Autoignition Of Mixtures Research Articles

Numerical study of laminar nonpremixed methane flames in coflow jets: Autoignited lifted flames with tribrachial edges and MILD combustion at elevated temperatures

Autoignition characteristics of laminar nonpremixed methane jet flames in high-temperature coflow air are studied numerically. Several flame configurations are investigated by varying the initial temperature and fuel mole fraction. At a relatively low initial temperature, a non-autoignited nozzle-attached flame is simulated at relatively low jet velocity. When the initial temperature is higher than that required for autoignition, two regimes are investigated: an autoignited lifted flame with tribrachial edge structure and an autoignited lifted flame with Mild combustion. The autoignited lifted flame with tribrachial edge exhibited three branches: lean and rich premixed flame wings and a trailing diffusion flame. Characteristics of kinetic structure for autoignited lifted flames are discussed based on the kinetic structures of homogeneous autoignition and flame propagation of stoichiometric mixture. Results showed that a transition from autoignition to flame propagation modes occurs for reasonably stoichiometric mixtures. The autoignited lifted flame with Mild combustion occurs when methane fuel is highly diluted with nitrogen. The kinetic structure analysis shows that the characteristics of Mild combustion can be treated as an autoignited lean premixed lifted flame. Transition behavior from Mild combustion to nozzle-attached flame was investigated by increasing the fuel mole fraction. As the maximum flame temperature increases with decreasing liftoff height, the kinetic structure showed a transition behavior from autoignition to flame propagation of a lean premixed flame.

Low-temperature autoignition of binary mixtures of methane with C3–C5 alkanes

The influence of C3–C5 alkanes on the ignition of their binary mixtures with methane in air at a temperature of 523–1000 K and a pressure of 1 atm is studied. It is shown that the presence of only 1% C3–C5 alkanes considerably reduces the ignition delay of methane. At a concentration of 10–20%, the ignition delay practically corresponds to the autoignition delay of the added alkane. The effect of additives of heavy alkanes becomes less noticeable with increasing initial temperature. These results can be used to estimate the permissible content of C5+ heavy species in gas turbine engine fuel at which their influence on the fuel knock resistance is sufficiently low. It is only 0.5%.

Low–Temperature Autoignition of Binary Mixtures of Methane with Alkanes С3–С5

Low–Temperature Autoignition of Binary Mixtures of Methane with Alkanes С3–С5

Numerical simulation of the autoignition of hydrogen-air mixtures behind shock waves

Problems related to the autoignition of hydrogen-air mixtures are highly important for the operation safety of nuclear reactors and for hydrogen power engineering. In spite of extensive studies in this area, there are still many problems directly concerned with the ignition delay times of H2/O2 mixtures and with the conditions under which these processes occur. This paper deals with the numerical analysis of the data available in the literature on O, H, and OH yields in order to determine the influence of the primary channels of the initiation of H2/Air mixtures. The numerical modeling of the available literature data on the ignition delays of hydrogen-air mixtures made it possible to describe the shock tube measurements of ignition delays within the framework of a unified kinetic mechanism over a temperature range of 930-2500 K at pressures from 0.1 to 8.7 MPa.

The influence of different auto-ignition modes on the behavior of pressure waves

For internal combustion engines, the knock of Homogeneous Charge Compression Ignition engines, the conventional knock of gasoline engines and the super knock are all caused by the auto-ignition of unburned mixture which leads to the oscillation burning, but their Maximal Pressure Oscillation Amplitude (MPOA) and Maximum Pressure Rising Rate (MPRR) are totally different. In order to explore the reason, we propose three typical auto-ignition modes and then bring up the method of “Energy Injected” (EI) which is based on the experiment measured heat release rate. Through changing the heat source term in the energy equation for different auto-ignition modes, we conducted a series of numerical simulations for these three modes. After that, the following pressure oscillations can be compared and analyzed. The numerical simulation results show that different combustion pressure waves with different oscillation characteristics come from different auto-ignition modes, thus the macroscopic MPRR and MPOA are totally different. Furthermore, the method of “EI” based on the experiment measured heat release rate can accurately and rapidly help to research the formation and propagation of pressure waves in the engine combustion chamber.

H2/air autoignition: The nature and interaction of the developing explosive modes

Appropriate algorithmic tools are employed for the analysis of the explosive modes developing during the autoignition of homogeneous mixtures. The ability of these tools to provide significant physical understanding is demonstrated in the case of the homogeneous ignition of a stoichiometric H2/air mixture, modelled by two different chemical kinetics mechanisms. It is shown that the ignition process evolves in two stages. The first stage is characterised by the development of two explosive timescales (one fast and one slow), that lead the system away from equilibrium. As the end of the first stage is approached, the two explosive timescales converge, they merge and then they disappear. In the second stage only dissipative timescales develop, which drive the system all the way to equilibrium. It is shown that throughout the first stage the fast explosive timescale is generated by chain reactions. The slow explosive timescale is initially generated by an initiation reaction that produces the radicals required for the start-up of the fast mode, while later on it is generated by reactions that are responsible for the heat released. These findings are validated with sensitivity analysis results for the ignition delay time and are employed in order to clarify the discrepancies in the solution provided by the two different chemical kinetics mechanisms considered.

Promotion of the high-temperature autoignition of hydrogen-air and methane-air mixtures by normal alkanes

Numerical simulations are performed to examine the effect of small additives of heavy hydrocarbons on the high-temperature autoignition of homogeneous hydrogen-air and methane-air mixtures. The kinetic calculations are carried out using a previously developed detailed mechanism of the oxidation and combustion of normal alkanes. It is shown that the behavior of hydrogen-hydrocarbon-air ternary mixtures is ambiguous. Large hydrocarbon additives of n-heptane and n-hexadecane to a hydrogen-air mixture promote its autoignition at low temperatures and inhibits it at high temperatures, whereas in a definite high-temperature range, small additives of hydrocarbons can promote rather than inhibit the autoignition. The autoignition of methane-air mixtures is promoted by additives of heavy hydrocarbons in all cases.

The initiation of methane/air autoignition: the important chemical components for various initial conditions

A reactive system will be considered, the slow dynamics of which is characterized by time scales that are of explosive character; i.e., the components of the system that generate them tend to lead the system away from equilibrium. In particular, the initiation of a methane/air mixture autoignition will be considered for various initial temperatures and pressures. The species that relate to the explosive time scales and reactions that are responsible for the generation of these time scales will be identified. The analysis will be based on the Computational Singular Perturbation (CSP) algorithm, which is employed for the construction of the the reduced system that governs the long range (slow) evolution of the system. It will be demonstrated that an excellent agreement with the existing literature is obtained.

Investigation of Auto-Ignition of Heptane-CNG Mixture in HCCI Engine

Homogeneous Charge Compression Ignition (HCCI) operating principals have been widely investigated yet the uncontrollable combustion of HCCI is the major obstacle in its development. This paper is trying to increase the understanding on the auto-ignition and combustion process of several fuels to be applied in HCCI combustion system. This paper investigates variation of fuel composition between heptane and compressed natural gas (CNG) with the composition ranges from 10-100% heptane/CNG. The investigation was done in a constant volume chamber with elevated temperature (800°C). Three lambdas were tested for each fuel, namely 0.8, 1, 1.2 and 2. From the results, it is found that the mixture composition highly affects the output of the combustion of dual fuel where CNG is able to suppress heptane combustion at CNG percentage more than 40%. Furthermore, homogeneity level highly determines the types of combustion produced by the mixture, distributed or propagation flame.

Autoignition of n-Decane/n-Butylbenzene/n-Propylcyclohexane Mixtures and the Effects of the Exhaust Gas Recirculation

In the present work, the autoignition of single-component and binary mixtures composed of n-decane, n-butylbenzene, and/or n-propylcyclohexane has been investigated using shock-tube techniques. In addition, the effects of the presence of the exhaust gas recirculation (EGR) components on the autoignition behavior of a 4:3:3 molar n-decane: n-butylbenzene:n-propylcyclohexane surrogate mixture have been investigated experimentally. The experiments have been performed at highly diluted conditions in argon bath gas, over a wide range of temperatures (1250–1750 K), equivalence ratios (Φ = 0.2–1.5), and nominal pressures of 10–20 bar. A chemical kinetic model was developed to simulate the newly obtained experimental data by blending sub-models from the literature. In particular, the experimental and numerical analyses suggest that the observed increase in the ignition delay times in the presence of EGR is mainly due to the dilution levels and not to the chemistry of the fuel. Additional kinetic analyses were performed to compare the high-temperature autoignition properties of the different components considered herein.

The global relaxation redistribution method for reduction of combustion kinetics.

An algorithm based on the Relaxation Redistribution Method (RRM) is proposed for constructing the Slow Invariant Manifold (SIM) of a chosen dimension to cover a large fraction of the admissible composition space that includes the equilibrium and initial states. The manifold boundaries are determined with the help of the Rate Controlled Constrained Equilibrium method, which also provides the initial guess for the SIM. The latter is iteratively refined until convergence and the converged manifold is tabulated. A criterion based on the departure from invariance is proposed to find the region over which the reduced description is valid. The global realization of the RRM algorithm is applied to constant pressure auto-ignition and adiabatic premixed laminar flames of hydrogen-air mixtures.

The Onset of Detonation Behind Shock Waves of Moderate Intensity in Gas Phase

The shock-to-detonation transition (SDT) in gaseous n-heptane/oxygen/argon mixtures has been experimentally studied, using a shock tube, at low initial pressure (2–4 kPa) for a better understanding of the deflagration-to-detonation transition process. The detonation is generated by a precursory shock wave (PSW), with a Mach number smaller than that of the self-sustained detonation. Pressure (P2) and temperature (T2) behind incident shock waves have been accurately determined from the PSW velocity. The transition occurs in the measurement zone located between 3.20 m and 3.65 m from the shock tube diaphragm. The auto-ignition of mixture behind PSW is immediately followed by the onset of a combustion wave, which propagates at near Chapman–Jouguet (CJ) detonation velocity in the mixture carried at P2, T2 conditions. Consequently, the pressure peak can reach 350 times the initial pressure during the transition. The combustion wave merges with the PSW to form an overdriven detonation propagating in the initial mixture at velocity that progressively decreases to a value close to CJ value, of the order of 2 000 m.s−1. The experimental particles heating times are compared with the computed ignition delay times, τI,2, by using a detailed kinetic model of n-heptane oxidation. For stoichiometric n-heptane/oxygen mixtures diluted by 50% Ar, the experimental delay times are compatible with those computed. In this case, τi,2 is governed by the so-called “high temperature mechanism” (T2 > 1000 K) and varies exponentially with temperature. For undiluted mixtures, the particle heating times is much shorter than τi, 2. Then, the chemistry is governed by the “low temperature mechanism” (T2 < 950 K). Between 750 K and 950 K, τi, 2 decreases or small changes with temperature decrease but is sensitive to pressure. Turbulence in boundary layer could also promote the SDT.

The role of global and detailed kinetics in the first-stage ignition delay in NTC-affected phenomena

Using n-heptane as a representative fuel exhibiting NTC (negative temperature coefficient) chemistry, a comprehensive computational and mechanism study was conducted on the role and controlling chemistry of the first-stage ignition delay in the superficially dissimilar systems of the auto-ignition of homogeneous mixtures and the nonpremixed counterflow ignition of fuel versus heated air. It is first shown that the first-stage auto-ignition delay time, τ1, possesses a minimum value, τ1,min, with increasing temperature, and that for temperatures below the range corresponding to τ1,min, τ1 is largely insensitive to the equivalence ratio (ϕ) and pressure (p) of the mixture. Furthermore, in this regime the global reaction order was found to be close to unity, hence supporting the notion that the limiting steps in this temperature regime are the RO2 isomerization reactions, which in turn explains the insensitivity of τ1 on ϕ and p in this temperature regime. However, when the temperature approaches that of τ1,min, competition of QOOH decomposition and the β scission reactions of the alkyl radicals with the low-temperature chemistry chain reactions, as well as the equilibrium shift of the oxygen addition reactions, increases τ1 and consequently results in τ1,min. The corresponding global reaction order also increases, to about two, indicating the progressive importance of the oxygen addition reactions. Extracted values of the global activation energy are also close to those of the controlling reactions in these temperature regimes. Results from the counterflow show the same global kinetic responses by identifying the reciprocal of the counterflow strain rate as the relevant ignition delay time and the temperature of the heated air stream as the homogeneous mixture temperature. It is further found that in the temperature range corresponding to τ1,min, the diminished heat release causes the counterflow to lose its characteristic, non-monotonic S-curve response and consequently distinct, abrupt ignition–extinction transition events.

The application of the Global Quasi-Linearisation technique to the analysis of the cyclohexane/air mixture autoignition

The paper is focused on the application of the recently developed Global-Quasi-Linearization (GQL) method to a specific problem of the cyclohexane/air autoignition in a rapid compression machine environment. A simplified autoignition mechanism including 50 species has been used in the analysis based on the customised computational fluid dynamics (CFD) package FLUENT and the new GQL method. The results predicted by FLUENT are shown to be very close to the results predicted by the zero dimensional code SPRINT, developed at the University of Leeds. The application of the GQL method shows that further substantial reduction of the mechanism is possible without significant loss of accuracy. The method is shown to be an efficient tool for identification and approximations of low-dimensional invariant system manifolds.

On the role of turbulence and compositional fluctuations in rapid compression machines: Autoignition of syngas mixtures

With the increasing interest in utilizing syngas in gas turbine applications, the characterization of H2/CO combustion and reaction chemistry under high-pressure and moderate-temperature operating conditions has been the focus of recent investigations. Different chemical-kinetics and hydrodynamic processes have been identified as being responsible for the discrepancies between experimental measurements and kinetic predictions of syngas ignition delay times. This contribution complements previous studies, and provides improved understanding about the role of turbulence and fluctuations in temperature and mixture composition on the syngas combustion process in rapid compression machines (RCMs). To this end, a self-contained model is developed that describes the ignition and combustion process by considering the interaction between turbulence, detailed reaction chemistry, and wall heat loss effects. Different mechanisms can be identified as being responsible for the generation of inhomogeneities in flow-field, temperature, and mixture composition, including turbulence-generation during the filling process, corner vortices, wall-generated turbulence, and mixing between fluid in the core region and the boundary layer. In the present model, these contributions are parametrically represented in terms of initial turbulence levels, and the mean-strain amplification of these perturbations during the compression phase is described using rapid distortion theory. The model is applied to different syngas mixtures and operating conditions, including pressures up to 20atm and temperatures between 600 and 1300K. Parametric studies show that the model captures experimentally observed trends of reduced ignition delay and prolonged reaction progress during the ignition phase. A Damköhler criterion is proposed in order to characterize the sensitivity of the induction chemistry to turbulence fluctuations. Results suggest that syngas mixtures with Damköhler numbers below 50 exhibit increasing sensitivity to turbulence and mixture fluctuations. This parametric study indicates that the turbulence/chemistry interaction can play an equally important role in affecting the syngas ignition chemistry, and requires consideration in addition to chemical-kinetics and hydrodynamic processes previously identified as leading mechanisms for the observed discrepancy in syngas ignition.

Numerical simulation of flame propagation and localized preflame autoignition in enclosures

A novel computational approach based on the coupled 3D Flame-Tracking–Particle (FTP) method is used for numerical simulation of confined explosions caused by preflame autoignition. The Flame-Tracking (FT) technique implies continuous tracing of the mean flame surface and application of the laminar/turbulent flame velocity concepts. The Particle method is based on the joint velocity–scalar probability density function approach for simulating reactive mixture autoignition in the preflame zone. The coupled algorithm is supplemented with the database of tabulated laminar flame velocities as well as with reaction rates of hydrocarbon fuel oxidation in wide ranges of initial temperature, pressure, and equivalence ratio. The main advantage of the FTP method is that it covers both possible modes of premixed combustion, namely, frontal and volumetric. As examples, combustion of premixed hydrogen–air, propane–air, and n-heptane–air mixtures in enclosures of different geometry is considered. At certain conditions, volumetric hot spots ahead of the propagating flame are identified. These hot spots transform to localized exothermic centers giving birth to spontaneous ignition waves traversing the preflame zone at very high apparent velocities, i.e., nearly homogeneous preflame explosion occurs. The abrupt pressure rise results in the formation of shock waves producing high overpressure peaks after reflections from enclosure walls.

Effect of syngas composition on combustion and exhaust emission characteristics in a pilot-ignited dual-fuel engine operated in PREMIER combustion mode

The objective of this study was to investigate the performance and emissions of a pilot-ignited, supercharged, dual-fuel engine powered by different types of syngas at various equivalence ratios. It was found that if certain operating conditions were maintained, conventional engine combustion could be transformed into combustion with two-stage heat release. This mode of combustion has been investigated in previous studies with natural gas, and has been given the name PREmixed Mixture Ignition in the End-gas Region (PREMIER) combustion. PREMIER combustion begins as premixed flame propagation, and then, because of mixture autoignition in the end-gas region, ahead of the propagating flame front, a transition occurs, with a rapid increase in the heat release rate. It was determined that the mass of fuel burned during the second stage affected the rate of maximum pressure rise. As the fuel mass fraction burned during the second stage increased, the rate of maximum pressure rise also increased, with a gradual decrease in the delay between the first increase in the heat release rate following pilot fuel injection and the point when the transition to the second stage occurred. The H 2 and CO 2 content of syngas affected the engine performance and emissions. Increased H 2 content led to higher combustion temperatures and efficiency, lower CO and HC emissions, but higher NOx emissions. Increased CO 2 content influenced performance and emissions only when it reached a certain level. In the most recent studies, the mean combustion temperature, indicated thermal efficiency, and NOx emissions decreased only when the CO 2 content of the syngas increased to 34%. PREMIER combustion did not have a major effect on engine cycle-to-cycle variation. The coefficient of variation of the indicated mean effective pressure (COV IMEP) was less than 4% for all types of fuel at various equivalence ratios, indicating that the combustion was within the stability range for engine operation.

Combustion visualization and experimental study on spark induced compression ignition (SICI) in gasoline HCCI engines

Spark induced compression ignition (SICI) is a relatively new combustion control technology and a promising combustion mode in gasoline engines with high efficiency. SICI can be divided into two categories, SACI and SI–CI. This paper investigated the SICI combustion process using combustion visualization and engine experiment respectively. Ignition process of SICI was captured by high speed photography in an optical engine with different compression ratios. The results show that SICI is a combustion mode combined with partly flame propagation and main auto-ignition. The spark ignites the local mixture near spark electrodes and the flame propagation occurs before the homogeneous mixture is auto-ignited. The heat release from central burned zone due to the flame propagation increases the in-cylinder pressure and temperature, resulting in the unburned mixture auto-ignition. The SICI combustion process can be divided into three stages of the spark induced stage, the flame propagation stage and the compression ignition stage. The SICI combustion mode is different from the spark ignition (SI) knocking in terms of the combustion and emission characteristics. Furthermore, three typical combustion modes including HCCI, SICI, SI, were compared on a gasoline direct injection engine with higher compression ratio and switchable cam-profiles. The results show that SICI has an obvious combustion characteristic with two-stage heat release and lower pressure rise rate. The SICI combustion mode can be controlled by spark timings and EGR rates and utilized as an effective method for high load extension on the gasoline HCCI engine. The maximum IMEP of 0.82 MPa can be achieved with relatively low NO x emission and high thermal efficiency. The SICI combustion mode can be applied in medium–high load region for high efficiency gasoline engines.

06/02674 Asymptotic analysis on autoignition and explosion limits of hydrogen-oxygen mixtures in homogeneous systemsLiu, Y. and Pei, P. International Journal of Hydrogen Energy, 2006, 31, (5), 639–647

06/02674 Asymptotic analysis on autoignition and explosion limits of hydrogen-oxygen mixtures in homogeneous systemsLiu, Y. and Pei, P. International Journal of Hydrogen Energy, 2006, 31, (5), 639–647

Autoignition of surrogate fuels at elevated temperatures and pressures

Autoignition of Jet-A and mixtures of benzene, hexane, and decane in air has been studied using a heated shock tube at mean post-shock pressures of 8.5 ± 1 atm within the temperature range of 1000–1700 K with the objective of identifying surrogate fuels for aviation kerosene. The influence of each component on ignition delay time and on critical conditions required for strong ignition of the mixture has been deduced from experimental observations. Correlation equation for Jet-A ignition times has been derived from the measurements. It is found that within the scatter of experimental data dilution of n-decane with benzene and n-hexane leads to slight increase in ignition times at low temperatures and does not change critical temperatures required for direct initiation of detonations in comparison with pure n-decane/air mixtures. Ignition times in 20% hexane/80% decane (HD), 20% benzene/80% decane (BD) and 18.2% benzene/9.1% hexane/72.7% decane (BHD) mixtures at temperature range of T ≅ 1450–1750 K correlate well with induction time of Jet-A fuel suggesting that these mixtures could serve as surrogates for aviation kerosene. At the same time, HD, BD and BHD surrogate fuels demonstrate a stronger autoignition and peak velocities of reflected shock front in comparison with Jet-A and n-decane/air mixtures.