Combustion and Flame (original) (raw)
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Characterisation of reaction propagation from auto-ignition
2016
Finite supplies of fossil fuels and their current dominance in energy production and usage make their efficient usage and the search for viable alternatives of critical importance. A large part of this is the understanding of the combustion of fuels, both existing and novel and of the engines in which they are consumed. One fundamental parameter that is not sufficiently understood is excitation time, the almost instantaneous heat release period at the end of an ignition delay period. A reduced thermokinetic model is applied to an attempt to simulate excitation without a large comprehensive model. The failure of the model in this task indicates differences in chemistry between excitation and ignition delay periods that are too large for a simple scheme to overcome with a single set of rate parameters. This work will present a full and fundamental characterisation through the use of two complimentary diagrams, one an existing diagram for the identification of developing detonation, th...
An experimental study of flame and autoignition interactions of iso-octane and air mixtures
Recent modifications to the University of Michigan rapid compression facility (UM RCF) were made to allow direct imaging of flame/autoignition interactions using compression to initiate autoignition chemistry and a spark plug to initiate simultaneous flame development. The experimental data in this study quantify the effects of spark-initiated flame propagation on autoignition of iso-octane/O2/inert gas mixtures at well-defined initial conditions. The work leveraged the controlled environment of the UM RCF, in which temperature, pressure, and composition are nominally uniform and well-known at the end of compression. Flame initiation by the spark plasma, flame propagation, and autoignition were monitored using high-speed optical imaging of chemiluminescence and in situ pressure time histories. End-of-compression temperatures from TEOC = 942–1012 K were considered, while the end-of-compression pressures were nominally constant within the range of PEOC = 7.8–9.5 atm. The fuel-to-O2 molar equivalence ratio was varied from / = 0.20–0.99 and dilution, defined as the molar ratio of inert gases to O2 in the reactant mixture, was varied from inert:O2 = 3.76–7.47 to determine the effects on flame/autoignition interactions as well as to identify the lean flammability limit of the mixtures as a function of dilution. Flame propagation is generally expected to decrease autoignition delay times by compression heating the unburned portion of the mixture. The effect of flame propagation was maximized in these experiments by igniting the mixtures early during the autoignition process. Later spark timings had small to negligible effect on the autoignition delay time. Dilution had significant effect on the lean flammability limits, increasing from a lean limit of / = 0.35 at air levels of dilution to / = 0.65 at inert:O2 dilution of 7.5. The flammability limit was well correlated with the theoretical adiabatic flame temperature of each experiment. The propagation rates of flames successfully initiated by the spark plasma were determined from the imaging data and were 1 to 12 m/s. The magnitude of the propagation rates and the effect on the time integrated temperature scaled with the energy content of the mixtures as indicated by the theoretical adiabatic flame temperature.
Paper # 070RK-0008 Topic: Reaction Kinetics 8 th U. S. National Combustion Meeting
2016
Ignition delay times for five low-vapor-pressure biodiesel surrogates were measured behind reflected shock waves, using an aerosol shock tube. These fuels included methyl decanoate (C 11 H 22 O 2 , CAS: 110-42-9), methyl laurate (C 13 H 26 O 2 , CAS: 111-82-0), methyl myristate (C 15 H 30 O 2 , CAS: 124-10-7), and methyl palmitate (C 17 H 34 O 2 , CAS: 112-39-0), all of which have a fully saturated alkane chemical structure. This study also examined a methyl oleate (C 19 H 36 O 2 , CAS: 112-62-9) / Fatty Acid Methyl Ester (FAME) blend. Experiments were conducted in 4% oxygen/argon mixtures with the exception of methyl decanoate which was studied in 1% and 21% oxygen/argon blends. Reflected shock conditions covered temperatures from 1026 to 1388 K, at pressures of 3.5 and 7.0 atm, and equivalence ratios from 0.3 to 1.4. Arrhenius expressions describing the experimental ignition delay time data are given and compared to those derived from applicable mechanisms available in the literature. Graphical comparisons between experimental data and mechanism predictions are also provided. Experiments of methyl laurate, methyl myristate, and methyl palmitate represent the first shock tube ignition delay time measurements for these fuels. Finally, experiments with methyl palmitate represent, to the authors' knowledge, the first neat fuel/oxidizer/diluent gas-phase experiments involving a fuel which is a waxy solid at room temperature.
Transported PDF modelling with detailed chemistry of pre- and auto-ignition in CH 4/air mixtures
Proceedings of The Combustion Institute, 2007
The pre-and auto-ignition behavior of methane under varying levels of preheat in a turbulent flow field has been studied through the combination of detailed chemistry with a transported PDF approach closed at the joint-scalar level. The study considers the Cabra Burner configuration, which consists of a central methane/air jet issuing into a vitiated co-flow. The aim of the work is to explore the detailed thermochemical flow structure and to substantially reduce uncertainties associated with the chemical kinetics. The applied chemistry features 44 solved species and 256 reactions and includes low temperature oxygen adducts. The mechanism has, in related work, been shown to reproduce the spontaneous temperature limit for methane and ethane along with ignition delays times at higher temperatures. Radiation is accounted for through the RADCAL method and the inclusion of enthalpy into the joint-scalar PDF. Molecular mixing is closed using the modified Curl's model and a set of time-scale ratios (C / = 2.3, 2.5, 3.0 and 4.0) have been used to explore the model sensitivity. The impact on predictions of variations in the pilot stream composition have been explored by varying concentrations of OH and H 2 over a wide range. A detailed analysis of the flame structure, focusing on the chemical processes occurring before and during the ignition, suggests that the burner conditions lead to a classical auto-ignition pattern with the early formation of HO 2 and CH 2 O prior to ignition. The work suggests that, under the current conditions, flame stabilization is dominated by turbulence-chemistry interactions rather than by specific modes of flame propagation. The work shows a significant sensitivity to the pilot stream composition and that residual H 2 acts as an ignition promoter. However, the sensitivity to the time-scale ratio C / is shown to be less than can be expected from studies of flame extinction using the same methodology.
Autoignition and structure of nonpremixed CH4/H2 flames: Detailed and reduced kinetic models
Combustion and Flame, 2006
The ignition dynamics and subsequent flame evolution of hydrogen-enriched methane mixtures are investigated numerically in a reacting vortex ring configuration. The CH 4 /H 2 combustion is studied using a detailed reaction mechanism (GRI-Mech v3.0) and two augmented reduced mechanisms (11-step and 12-step). The main objective of this study is to identify the extent that the current reduced mechanisms can go in replicating the dynamics of the ignition process and flame structure in an unsteady nonpremixed configuration. The parameters of the numerical simulations are adjusted such that flame ignition occurs during either the formation or the postformation of the ring. The quasi-steady state assumption for O in the 12-step reduced kinetic model leads to shorter ignition delay times than those in the other kinetic models. For formation-phase ignition runs, the flame structure near the stoichiometric region is captured well by the 12-step model compared to GRI-Mech 3.0. For postformation ignition runs, the 12-step model predicts larger heat release rates and main species mole fractions compared to GRI-Mech 3.0. The 11-step model predicts well the ignition delay time. At later times the fuel-rich side of the flame predicted by this reduced mechanism exhibits differences from the detailed model. Counterflow diffusion flame results are used to further compare the fuel-rich chemistry for the detailed and augmented reduced kinetic models.
Combustion Theory and Modelling
In order to obtain physical insights on ammonia combustion, which is characterised by exceptionally long ignition delays and increased NOx emissions, the autoignition dynamics of an ammonia/air mixture is analysed using the diagnostics tools derived from the Computational Singular Perturbation (CSP) methodology. The results are compared to the autoignition dynamics of a methane/air mixture of same initial conditions. Methane was chosen for comparison because, even though the two molecules have a formal similarity, the ignition delay of methane is more than 10 times shorter than the one of ammonia. By using the CSP diagnostics tools, we identified the dominant chemical pathways that relate to the explosive components that drive the system towards ignition for both cases. Furthermore, the reactions that hinder the ammonia ignition were identified. This led to the determination of an interesting difference in the electronic configuration of the molecules of the two fuels, which is the root of their drastically different oxidation dynamics. In particular, it was shown that the autoignition process starts with the formation of methyl (CH 3) and amine (NH 2) radicals, through dehydrogenation of methane and ammonia, respectively. In the methane case, the methyl-peroxy radical (CH 3-O-O-) then forms, which initiates a chemical runaway that lasts for approximately 2/3 of the ignition delay and leads to the gradual oxidation of carbon to CO 2. In the ammonia case, though, the structure of NH 2 is such that it is not possible to form NH 2-O-O-. As a result, the chemical runaway is suspended.
Combustion and Flame, 2019
1,3-cyclohexadiene (1,3-CHD) can be transformed into cis-1,3,5-hexatriene (1,3,5-HT) upon light irradiation, which makes it a potential additive able to change the reactivity of a conventional fuel. This paper presents the development of a detailed chemical kinetic model for the low-temperature (50 0-120 0 K) combustion of 1,3-cyclohexadiene and 1,3,5-hexatriene. Theoretical calculations were performed to compute the thermochemistry of a large number of intermediates involved in the reaction mechanism, and for several kinetic parameters. In particular, the pericyclic reactions of 1,3-cyclohexadiene, linking it to cis-1,3,5-hexatriene, were studied theoretically. It was shown that 1,3,5-HT is inherently a secondary molecule of the 1,3-CHD mechanism and a comprehensive set of its oxidation reactions were included. Simulation of literature data (ignition delays and products speciation) measured in rapid compression machines for 1,3-CHD were performed using the newly developed kinetic model. A good agreement with experiments was found, and kinetic analyses highlighted the decomposition mechanism of 1,3-CHD and the most sensitive reactions affecting the auto-ignition delay times. Simulations of cis-1,3,5-HT auto-ignition in an RCM were also performed and compared to the ignition behaviour of 1,3-CHD. The simulation results showed that 1,3,5-HT combustion involves an induction period characterized by the predominant formation of 1,3-CHD, whose decomposition starts the radical chain mechanism.
CH4/air homogeneous autoignition: A comparison of two chemical kinetics mechanisms
Fuel, 2018
Reactions contributing to the generation of the explosive time scale that characterise autoignition of homogeneous stoichiometric CH 4 /air mixture are identified using two different chemical kinetics models; the well known GRI-3.0 mechanism (53/325 species/reactions with N-chemistry) and the AramcoMech mechanism from NUI Galway (113/710 species/reactions without N-chemistry; Combustion and Flame 162:315-330, 2015). Although the two mechanisms provide qualitatively similar results (regarding ignition delay and profiles of temperature, of mass fractions and of explosive time scale), the 113/710 mechanism was shown to reproduce the experimental data with higher accuracy than the 53/325 mechanism. The present analysis explores the origin of the improved accuracy provided by the more complex kinetics mechanism. It is shown that the reactions responsible for the generation of the explosive time scale differ significantly. This is reflected to differences in the length of the chemical and thermal runaways and in the set of the most influential species.
Reactions contributing to the generation of the explosive time scale that characterise autoignition of homogeneous stoichiometric CH 4 /air mixture are identified using two different chemical kinetics models; the well known GRI-3.0 mechanism (53/325 species/reactions with N-chemistry) and the AramcoMech mechanism from NUI Galway (113/710 species/reactions without N-chemistry; Combustion and Flame 162:315-330, 2015). Although the two mechanisms provide qualitatively similar results (regarding ignition delay and profiles of temperature, of mass fractions and of explosive time scale), the 113/710 mechanism was shown to reproduce the experimental data with higher accuracy than the 53/325 mechanism. The present analysis explores the origin of the improved accuracy provided by the more complex kinetics mechanism. It is shown that the reactions responsible for the generation of the explosive time scale differ significantly. This is reflected to differences in the length of the chemical and thermal runaways and in the set of the most influential species.
Ignition delay studies on hydrocarbon fuel with and without additives
Shock Waves, 2009
Single pulse shock tube facility has been developed in the High Temperature Chemical Kinetics Lab, Aerospace Engineering Department, to carry out ignition delay studies and spectroscopic investigations of hydrocarbon fuels. Our main emphasis is on measuring ignition delay through pressure rise and by monitoring CH emission for various jet fuels and finding suitable additives for reducing the delay. Initially the shock tube was tested and calibrated by measuring the ignition delay of C2H6 − O2 mixture. The results are in good agreement with earlier published works. Ignition times of exo-tetrahdyrodicyclopentadiene (C10H16), which is a leading candidate fuel for scramjet propulsion has been studied in the reflected shock region in the temperature range 1250-1750 K with and without adding Triethylamine (TEA). Addition of TEA results in substantial reduction of ignition delay of C10H16.