Minimum ignition energy of hydrogen-air and methane-air mixtures at temperatures as low as 200 K

The primary objective of this study is to measure the minimum ignition energy (MIE) of methane-air and hydrogen-air mixtures at low temperatures and atmospheric pressure. Initial fuel-air mixture temperatures as low as 200 K were considered, for a constant equivalence ratio of 1.0 for methane-air and 0.16 for hydrogen-air. The ignition source was a spark, generated by a high-voltage pulse of 100 μs duration, applied on two pin electrodes of 0.1-mm diameter, separated by a gap distance of 1 mm. The experimental methodology was validated by comparing the results obtained with those from previous studies available in the literature. First, for methane-air mixtures, the MIE as a function of the equivalence ratio followed the same trend at 295 K and 255 K, i.e., its lowest value was obtained for a stoichiometric mixture. Second, when the temperature of the mixture was decreased, the MIE increased linearly for both fuels. The rate at which the MIE changed was higher for hydrogen-air (?7.9 μJ/K) than for methane-air (?3.4 μJ/K). Overall, this study provides valuable information on the MIE of methane-air and hydrogen-air mixtures at low temperatures, which can be useful for the design of cryogenic fuel storage systems.     

Laminar burning characteristics of ammonia/hydrogen/air mixtures with laser ignition

Ammonia, as a zero-carbon fuel, is drawing more and more attention. The major challenge of using ammonia as a fuel for the combustion engines lies in its low chemical reactivity, and therefore more fundamental researches on the combustion characteristics of ammonia are required to explore effective ways to burn ammonia in engines. In this study, the laminar burning characteristics of the premixed ammonia/hydrogen/air mixtures are investigated. In the experiment, the laser ignition was used to achieve stable ignition of the ammonia/air mixtures with an equivalence ratio range from 0.7 to 1.4. The propagating flame was recorded with the high-speed shadowgraphy. Three different processing methods were introduced to calculate the laminar burning velocity with a consideration of the flame structure characteristics induced by the laser ignition. The effects of initial pressure (0.1 MPa–0.5 MPa), equivalence ratio (0.7–1.4), hydrogen fraction (0–20%) on the laminar burning velocity were investigated under the initial ambient temperature of 360 K. The state-of-the-art kinetic models were used to calculate the laminar burning velocities in the CHEMKIN-pro software. Both the simulation and experimental results show that the laminar burning velocity of the ammonia mixtures increases at first, reaches the peak around ϕ of 1.1, and then decreases with the equivalence ratio increasing from 0.7 to 1.4. The peak laminar burning velocities of the ammonia mixture are lower than 9 cm/s and are remarkably lower than those of hydrocarbon fuels. The laminar burning velocity of the ammonia mixture decreases with the increase of the initial ambient pressure, and it can be drastically speeded up with the addition of hydrogen. While the models except for those by Miller and Bian can give reasonable predictions compared to the experimental results for the equivalence ratio from 0.7 to 1.1 in the ammonia (80%)/hydrogen (20%)/air mixtures, all the kinetic models overpredict the experiments for the richer mixtures, indicating further work necessary in this respect.     

Numerical study on the spark ignition characteristics of hydrogen-air mixture using detailed chemical kinetics

Hydrogen is a promising fuel and is expected to replace hydrocarbon fuels for its significant potentials to reduce the pollutants and greenhouse gases. It is very important to investigate Minimum ignition energy (MIE) on safety standards and ignition process of hydrogen-air mixtures. Even though the formation of flame kernels in quiescent hydrogen-air mixtures has been researched numerically and experimentally, the details of ignition mechanism have never been satisfactorily explained. In this study, the spark ignition of hydrogen-air mixture is investigated by using detailed chemical kinetics and considering the heat loss to the electrode. The purpose of this study is emphasized in the effects of the energy supply procedure, the radius of the spark channel, electrode size and electrode gap distance on the MIE. In addition, the effects of mixture temperature, electrode gap distance and electrode size on relationship between the equivalence ratio and the MIE are examined.     

A computational study and correlation of premixed isooctane–air laminar reaction front properties under spark ignited and spark assisted compression ignition engine conditions

To address the need for reliable premixed laminar burning velocity and thickness information within the spark assisted compression ignition (SACI) combustion regime, a large dataset of simulated reaction fronts has been generated in this work. A transient one dimensional premixed laminar flame simulation was applied to isooctane–air mixtures using a 215 species chemical kinetic mechanism. The simulation was exercised over fuel–air equivalence ratios, unburned gas temperatures and pressures ranging from 0.1 to 1.0, 298 to 1000 K and 1 to 250 bar, respectively, a range that extends beyond that of previous researchers. Steady reaction fronts with burning velocities in excess of 5 cm/s could not be established under all of these conditions, especially when burned gas temperatures were below 1500 K and/or when characteristic reaction front times were on the order of the unburned gas ignition delay. Steady premixed laminar burning velocities were correlated using a modified two-equation form based upon the asymptotic structure of a laminar flame, which produced an average error of 2.5% between the simulated and correlated laminar burning velocities, with a standard deviation of 3.0%. Additional correlations were constructed for reaction front thickness and adiabatic flame temperature. The resulting premixed laminar burning velocity correlation showed good agreement with experiments and existing correlations within the spark-ignited (SI) regime. Analysis of the simulated characteristic reaction front times and ignition delays suggests that homogeneous SACI combustion is most useful under medium and high load operating conditions.     

Effect of concentration,obstacles, and ignition location on the explosion overpressure of hydrogen-air in a closed-vessel

The report deals with the investigation of explosion safety parameters of hydrogen-air mixtures in a 17.17 L cylindrical closed-vessel with different concentrations, obstacles, and ignition locations. The experimental data including the maximum explosion pressure, laminar burning velocity, and corresponding flame radius were confirmed by using GASEQ code and theoretical calculation, respectively. The report shows the orifice plate reduced the maximum explosion pressure of the low-concentration hydrogen (φ＜20% v/v), while the maximum explosion pressure of high-concentration hydrogen (φ＞20% v/v) was increased, and the oscillation of the explosion pressure in the closed-vessel was obvious. The effect of the ignition location on the maximum explosion pressure was related to the interaction between the flame instability and the orifice plate for the φ = 30% v/v hydrogen-air mixture.     

Premixed hydrogen-air flame front dynamics in channels with central and peripheral ignition

Experimental and analytical study of burning hydrogen-air mixtures with 12, 13, and 15 vol% hydrogen concentrations in channels with central and peripheral ignition was performed. Flame propagation speeds were determined by shadow and infrared high-speed imaging in the transverse and longitudinal directions, respectively. It was found that the increase in the flame front speed during the peripheral ignition reaches up to 1.7 times compared to the central ignition depending on mixture content. The pressure growth rate was examined in a closed channel. It was estimated that the time to reach a maximum pressure is 1.1 times less in the case or peripheral ignition than the central one. An analytical model was formed to describe the dynamics of the flame front in both cases. The model of a “reversed finger-flame” generated by a peripheral ignition was presented. The obtained results could be used in designing hydrogen-fueled combustible engines with the reduced knock-effect.     

Experimental and modeling study of laminar burning velocity of biomass derived gases/air mixtures

Laminar burning velocities of four biomass derived gases have been measured at atmospheric pressure over a range of equivalence ratios and hydrogen contents, using the heat flux method on a perforated flat flame burner. The studied gas mixtures include an air-blown gasification gas from an industrial gasification plant, a model gasification gas studied in the literature, and an upgraded landfill gas (bio-methane). In addition, co-firing of the industrial gasification gas (80% on volume basis) with methane (20% on volume basis) is studied. Model simulations using GRI mechanisms and detailed transport properties are carried out to compare with the measured laminar burning velocities. The results of the bio-methane flame are generally in good agreement with data in the literature and the prediction using GRI-Mech 3.0. The measured laminar burning velocity of the industrial gasification gas is generally higher than the predictions from GRI-Mech 3.0 mechanism but agree rather well with the predictions from GRI-Mech 2.11 for lean and moderate rich mixtures. For rich mixtures, the GRI mechanisms under-predict the laminar burning velocities. For the model gasification gas, the measured laminar burning velocity is higher than the data reported in the literature. The peak burning velocities of the gasification gases/air and the co-firing gases/air mixtures are in richer mixtures than the bio-methane/air mixtures due to the presence of hydrogen and CO in the gasification gases. The GRI mechanisms could well predict the rich shift of the peak burning velocity for the gasification gases but yield large discrepancy for the very rich gasification gas mixtures. The laminar burning velocities for the bio-methane/air mixtures at elevated initial temperatures are measured and compared with the literature data.     

Modelling of hydrogen-blended dual-fuel combustion using flamelet-generated manifold and preferential diffusion effects

In the present study, Reynolds-Averaged Navier-Stokes simulations together with a novel flamelet generated manifold (FGM) hybrid combustion model incorporating preferential diffusion effects is utilised for the investigation of a hydrogen-blended diesel-hydrogen dual-fuel engine combustion process with high hydrogen energy share. The FGM hybrid combustion model was developed by coupling laminar flamelet databases obtained from diffusion flamelets and premixed flamelets. The model employed three control variables, namely, mixture fraction, reaction progress variable and enthalpy. The preferential diffusion effects were included in the laminar flamelet calculations and in the diffusion terms in the transport equations of the control variables. The resulting model is then validated against an experimental diesel-hydrogen dual-fuel combustion engine. The results show that the FGM hybrid combustion model incorporating preferential diffusion effects in the flame chemistry and transport equations yields better predictions with good accuracy for the in-cylinder characteristics. The inclusion of preferential diffusion effects in the flame chemistry and transport equations was found to predict well several characteristics of the diesel-hydrogen dual-fuel combustion process: 1) ignition delay, 2) start and end of combustion, 3) faster flame propagation and quicker burning rate of hydrogen, 4) high temperature combustion due to highly reactive nature of hydrogen radicals, 5) peak values of the heat release rate due to high temperature combustion of the partially premixed pilot fuel spray with entrained hydrogen/air and then background hydrogen-air premixed mixture. The comparison between diesel-hydrogen dual-fuel combustion and diesel only combustion shows early start of combustion, longer ignition delay time, higher flame temperature and NOx emissions for dual-fuel combustion compared to diesel only combustion.     

Flammability limits and laminar flame speed of hydrogen–air mixtures at sub-atmospheric pressures

Hydrogen behavior at elevated pressures and temperatures was intensively studied by numerous investigators. Nevertheless, there is a lack of experimental data on hydrogen ignition and combustion at reduced sub-atmospheric pressures. Such conditions are related to the facilities operating under vacuum or sub-atmospheric conditions, for instance like ITER vacuum vessel. Main goal of current work was an experimental evaluation of such fundamental properties of hydrogen–air mixtures as flammability limits and laminar flame speed at sub-atmospheric pressures. A spherical explosion chamber with a volume of 8.2 dm³ was used in the experiments. A pressure method and high-speed camera combined with schlieren system for flame visualization were used in this work. Upper and lower flammability limits and laminar flame velocity have been experimentally evaluated in the range of 4–80% hydrogen in air at initial pressures 25–1000 mbar. An extraction of basic flame properties as Markstein length, overall reaction order and activation energy was done from experimental data on laminar burning velocity.     

Outward propagation velocity and acceleration characteristics in hydrogen-air deflagration

Propagation characteristics of hydrogen-air deflagration need to be understood for an accurate risk assessment. Especially, flame propagation velocity is one of the most important factors. Propagation velocity of outwardly propagating flame has been estimated from burning velocity of a flat flame considering influence of thermal expansion at a flame front; however, this conventional method is not enough to estimate an actual propagation velocity because flame propagation is accelerated owing to cellular flame front caused by intrinsic instability in hydrogen-air deflagration. Therefore, it is important to understand the dynamic propagation characteristics of hydrogen-air deflagration. We performed explosion tests in a closed chamber which has 300 mm diameter windows and observed flame propagation phenomena by using Schlieren photography. In the explosion experiments, hydrogen-air mixtures were ignited at atmospheric pressure and room temperature and in the range of equivalence ratio from 0.2 to 1.0. Analyzing the obtained Schlieren images, flame radius and flame propagation velocity were measured. As the result, cellular flame fronts formed and flame propagations of hydrogen–air mixture were accelerated at the all equivalence ratios. In the case of equivalent ratio φ = 0.2, a flame floated up and could not propagate downward because the influence of buoyancy exceeded a laminar burning velocity. Based upon these propagation characteristics, a favorable estimation method of flame propagation velocity including influence of flame acceleration was proposed. Moreover, the influence of intrinsic instability on propagation characteristics was elucidated.     

Sub-critical stable hydrogen-air premixed laminar flames in micro gaps

Sub-critical burning of lean hydrogen-air mixtures in micro gaps between two quartz disks was investigated both experimentally and numerically. Stationary regimes for different compositions and gap sizes were found when sub-critical flames remained in a stable position relative to the disk surfaces. The burning velocity in the micro gaps was observed to reach values much larger than the laminar burning velocity. A reaction-diffusion numerical model was proposed to corroborate experimental results. Different factors, such as boundary conditions for velocity, irradiation of the disk surfaces contacting the gas, and an increase in the chemical reaction rate near disk surfaces were modeled numerically in order to explain the increase in burning velocities. The best correlation between the numerical results and experimental data was observed in the scenario proposing as increased chemical reaction rate near the disk surfaces. Numerical simulations also showed that for large flame front velocities and wider sub-critical gaps, the flame front becomes unstable. The reason for this instability is the asynchronization of the combustion near the disk surfaces and the subsequent turbulization of the flame.     

Explosion bomb measurements of ethanol-air laminar gaseous flame characteristics at pressures up to 1.4 MPa

The principal burning characteristics of a laminar flame comprise the fuel vapour pressure, the laminar burning velocity, ignition delay times, Markstein numbers for strain rate and curvature, the stretch rates for the onset of flame instabilities and of flame extinction for different mixtures. With the exception of ignition delay times, measurements of these are reported and discussed for ethanol-air mixtures. The measurements were in a spherical explosion bomb, with central ignition, in the regime of a developed stable, flame between that of an under or over-driven ignition and that of an unstable flame. Pressures ranged from 0.1 to 1.4 MPa, temperatures from 300 to 393 K, and equivalence ratios were between 0.7 and 1.5. It was important to ensure the relatively large volume of ethanol in rich mixtures at high pressures was fully evaporated. The maximum pressure for the measurements was the highest compatible with the maximum safe working pressure of the bomb. Many of the flames soon became unstable, due to Darrieus-Landau and thermo-diffusive instabilities. This effect increased with pressure and the flame wrinkling arising from the instabilities enhanced the flame speed. Both the critical Peclet number and the, more rational, associated critical Karlovitz stretch factor were evaluated at the onset of the instability. With increasing pressure, the onset of flame instability occurred earlier. The measured values of burning velocity are expressed in terms of their variations with temperature and pressure, and these are compared with those obtained by other researchers. Some comparisons are made with the corresponding properties for iso-octane-air mixtures.     

Experimental study and proposed power correlation for laminar burning velocity of hydrogen-diluted methane with respect to pressure and temperature variation

The aim of the present work is to contribute to the better understanding of the combustion process and the laminar flame properties of methane/hydrogen-air flames at elevated temperatures and pressures. The heat flux method provides an accurate and direct measurement of laminar burning velocities (LBV) at elevated temperatures, while the constant volume chamber method provides measurements at elevated pressures. In the present work, a database of more than 250 experimental points for the range of temperature (298–373 K) and pressure conditions (1–5 bar) for mixtures up to 50% hydrogen in methane was generated using these two methods. Comparison with the sparse literature data shows quite good agreement. A power-law correlation for temperature and pressure is proposed for methane/hydrogen-air mixtures, which has a practical application in estimating the LBV of a natural gas/hydrogen mixture intended to replace pure natural gas in different processes. The power-law temperature exponent, α, and the pressure exponent, β, show inverse trends. The former decreases almost linearly and the latter increases approximately linearly when the hydrogen content is increased. The power-law exponents are highly affected by the mixture equivalence ratio, ?, showing a parabola like trend. However, for the pressure exponent this trend becomes almost linear for 50% H₂ in the mixture. The power-law correlation has been validated against experimental data for a wide range of temperature (up to 573 K), pressure (1–7.5 bar), equivalence ratios (? between 0.7 and 1.3) and H₂ contents up to 50%.     

Numerical study on the spark ignition characteristics of a methane-air mixture using detailed chemical kinetics: Effect of equivalence ratio, electrode gap distance, and electrode radius on MIE, quenching distance, and ignition delay

The minimum ignition energy (MIE) is an important property for designing safety standards and understanding the ignition process of combustible mixtures. Even though the formation of flame kernels in quiescent methane-air mixtures has been simulated numerically, the ignition mechanism has never been satisfactorily explained. This study investigated the spark ignition of methane-air mixtures through a numerical analysis using detailed chemical kinetics consisting of 53 species and 325 elementary reactions while considering the heat loss to the electrode. The simulation was used to investigate the quenching distance and the effects on the MIE of the electrode size, electrode gap distance, ignition duration, and equivalence ratio. The effect of the equivalence ratio on the ignition delay time was also examined. The simulated results showed the same trend as previous experimental results.     

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.     

Calculation of the upper flammability limit of methane/hydrogen/air mixtures at elevated pressures and temperatures

The results of three different numerical methods to calculate flammability limits—namely (1) the calculation of planar flames with the inclusion of a (radiation) heat loss term in the energy conservation equation, and the application of (2) a limiting burning velocity and of (3) a limiting flame temperature—are compared with experimental data on the upper flammability limit (UFL) of methane/hydrogen/air mixtures with hydrogen fuel molar fractions of 20% and 40%, at initial pressures up to 10 bar and initial temperatures up to 200 °C. The application of a limiting burning velocity is found to predict the pressure dependence of the UFL well, while the application of a limiting flame temperature generally is found to slightly underestimate the temperature dependence of the UFL.     

Laminar burning velocities and flame instabilities of diluted H2/CO/air mixtures under different hydrogen fractions

An experimental study was conducted using outwardly propagating flame to evaluate the laminar burning velocity and flame intrinsic instability of diluted H₂/CO/air mixtures. The laminar burning velocity of H₂/CO/air mixtures diluted with CO₂ and N₂ was measured at lean equivalence ratios with different dilution fractions and hydrogen fractions at 0.1 MPa; two fitting formulas are proposed to express the laminar burning velocity in our experimental scope. The flame instability was evaluated for diluted H₂/CO/air mixtures under different hydrogen fractions at 0.3 MPa and room temperature. As the H₂ fraction in H₂/CO mixtures was more than 50%, the flame became more unstable with the decrease in equivalence ratio; however, the flame became more stable with the decrease in equivalence ratio when the hydrogen fraction was low. The flame instability of 70%H₂/30%CO premixed flames hardly changed with increasing dilution fraction. However, the flames became more stable with increasing dilution fraction for 30%H₂/70%CO premixed flames. The variation in cellular instability was analyzed, and the effects of hydrogen fraction, equivalence ratio, and dilution fraction on diffusive-thermal and hydrodynamic instabilities were discussed.     

Release of chemical energy by combustion in a supersonic mixing layer of hydrogen and air

The process involved in chemical energy release by combustion in a supersonic, constant pressure, hydrogen-air laminar mixing layer was studied computationally, with a chemical kinetics model involving nineteen reactions and eight species. To try to find out the physical reason for the different trends of the pressure curves observed in an experimental supersonic combustor at two different initial air stream temperatures. Two initial air stream temperatures corresponding to the two experimental cases are chosen such that the higher temperature yielded a shorter ignition distance, and the lower temperature yielded a longer ignition distance. For both cases the stream wise rate of energy release rises rapidly to a peak after ignition then falls to a post-ignition value which decreases very slowly with distance. A single premixed flame occurs at ignition for both cases, but then develops into a triple flame structure in the high temperature case, and a flame with only two branches in the low temperature case. The flames move from the airside to hydrogen side consuming the oxygen as they go, until the post-ignition phase is reached. There the dominant energy release arises from the formation of a diffusion flame. In the high temperature case a narrow lean premixed flame accompanies this diffusion flame on the airside. The flame structure, but not the energy release, is effected by the initial temperature distribution across the mixing layer, which is found to be influenced by the velocity difference between the faster air stream and the slower hydrogen stream. Increasing the concentration of oxygen atoms in the oncoming air stream was found to cause substantial reduction in the ignition distance, but did not significantly effect the flame structure, or the rate of heat release in the post-ignition phase. Finally, the different trends of pressure curves observed in the experiment can be reconstructed when pressure variation was considered in this model. Thus we can conclude that the difference in the trends of the pressure curves is caused by the difference in the initial air stream temperature.     

天然气-氢气-空气混合气的层流燃烧速度测定

在定容燃烧弹内研究了常温常压下天然气-氢气-空气混合气的火焰传播规律,得到了不同掺氢比例（氢气在天然气中的体积掺混比例为0％～100％）和燃空当量比（0．6～1．4）下混合气的层流燃烧速率和马克斯坦长度,通过对马克斯坦长度的测量,分析了拉伸对火焰传播的影响。结果表明,随着天然气中掺氢比例的增加,混合气的燃烧速率呈指数规律增加,马克斯坦长度值减小,火焰的稳定性下降。各掺氢比例下,随当量比的增加,马克斯坦长度值增加,火焰的稳定性增强。通过对试验结果的数据拟合,得到了计算天然气-氢气-空气混合气层流燃烧速率的关系式。     

Effects of hydrogen addition on the premixed laminar-flames of ethanol–air gaseous mixtures: An experimental study

The effects of hydrogen addition on the laminar premixed-flame characteristics of ethanol–air gaseous mixtures were investigated experimentally by using outwardly propagating spherical flames. The experiments were conducted in a constant-volume combustion vessel with a central ignition at an initial temperature of 383 K, a pressure of 0.1 MPa, a hydrogen fraction from 0% to 100%, and an equivalence ratio from 0.6 to 1.6, and the flame images were obtained by a high-speed schlieren camera system. The results show that the unstretched flame propagation speeds and burning velocities increase exponentially with the increase in hydrogen fraction for a constant equivalence ratio. When the hydrogen fraction is equal to or less than 60%, the burned gas Markstein length reduces with the increase of equivalence ratio, indicating a positive correlation between the flame instability and hydrogen fraction, while the opposite effect is observed when the hydrogen fraction is greater than 60%. At an equivalence ratio below 1.4, the Markstein length decreases with increased hydrogen fraction, indicating that the flame instability is exacerbated with hydrogen addition, while the reverse holds in the case of equivalence ratio above 1.4. Finally, an empirical formula is developed to estimate the laminar burning velocity of ethanol–hydrogen–air flames on the basis of present experimental data.