•Ignition of methane, n-butane and n-decane were studied numerically.•Effects of water vapor dilution on ignition were examined.•The minimum ignition energy increases greatly with the water vapor dilution.•The minimum ignition energy changes inversely with the pressure.Water vapor dilution has great impact on fundamental combustion processes such as ignition, flame propagation and extinction. In the literature, there are many studies on how water vapor addition affects flame propagation and extinction limit. However, the influence of water vapor addition on ignition receives little attention. In this study, numerical simulations considering detailed chemical mechanisms are conducted for the ignition of methane, n-butane and n-decane/air/water vapor mixtures. The emphasis is spent on examining the effects of water vapor dilution on the ignition of these fuels at normal and reduced pressures. The minimum ignition energies (MIE) at different dilution ratios and initial pressures are obtained. It is found that at normal and reduced pressures, the MIE is proportional to the inverse of pressure and it increases exponentially with water vapor dilution ratio. A general correlation among the MIE, pressure and dilution ratio is proposed for each fuel. Furthermore, for stoichiometric methane/air/water vapor mixtures, the chemical and radiation effects of water vapor dilution are isolated and quantified. It is found that the three-body recombination reaction greatly increases the MIE and reduces the dilution limit.

According to the reactivity gradient theory of Zel'dovich, the non-uniformity in temperature or concentration can lead to detonation development under certain conditions. In the literature, there are many studies on detonation development caused by temperature gradient or hot spot. However, the modes of supersonic reaction front propagation and detonation development regime caused by concentration non-uniformity have not been investigated previously. In this study, one-dimensional simulations were conducted to investigate the transient autoignition and reaction front propagation processes in n-heptane/air mixture with concentration non-uniformity. With the increase of equivalence ratio gradient, three modes (supersonic autoignitive reaction front, developing detonation and subsonic reaction front) of reaction front propagation induced by concentration non-uniformity were identified. The effects of heat conduction and mass diffusion on these three modes were examined and it was demonstrated that molecular diffusion has little influence on the first two modes. The detonation development regime caused by concentration non-uniformity was reported in this paper. This regime was found to be similar to the one caused by temperature gradient. A non-dimensional parameter was proposed to characterize the lower limit of the detonation regime. Furthermore, the effects of initial temperature on the detonation development regime were examined. It was found that the detonation development regime becomes wider as the initial temperature decreases. The initial temperature was shown to only have great impact on the upper limit of the detonation development regime while it has little influence on the lower limit. The influence of initial temperature was explained using the volumetric energy density of the mixture.

Laminar flame speed is one of the most important intrinsic properties of a combustible mixture. Due to its importance, different methods have been developed to measure the laminar flame speed. This paper reviews the constant-volume propagating spherical flame method for laminar flame speed measurement. This method can be used to measure laminar flame speed at high pressures and temperatures which are close to engine-relevant conditions. First, the propagating spherical flame method is introduced and the constant-volume method (CVM) and constant-pressure method (CPM) are compared. Then, main groups using the constant-volume propagating spherical flame method are introduced and large discrepancies in laminar flame speeds measured by different groups for the same mixture are identified. The sources of discrepancies in laminar flame speed measured by CVM are discussed and special attention is devoted to the error encountered in data processing. Different correlations among burned mass fraction, pressure, temperature and flame speed, which are used by different researchers to obtain laminar flame speed, are summarized. The performance of these correlations are examined, based on which recommendations are given. Finally, recommendations for future studies on the constant-volume propagating spherical flame method for laminar flame speed measurement are presented. 层流火焰速度是预混燃气的重要燃烧特性。目前已有多种方法被用来测量层流火焰速度。定容球形传播火焰方法能够测量接近于内燃机燃烧工况下的层流火焰速度,本文对该方法进行了综述。首先介绍并比较了定容和定压球形传播火焰方法;接下来讨论了导致定容球形传播火焰方法测量误差的各种因素,重点分析比较了各种数据处理模型的优缺点,并对实验数据处理给出了建议;最后给出了关于定容球形传播火焰方法的研究展望。

•Flame propagation and ignition properties of iC8H18/H2/air are studied.•Effects of hydrogen addition on laminar flame speed are examined.•The Markstein length changes non-monotonically with hydrogen blending.•Small amount of hydrogen addition greatly reduces minimum ignition energy.Numerical simulations of one-dimensional planar and spherical flame propagation of iso-octane/hydrogen/air mixtures at different equivalence ratios and hydrogen blending levels are conducted considering detailed chemistry. Our focus is on the effects of hydrogen addition on laminar flame propagation and ignition of iso-octane/air mixtures. Specifically, the laminar flame speed, Markstein length, and minimum ignition energy of iso-octane/hydrogen binary fuel blends are investigated. The laminar flame speed is found to increase first slightly and then exponentially with the molar ratio of hydrogen in the iso-octane/hydrogen binary fuel blends. However, a nearly linear trend is observed when the mass ratio instead of molar ratio of hydrogen blending is considered. Similar trend holds for hydrogen addition to other hydrocarbon fuels such as methane and propane. The thermal and chemical effects involved in laminar flame speed enhancement by hydrogen addition are quantified and it is found that the chemical effect prevails over the thermal effect. Unlike the laminar flame speed, the Markstein length is found to change non-monotonically with hydrogen blending ratio. Blending hydrogen to iso-octane/air and blending iso-octane to hydrogen/air both promote diffusive-thermal instability. Moreover, the minimum ignition energy of iso-octane/air is shown to be reduced by a small amount of hydrogen addition, especially for the fuel lean case. When the hydrogen blending molar ratio is above 60%, the minimum ignition energy is found to be insensitive to hydrogen addition.

•Ignition enhancement of CH4 by H2 and DME addition is studied.•Different ignition enhancement trends are found for H2 and DME addition.•The kinetic effects and transport effects on the ignition enhancement are examined.•The non-premix ignition is greatly affected by the mass diffusivity of each fuel.Premixed and non-premixed ignition of methane/hydrogen and methane/dimethyl ether (DME) binary fuel blends with hot air is studied through numerical simulation with detailed chemistry and variable thermodynamic and transport properties. The emphasis is spent on assessing the kinetic and transport effects involved in CH4 ignition enhancement caused by H2 and DME addition. Two configurations are considered: a premixed homogeneous configuration to examine the chemical kinetics and a non-premixed counterflow configuration to assess the transport effects. For the homogeneous ignition process, small amount of DME addition is found to be more effective than H2 addition in terms of promoting the ignition of CH4/air mixture. Sensitivity analysis and reaction path analysis are conducted and key elementary reactions involved in CH4 ignition enhancement by H2 and DME addition are identified. For the non-premixed ignition process, H2 addition is shown to be always more effective than DME addition in terms of CH4 ignition enhancement. It is found that the preferential mass diffusion of H2 over CH4 and that of CH4 over DME have great influence on the local blending ratio at the ignition kernel, which controls the non-premixed ignition process. Therefore, non-premixed ignition of binary fuel blends is significantly affected by the mass diffusivity of each fuel component. Moreover, the effects of strain rate on the non-premixed ignition of CH4/H2 and CH4/DME binary fuel blends with hot air are discussed.

Correlations for the laminar burning velocity of premixed CH4/H2/O2/N2 mixtures were developed using the method of High Dimensional Model Representation (HDMR). Based on experiment data over a wide range of conditions reported in the literature, two types of HDMR correlation (i.e. global and piecewise HDMR correlations) were obtained. The performance of these correlations was assessed through comparison with experimental results and the correlation reported in the literature. The laminar burning velocity predicted by the piecewise HDMR correlations was shown to agree very well with those from experiments. Therefore, the piecewise HDMR correlations can be used as an effective replacement for the full chemical mechanism when the prediction of the laminar burning velocity is needed in certain combustion modeling.Highlights►HDMR correlations for the laminar burning velocity of CH4/H2/O2/N2 were developed. ►The performance of these correlations was assessed. ►The piecewise HDMR correlations can accurately predict the laminar burning velocity.

Fuel blends are widely utilized in high-performance combustion engines and surrogate fuel models. It is essential to understand thoroughly the fundamental combustion properties such as the ignition delay time, laminar flame speed, and extinction strain rate of fuel blends. In this study, the unsteady ignition process of n-decane/toluene binary fuel blends is investigated numerically with detailed reaction mechanism and transport properties. The emphasis is spent on assessing the kinetic and transport effects of toluene addition on the premixed and nonpremixed ignition of n-decane with air. Two configurations are considered: a static premixed homogeneous configuration to examine the chemical kinetics and a nonpremixed counterflow configuration to assess the effects of kinetics as well as transport. For the homogeneous ignition process, the ignition delay time is found to be strongly affected by the toluene molar fraction in the binary fuel blends. Sensitivity analysis and reaction path analysis are conducted and key elementary reactions involved in the ignition inhibition by toluene addition are identified. For the nonpremixed ignition process, the ignition delay time is shown to be strongly affected by the strain rate as well as the toluene blending ratio. The transport effects on the nonpremixed ignition process are examined with the help of scalar dissipation rate and sensitivity analysis. It is demonstrated that the diffusion transport plays a very important role in the nonpremixed ignition process.

Recent studies have demonstrated promising performance of adding hydrogen to methane in internal combustion engines and substantial attention has been devoted to binary fuel blends. Due to the strong nonlinearity of chemical reaction process, the laminar flame speed of binary fuel blends cannot be obtained from linear combination of the laminar flame speed of each individual fuel constituent. In this study, theoretical analysis is conducted for a planar premixed flame of binary fuel blends and a model for the laminar flame speed is developed. The model shows that the laminar flame speed of binary fuel blends depends on the square of the laminar flame speed of each individual fuel component. This model can predict the laminar flame speed of binary fuel blends when three laminar flame speeds are available: two for each individual fuel component and the third one for the fuel blends at one selected blending ratio. The performance of this model as well as models reported in the literature is assessed for methane/hydrogen mixtures. It is demonstrated that good agreements with calculations or measurements can be achieved by the present model prediction. Moreover, it is found that the present model also works for other binary fuel blends besides methane/hydrogen.Highlights► Theoretical analysis is conducted for a planar premixed flame of binary fuel blends. ► A model for the laminar flame speed of binary fuel blends is proposed. ► This model works for methane/hydrogen and other binary fuel blends.

Asymptotic analysis is conducted for outwardly propagating spherical flames with large activation energy. The spherical flame structure consists of the preheat zone, reaction zone, and equilibrium zone. Analytical solutions are separately obtained in these three zones and then asymptotically matched. In the asymptotic analysis, we derive a correlation describing the spherical flame temperature and propagation speed changing with the flame radius. This correlation is compared with previous results derived in the limit of infinite value of activation energy. Based on this correlation, the properties of spherical flame propagation are investigated and the effects of Lewis number on spherical flame propagation speed and extinction stretch rate are assessed. Moreover, the accuracy and performance of different models used in the spherical flame method are examined. It is found that in order to get accurate laminar flame speed and Markstein length, non-linear models should be used.

Correlations for the ignition delay times of hydrogen/air mixtures were developed using the method of High Dimensional Model Representation (HDMR). The hydrogen/air ignition delay times for initial conditions over a wide range of temperatures from 800 to 1600 K, pressures from 0.1 to 100 atm, and equivalence ratios from 0.2 to 10 were first calculated utilizing the full chemical mechanism. Correlations were then developed based on these ignition delay times. Two forms of correlations were constructed: the first one is an overall general model covering the whole range of the initial conditions; while the second one is a piecewise correlation model valid for initial conditions within different sub-domains. The performance of these correlations was studied through comparison with results from the full chemical mechanism as well as experimental data. It was shown that these correlations work well over the whole range of initial conditions and that the accuracy can be significantly improved by using different piecewise correlations for different sub-domains. Therefore, piecewise correlations can be used as an effective replacement for the full mechanism when the prediction of chemical time scale is needed in certain combustion modeling.

A computational study is performed to investigate the effects of hydrogen addition on the fundamental characteristics of propagating spherical methane/air flames at different conditions. The emphasis is placed on the laminar flame speed and Markstein length of methane/hydrogen dual fuel. It is found that the laminar flame speed increases monotonically with hydrogen addition, while the Markstein length changes non-monotonically with hydrogen blending: it first decreases and then increases. Consequently, blending of hydrogen to methane/air and blending methane to hydrogen/air both destabilize the flame. Furthermore, the computed results are compared with measured data available in the literature. Comparison of the computed and measured laminar flame speeds shows good agreement. However, the measured Markstein length is shown to strongly depend on the flame radii range utilized for data processing and have very large uncertainty. It is found that the experimental results cannot correctly show the trend of Markstein length changing with the hydrogen blending level and pressure and hence are not reliable. Therefore, the computed Markstein length, which is accurate, should be used in combustion modeling to include the flame stretch effect on flame speed.

Laminar flame speed is one of the most important intrinsic properties of a combustible mixture. Due to its importance, different methods have been developed to measure the laminar flame speed. This paper reviews the constant-volume propagating spherical flame method for laminar flame speed measurement. This method can be used to measure laminar flame speed at high pressures and temperatures which are close to engine-relevant conditions. First, the propagating spherical flame method is introduced and the constant-volume method (CVM) and constant-pressure method (CPM) are compared. Then, main groups using the constant-volume propagating spherical flame method are introduced and large discrepancies in laminar flame speeds measured by different groups for the same mixture are identified. The sources of discrepancies in laminar flame speed measured by CVM are discussed and special attention is devoted to the error encountered in data processing. Different correlations among burned mass fraction, pressure, temperature and flame speed, which are used by different researchers to obtain laminar flame speed, are summarized. The performance of these correlations are examined, based on which recommendations are given. Finally, recommendations for future studies on the constant-volume propagating spherical flame method for laminar flame speed measurement are presented.

Usually different autoignition modes can be generated by a hot spot in which ignition occurs earlier than that in the surrounding mixture. However, for large hydrocarbon fuels with negative temperature coefficient (NTC) behavior, ignition happens earlier at lower temperature than that at higher temperature when the temperature is within the NTC regime. Consequently, a cool spot may also result in different autoignition modes. In this study, the modes of reaction front propagation caused by temperature gradient in a one dimensional planar configuration are investigated numerically for n-heptane/air mixture at initial temperature within and below the NTC regime. For the first time, different supersonic autoignition modes caused by a cool spot with positive temperature gradient are identified. It is found that the initial temperature gradient has strong impact on autoignition modes. With the increase of the positive temperature gradient of the cool spot, supersonic autoignitive deflagration, detonation, shock-detonation, and shock-deflagration are sequentially observed. It is found that shock compression of the mixture between the deflagration wave and leading shock wave produces an additional ignition kernel, which determines the autoignition modes. Furthermore, the cool spot is compared with the hot spot with temperature below the NTC regime. Similar autoignition modes are observed for the hot and cool spots. Different autoignition modes in the considered simplified configuration are summarized in terms of the normalized temperature gradient and acoustic-to-excitation time scale ratio. It is shown that the transition between different autoignition modes is not greatly affected by the NTC behavior. Therefore, our 1-D simulation indicates that like hot spot, the cool spot may also generate knock in engines when fuels with NTC behavior is used and the temperature is within the NTC regime.

The effects of Soret diffusion on premixed syngas/air flames at normal and elevated temperatures and pressures are investigated numerically including detailed chemistry and transport. The emphasis is placed on assessing and interpreting the influence of Soret diffusion on the unstretched and stretched laminar flame speed and Markstein length of syngas/air mixtures. The laminar flame speed and Markstein length are obtained by simulating the unstretched planar flame and positively-stretched spherical flame, respectively. The results indicate that at atmospheric pressure the laminar flame speed of syngas/air is mainly reduced by Soret diffusion of H radical while the influence of H2 Soret diffusion is negligible. This is due to the facts that the main reaction zone and the Soret diffusion for H radical (H2) are strongly (weakly) coupled, and that Soret diffusion reduces the H concentration in the reaction zone. Because of the enhancement in the Soret diffusion flux of H radical, the influence of Soret diffusion on the laminar burning flux increases with the initial temperature and pressure. Unlike the results at atmospheric pressure, at elevated pressures the laminar flame speed is shown to be affected by the Soret diffusion of H2 as well as H radical. For stretched spherical flame, it is shown that the Soret diffusion of both H and H2 should be included so that the stretched flame speed can be accurately predicted. Similar to the laminar flame speed, the Markstein length is also reduced by Soret diffusion. However, the reduction is found to be mainly caused by Soret diffusion of H2 rather than that of H radical. Moreover, the influence of Soret diffusion on the Markstein length is demonstrated to decrease with the initial temperature and pressure.