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At least 649 records · Page 36

Measurements of Propanal Ignition Delay Times and Species Time Histories Using Shock Tube and Laser Absorption

Propanal is an aldehyde intermediate formed during the hydrocarbon combustion process. Potentially, the use of oxygenated biofuels reduces greenhouse gas emissions; however, it also results in increased toxic aldehyde by-products, mainly formaldehyde, acetaldehyde, acrolein, and propanal. These aldehydes are carcinogenic, and therefore it is important to understand their formation and destruction pathways in combustion systems. In this work, ignition delay times were measured behind reflected shock waves for stoichiometric (Φ = 1) mixtures of propanal (CH 3 CH 2 CHO) and oxygen (O 2 ) in argon bath gas at temperatures of 1129 K < T < 1696 K and pressures around 1 and 6 atm. Measurements were conducted using the kinetics shock tube facility at the University of Central Florida. Current results were compared to available data in the literature as well as to the predictions of three propanal combustion kinetic models: Politecnico di Milano (POLIMI), National University of Ireland at Galway, and McGill mechanisms. In addition, a continuous wave-distributed feedback interband cascade laser centered at 3403.4 nm was used for measuring methane (CH 4 ) and propanal time histories behind the reflected shock waves during propanal pyrolysis. Concentration time histories were obtained at temperatures between 1192 and 1388 K near 1 atm. Sensitivity analysis was carried for both ignition delay time and pyrolysis measurements to reveal the important reactions that were crucial to predicting the current experimental results. Adjustments to the POLIMI mechanism were adopted to better match the experimental data. Further research was suggested for the H abstraction reaction rates of propanal. Finally, in addition to extending the temperature and pressure region of literature ignition delay times, we provide the first high-temperature species concentration time histories during propanal pyrolysis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Relating microstructure, temperature, and chemistry to explosive ignition and shock sensitivity

An analysis is put forth that relates explosive properties including microstructure, temperature, and chemistry to explosive ignition and sensitivity. Unlike approaches that focus only on elucidating mesoscopic mechanisms important to ignition, the present analysis seeks a methodology directly applicable to continuum explosive burn models. Because the Scaled Uniform Reactive Front (SURF) burn model has a microstructural basis, it is chosen as the starting point of the present analysis. To build upon the SURF framework, a literal translation is performed of the high-level conceptual notions for which SURF is based, to concrete ignition–combustion parameters, a statistical description of the microstructure, and other thermo-chemical data acquired by non-shock experiments. The analysis requires a void volume fraction distribution acquired from ultra-small angle neutron scattering (USANS). The shock response of PBX 9502 is used to illustrate the theory. While the analysis requires calibration to a shock experiment (the pop plot, for example), the results are a self-consistent set of realistic physical quantities that contribute to the shock initiation process including, initial temperature, chemical kinetics, a statistical description of the microstructure, and the hot spot size, spacing, and temperature. The utility of having a mesoscale based theory is that once the critical hot spot temperature, as a function of shock pressure, is known for a given explosive and initial porosity distribution, the entire set of meso- and macro-scale results, including the pop plot can be calculated for any other porosity. Thus, one can understand changes in sensitivity at different densities (void size distributions), relying only on the assumption that the hot spot temperature curve would not differ significantly if the morphology of the hots spots were similar. Another important utility of the present analysis is to address the question of the role of initial temperature on the observed shock sensitivity of PBX 9502 and other explosives.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

A chemical pathway perspective on the kinetics of low-temperature ignition of propane

In this work, the chemistry of low-temperature ignition in propane/air mixtures is analyzed using a recently developed pathway representation of the chemical kinetics. The "Sum Over Histories Representation" allows time dependent kinetic observables to be computed using an expansion over global chemical pathways that follow chemical moieties as they move through a complex reaction network. This methodology assigns probabilities to complete chemical pathways through which specific intermediate or product species are generated. The growth of the radical pool during the ignition process is analyzed by enumerating chemical pathways that constitute catalytic cycles, in particular the catalyzed production of the highly reactive OH-radical. In addition to the well-known reaction route followed in low-temperature ignition of hydrocarbons which involves the QOOH and keto-hydroperoxide species, we have explicitly identified several other cycles that are responsible for most of the remaining OH-production.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Ignition delay time and H 2 O measurements during methanol oxidation behind reflected shock waves

To improve detailed chemical kinetics models, the oxidation of methanol was investigated behind reflected shock waves in shock tubes. Ignition delay times of methanol–air mixtures, with Ar as diluent, were studied between 940 and 1540 K in a heated shock tube, for pressures up to 14.9 atm and for equivalence ratios of 0.5, 1.0, and 2.0. Water profiles were measured by utilizing a laser absorption technique in the 1350-to-1600-K temperature range, at an average pressure of 1.3 atm and for similar equivalence ratios. The present study shows here the ignition delay times of methanol to be in very good agreement with results from the literature (Fieweger et al., 1997), whereas the other conditions have never been investigated before. The ignition delay time data are also in good agreement with modern detailed kinetics mechanisms such as the AramcoMech 3.0 model. The water time-history profiles were modeled using well-known literature mechanisms. Discrepancies were observed between these kinetics mechanisms, and poor predictions were observed for the lower temperatures investigated. Sensitivity and rate-of-production analyses were performed using 3 literature mechanisms (namely, AramcoMech 3.0, Princeton, and JetSurfII). Discrepancies were found among the models when predicting important reactions dominating the oxidation of methanol as well as the rate-of-production of H 2 O.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

High-pressure shock tube study of ethanol oxidation: Ignition delay time and CO time-history measurements

Ethanol oxidation was studied by measuring CO time-histories and ignition delay times behind reflected shockwaves at elevated pressures. In this study, experimental conditions included a temperature range of 960–1580 K, pressure range of 17.8–23.9 atm, and initial fuel concentrations of 6.54% and 0.25% with nitrogen and argon used as bath gasses, respectively. The equivalence ratio for high fuel loading was kept constant at 1.0, and for low fuel loading equivalence ratios were 1.0 and 0.5. For high fuel loading, early heat/energy release was observed in nearly all ignition delay time measurements, indicative of preignition; however, data collected are in good agreement with model predictions. These events are interpreted as a transition from mild to strong ignition. For the low fuel loading cases (Φ = 1.0 and 0.5), no early heat/energy release was observed. Comparisons of measured CO concentration profiles with the predictions from kinetic mechanisms of Metcalfe et al. (2013), Mittal et al. (2014), and Zhang et al. (2018) were made assuming constant internal energy and volume for the test gas. Such comparisons in addition with performed sensitivity and pathway analyses for CO formation revealed lower temperature sensitivity to the bimolecular reaction of methyl and hydroperoxyl radicals given by CH 3 + HO 2 CH 3 O + OH, and H-atom abstraction by the hydroperoxyl radical at the alpha site on ethanol given by elementary reaction C 2 H 5 OH + HO 2 =sC 2 H 4 OH + H 2 O 2 . Furthermore, the current study provides important validation targets for ethanol chemical kinetic mechanisms and highlights the benefits of time-history measurements at various temperatures and pressures in a shock tube, which are scare in the literature.

09 BIOMASS FUELS↗

In situ multi-tier auto-ignition detection applied to dual-fuel combustion simulations

Here we use an anomaly detection methodology that is centered on analyzing fourth-order joint moments (co-kurtosis), particularly focusing on its application in auto-ignition of combustion problems with large numbers of species. Unsupervised anomaly detection is challenging to generalize across problem types and domains. A recent technique, centered on analyzing information in the fourth-order joint moment co-kurtosis, has shown promise, especially for high-dimensional scientific data. In this work we present developments to the co-kurtosis based anomaly detection method needed to make it effective and scalable for large-scale distributed scientific data, such as those generated by massively parallel simulations. An in situ co-kurtosis algorithm is employed as the anomaly detection method for identifying ignition kernels in simulations of turbulent combustion. Here, we extend an existing methodology which identifies regions of the domain where anomalies are present, and add another tier of anomaly detection where the individual samples contributing to the anomaly are identified. We apply this algorithm on-the-fly to a variety of turbulent reacting flow problems and compare it to the widely used (but significantly more expensive) chemical explosive mode analysis (CEMA). We demonstrate the ability of the method to detect and identify the onset of low and high temperature ignition which can be used for computational steering, as chemical and combustion anomalies occur intermittently at spatio-temporal locations unknown a priori. Finally, we apply our lightweight in situ algorithm to an exascale high-fidelity simulation with a total of 2.4 Trillion degrees of freedom, performed using an adaptive mesh refinement solver. Furthermore, through a scalability analysis, we show that the relative computational cost of this in-situ anomaly detection algorithm compared to an iteration of the reacting flow solver is negligible.

97 MATHEMATICS AND COMPUTING↗

Data assimilation for combustion ignition delay time simulation using schlieren image velocimetry

This study sought to improve the accuracy of simulating spray penetration and combustion ignition delay by means of data assimilation (DA). The simulations were conducted using the Reynolds-averaged Navier–Stokes (RANS) equations and assimilating the schlieren image data. In DA, an ensemble square root filter (EnSRF) was used to build the statistical model, making the simulation results more accurate without any change in the governing equations. Recognizing that the spray-cone injection angle has a large effect on penetration, we created ensemble members with different injection angles. And we applied the two-component velocity distribution calculated via SIV and updated both velocity and temperature by using a DA statistical model derived from RANS ensemble simulations. The ignition delay time is generally known to vary even under the same experimental conditions because it is influenced by many factors. In this study, we attempted the transient DA-assisted RANS simulation to predict the ignition delay time even when the temporal resolution and accuracy of the observation data ware insufficient. Our trials offer an example of how a combination of techniques can be effectively used to assimilate experimental data obtained under restricted conditions.

Combustion simulation↗

Scaling relation for high-temperature biodiesel surrogate ignition delay times

High-temperature Arrhenius ignition delay time correlations are useful for revealing the underlying parameter dependencies of combustion models, for simplifying and optimizing combustion mechanisms for use in engine simulations, for scaling experimental data to new conditions for comparison purposes, and for guiding in experimental design. Here, we have developed a scaling relationship for Fatty Acid Methyl Ester (FAME) ignition time data taken at high temperatures in 4%O 2 /Ar mixtures behind reflected shocks using an aerosol shock tube: τ ign [ms] = 2.24 x 10 -6 [ms] (P [atm]) -.41 ($\phi$) 0.30 (C n ) -.61 x exp $$ \left(\frac{37.1 [kcal/mol]}{\hat{R}_u [kcal / mol K] T [K]}\right) $$ In addition, we have combined our ignition delay time data for methyl decanoate, methyl palmitate, methyl oleate, and methyl linoleate with other experimental results in the literature in order to derive fuel-specific oxygen-mole-fraction scaling parameters for these surrogates. In conclusion, in this article, we discuss the significance of the parameter values, compare our correlation to others found in the literature for different classes of fuels, and contrast the above expression’s performance with correlations obtained using leading FAME kinetic models in 4%O 2 /Ar mixtures.

09 BIOMASS FUELS↗

High-pressure ignition delay time measurements of a four-component gasoline surrogate and its high-level blends with ethanol and methyl acetate

Ignition delay times of a gasoline surrogate (iso-octane/toluene/n-heptane/1-hexene at 55/25/15/5% by liquid volume, developed in McCormick et al. 2017) with high-level bioblendstock-gasoline surrogate blends (50% and 85% biofuel by liquid volume of each ethanol and methyl acetate) were collected behind reflected shock waves. Post-reflected-shock temperatures ranged from 968 to 1361 K at pressures of about 4, 10, 25, and 50 atm. Data were collected for real fuel–air mixtures at fuel lean and stoichiometric conditions (φ = 0.5, 1.0) with focus on the more-practical, fuel-lean conditions. Ignition delay times were measured from OH* chemiluminescence around 307 nm using an endwall diagnostic. The RON and MON of the surrogate were 90.3 and 84.7, respectively. Ethanol was chosen due to its wide use in flex-fuel vehicles and availability, while also providing an increased octane rating. Methyl acetate was chosen for its especially high octane rating to investigate the effect of an extreme case. To validate the gasoline surrogate, the data are compared to real gasoline (RD387) and several gasoline surrogate experiments from the literature. Using the wide range of pressures studied, the gasoline surrogate’s pressure dependence was quantified to account for test-to-test variations using regression analysis. Similarly, a global correlation for gasoline and its surrogates was developed using all available data from the literature. Two modern chemical kinetics models targeting gasoline and its surrogates are compared to the ignition delay time measurements. These new high-pressure, high-bioblendstock concentration tests provide required chemical kinetic data for optimizing fuel and engine design.

42 ENGINEERING↗

The impact of the third O 2 addition reaction network on ignition delay times of neo-pentane

In this work, we studied the oxidation of neo-pentane by combining experiments, theoretical calculations, and mechanistic developments to elucidate the impact of the 3rd O 2 addition reaction network on ignition delay time predictions. The experiments are based on photoionization mass spectrometry in jet-stirred and time-resolved flow reactors allowing for sensitive detection of the keto-hydroperoxide (KHP) and keto-dihydroperoxide (KDHP) intermediates. With neo-pentane exhibiting a unique symmetric molecular structure, which consequently results only in single KHP and KDHP isomers, theoretical calculations of ionization and fragment appearance energies and of absolute photoionization cross sections enabled the unambiguous identification and quantification of the KHP intermediate. Its temperature and time-resolved profiles together with calculated and experimentally observed KHP-to-KDHP signal ratios were compared to simulation results based on a newly developed mechanism that describes the 3rd O 2 addition reaction network. A satisfactory agreement has been observed between the experimental data points and the simulation results, thus adding confidence to the model's overall performance. Finally, this mechanism was used to predict ignition delay times reported previously in shock tube and rapid compression machine experiments (J. Bugler et al., Combust. Flame 163 (2016) 138–156). While the model accurately reproduces the experimental data, simulations with and without the 3rd O 2 addition reaction network included reveal only a negligible effect on the predicted ignition delay times at 10 and 20 atm. According to model calculations, low temperatures and high pressures promote the importance of the 3rd O2 addition reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Ion beam requirements for fast ignition of inertial fusion targets

Ion beam requirements for fast ignition are investigated by numerical simulation taking into account new effects, such as ion beam divergence, not included before. We assume that ions are generated by the TNSA scheme in a curved foil placed inside a re-entrant cone and focused on the cone apex or beyond. From the focusing point to the compressed core, ions propagate with a given divergence angle. Ignition energies are obtained for two compressed fuel configurations heated by proton and carbon ion beams. The dependence of the ignition energies on the beam divergence angle and on the position of the ion beam focusing point has been analyzed. Comparison between TNSA and quasi-monoenergetic ions is also shown.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

The ignition design space of magnetized target fusion

The simple magnetized target implosion model of Lindemuth and Kirkpatrick [Nucl. Fusion 23, 263 (1983)] has been extended to survey the potential parameter space in which three types of magnetized targets—cylindrical with axial magnetic field, cylindrical with azimuthal magnetic field, and spherical with azimuthal magnetic field—might achieve ignition and produce large gain at achievable radial convergence ratios. The model has been used to compute the dynamic, time-dependent behavior of many initial parameter sets that have been based upon projected ignition conditions using the quasi-adiabatic and quasi-flux-conserving properties of magnetized target implosions. The time-dependent calculations have shown that energy gains greater than 30 can potentially be achieved for each type of target. By example, it is shown that high gain may be obtained at extremely low convergence ratios, e.g., less than 15, for appropriate initial conditions. It is also shown that reaching the ignition condition, i.e., when fusion deposition rates equal total loss rates, does not necessarily lead to high gain and high fuel burn-up. At the lower densities whereby fusion temperatures can be reached in magnetized targets, the fusion burn rate may be only comparable with the hydrodynamic heating/cooling rates. On the other hand, when the fusion burn rates significantly exceed the hydrodynamic rates, the calculations show a characteristic rapid increase in temperature due to alpha particle deposition with a subsequent increased burn rate and high gain. A major result of this paper is that each type of target operates in a different initial density-energy-velocity range. The results of this paper provide initial target plasma parameters and driver parameters that can be used to guide plasma formation and driver development for magnetized targets. The results indicate that plasmas for spherical, cylindrical with azimuthal field, and cylindrical with axial field targets must have an initial density greater than approximately 10{sup 17}/cm{sup 3}, 10{sup 18}/cm{sup 3}, and 10{sup 20}/cm{sup 3}, respectively, implying constraints on target plasma formation research.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗

Convolutional Non-Homogeneous Poisson Process and its Application to Wildfire Ignition Risk Quantification for Power Delivery Networks

To quantify wildfire ignition risks on power delivery networks, the current practice predominantly relies on the empirically calculated fire danger indices, which may not well capture the effects of dynamically changing environmental factors. This article proposes a spatio-temporal point process model, known as the Convolutional Non-homogeneous Poisson Process (cNHPP), and applies the model to quantify wildfire ignition risks for power delivery networks. The proposed model captures both the current (i.e., instantaneous) and cumulative (i.e., historical) effects of key environmental processes (i.e., covariates) on wildfire risks, as well as the spatio-temporal dependency among different segments of the power delivery network. The computation and interpretation of the intensity function are thoroughly investigated. We apply the proposed approach to estimate wildfire ignition risks on major transmission lines in California, using historical fire data, meteorological and vegetation data obtained from the National Oceanic and Atmospheric Administration and National Aeronautics and Space Administration. Here, a comprehensive comparison study is performed to show the applicability and predictive capability of the proposed approach.

Non-homogeneous Poisson Process↗

Mathematical Models and Numerical Methods for High-Fidelity Simulation of Ignition of Reactive Mixtures by Nanosecond Plasma Discharges in Realistic Configurations

We present a newly developed framework for the numerical simulation of ignition of reactive mixtures using single or repeated nanosecond discharge pulses. The framework builds upon the AMReX library, using the existing compressible solver PeleC and low-Mach solver PeleLMeX and allowing for adaptive mesh refinement, complex geometries, and execution on next-generation high-performance computing (HPC) systems. High-fidelity elementary models are adopted for weakly-ionised plasma discharges with significant energy deposition, consistent with nanosecond discharge pulses, and then implemented in the solver. The treatment of non-thermal electrons and charged species, thermodynamics of non-equilbrium species, plasma kinetics, limiting time scales, and boundary conditions for charged species are discussed and addressed for computational efficiency. The framework is demonstrated for three relevant applications: single and multi-pulse discharges in air, single pulse ignition of an ethylene/air mixture, and a three-dimensional plasma discharge in air with temperature stratification. The successful application of the framework demonstrates the feasibility of high-fidelity simulation of ignition of air/hydrocarbon mixtures in three-dimensions with multiple discharge pulses.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Computational study of ignition behavior and hotspot dynamics of a potential class of aluminized explosives

The ignition behavior and hotspot dynamics of a potential class of aluminized energetic materials are studied computationally. The materials consist of HMX (octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) grains embedded in an aluminum matrix and, henceforth referred to as metal–matrix explosives (MMXs). For the analysis, two different MMXs, the soft MMX with a matrix of 1100 Al alloy and the hard MMX with a matrix of 7075 T651 Al alloy are considered. The thermo-mechanical response of the MMXs are computationally analyzed by subjecting them to monotonic impact loading using a Lagrangian cohesive finite element framework, with their ignition behavior analyzed through characterization of hotspots. For comparison, a polymer-bonded explosive (PBX) consisting of HMX and Estane is also analyzed under the same conditions. Here, the results show that the MMXs have significantly lower propensity for ignition and higher structural integrity than the PBX over the loading velocity range of 200–500 m s -1 .

45 MILITARY TECHNOLOGY, WEAPONRY, AND NATIONAL DEF↗

Experimental Ignition Delay Time Measurements and Chemical Kinetics Modeling of Hydrogen/Ammonia/Natural Gas Fuels

In recent years, hydrogen-carrying compounds have accrued interest as an alternative to traditional fossil fuels due to their function as zero-emission fuels. As such, there is interest in investigating hydrogen-carrying compounds to improve understanding of the fuels' characteristics for use in high pressure systems. In the current study, the oxidation of ammonia/natural gas/hydrogen mixtures was carried out to study carbon monoxide (CO) formation profiles as well as the ignition delay times (IDTs) behind reflected shock waves in order to refine chemical kinetic models. Experiments were carried out in the University of Central Florida's shock tube facility by utilizing chemiluminescence to obtain OH* emission and laser absorption spectroscopy to obtain CO profiles over a temperature range between 1200 K and 1800 K with an average pressure of 2.2 atm. Experimental mixtures included both neat and combination natural gas/hydrogen with ammonia addition, with all mixtures except one having an equivalence ratio of 1. Results were then compared with the GRI 3.0 mechanism, as well as the newly developed UCF 2022 mechanism utilizing chemkin-pro software. In general, both models were able to capture the trend in auto-ignition delay times and CO time histories for natural gas and ammonia mixtures. However, for ammonia–hydrogen mixtures, GRI 3.0 failed to predict ignition delay times, whereas the UCF 2022 mechanism was able to capture the IDTs within the uncertainty limits of the experiments. A sensitivity analysis was conducted for different mixtures to understand the important reactions at the experimental conditions. Lastly, a reaction pathway analysis was carried out to understand important ammonia decomposition pathways in the presence of hydrogen and natural gas.

33 ADVANCED PROPULSION SYSTEMS↗

Spectral-Kinetic Characteristics of Laser Ignition of Pulverized Brown Coal

In this study, we measured the spectral-kinetic characteristics of ignition of dustlike (pulverized) coal particles with a particle size of ≤100 μm using an yttrium-aluminum garnet laser with an admixture of neodymium operating in the free-running mode (120 μs). We discovered three stages of the process, each of which has a threshold character. During the laser pulse in the energy density range of 0.3–1.5 J/cm{sup 2}, a rapid heating of the sample surface to 3000 K occurs due to the absorption of laser energy and initiation of a chemical reaction in the coal particles. In the energy density range of 1.6–2.1 J/cm{sup 2}, the yield and ignition of volatile matter above the sample surface are observed in the time interval of 0.5–2 ms. In the emission spectra of the flame, we are able to distinguish bands associated with the combustion of volatile matter including the CO flame and the emission of excited H{sub 2}O and H{sub 2} molecules. At H > 2 J/cm{sup 2}, ignition and burning of the coke residue takes place in a time interval of 10–100 ms. Its emission spectrum corresponds to the emission spectrum of a black body at 1800 K.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗

Fast ignition when heating the central part of an inertial confinement fusion target by an ion beam

We investigate the ignition and burning of a precompressed laser fusion target when it is rapidly heated by an ion beam with the formation of a temperature peak in the central part of the target. We present the results of our comprehensive numerical simulations of the problem that include the following components: (1) the target compression under the action of a profiled laser pulse, (2) the heating of the compressed target with spatially nonuniform density and temperature distributions by a beam of high-energy ions, and (3) the burning of the target with the initial spatial density distribution formed at the instant of maximum target compression and the initial spatial temperature distribution formed as a result of the compressed-target heating by an ion beam. The dependences of the threshold energies of the igniting ion beam and the thermonuclear gain on the width of the Gaussian beam ion energy spectrum have been established. The peculiarities of fast ignition by an ion beam related to the spatial distribution of parameters for the target precompressed by a laser pulse are discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗