Numerical study on laminar burning velocity and NO formation of premixed methane–hydrogen–air flames

Numerical study on laminar burning velocity and NO formation of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity, adiabatic flame temperature, and radical mole fractions of H, OH and NO are obtained at various equivalence ratios and hydrogen fractions. The results show that the unstretched laminar burning velocity is increased with the increase of hydrogen fraction. Methane-dominated combustion is presented when hydrogen fraction is less than 40%, where laminar burning velocity is slightly increased with the increase of hydrogen addition. When hydrogen fraction is larger than 40%, laminar burning velocity is exponentially increased with the increase of hydrogen fraction. A strong correlation exists between burning velocity and maximum radical concentration of H + OH radicals in the reaction zone of premixed flames. High burning velocity corresponds to high radical concentration in the reaction zone. With the increase of hydrogen fraction, the overall activation energy of methane–hydrogen mixture is decreased, and the inner layer temperature and Zeldovich number are also decreased. All these factors contribute to the enhancement of combustion as hydrogen is added. The curve of NO versus equivalence ratio shows two peaks, where they occur at the stoichiometric mixture due to Zeldovich thermal-NO mechanism and at the rich mixture with equivalence ratio of 1.3 due to the Fenimore prompt-NO mechanism. In the stoichiometric flames, hydrogen addition has little influence on NO formation, while in rich flames, NO concentration is significantly decreased. Different NO formation responses to stretched and unstretched flames by hydrogen addition are discussed.     

Turbulent burning velocity of hydrogen–air premixed propagating flames at elevated pressures

Lewis number represents the thermo-diffusive effects on laminar flames. That of hydrogen–air mixture varies extensively with the equivalence ratio due to the high molecular diffusivity of hydrogen. In this study, the influences of pressure and thermo-diffusive effects on spherically propagating premixed hydrogen–air turbulent flames were studied using a constant volume fan-stirred combustion vessel. It was noted that the ratio of the turbulent to unstretched laminar burning velocity increased with decreasing equivalence ratio and increasing mixture pressure. Turbulent burning velocity was dominated by three factors: (1) purely hydrodynamic factor, turbulence Reynolds number, (2) relative turbulence intensity to reaction speed, the ratio of turbulence intensity to unstretched laminar burning velocity, and (3) sensitivity of the flame to the stretch due to the thermo-diffusive effects, Lewis and Markstein numbers. A turbulent burning velocity correlation in terms of Reynolds and Lewis numbers is presented.     

Experimental and numerical study on laminar burning characteristics of premixed methane–hydrogen–air flames

An experimental and numerical study on laminar burning characteristics of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity and the Markstein length were obtained over a wide range of equivalence ratios and hydrogen fractions. Moreover, for further understanding of the effect of hydrogen addition on the laminar burning velocity, the sensitivity analysis and flame structure were performed. The results show that the unstretched laminar burning velocity is increased, and the peak value of the unstretched laminar burning velocity shifts to the richer mixture side with the increase of hydrogen fraction. Three regimes are identified depending on the hydrogen fraction in the fuel blend. They are: the methane-dominated combustion regime where hydrogen fraction is less than 60%; the transition regime where hydrogen fraction is between 60% and 80%; and the methane-inhibited hydrogen combustion regime where hydrogen fraction is larger than 80%. In both the methane-dominated combustion regime and the methane-inhibited hydrogen combustion regime, the laminar burning velocity increases linearly with the increase of hydrogen fraction. However, in the transition regime, the laminar burning velocity increases exponentially with the increase of hydrogen fraction in the fuel blends. The Markstein length is increased with the increase of equivalence ratio and is decreased with the increase of hydrogen fraction. Enhancement of chemical reaction with hydrogen addition is regarded as the increase of H, O and OH radical mole fractions in the flame. Strong correlation is found between the burning velocity and the maximum radical concentrations of H and OH in the reaction zone of the premixed flames.     

Effects of hydrogen concentration on premixed laminar flames of hydrogen–methane–air

The unstretched laminar burning velocities and Markstein numbers of spherically propagating hydrogen–methane–air flames were studied at a mixture pressure of 0.10 MPa and a mixture temperature of 350 K. The fraction of hydrogen in the binary fuel was varied from 0 to 1.0 at equivalence ratios of 0.8, 1.0 and 1.2. The unstretched laminar burning velocity increased non-linearly with hydrogen fraction for all the equivalence ratios. The Markstein number varied non-monotonically at equivalence ratios of 0.8 and 1.0 and increased monotonically at equivalence ratio of 1.2 with increasing hydrogen fraction. Analytical evaluation of the Markstein number suggested that the trends could be due to the effective Lewis number, which varied non-monotonically with hydrogen fraction at equivalence ratios of 0.8 and 1.0 and increased monotonically at 1.2. The propensity of flame instability varied non-monotonically with hydrogen fraction at equivalence ratios of 0.8 and 1.0.     

The effect of hydrogen addition on premixed laminar acetylene–hydrogen–air and ethanol–hydrogen–air flames

In the present work, the laminar premixed acetylene–hydrogen–air and ethanol–hydrogen–air flames were investigated numerically. Laminar flame speeds, the adiabatic flame temperatures were obtained utilizing CHEMKIN PREMIX and EQUI codes, respectively. Sensitivity analysis was performed and flame structure was analyzed. The results show that for acetylene–hydrogen–air flames, combustion is promoted by H and O radicals. The highest flame speed (247 cm/s) was obtained in mixture with 95% H₂–5% C₂H₂ at λ = 1.0. The region between 0.95 < X_H2 < 1.0 was referred to as the acetylene-accelerating hydrogen combustion since the flame speed increases with increase the acetylene fraction in the mixture. Further increase in the acetylene fraction decreases the H radicals in the flame front. In ethanol–hydrogen–air mixtures, the mixture reactivity is determined by H, OH and O radicals. For X_H2 < 0.6, the flame speed in this regime increases linearly with increasing the hydrogen fraction. For X_H2 > 0.8, the hydrogen chemistry control the combustion and ethanol addition inhibits the reactivity and reduces linearly the laminar flame speed. For 0.6 < X_H2 < 0.8, the laminar flame speed increases exponentially with the increase of hydrogen fraction.     

Measurements of laminar burning velocities and onset of cellular instabilities of methane–hydrogen–air flames at elevated pressures and temperatures

An experimental study on laminar burning velocities and onset of cellular instabilities of the premixed methane–hydrogen–air flames was conducted in a constant volume combustion vessel at elevated pressures and temperatures. The unstretched laminar burning velocity and Markstein length were obtained over a wide range of hydrogen fractions. Besides, the effects of hydrogen addition, initial pressure and initial temperature on flame instabilities were analyzed. The results show that the unstretched flame propagation speed and the unstretched laminar burning velocity are increased with the increase of initial temperature and hydrogen fraction, and they are decreased with the increase of initial pressure. Early onset of cellular instability is presented and the critical radius and Markstein length are decreased with the increase of initial pressure, indicating the increase of hydrodynamic instability with the increase of initial pressure. Flame instability is insensitive to initial temperature compared to initial pressure. With the increase of hydrogen fraction, significant decrease in critical radius and Markstein length is presented, indicating the increase in both diffusional-thermal and hydrodynamic instabilities as hydrogen fraction is increased.     

Effects of hydrogen addition and nitrogen dilution on the laminar flame characteristics of premixed methane–air flames

The effect of hydrogen addition and nitrogen dilution on laminar flame characteristics was investigated. The spherical expanding flame technique, in a constant volume bomb, was employed to extract laminar flame characteristics. The mole fraction of hydrogen in the methane–hydrogen mixture was varied from 0 to 1 and the mole fraction of nitrogen in the total mixture (methane–hydrogen–air–diluent) from 0 to 0.35. Measurements were performed at an initial pressure of 0.1 MPa and an initial temperature of 300 K. The mixtures investigated were under stoichiometric conditions. Based on experimental measurements, a new correlation for calculating the laminar burning velocity of methane–hydrogen–air–nitrogen mixtures is proposed. The laminar burning velocity was found to increase linearly with hydrogen mass fraction for all dilution ratios while the burned gas Markstein length decreases with the increase in hydrogen amount in the mixture except for high hydrogen mole fractions (>0.6). Nitrogen dilution has a nonlinear reducing effect on the laminar burning velocity and an increasing effect on the burned gas Markstein length. The experimental results and the proposed correlation obtained are in good agreement with literature values.     

Effects of hydrogen and steam addition on laminar burning velocity of methane–air premixed flame: Experimental and numerical analysis

Effects of hydrogen enrichment and steam addition on laminar burning velocity of methane–air premixed flame were studied both experimentally and numerically. Measurements were carried out using the slot burner method at 1 bar for fresh gases temperatures of 27 °C and 57 °C and for variable equivalence ratios going from 0.8 to 1.2. The hydrogen content in the fuel was varied from 0% to 30% in volume and the steam content in the air was varied from 0 to 112 g/kg (0–100% of relative humidity). Numerical calculations were performed using the COSILAB code with the GRI-Mech 3.0 mechanism for one-dimensional premixed flames. The calculations were implemented first at room temperature and pressure and then extended to higher temperatures (up to 917 K) and pressures (up to 50 bar). Measurements of laminar burning velocities of methane–hydrogen–air and methane–air–steam agree with the GRI-Mech calculations and previous measurements from literature obtained by different methods. Results show that enrichment by hydrogen increases of the laminar burning velocity and the adiabatic flame temperature. The addition of steam to a methane–air mixture noticeably decreases the burning velocity and the adiabatic flame temperature. Modeling shows that isentropic compression of fresh gases leads to the increase of laminar burning velocity.     

New insights into the peculiar behavior of laminar burning velocities of hydrogen–air flames according to pressure and equivalence ratio

Hydrogen is a clean and energetic fuel, and its oxidation mechanism is a subset of the oxidation mechanisms of all hydrocarbons. Therefore, the validation of the available kinetic schemes is of great importance. In the current study, experimental measurements of laminar flame speeds and modeling studies were performed for H₂–air premixed flames over a wide range of equivalence ratios (0.5–4.0) and pressures (0.2–3 bar). The large scale in mixture and thermodynamic conditions allows a better understanding of the peculiar behavior of hydrogen flame speeds with pressure. Two very recent detailed chemical kinetic mechanisms for hydrogen combustion were selected. Excellent agreement was observed between calculations and experimental results, confirming the validity of the kinetic schemes selected. The kinetic analyses performed allow proposing an explanation for the nonmonotonic variation of hydrogen/air flame speed with pressure observed in the experiments.     

A study on methane–air premixed flames interacting with syngas–air premixed flames

Numerical study on the interaction between methane–air and syngas–air premixed flames is conducted according to equivalence ratio and global strain rate in detailed chemistry. This study targets at understanding of an interacting combustion system as an alternative retrofit concept where one can modify the existing facilities minimally in industrial and power plant burners in order to reduce the emission of carbon dioxide. It is seen that methane premixed flame interacting with syngas premixed flame can be sustained even over the rich flammable limit of single methane premixed flame. The inspection of detailed flame structure such as the distributions of major species and chain carrier radicals, flame separation distance, spatial flow velocity, and spatial distribution of the rate of production and consumption of CH₄, H₂, and CO is also conducted to depict the flame interactions. The importance of global strain rate and thus the flame separation distance in the enhanced burning of methane premixed flame is also stressed through the inspection of the consumption rates of H₂, CO, and CH₄. Anomalous phenomena such as the migration of premixed flame cross the stagnation plane and the nearly constant flame separation distance are also discussed.     

Effect of initial pressure on flame–shock interaction of hydrogen–air premixed flames

By utilizing a newly designed constant volume combustion bomb (CVCB), turbulent flame combustion phenomena are investigated using hydrogen–air mixture under the initial pressures of 1 bar, 2 bar and 3 bar, including flame acceleration, turbulent flame propagation and flame–shock interaction with pressure oscillations. The results show that the process of flame acceleration through perforated plate can be characterized by three stages: laminar flame, jet flame and turbulent flame. Fast turbulent flame can generate a visible shock wave ahead of the flame front, which is reflected from the end wall of combustion chamber. Subsequently, the velocity of reflected shock wave declines gradually since it is affected by the compression wave formed by flame acceleration. In return, the propagation velocity of turbulent flame front is also influenced. The intense interaction between flame front and reflected shock can be captured by high-speed schlieren photography clearly under different initial pressures. The results show that the propagation velocity of turbulent flame rises with the increase of initial pressure, while the forward shock velocities show no apparent difference. On the other hand, the reflected shock wave decays faster under higher initial pressure conditions due to the faster flame propagation. Moreover, the influence of initial pressure on pressure oscillations is also analyzed comprehensively according to the experimental results.     

Effects of hydrogen and carbon dioxide on the laminar burning velocities of methane–air mixtures

The effects of different mole fractions of hydrogen and carbon dioxide on the combustion characteristics of a premixed methane–air mixture are experimentally and numerically investigated. The laminar burning velocity of hydrogen-methane-carbon dioxide-air mixture was measured using the spherically expanding flame method at the initial temperature and pressure of 283 K and 0.1 MPa, respectively. Additionally, numerical analysis is conducted under steady 1D laminar flow conditions to investigate the adiabatic flame temperature, dominant elementary reactions, and NO formation. The measured velocities correspond with those estimated numerically. The results show that increasing the carbon dioxide mole fraction decreases the laminar burning velocity, attributed to the carbon dioxide dilution, which decreases the thermal diffusivity and flame temperature. Conversely, the velocity increases with the thermal diffusivity as the hydrogen mole fraction increases. Moreover, the hydrogen addition leads to chain-branching reactions that produce active H, O, and OH radicals via the oxidation of hydrocarbons, which is the rate-determining reaction. Furthermore, an increase in the mole fractions of hydrogen and carbon dioxide decreases the NO production amount.     

Flame characteristics of hydrogen-enriched methane–air premixed swirling flames

The effect of hydrogen addition in methane–air premixed flames has been examined from a swirl-stabilized combustor under unconfined flame conditions. Different swirlers have been examined to investigate the effect of swirl intensity on enriching methane–air flame with hydrogen in a laboratory-scale premixed combustor operated at 5.81 kW. The hydrogen-enriched methane fuel and air were mixed in a pre-mixer and introduced into the burner having swirlers of different swirl vane angles that provided different swirl strengths. The combustion characteristics of hydrogen-enriched methane–air flames at fixed thermal load but different swirl strengths were examined using particle image velocimetry (PIV), OH chemiluminescence, gas analyzers, and micro-thermocouple diagnostics to provide information on flow field, combustion generated OH radical and gas species concentration, and temperature distribution, respectively. The results show that higher combustibility of hydrogen assists to promote faster chemical reaction, raises temperature in the reaction zone and reduces the recirculation flow in the reaction zone. The upstream of flame region is more dependent on the swirl strength than the effect of hydrogen addition to methane fuel. At lower swirl strength condition the NO concentration in the reaction zone reduces with increase in hydrogen content in the fuel mixture. Higher combustibility of hydrogen accelerates the flow to reduce the residence time of hot product gases in the high temperature reaction zone. At higher swirl strength the NO concentration increases with increase in hydrogen content in the fuel mixture. The effect of dynamic expansion of the gases with hydrogen addition appears to be more dominant to reduce the recirculation of relatively cooler gases into the reaction zone. NO concentration also increases with decrease in the swirl strength.     

A study on flame interaction between methane/air and nitrogen-diluted hydrogen–air premixed flames

Numerical and experimental studies are conducted to grasp downstream interactions between premixed flames stratified with two different kinds of fuel mixture. The selected fuel mixtures are methane and a nitrogen-diluted hydrogen with composition of 30% H₂ + 70% N₂. Extinction limits are determined for methane/air and (30% H₂ + 70% N₂)/air over the entire range of mixture concentrations. These extinction limits are shown to be significantly modified due to the interaction such that a mixture much beyond the flammability limit can burn with the help of a stronger flame. The lean extinction limit shows both the slanted segments of lower and upper branches due to the strong interaction with Lewis numbers of deficient reactant less than unity, while the rich extinction limit has a square shape due to the weak interaction with Lewis numbers of deficient reactant larger than unity. The regimes of negative flame speed show an asymmetric aspect with a single wing shape. The negative flame always appears only when methane is weak. The extent of interaction depends on the separation distance between the flames, which are the functions of the mixtures’ concentrations, the strain rate, the Lewis numbers, and the preferential diffusions of the penetrated hydrogen from the nitrogen-diluted hydrogen flame. The important role of preferential diffusion effects of hydrogen in the flame interaction is also discussed.     

Large-eddy simulation of lean hydrogen–methane turbulent premixed flames in the methane-dominated regime

The application of large-eddy simulation (LES) to the prediction of H₂-enriched lean methane–air turbulent premixed combustion is considered. A presumed conditional moment (PCM) subfilter-scale combustion model is coupled with the flame prolongation of intrinsic low-dimensional manifold (FPI) chemistry tabulation technique. The LES and PCM-FPI modelling procedures are then applied to the prediction of laboratory-scale axisymmetric Bunsen-type turbulent premixed flames. Both premixed methane–air and H₂-enriched methane–air flames are considered and the predicted solutions are examined and compared to available experimental data. The enriched flame has 20% H₂ in terms of mole fraction and lies in the methane-dominated regime of hydrogen–methane mixtures. The LES simulations predict similar qualitative trends to those found in the experiments for flame height and curvature. The addition of H₂ decreases the flame height and broadens the curvature probability density functions, which show a Gaussian-type shape centred around zero. Moreover, the enriched flame displays a higher degree of wrinkling with sharper ridges of negative curvature and larger pockets of positive curvature. Overall, the proposed treatment for the PCM-FPI combustion model, in terms of progress variable and tabulated data, seems to perform well for the H₂-enriched methane flame in the methane-dominated regime.     

Optimization of flame stabilization methods in the premixed microcombustion of hydrogen–air mixture

The premixed combustion of a lean hydrogen–air mixture is analyzed in this study to examine various properties and flame stabilization. A two-dimensional (2D) analysis of a microscale combustor is performed with various shapes of bluff bodies (e.g., circular and triangular). Nine bluff bodies are placed at the entrance of the microscale combustor and solved with 2D governing equations. The analysis is performed with the three velocities of 10, 20, and 30 m/s, but the equivalence ratio is fixed in all cases. The various characteristics of the microscale combustor are studied such as the temperature of the wall, difference in peak temperature, the mean velocity at the outlet, and temperature of the exhaust gases. Flame stabilization depends on various factors such as bluff body shape and size, and the velocity of the fuel–air mixture at the inlet and recirculation zone. In comparison to all bluff body cases, we observe that the wall blade bluff body is the most efficient (low exhaust gas temperature, large recirculation zone, low mean velocity at the outlet of the microcombustor, and high wall temperature) compared with all eight other bluff body cases. Combustion efficiency is directly proportional to the wall temperature, meaning that the microcombustor with wall blade bluff bodies is more efficient with a stabilized flame. The simulation results are compared with published data on an L/D ratio of 15.     

Analysis of entropy generation in counter-flow premixed hydrogen–air combustion

In this paper, entropy generation in counter-flow premixed hydrogen–air combustion confined by planar opposing jets is investigated for the first time. The effects of the equivalence ratio and the inlet Reynolds number (corresponding to the global stretch rate) on entropy generation are studied by numerical evaluating the entropy generation equation. The lattice Boltzmann model proposed in our previous work, instead of traditional numerical methods, is used to solve the governing equations for combustion process. Through the present study, three interesting features of this kind of combustion, which are quite different from that reported in previous literature on entropy generation analysis for reactive flows, are revealed. Moreover, it is observed that the whole investigated domain can be divided into two parts according to the predominant irreversibilities. The total entropy generation number can be approximated as a linear increasing function of the equivalence ratio and the inlet Reynolds number for all the cases under the present study.     

A mechanistic study of Soret diffusion in hydrogen–air flames

The separate and combined effects of Soret diffusion of the hydrogen molecule (H₂) and radical (H) on the structure and propagation speed of the freely-propagating planar premixed flames, and the strain-induced extinction response of premixed and nonpremixed counterflow flames, were computationally studied for hydrogen–air mixtures using a detailed reaction mechanism and transport properties. Results show that, except for the conservative freely-propagating planar flame, Soret diffusion of H₂ increases the fuel concentration entering the flame structure and as such modifies the mixture stoichiometry and flame temperature, which could lead to substantial increase (decrease) of the flame speed for the lean (rich) mixtures respectively. On the other hand, Soret diffusion of H actively modifies its concentration and distribution in the reaction zone, which in turn affects the individual reaction rates. In particular, the reaction rates of the symmetric, twin, counterflow premixed flames, especially at near-extinction states, can be increased for lean flames but decreased for rich flames, whose active reaction regions are respectively located at, and away from, the stagnation surface. However, such a difference is eliminated for the single counterflow flame stabilized by an opposing cold nitrogen stream, as the active reaction zone up to the state of extinction is always located away from the stagnation surface. Finally, the reaction rate is increased in general for diffusion flames because the bell-shaped temperature distribution localizes the H concentration to the reaction region which has the maximum temperature.     

Study of interaction of entrained coal dust particles in lean methane–air premixed flames

This study investigates the interaction of micron-sized coal particles entrained into lean methane–air premixed flames. In a typical axisymmetric burner, coal particles are made to naturally entrain into a stream of the premixed reactants using an orifice plate and a conical feeder setup. Pittsburgh seam coal dust, with particle sizes in the ranges of 0–25 μm, 53–63 μm, and 75–90 μm, is used. The effects of different coal dust concentrations (10–300 g/m³) entrained into the mixture of methane–air at three lean equivalence ratios, ?, of 0.75, 0.80 and 0.85, on the laminar burning velocity are studied experimentally. The laminar burning velocity of the coal dust–methane–air mixture is determined by taking high quality shadowgraph images of the resulting flames and processing them using the cone-angle method. The results show that the laminar burning velocity reduces with the addition of coal dust having particle sizes in the ranges of 53–63 μm and 75–90 μm, irrespective of the equivalence ratio values. However, burning velocity promotion is observed for one case with particle size in the range of 0–25 μm at an equivalence ratio of 0.75. Two competing effects are considered to explain these trends. The first effect is due to volatile release, which increases the overall equivalence ratio and thus, the flame temperature and burning velocity. The second is the heat sink effect that the coal particles take up to release the volatiles. This process reduces the flame temperature and accordingly the burning velocity also. A mathematical model is developed considering these effects and it is seen to successfully predict the change of laminar burning velocity for various cases with different dust concentrations and equivalence ratios of the gas mixture. Furthermore, the implication of this study to coal mine safety is discussed.