagation during ignition. The increase in exothermic and heat transfer efficiency for fuel molecules can reduce MIE, whereas prolonging the flame induction period can increase MIE.

Investigation of C2H6, C2H4, CO and H2 on the explosion pressure behavior of methane/blended fuels

The introduction of other combustible gases to methane could change accident consequences. This work aimed to experimentally compare the effects of blended fuels (C2H6, C2H4, CO, and H2 mix in different proportions) on the explosion pressure behavior of methane and blended fuels in air. The explosion pressure properties of the mixtures were tested. Principal component analysis was employed, and multiple regression models were developed to predict the influences of principal components on the maximum explosion pressure of CH4. In addition, the software was used to analyze and compare the sensitivity analysis. For the fuel-lean CH4-air mixture(φ = 0.72), the results demonstrated that the value of Pmax and (dP/dt)max of the mixed gases increased, as the volume fraction of the blended fuels increased. The effects of other gases followed the order C2H6>C2H4>H2>CO. For stoichiometric concentrations (φ = 1), the order of the influence degree was H2>CO > C2H6>C2H4, and the value of Pmax and (dP/dt)max of the mixed gases tended to decrease slightly. As the volume fraction of the blended fuels was increased to 2%, the Pmax and (dP/dt)max of CH4 showed a decreasing trend for the fuel-rich CH4-air mixture. The effects of blended fuels on the equivalence ratio of φ = 1.1 followed the order C2H4>H2>CO > C2H6. The sensitivity variation analysis and normalized sensitivity variation analysis indicated that the chain branching reaction 2CH3(+M)<=>C2H6(+M) was the most sensitive reaction under various conditions. However, it was not very susceptible to changes in the blended fuel ratio. Most reactions were susceptible to changes in the blended fuel ratio. The 10 most sensitive reactions were generally much more susceptible to changes in the blended fuel ratio in the 2% blended fuels than they were in the 0.4% blended fuels.

Thermoacoustic Combustion Stability Analysis of a Bluff Body-Stabilized Burner Fueled by Methane–Air and Hydrogen–Air Mixtures

Hydrogen can play a key role in the gradual transition towards a full decarbonization of the combustion sector, e.g., in power generation. Despite the advantages related to the use of this carbon-free fuel, there are still several challenging technical issues that must be addressed such as the thermoacoustic instability triggered by hydrogen. Given that burners are usually designed to work with methane or other fossil fuels, it is important to investigate their thermoacoustic behavior when fueled by hydrogen. In this framework, the present work aims to propose a methodology which combines Computational Fluid Dynamics CFD (3D Reynolds-Averaged Navier-Stokes (RANS)) and Finite Element Method (FEM) approaches in order to investigate the fluid dynamic and the thermoacoustic behavior introduced by hydrogen in a burner (a lab-scale bluff body stabilized burner) designed to work with methane. The case of CH4-air mixture was used for the validation against experimental results and benchmark CFD data available in the literature. Numerical results obtained from CFD simulations, namely thermofluidodynamic properties and flame characteristics (i.e., time delay and heat release rate) are used to evaluate the effects of the fuel change on the Flame Response Function to the acoustic perturbation by means of a FEM approach. As results, in the H2-air mixture case, the time delay decreases and heat release rate increases with respect to the CH4-air mixture. A study on the Rayleigh index was carried out in order to analyze the influence of H2-air mixture on thermoacoustic instability of the burner. Finally, an analysis of both frequency and growth rate (GR) on the first four modes was carried out by comparing the two mixtures. In the H2-air case the modes are prone to become more unstable with respect to the same modes of the case fueled by CH4-air, due to the change in flame topology and variation of the heat release rate and time delay fields.

Chemical kinetics properties and the influences of different hydrogen blending ratios on reactions of natural gas

Hydrogen is blended with natural gas to form hydrogenated natural gas (HCNG), which is a new, efficient and clean energy. CHEMKIN-PRO 19.0 software was combined with the GRI-Mech 3.0 mechanism to evaluate the capacity of H2 blending in reducing CO and CO2 emissions. Influences of H2 blending on combustion reactions of the CH4-air mixture were investigated. The results showed that the main reactants and products (CH4, CO, and CO2) decreased in gradient with increasing H2 blending ratio, accompanied by a shorter reaction duration and a faster reaction rate. After adding H2, important key radicals H, O, and OH increase significantly, so that the combustion reactions become more violent. Sensitivity analysis reveals that among relevant elementary reactions of CO and CO2, R38 (with its promotional effect) and R158 (with its inhibitory effect) show the greatest sensitivity. As the H2 concentration increases, the sensitivity of the two reactions (separately with promotional and inhibitory effects) decreases. Blending H2 in the natural gas can improve the combustion rate and reduce the generation of emissions CO and CO2, which is of important significance for realizing low-carbon goals and reducing air pollution.

Effect of Hydrogen Enhancement on Natural Flame Luminosity of Tri-Fuel Combustion in an Optical Engine

A novel combustion mode, namely tri-fuel (TF) combustion using a diesel pilot to ignite the premixed methane–hydrogen–air (CH4–H2–air) mixtures, was experimentally investigated under various H2 fractions (0%, 10%, 20%, 40%, 60%) and ultra-lean conditions (equivalence ratio of φ= 0.5). The overarching objective is to evaluate the effect of H2 fraction on flame characteristics and engine performance. To visualize the effect of H2 fraction on the combustion process and flame characteristics, a high-speed color camera (Photron SA-Z) was employed for natural flame luminosity (NFL) imaging to visualize the instantaneous TF combustion process. The engine performance, flame characteristics, and flame stability are characterized based on cylinder pressure and color natural flame images. Both pressure-based and optical imaging-based analyses indicate that adding H2 into the CH4–air mixture can dramatically improve engine performance, such as combustion efficiency, flame speed, and flame stability. The visualization results of NFL show that the addition of H2 promotes the high-temperature reaction, which exhibits a brighter bluish flame during the start of combustion and main combustion, however, a brighter orangish flame during the end of combustion. Since the combustion is ultra-lean, increasing the H2 concentration in the CH4–air mixture dramatically improves the flame propagation, which might reduce the CH4 slip. However, higher H2 concentration in the CH4–air mixture might lead to a high-temperature reaction that sequentially promotes soot emissions, which emit a bright yellowish flame.

Influence of pressure and dilution gas on the explosion behavior of methane-oxygen mixtures

This study investigates the influence of initial pressure (P0) and dilution gas (N2, CO2, He, or Ar) on the explosion behavior of CH4-O2 mixtures. Spherical premixed flames in a 20 L spherical vessel are recorded by high-speed schlieren photography. The explosion parameters, explosion duration (τe), maximum explosion pressure (Pmax), maximum explosion pressure change rate ((dP/dt)max), flame speed, burning velocity, and Markstein length, are experimentally investigated at various pressures up to 250 kPa and at room temperature (280 ± 3 K). The experimental results show that Pmax and (dP/dt)max increase linearly with P0. The influence on the explosion intensities, (dP/dt)max, and flame speed, from high to low with the dilution is in the order of CO2, N2, Ar, and He, which indicates that CO2 and N2 more significantly inhibit CH4-O2 explosion. The results suggest that CO2 not only has a strong endothermic ability and reacts with H, but also changes the active free radicals to molecules due to three-element collision. The experimental phenomena are explained and validated by the results from the Chemkin package using two detailed mechanisms, indicating that (dP/dt)max is closely related to the content of H, O, OH, and CH3. The cellular instability is analyzed by the flame thickness and Markstein length, and it becomes more obvious as the flame thickness and Markstein length decrease. The buoyancy instability, Rayleigh-Taylor instability, and a series of phenomena due to the dilution of CO2 are analyzed. The maximum explosion pressure and flame speed decrease significantly with the addition of CO2 into CH4-air mixtures, and CO2 affects the generation of flame. The buoyancy instability of CH4-air mixtures with CO2 addition is examined. The experimental results contribute to a deeper understanding of the explosion behavior of CH4-O2 mixtures and the dilution effect of other gases on it.

Laminar burning velocity measurement of CH4/H2/NH3-air premixed flames at high mixture temperatures

The development of carbon-free fuels is essential for establishing sustainable energy growth with ultra-low carbon emissions. In this quest, development of newer fuels with efficient combustion characteristics equivalent to hydrocarbon fuels needs to be encouraged. With this consideration, the present work aims at the measurement of laminar burning velocity of CH4/H2/NH3-air mixture at higher temperatures using the externally heated diverging channel technique. The volume fraction of ammonia is varied from 0 to 30 % by volume in a 4:1 ratio CH4:H2 fuel blend. The proposed CH4/H2/NH3 mixture is analogous to CH4-air mixture in terms of burning velocity and adiabatic flame temperature. The measurements are obtained for elevated mixture temperature (300–750 K) and equivalence ratio range of ϕ = 0.7–1.2. The measured burning velocities of CH4/H2/NH3 mixtures are nearly similar to CH4-air mixtures for mixture temperatures >550 K. Temperature exponent remains nearly constant irrespective of the NH3 mole-fraction. Predictions of Okafor and Li kinetic-models compare well with the present experimental measurements up to 750 K, whereas Shrestha mechanism over-predicts the burning velocity values. The burning velocity is observed to decrease with an increase in NH3 mole-fraction due to the presence of nitroxyl radical. The present work ably demonstrates the application of CH4/H2/NH3-air mixtures as an efficient alternative to achieve the reduction in carbon-emission targets and develop efficient combustion systems for these derived fuels.

Laser-induced ignition of methane-air mixtures by a four-beam, pulse-burst mode passively Q-switched Nd:YAG/Cr4+:YAG laser

In this work we report on the characteristics of laser ignition of CH4-air mixtures, using multi-point (up to four points) ignition with single-pulse beams as well as with trains of two pulses. A compact Nd:YAG/Cr4+:YAG laser with four beams, each beam being operated in single-pulse or burst mode (with up to five pulses), each pulse with an energy of 3.8 mJ and 0.9 ns duration was employed in the experiments. A combustion chamber with spherical geometry was used to ignite CH4-air mixtures with various values of the air–fuel equivalence ratio, λ and at different initial pressures of the fuel mixtures. Analysis of pressure curves indicates that using multiple ignition points leads to an acceleration of the flame speed in the early kernel development phase. It has been observed that for an initial pressure of 1 bar in the combustion chamber, ignition at a point with two laser pulses decreases the duration of combustion by about 7 % compared to ignition at a point with a single laser pulse. For ignition in four points with one or two pulses, the combustion is 10 % and 20 % faster, respectively. The combustion efficiency is also analyzed, and we report that a multi-point laser ignition source leads to a slight increase in the efficiency of burning the fuel near the lean limit. The ignition limits, for both lean and rich CH4-air mixtures, have also been determined for initial pressures in the combustion chamber of up to 9 bars and each ignition method, and a discussion of these is presented.

Flame propagation and spectrum characteristics of CH4-air gas mixtures in a vertical pressure relief pipeline

The explosion characteristics, flame propagation mechanism, and spectrum characteristics of five free radicals (C2·, CHO·, CH·, O·, and OH·), formaldehyde (CH2O), and one reagent (O2) under various concentrations (w) of methane were investigated using a vertical rectangular pressure relief pipeline. The experimental results indicated that the instantaneous flame propagation speed (v), maximum flame temperature (Tex), maximum explosion pressure, and relative spectrum intensity (I) first increased and subsequently decreased with methane concentration. The maximum instantaneous flame propagation speed (vmax), maximum flame temperature (Tmax), maximum explosion pressure (Pmax), and maximum relative spectrum intensity (Imax) were achieved, and the time taken to form a tulip-shaped flame was the shortest at w = 10 vol%. The experiment also revealed that Tex and w had a polynomial relationship. In the explosion process, positive correlations were noted among the T, P, and I. Furthermore, the spectrum signal characteristic times of the intermediate species was considerably influenced by v. When the methane concentration was 9.5 vol%, the I of the seven intermediate generation rates exhibited two distinct trends. This finding contributes to the analysis and validation of the microscopic mechanism underlying gas explosion flame propagation. Further, a reference for selecting new gas explosion suppressants based upon the attenuation level of free radical spectrum intensity.

Quantifying the impact of heat loss, stretch and preferential diffusion effects to the anchoring of bluff body stabilized premixed flames

The response of a premixed flame subjected to either flame stretch (and associated Lewis number effects) or heat loss has been well documented in the literature and has enabled a good understanding of canonical configurations such as flat burner-stabilized, counter flow and tubular flames. However, in practical burners, flames are simultaneously subjected to stretch, heat transfer with the flame holder and preferential diffusion effects. For such flames, usually the collective effect of underlying contributions is studied and individual effects are only treated in a qualitative manner. In this paper, our objective is to use flame stretch theory to separate and quantify the underlying contributions from flame stretch, preferential diffusion and heat transfer with the flame holder to the flame speed of bluff body stabilized flames. It is shown that the theory adequately predicts the flame displacement speed in comparison to the results from the numerical simulations. Using the quantification of contributions, an overall stabilization mechanism for H2 enriched CH4-air mixtures is discussed. The role of competing contributions from preferential diffusion and heat loss is highlighted especially near the flame base region where the flame speed is heavily impacted by all the effects. Insights are also given for low Lewis number flashback prone flames.