Shock-tube investigation of ignition in methane-oxygen-argon mixtures
The ignition of methane-oxygen mixtures highly diluted with argon was examined in the region behind a reflected shock wave in a single-pulse shock tube. The measurements covered a temperature range of 1500–2150°K at pressures varying from 2 to 10 atm for mixture equivalence ratios of 0.5–2.0. For these conditions, observed induction times ranged from 10 to 600 μsec. Mixture compositions and test conditions in this investigation were selected in such a manner that the influence of significant parameters on the ignition delay times could be clearly delineated. It was found that the experimental results of more than 60 tests were correlated extremely well by a relationship of the form
in which concentrations are expressed in moles per cubic centimeter.
An additional 40 shocks were run in order to test the influence of small amounts of hydrogen and propane on the ignition delay time of the mixture. The amount of additives varied from 2 to 15% of the total fuel content of the mixture. If was found that the decrease in the ignition delay time could be correlated with the concentration of the additive. It is also directly related to the heat released by the combustion of the additive, which acts as a booster.
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Cited by (209)
This study presents new ignition delay time data for two multi-component natural gas (NG) blends composed of C1–C7 n-alkanes with methane as the major component. New experimental data were recorded using a high-pressure shock tube (HPST) at reflected shock pressures (p5) of 10–30 bar, at temperatures (T5) in the range 770–1480 K, and at equivalence ratios (φ) of 0.5–1.5 in ‘air’. The current results together with published rapid compression machine (RCM) measurements show that higher concentrations of larger molecular weight n-alkanes in the NG blends increase fuel reactivity by more than an order of magnitude with mixtures that have ∼18.75% of C3–C5 components compared to mixtures that have ∼44.4% of C3–C7 components at temperatures below 1000 K. On the contrary, at higher temperatures the effect of increasing reactivity is reduced. NUIGMech1.2 is used to simulate the conditions studied and shows good agreement with the experimental ignition delay time data. A correlation equation is developed through regression analyses using NUIGMech1.2 to predict ignition delay times for a wide range of C1–C7 NG mixtures, in the pressure range 10–50 bar, at temperatures in the range 950–2000 K, and at φ = 0.3–3.0 in air. The proposed correlation expression that employs a traditional Arrhenius form is successfully validated against the new experimental data as well as previously published HPST experimental data.
Effect of oxygen enrichment on methane ignition
2023, Combustion and FlameOxygen-enriched combustion has attracted interest in the energy sector due to its increased thermal efficiency and low carbon capture cost compared to ‘air’ combustion. Experimental data on the ignition of oxygen-enriched mixtures are limited in literature. In this work, ignition delay times (IDTs) of various methane/oxygen mixtures diluted in argon/nitrogen were measured using a low- and a high-pressure shock tube over a temperature range of 1200 – 1700 K, three pressures of 1, 10, and 20 bar and an equivalence ratio range of 0.21 - 4. The oxygen mole fraction in the mixtures was varied from 19% (‘air’) to 90.5%, and dilution levels from 71% to zero. The reported IDTs were extracted from pressure and OH* emission profiles. To the best of our knowledge, this is the first comprehensive IDT study on the effect of oxygen enrichment on methane ignition. High-speed imaging experiments were performed to determine the possible presence of non-ideal ignition in these unconventional mixtures. The Bifurcation Damköhler number was found to be a good indicator of non-ideal ignition observed in some imaging experiments. Measured IDTs were compared with the predictions of AramcoMech 3.0 model as well as with GRIMech 3.0 and NUIGMech 1.1 for few cases. In general, AramcoMech 3.0 overestimated IDTs for the investigated methane mixtures. Brute force sensitivity analyses were conducted with AramcoMech 3.0 to identify reactions with a strong influence on IDT prediction for the investigated mixtures. Minor modifications were made to AramcoMech 3.0 resulting in improved predictions of ignition behavior in oxygen-enriched methane mixtures.
Neural network approach to response surface development for reaction model optimization and uncertainty minimization
2023, Combustion and FlameWe examine the state-of-the-art neural network (NN) approach and its flexible implementations in combustion reaction model uncertainty quantification (UQ), optimization, and uncertainty minimization (UM). The work is motivated by addressing the problem of limited scalability of the traditional polynomial response surface methodology in handling large size of rate parameters and target data sets. Features of the NN training, accuracy, and trade-offs in several key aspects of the NN application are discussed. We show that for high-dimensional reaction model optimization and UM, a shallow NN with only one hidden layer is more robust and accurate than the polynomial response methodology. Further, we demonstrate that NN allows for adaptive training. New neural networks that augment new input parameters or updates in a trial reaction model can be adapted from the existing networks with much smaller training efforts. In addition, deep neural networks are capable of covering functional dependencies of initial thermodynamic conditions and boundary conditions, thus yielding generalized response surfaces with rate parameters and thermodynamic/mixture conditions as the input for a given combustion property. The NN approach can be readily integrated into the framework of the Method of Uncertainty Minimization using Polynomial Chaos Expansions (MUM-PCE) developed earlier. We present a test case that uses the trial Foundational Fuel Chemistry Model Version 2.0 (FFCM-2, a model consisting of 96 species and 1054 reactions for combustion of relevant C0–4 species), optimizing it against FFCM-1 targets, to illustrate the efficiency and accuracy of the NN method.
On fluidising a heated bed of sand with a mixture of methane and air: When do the bubbles ignite and when does combustion occur between the sand particles?
2023, Combustion and FlameCombustion in a heated bed of sand, fluidised by mixtures of CH4 and air, is inhibited. It was recently suggested that this is partly because a bubble only ignites, when the longest residence time of the fluidising gas passing through this bubble exceeds the ignition delay, tig, of the gas. This idea led to: (db/3Umf) ≥ tig as the condition for a bubble of diameter, db, to ignite. To test this, beds of sand (3 different sizes) were fluidised by different mixtures of CH4 and air. The bubbling bed was electrically heated slowly up to ∼ 1100 °C, whilst monitoring the bed's temperature and the composition of the gas at a fixed point in the bed. The temperature, at which bubbles ignited at this observation point, was identified as that of the bed, when the local concentration of CO reached a maximum. The mean size of the igniting bubbles was calculated from the flowrate of gas through the bed and the height above the distributor. Using tig (from the literature) at the measured ignition temperature, the above equation described the observations very satisfactorily. Thus, the fuel-richness of the fluidising gas and its temperature affect tig, Umf and db. Combustion of CO (or CH4) in the gas passing interstitially between fluidised sand particles was found to be unlikely, but can be promoted by fluidising large particles in a very hot bed, in which CO has a concentration larger than ≈ 10 vol.% at 1 at.
When hydrogen is slower than methane to ignite
2023, Proceedings of the Combustion InstituteHydrogen (H2) is known to be the fastest fuel to ignite among all practical combustion fuels. In this study, for the first time, longer ignition delay times (IDTs) for the H2 and H2 blended CH4 mixtures were measured compared to those for pure CH4. This work investigates the ignition characteristics of H2, CH4, and 50% CH4/50% H2 mixtures using a rapid compression machine at pressures ranging from 20 to 50 bar and at equivalence ratios (φ) from 0.5 to 2.0 in air in the temperature range 858–1080 K. The experimental IDTs are simulated using a newly updated kinetic mechanism, NUIGMech1.3, and good agreement is observed. At lower temperatures the IDTs of H2, CH4, and the 50% CH4/50% H2 mixtures are similar to one another, and the IDTs of the 50% CH4/50% H2 mixtures are longer than those for pure CH4 at temperatures below 930 K. At temperatures below 890–925 K, depending on the operating pressure and equivalence ratio, the hydrogen mixtures are the slowest to ignite, with IDTs being 2.5 times longer than those recorded for CH4 at a pressure of 40 bar at 890 K for φ = 1.0, and at 875 K for φ = 2.0. At low temperatures alkyl (Ṙ = ĊH3 and Ḣ) radicals add to O2 producing RȮ2 radicals, which then react with HȮ2 radicals forming ROOH (H2O2 and CH3OOH) and O2. For H2, the self-recombination of HȮ2 radicals leads to chain propagation which inhibits reactivity, whereas for CH4, the reaction between RȮ2 (CH3OȮ) and HȮ2 leads to chain branching, increasing reactivity. Furthermore, CH3OOH decomposes more easily to produce CH3Ȯ and ȮH radicals than does H2O2 to produce two ȮH radicals. Thus, mixtures containing higher H2 concentrations are slower to ignite compared to those with higher CH4 concentrations at low temperatures.
Analysis of the ignition induced by shock wave focusing equipped with conical and hemispherical reflectors
2022, Combustion and FlameThe interaction of a plane shock wave in air with concave profiles has been used in the past to understand the nature of shock wave focusing. The current study examines the complex two-dimensional flow field resulting from the interaction of a plane shock wave entering a symmetrical cavity with a curved end wall. The development of reflection patterns of the incident shock wave at the profile wall and the process of gas dynamic focus are of particular interest. In this study, numerical and experimental studies are performed to understand the ignition behavior in a stoichiometric methane-oxygen-argon mixture due to shock wave reflection from a variety of shapes, including planar, 60° and 90° conical and hemispherical reflectors. The numerical simulation reveals the complex two-dimensional flow field when the shock wave collides head-on with different reflectors and the propagation behavior of the reflected waves in the process of focusing. The instantaneous maximum temperature and pressure as a function of shock wave intensity in four reflectors are given and analyzed. The experimental results show that two ignition modes, namely, weak ignition and strong ignition, occur in those reflectors when the incident shock wave velocity (Vi) is greater than a critical value, and the separatrix between the weak and strong ignition for each reflector is demonstrated to be highly dependent on Vi. The critical values of Vi for the 60° conical, 90° conical and hemispherical reflectors are 740, 775 and 780 m/s, respectively. The ignition delay time (tign) of the combustible mixture under shock wave focusing for various reflectors is provided. The results show that in the weak ignition mode, tign is shorter in the hemispherical reflector than in the others, and therefore, its Pign (the maximum pressure after ignition) is higher. However, the result is completely opposite in the strong ignition mode, which is mainly because the ignition temperature of hot spots in the hemispherical reflectors is much lower than those in the conical reflectors. This study provides new simulation and experimental evidence of the ignition behavior in various reflectors, supporting the effect of enhancing shock wave focusing on the ignition performance.
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Ohio State University Research Foundation Visiting Research Associate under Contract F33615 57 C1758.
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Permanent address: Department of Physical Chemistry, The Hebrew University, Jerusalem, Israel.