Search

The sensitivity coefficients of ignition delay time with respect to chemical kinetic rate parameters is traditionally calculated using a brute force approach wherein the number of initial value problems solved is no fewer than the number of rate parameters. By reformulating the underlying initial value problem as a boundary value problem, it is shown that the sensitivity coefficients can be extracted directly, accelerating thus the computational time by at least an order of magnitude. This has far reaching consequences in kinetic model development and optimization, especially when a large number of rate constants are involved. [ABSTRACT FROM AUTHOR]

The counterflow configuration is widely used to study experimentally premixed and non-premixed flame ignition, with the advantage being that the data can be modeled using quasi one-dimensional codes. In this study, experiments and direct numerical simulations were carried out in order to assess the validity of the assumptions of the one-dimensional formulation. Experimentally, particle image velocimetry, shadowgraph, and a high-speed camera were employed to characterize the flow field before ignition, and to capture the ignition position and further evolution of the flame. The modeling involved axisymmetric numerical simulations using detailed molecular transport and chemical kinetic models. Both experiments and simulations revealed that if solid surfaces are present in the vicinity of the jets exit, the flow separates generating recirculation zones that are unstable and result in the bifurcation of the flow field. As a result, for a given set of boundary conditions at the burners’ exits, there exists two possible stable states of the flow field which have different velocity and scalars distribution, and the fuel concentration at which ignition occurs was determined to differ for these two states. A novel approach is proposed to correct for the unavoidable radial non-uniformity of the temperature profile at the exit of the heated jet and the conditions that do not result in bifurcation are outlined, so that the results from one-dimensional codes can be compared to the data with confidence. [ABSTRACT FROM AUTHOR]

A computational investigation was carried out to assess the effects of highly energetic vortices on the local structure of laminar premixed flames. The study involved a lean atmospheric methane-air flame interacting with vortex sizes ranging from five times to half of the laminar flame thickness. The characteristic velocities of the vortices were chosen to correspond to extreme turbulence intensities. Vortices smaller than the flame thickness were found to have a minimal effect on the flame structure due to the high rate of dissipation. Vortices with sizes greater than or equal to the flame thickness were found to induce flame siphoning and/or product-side local flame annihilation and as a result transport considerable amounts of formaldehyde and thermal energy to the unburned side thereby modifying the state of the reactants. The results of the present study for high activation energy reacting flows raise questions regarding the general validity of: (1) conjectured mechanisms of the so-called flame broadening under intense turbulence conditions; and (2) qualitative arguments regarding sub-flame-thickness vortex effects on the preheat zone in the context of the Borghi–Peters regime diagram, as originally postulated by Damköhler. [ABSTRACT FROM AUTHOR]

Thedecomposition of silane (SiH4), as a model silicon-containingtrace compound in renewable natural gas (RNG), has been studied duringRNG combustion at ambient pressure conditions, using the opposed-jet,flat-flame experimental configuration. Silane flame concentrationprofiles were obtained, which indicate that complete SiH4oxidation occurs in the preflame and luminous flame regions. Thelaser-extinction technique was used to measure particle volume fractionsin the flame, and scanning electron microscopy and energy dispersiveanalysis by X-ray were used to study the morphology (size and shape)and the elemental composition of the particles formed. It was shownthat pure solid silica (SiO2) particles are generated andcarried out with the gaseous combustion products into the postflameregion. The experimental data were modeled using two detailed SiH4decomposition models, and the major SiO2productionchannels have been identified for both these models via sensitivityand reaction path analyses. It was determined further that a first-orderglobal decomposition reaction rate can describe adequately the experimentaldata. [ABSTRACT FROM AUTHOR]

Abstract: The phenomena of propagation and extinction of flames of saturated C3 alcohols and propane were studied experimentally and numerically in order to assess the effects of the presence and location of the hydroxyl radical in the fuel molecular structure. The experiments were carried out in the counterflow configuration under atmospheric pressure and for unreacted fuel-carrying stream temperature of 343K. The simulations included detailed descriptions of molecular transport and chemical kinetics using a recently developed kinetic model for C3 alcohols. The experimental results revealed that the laminar flame speeds and extinction strain rates of n-propanol/air and propane/air flames are close to each other whereas those of iso-propanol/air flames are consistently lower. Similar behavior was observed also for the extinction strain rates of non-premixed n-propanol and iso-propanol flames. It was shown through sensitivity and reaction path analyses that there are two major differences between the intermediates of n-propanol/air and iso-propanol/air flames. In iso-propanol/air flames there are notably higher concentrations of propene whose consumption pathway results in the relatively unreactive allyl radicals, retarding thus the overall reactivity. In n-propanol/air flames there are notably higher concentrations of formaldehyde that reacts readily to form formyl radicals whose subsequent reactions enhance the overall reactivity. The kinetic model used in this study was found to overpredict the experimental results for rich n-propanol/air and propane/air flames. Analysis revealed that those discrepancies are most likely caused by deficiencies in the C3 alkane kinetics. Through sensitivity analysis, it was determined also that the propagation and extinction of n-propanol/air and iso-propanol/air flames are sensitive largely to hydrogen, carbon monoxide, and C1–C3 kinetics and not to fuel-specific reactions. Finally, the relative sooting propensities of flames of these three fuels were assessed computationally. [Copyright &y& Elsevier]

The spherically expanding flame under constant volume method was introduced in 1934 by Lewis and von Elbe as a means to study laminar flame propagation at engine-relevant conditions. Despite its potential, this method has not been utilized extensively due to concerns regarding the underlying assumptions and data uncertainty. In the present study, the intricacies of the experimental approach as well as the models and assumptions involved during data interpretation were reassessed with the aid of direct numerical simulations. Results confirmed that stretch effects are negligible during the compression stage of the experiment for a wide range of Lewis numbers. Additionally, it was shown that the laminar flame speed is sensitive to the flame radius stressing thus the requirement that the modeling of flame radius needs to be done with the highest possible accuracy by accounting properly for product dissociation and thermal radiation from the burned gases. It was also shown, that the equilibrium assumption is valid for modeling the flame radius as a function of pressure and, as expected, the kinetic and transport effects are negligible. Subsequently, laminar flame speeds were measured for methane, ethane, ethylene, propane, propylene, n -butane, 1-butene, and isobutene flames for 8–30 atm pressures and 400–520 K unburned mixture temperatures. A hybrid thermodynamic/radiation model was utilized to interpret the experimental observables and derive the laminar flame speed by accounting for spectrally dependent emission from and absorption by the burned gases as well as product dissociation during compression. The data were found to be consistent with measurements obtained in spherically expanding flame experiments under constant pressure conditions and predictions using a number of current kinetic models. [ABSTRACT FROM AUTHOR]

Measurements of flame propagation velocities and extinction states in counterflow provide a valuable source of flame data that contain information about fundamental combustion physics. The approach to properly account for stretch effects in counterflow flame measurements through non-intrusive laser-based local velocity characterization was advanced in the mid-80s by Law and coworkers at atmospheric conditions with simple fuels. Subsequently, several research groups have extended the measurements to elevated pressures and complex fuels. However, counterflow flame data at subatmospheric pressures are limited. In the present study, a method is introduced for measuring laminar flame speeds and extinction strain rates in subatmospheric counterflow flames. A numerical study was performed to assess the dynamics of tracer particles used to facilitate measurements. It was found that the particle phase dynamics used in particle velocimetry measurements are not always representative of the underlying gas phase motion due to thermophoresis and insufficient drag, especially at low pressures. A numerical scheme was implemented whereby the computed particle phases were used for proper comparison with measurements and, based on the computed results, to infer the corresponding values of the gas phase. The method was applied to premixed methane/air and non-premixed methane–nitrogen/oxygen flames at pressures as low as 0.1 atm. Complimentary flame structure simulations were carried out which show that the kinetics of formyl radical prompt dissociation strongly impact the computed subatmospheric flames and may influence the validation of unimolecular and bimolecular reactions rate constants when tested against laminar flame data. [ABSTRACT FROM AUTHOR]

The opposed-jet counterflow configuration is widely used to measure fundamental flame properties that are essential targets for validating chemical kinetic models. The main and key assumption of the counterflow configuration in laminar flame experiments is that the flow field is steady and quasi-one-dimensional. In this study, experiments and numerical simulations were carried out to investigate the behavior and controlling parameters of counterflowing isothermal air jets for various nozzle designs, Reynolds numbers, and surrounding geometries. The flow field in the jets’ impingement region was analyzed in search of instabilities, asymmetries, and two-dimensional effects that can introduce errors when the data are compared with results of quasi-one-dimensional simulations. The modeling involved transient axisymmetric numerical simulations along with bifurcation analysis, which revealed that when the flow field is confined between walls, local bifurcation occurs, which in turn results in asymmetry, deviation from the one-dimensional assumption, and sensitivity of the flow field structure to boundary conditions and surrounding geometry. Particle image velocimetry was utilized and results revealed that for jets of equal momenta at low Reynolds numbers of the order of 300, the flow field is asymmetric with respect to the middle plane between the nozzles even in the absence of confining walls. The asymmetry was traced to the asymmetric nozzle exit velocity profiles caused by unavoidable imperfections in the nozzle assembly. The asymmetry was not detectable at high Reynolds numbers of the order of 1000 due to the reduced sensitivity of the flow field to boundary conditions. The cases investigated computationally covered a wide range of Reynolds numbers to identify designs that are minimally affected by errors in the experimental procedures or manufacturing imperfections, and the simulations results were used to identify conditions that best conform to the assumptions of quasi-one-dimensional modeling. [ABSTRACT FROM AUTHOR]

The effect of fuel decomposition on fundamental flame properties was investigated computationally for atmospheric-pressure n- dodecane/air mixtures. The fuel decomposition was modeled under isobaric and adiabatic conditions for initial temperatures of 1100, 1200 and 1300 K, and equivalence ratios of 0.7, 1.0, and 1.4. For various extents of n- dodecane oxidative thermal decomposition, the combustion characteristic of mixtures of the resulting products with air were investigated by keeping the total enthalpy constant and equal to that of the n- dodecane/air mixture. The endothermic n- dodecane decomposition was found, to a large extent, to be decoupled from the subsequent oxidation of the attendant products that include largely hydrogen, ethylene, methane, and other small alkenes. The mass burning rates in freely propagating flames were found to increase with an increase in the extent of n -dodecane decomposition, but the change is limited to 15%, which occurs in the highest extent of decomposition. On the other hand, the extinction strain rate of decomposed, lean to stoichiometric mixtures increases notably compared to the corresponding un-decomposed fuel–air mixtures. Sensitivity analyses of mass burning rates and extinction strain rates to kinetics and binary diffusion coefficients reveal that the laminar flame speed is primarily sensitive to key heat release and radical branching reactions, and as such fuel decomposition has a small effect on the mass burning rate. On the other hand, the extinction strain rate of the fuel-lean mixtures is sensitive to the diffusivity of the fuel, and for this reason, fuel decomposition removes the difficulties associated with the transport of the large fuel molecules into the flame zone. [ABSTRACT FROM AUTHOR]

Laminar flame speeds of methyl formate/air mixtures were measured at sub-atmospheric pressures for which limited data exist. The experiments were carried out in the counterflow configuration at an unburned mixture temperature of 333 K. The flow velocities were measured using particle image velocimetry. Particle phase slip correction was applied to low-pressure data sets for which the density disparity between the flow tracers and the gaseous phase is notable. The data were modeled using two recently developed kinetic models of methyl formate oxidation, and significant disagreements were realized at all pressures especially under fuel-rich conditions. Additionally, the computed species profiles of CO and CO 2 in the burner-stabilized flame configuration using the two models were found to differ significantly. Reaction path analysis revealed that the kinetics of CH 2 OCHO that is produced directly from the fuel affects the overall reactivity, and the attendant rate constants differ between the two models. The variation of laminar flame speed with pressure revealed also a different behavior between experiments and simulations. Further insight into the sources causing the observed discrepancies were investigated and it was determined that reactions involving formyl radical, methanol, and formaldehyde could also be responsible for the reduction in reactivity specifically under fuel-rich conditions. [ABSTRACT FROM AUTHOR]

The next generation of advanced combustion devices is being developed to operate under ultra-high-pressure conditions. However, under such extreme conditions, flame tends to become unstable and measurement of fundamental properties such as the laminar flame speed becomes challenging. One potential method to resolve this issue is measuring the ignition-affected region during spherically expanding flame experiments. The flame in this region is more resistant to perturbations and remains smooth due to the high stretch rates (i.e. small radii). Stable flame propagation allows for improved flame measurement, however, the experimentally observed kernel propagation is a function of both inflammation and ignition plasma. Therefore, the goal of the present study is to better understand the plasma formation and propagation during the ignition process, which would allow for reliable laminar flame speed measurements. To accomplish this goal, thermal plasma operating at high pressures is studied with emphasis on the spark energy effects on the formation of the ignition kernel. The thermal effect of the plasma is experimentally observed using a high-speed Schlieren imaging system. The energy dissipated within the plasma is measured with the use of voltage and current probes with a measurement of plasma sheath voltage drop as an input to numerical modeling. The measured kernel propagation rate is used to assess the accuracy of the model. The experiments and modeling are conducted in dry air at 1, 3, and 5 atm as well as in CH4-N2 mixtures at 1 atm, and kernel radius, temperature, and mass are reported. The voltage-drop (as a non-thermal loss) is measured to be approximately 330 ± 5 V (dry air at 1 atm) for glow plasma with a large dependency on pressure, gas composition, electrode surface quality, electrode geometry, electrode shape, and current density. The same loss within the arc plasma is measured to be 15 ± 5 V, however the arc phase loss which agrees with arc propagation is significantly higher (∼45 V) which suggest additional unaccounted for phenomena occurring during the arc phase. With these losses, the modeling results are shown to predict the final kernel radius within 10%–20% of the observed kernel size. The difference found between the modeling and experimental results is determined to be a result of assuming that the primary loss mechanism (voltage drop across sheath formation) remains constant for the duration of glow discharge. The discrepancy for arc discharge is discussed with several potential sources, however, additional studies are required to better understand how the arc formation affects the kernel propagation. [ABSTRACT FROM AUTHOR]

The uncertainties associated with the extraction of laminar flame speeds through extrapolations from directly measured experimental data were assessed using one-dimensional direct numerical simulations with focus on the effects of molecular transport and thermal radiation loss. The simulations were carried out for counterflow and spherically expanding flames given that both configurations are used extensively for the determination of laminar flame speeds. The spherically expanding flames were modeled by performing high fidelity time integration of the mass, species, and energy conservation equations. The simulation results were treated as "data" for stretch rate ranges that are encountered in experiments and were used to perform extrapolations using formulas that have been derived based on asymptotic analyses. The extrapolation results were compared then against the known answers of the direct numerical simulations. The fuel diffusivity was varied in order to evaluate the flame response to stretch and to address reactant differential diffusion effects that cannot be captured based on Lewis number considerations. It was found that for large molecular weight hydrocarbons at fuel-rich conditions, the flame behavior is controlled by differential diffusion and that the extrapolation formulas can result in notable errors. Analysis of the computed flame structures revealed that differential diffusion modifies the fluxes of fuel and oxygen inside the flame and thus affect the reactivity as stretch increases. Radiation loss was found to affect notably the extracted laminar flame speed from spherically expanding flame experiments especially for slower flames, in agreement with recent similar studies. The effect of radiation could be eliminated however, by determining the displacement speed relative to the unburned gas. This can be achieved in experiments using high-speed particle image velocimetry to determine the flow velocity field within the few milliseconds duration of the experiment. In general, extrapolations were found to be unreliable under certain conditions, and it is proposed that the raw experimental data in either flame configurations are compared against results of direct numerical simulations in order to avoid potential falsifications of rate constants upon validation. [ABSTRACT FROM AUTHOR]

Abstract: Laminar flame speeds of iso-cetane/air and decalin/air mixtures were measured in the counterflow configuration at atmospheric pressure and an elevated unburned mixture temperature of 443K. Axial flow velocities were measured along the stagnation streamline using the digital particle image velocimetry. The laminar flame speeds were determined by determining the variation of a reference flame speed as a function of strain rate and computationally assisted non-linear extrapolations. The data are the first to be reported in the literature, and they were modeled using a recently developed kinetic model that includes 187 species and 6086 elementary reactions. In general, the computed results were found to be in close agreement with the data. In order to get insight into kinetic effects on flame propagation, detailed sensitivity and reaction path analyses were performed using the computed flame structures. The results revealed that at the same equivalence ratio, laminar flame speeds of iso-cetane/air mixtures are lower than those of n-hexadecane/air mixtures. Additionally, it was found that the laminar flame speeds of iso-cetane/air and decalin/air mixtures are sensitive largely to C0–C4 kinetic subset, and that the lower reactivity of iso-cetane compared to n-hexadecane could be attributed to the higher production of relatively stable intermediates. [Copyright &y& Elsevier]

Abstract: Ignition temperatures of non-premixed C3 and C5–C12 n-alkane flames were determined in the counterflow configuration at atmospheric pressure, a free-stream fuel/N2 mixture temperature of 448K, a local strain rate of 140s−1, and fuel mole fractions ranging from 1% to 12% in the fuel stream. The strain rate was measured on the fuel side using Laser Doppler Velocimetry. Simulations of the experiments were performed using the recently developed JetSurF 1.0 reaction model consisting of 194 species and 1459 reactions. In both experiments and simulations, the ignition temperatures of all n-alkane flames were found to decrease with increasing fuel concentration. The computed ignition temperatures are in close agreement with the experimental data for all C5–C12 n-alkanes, while for propane flames the data are slightly over-predicted. Detailed sensitivity analyses on both reaction rates and binary diffusion coefficients were performed, and the ignition temperature was determined to be sensitive both to fuel diffusion and the H2/CO and small hydrocarbon kinetics. The dependence of the ignition temperature on the fuel molecular weight was determined to differ at low and high fuel mole fractions, due to the competition between the fuel reactivity and diffusive transport. [Copyright &y& Elsevier]

Abstract: The oxidation of three model biodiesel fuels, namely methyl butanoate (C5H10O2, CAS No. 623-42-7), methyl crotonate (C5H8O2, CAS No. 623-43-8), and methyl decanoate (C11H22O2, CAS No. 110-42-9) was investigated in laminar premixed and non-premixed flames. The experiments were conducted in the counterflow configuration at atmospheric pressure, for a wide range of equivalence or inert-dilution ratios, and elevated reactant temperatures. Laminar flame speeds and local extinction strain rates were determined by measuring the flow velocities using digital particle image velocimetry. The experimental data were compared against those derived for flames of n-alkanes of similar carbon number, in order to assess the effects of saturation, the length of carbon chain, and the presence of the ester group. Several recent chemical kinetic models were tested against the experimental data, and major differences were identified and assessed. The accuracy of the Lennard–Jones potential parameters assigned to the methyl esters in the transport databases of the different models was evaluated and new values were estimated. Insight was provided into the high-temperature kinetic pathways of methyl esters in flame environments. Additionally, the reduced sooting propensity of methyl ester flames compared to n-alkane flames was investigated computationally. [Copyright &y& Elsevier]

In this study, the oxidation characteristics of biodiesel fuels are investigated with the goal of contributing toward the fundamental understanding of their combustion characteristics and evaluating the effect of using these alternative fuels on engine performance as well as on the environment. The focus of the study is on pure fatty acid methyl-esters (FAME,) that can serve as surrogate compounds for real biodiesels. The experiments are conducted in the stagnation-flow configuration, which allows for the systematic evaluation of fundamental combustion and emission characteristics. In this paper, the focus is primarily on the pollutant emission characteristics of two C4FAMEs, namely, methyl-butanoate and methyl-crotonate, whose behavior is compared with that of n-butane and n-pentane. To provide insight into the mechanisms of pollutant formation for these fuels, the experimental data are compared with computed results using a model with consistent C1−C4oxidation and NOxformation kinetics. [ABSTRACT FROM AUTHOR]

Abstract: Laminar flame speeds and extinction strain rates of premixed methanol, ethanol, and n-butanol flames were determined experimentally in the counterflow configuration at atmospheric pressure and elevated unburned mixture temperatures. Additional measurements were conducted also to determine the laminar flame speeds of their n-alkane/air counterparts, namely methane, ethane, and n-butane in order to compare the effect of alkane and alcohol molecular structures on high-temperature flame kinetics. For both propagation and extinction experiments the flow velocities were determined using the digital particle image velocimetry method. Laminar flame speeds were derived through a non-linear extrapolation approach based on direct numerical simulations of the experiments. Two recently developed detailed kinetics models of n-butanol oxidation were used to simulate the experiments. The experimental results revealed that laminar flame speeds of ethanol/air and n-butanol/air flames are similar to those of their n-alkane/air counterparts, and that methane/air flames have consistently lower laminar flame speeds than methanol/air flames. The laminar flame speeds of methanol/air flames are considerably higher compared to both ethanol/air and n-butanol/air flames under fuel-rich conditions. Numerical simulations of n-butanol/air freely propagating flames, revealed discrepancies between the two kinetic models regarding the consumption pathways of n-butanol and its intermediates. [ABSTRACT FROM AUTHOR]

O2 lancing is one of the industrial techniques for implementing oxygen-enhanced combustion (OEC), and can potentially result in stable combustion with reduced NOx emissions. The key technical challenge facing lancing (and OEC in general) is the significant cost of O2 enrichment. The present work reports early results from a study of CH4 OEC, in which the O2 enrichment of air is implemented in situ using dense, high-temperature mixed-conducting solid-oxide membranes. We report on the preparation and characterization of these membranes, and on the fundamental combustion studies of the relevant reactive mixtures, including the effect of O2 lancing on the NOx and CO emissions of fuel-lean and fuel-rich CH4/air premixed flames. [Copyright &y& Elsevier]

An experimental and numerical investigation of the thermochemical structure of piloted premixed jet flames was conducted, encompassing laser absorption tomography measurements and large-eddy simulations (LES). The investigation was performed holding laminar flame speed, jet Reynolds number, and surrounding flow conditions constant while considering three different fuel types, namely an alkene, a normal alkane, and an aromatic fuel. Quantitative spatially-resolved thermochemical profiles of carbon monoxide (CO), carbon dioxide (CO 2), and temperature obtained from laser absorption tomography were compared against profiles predicted by the simulations for premixed ethylene-, toluene-, and n -heptane-air flames. Variations in flow structure are observed for the different fuels, highlighting fuel-specific chemical effects on the spatial evolution of the flames. Quantitative agreement of laser absorption tomography and LES results is generally observed for all flames, with larger deviations observed in the nozzle-near region for the higher molecular-weight fuels, indicating potential deficiencies in the turbulent mixing models. To the authors' knowledge, these measurements represent the largest molecular-weight fuels for which quantitative thermochemical data have been reported in a canonical piloted premixed jet-flame configuration. The spatially-resolved experimental measurements of CO, CO 2 , and gas temperature provide valuable data which can be used as validation targets for the development of turbulent combustion models. [ABSTRACT FROM AUTHOR]

The HyChem (hy brid chem istry) approach is utilized for modeling the combustion behaviors of gasoline fuels. The approach combines an experimentally constrained, lumped-step model for fuel pyrolysis under the high-temperature combustion condition and a detailed foundation fuel chemistry model to describe the subsequent oxidation of the pyrolysis products. We present results obtained for two Shell gasoline fuels as examples. The results show that with the parameters in the lumped reactions determined by matching the experiment time history data of key products of gasoline pyrolysis, the HyChem reaction models capture the ignition delay times and laminar flame speeds over a wide range of thermodynamic conditions. The HyChem approach is also extended to model the negative-temperature coefficient (NTC) behaviors for the gasoline fuels. The results show that the NTC-enabled models are capable of capturing the ignition delays under both high-temperature conditions and the conditions under which the NTC behaviors are important. The relationship between fuel composition and combustion properties is analyzed. Finally, the HyChem models are reduced to about 40 species to enable turbulent combustion modeling of gasoline fuels in practical engine simulations. [ABSTRACT FROM AUTHOR]

The laminar flame speed is an important input in turbulent premixed combustion modelling of spark ignition engines. At engine-relevant temperatures and pressures, its measurement is challenging or not possible and thereby it is usually obtained from simulations based on chemical models or power-law correlations. This work aims to investigate the performance of different models and power-law correlations in terms of predicting laminar flame speeds of methane/air at engine conditions. The propagation of spherically expanding laminar flames in a closed chamber was simulated and laminar flame speeds were computed over a broad range of pressures (1-120 atm) and temperatures (300-1100 K) for methane/air mixtures based on seven kinetic models. It was found that at engine conditions, there are notable discrepancies among the predictions. GRI Mech. 3.0 and USC Mech. II respectively predict the largest and smallest values at high pressure conditions. This was explained by the difference in CH 3 oxidation and recombination according to reaction pathway analysis. Additionally, laminar flame speeds of methane flames were experimentally determined under engine-relevant conditions. It was shown that the recently developed Foundational Fuel Chemistry Model Version 1.0 model predicts closely the data at high pressures and temperatures. Therefore, it was chosen as the reference model for the comparisons. Thirteen published power-law correlations for laminar flame speeds of CH 4 /air were implemented, and their performance in predicting the laminar flame speeds at engine conditions was investigated. Most of these correlations have been derived for a narrow range of temperatures and pressures, which are lower than those encountered in engines. A new power-law correlation was derived based on predictions by the Foundational Fuel Chemistry Model Version 1.0. This new correlation is expected to provide reliable predictions at engine conditions for a stoichiometric methane/air mixture and thereby it is recommended to be used in modeling turbulent premixed combustion in spark-ignition engine simulations. [ABSTRACT FROM AUTHOR]

The confined spherical expanding flame method was used to measure laminar flame speeds and ignition delay times of blends of methane and hydrogen at engine-relevant conditions. Laminar flame speeds were measured for hydrogen addition to methane of up to 40 % on a molar basis, for pressures ranging from 8 to 30 atm, and for unburned mixture temperatures ranging from 420 to 530 K. It was found that the laminar flame speed increases with the hydrogen addition, as expected, and that the trend of the data is consistent with predictions resulting from using recently developed kinetic models. For properly chosen conditions, autoignition of the end gas was induced during the compression stage of the outward flame propagation, and ignition delay times were determined using the rapid changes in the pressure-time derivatives. The autoignition experiments were carried out for blends of methane, n -pentane, and hydrogen. The use of n -pentane was essential in order to achieve autoignition within the time scales available in the existing spherical facility. In the presence of n -pentane, a two-stage ignition behavior was observed, and the addition of hydrogen was found to reduce the ignition delay times. All data were modeled, and insight was provided into the mechanisms controlling flame propagation and the two stages of ignition. These results are expected to be of direct relevance to ongoing efforts in hydrogen utilization toward the decarbonization of the transportation sector. [ABSTRACT FROM AUTHOR]

Unsteady premixed and non-premixed counterflow laminar flame simulations were conducted in order to investigate extinction effects on observables commonly used in turbulent combustion. CH 4 and n -C 12 H 26 were the fuels studied, with air as the oxidizer at pressures of 1, 5, and 10 bar. It was determined that CH 2 O persists, compared to all other reactive species, during the extinction transient for both fuels and at all conditions, as the loss of OH concentration removes the dominant CH 2 O consumption pathway. The persistence of CH 2 O concentration is duplicated similarly in CH 4 and n -C 12 H 26 premixed flames. For non-premixed flames, the results indicate that the peak CH 2 O concentration reduction for n -C 12 H 26 flames is milder compared to CH 4 flames. Increasing the pressure causes an extension of reactivity, resulting in greater CH 2 O production and thus a delayed decay during the extinction transient. In addition, a change in the magnitude of the applied scalar dissipation rate for the non-premixed flames did not alter the trends of CH 2 O during extinction. Thus, caution is suggested when using CH 2 O in turbulent combustion experiments as a marker of the preheat zone thickness, given that increased levels of CH 2 O could be a result of multiple local extinction events. In addition, the product of OH and CH 2 O was found to scale well with the heat release rate for CH 4 and n -C 12 H 26 flames at multiple pressures. Finally, the CH* and OH* chemiluminescence was examined. CH* was found to extinguish slightly before the other species and more importantly, that once its concentration is reduced to a negligible level, the flame is on its way to extinction with no chance of recovery. OH* was determined to scale well with heat release at both 1 and 10 bar for both fuels and type of flames. [ABSTRACT FROM AUTHOR]

Highlights • A UV photodecomposition reactor (PhoR) studied for siloxane removal from biogas. • 3D model developed validated with experiments. • Model used for PhoR design/scale-up. Abstract The goal of this research was to develop a model for a UV photodecomposition reactor (PhoR), which is used for the removal of trace siloxane impurities from biogas. The model properly accounts for the phenomena taking place inside the reactor, including fluid flow, mass and heat transfer, the transmission of UV radiation, and the photodecomposition reaction. Lab-scale experimental data were used to experimentally validate the model, and to also provide a “proof-of-concept” of the proposed PhoR technology prior to its field-testing at a real landfill site. The model was then utilized in the design of the experimental system used in the field-testing component of this study. Two reactor configurations were modeled: In the first configuration (termed PhoRI), the UV lamps are not in direct contact with the biogas, whereas in the second one (PhoRII), the lamps are placed in direct contact with the biogas stream. The PhoRII configuration is substantially more efficient for siloxane degradation, but concerns exist about the long-term robustness of the UV lamps in such an environment. The same model also presently finds use for further process design and scale-up, and in the preliminary technical and economic analysis (TEA) of the PhoR technology. [ABSTRACT FROM AUTHOR]

Abstract The Hybrid Chemistry (HyChem) approach has been proposed previously for combustion chemistry modeling of real, liquid fuels of a distillate origin. In this work, the applicability of the HyChem approach is tested for single-component fuels using JP10 as the model fuel. The method remains the same: an experimentally constrained, lumped single-fuel model describing the kinetics of fuel pyrolysis is combined with a detailed foundational fuel chemistry model. Due to the multi-ring molecular structure of JP10, the pyrolysis products were found to be somewhat different from those of conventional jet fuels. The lumped reactions were therefore modified to accommodate the fuel-specific pyrolysis products. The resulting model shows generally good agreement with experimental data, which suggests that the HyChem approach is also applicable for developing combustion reaction kinetic models for single-component fuels. [ABSTRACT FROM AUTHOR]

Abstract A Hybrid Chemistry (HyChem) approach has been recently developed for the modeling of real fuels; it incorporates a basic understanding about the combustion chemistry of multicomponent liquid fuels that overcomes some of the limitations of the conventional surrogate fuel approach. The present work extends this approach to modeling the combustion behaviors of a two-component bio-derived jet fuel (Gevo, designated as C1) and its blending with a conventional, petroleum-derived jet fuel (Jet A, designated as A2). The stringent tests and agreement between the HyChem models and experimental measurements for the combustion chemistry, including ignition delay and laminar flame speed, of C1 highlight the validity as well as potential wider applications of the HyChem concept in studying combustion chemistry of complex liquid hydrocarbon fuels. Another aspect of the present study aims at answering a central question of whether the HyChem models for neat fuels can be simply combined to model the combustion behaviors of fuel blends. The pyrolysis and oxidation of several blends of A2 and C1 were investigated. Flow reactor experiments were carried out at pressure of 1 atm, temperature of 1030 K, with equivalence ratios of 1.0 and 2.0. Shock tube measurements were performed for the blended fuel pyrolysis at 1 atm from 1025 to 1325 K. Ignition delay times were also measured using a shock-tube. Good agreement between measurements and model predictions was found showing that formation of the products as well as combustion properties of the blended fuels were predicted by a simple combination of the HyChem models for the two individual fuels, thus demonstrating that the HyChem models for two jet fuels of very different compositions are "additive." [ABSTRACT FROM AUTHOR]

Real distillate fuels usually contain thousands of hydrocarbon components. Over a wide range of combustion conditions, large hydrocarbon molecules undergo thermal decomposition to form a small set of low molecular weight fragments. In the case of conventional petroleum-derived fuels, the composition variation of the decomposition products is washed out due to the principle of large component number in real, multicomponent fuels. From a joint consideration of elemental conservation, thermodynamics and chemical kinetics, it is shown that the composition of the thermal decomposition products is a weak function of the thermodynamic condition, the fuel-oxidizer ratio and the fuel composition within the range of temperatures of relevance to flames and high temperature ignition. Based on these findings, we explore a hybrid chemistry (HyChem) approach to modeling the high-temperature oxidation of real, distillate fuels. In this approach, the kinetics of thermal and oxidative pyrolysis of the fuel is modeled using lumped kinetic parameters derived from experiments, while the oxidation of the pyrolysis fragments is described by a detailed reaction model. Sample model results are provided to support the HyChem approach. [ABSTRACT FROM AUTHOR]

We propose and test an alternative approach to modeling high-temperature combustion chemistry of multicomponent real fuels. The hy brid chem istry (HyChem) approach decouples fuel pyrolysis from the oxidation of fuel pyrolysis products. The pyrolysis (or oxidative pyrolysis) process is modeled by seven lumped reaction steps in which the stoichiometric and reaction rate coefficients are derived from experiments. The oxidation process is described by detailed chemistry of foundational hydrocarbon fuels. We present results obtained for three conventional jet fuels and two rocket fuels as examples. Modeling results demonstrate that HyChem models are capable of predicting a wide range of combustion properties, including ignition delay times, laminar flame speeds, and non-premixed flame extinction strain rates of all five fuels. Sensitivity analysis shows that for conventional, petroleum-derived real fuels, the uncertainties in the experimental measurements of C 2 H 4 and CH 4 impact model predictions to an extent, but the largest influence of the model predictability stems from the uncertainties of the foundational fuel chemistry model used (USC Mech II). In addition, we introduce an approach in the realm of the HyChem approach to address the need to predict the negative-temperature coefficient (NTC) behaviors of jet fuels, in which the CH 2 O speciation history is proposed to be a viable NTC-activity marker for model development. Finally, the paper shows that the HyChem model can be reduced to about 30 species in size to enable turbulent combustion modeling of real fuels with a testable chemistry model. [ABSTRACT FROM AUTHOR]

The goal of this study is to assess the technical feasibility of remediating siloxane contaminants in biogas via a photochemical process. Specifically, we studied in the laboratory a process that involves the use of an ultraviolet (UV) photodecomposition reactor (PhoR) to convert siloxane trace impurities, commonly found in biogas produced in water treatment plants and landfills, into silica particulates. These can then be effectively removed from the reactor effluent with the use of a downstream filter. High siloxane conversions were obtained, which demonstrates the effectiveness of the technique. The proposed technology is presently being field-tested in a California landfill. [ABSTRACT FROM AUTHOR]

An exploratory study was performed on highly turbulent piloted premixed jet flames of straight chain (paraffin and olefin), branched, cyclo, and aromatic C 1 C 8 hydrocarbons. The goal was to investigate fuel effects on global and local flame observables under flow conditions corresponding to the transition from the thin to the broken reaction zone regimes. The experiments were performed at atmospheric pressure, reactant temperature of 298 K, jet Reynolds numbers of 12,500 and 25,000, and various equivalence ratios and fuels in order to assess the attendant effects on the flame characteristics. The effects of heat losses from the flame were considered also in order to sensitize kinetic effects. Measurements were performed using particle image velocimetry and spontaneous luminosity imaging. The averaged and fluctuating velocity components as well the average flame height were determined to scale in general with the laminar flame speed. The exception to this scaling was the height of methane flames that exhibit notable differences compared to large molecular weight fuels. Subtle differences were identified even among the large hydrocarbon flames and became evident from measurements of the instantaneous luminosity field. Finally, high-speed luminosity imaging revealed that there are apparent fuel effects on the local structure of the flame as evident by the distribution and size of luminous pockets. These results suggest that the fuel effects should be also the focus of further investigations by the community. [ABSTRACT FROM AUTHOR]

Laminar flame speed measurements were carried for mixture of air with eight C 3-4 hydrocarbons (propene, propane, 1,3-butadiene, 1-butene, 2-butene, iso -butene, n -butane, and iso -butane) at the room temperature and ambient pressure. Along with C 1-2 hydrocarbon data reported in a recent study, the entire dataset was used to demonstrate how laminar flame speed data can be utilized to explore and minimize the uncertainties in a reaction model for foundation fuels. The USC Mech II kinetic model was chosen as a case study. The method of uncertainty minimization using polynomial chaos expansions (MUM-PCE) (Sheen and Wang, 2011) was employed to constrain the model uncertainty for laminar flame speed predictions. Results demonstrate that a reaction model constrained only by the laminar flame speed values of methane/air flames notably reduces the uncertainty in the predictions of the laminar flame speeds of C 3 and C 4 alkanes, because the key chemical pathways of all of these flames are similar to each other. The uncertainty in model predictions for flames of unsaturated C 3 – 4 hydrocarbons remain significant without considering fuel specific laminar flames speeds in the constraining target data set, because the secondary rate controlling reaction steps are different from those in the saturated alkanes. It is shown that the constraints provided by the laminar flame speeds of the foundation fuels could reduce notably the uncertainties in the predictions of laminar flame speeds of C 4 alcohol/air mixtures. Furthermore, it is demonstrated that an accurate prediction of the laminar flame speed of a particular C 4 alcohol/air mixture is better achieved through measurements for key molecular intermediates formed during the pyrolysis and oxidation of the parent fuel. [ABSTRACT FROM AUTHOR]

The potential for the use of chemical weapons has increased in recent times. The successful use of a flow-through catalytic membrane reactor (FTCMR) as a protection system against a chemical warfare agent (CWA) was recently reported by this group. This FTCMR, employing a single catalytically active mesoporous membrane, was applied for the destruction of dimethyl methylphosphonate (DMMP), which is known as a chemical precursor (and used to simulate its characteristics) for Sarin (GB), a toxic CWA. In this paper, results are reported of efforts to scale-up this lab-scale FTCMR towards practical individual protection (IP) and collective protection (CP) applications. A multi-tubular FTCMR (MFTCMR) has been developed and tested towards the destruction of DMMP. The MFTCMR has been shown quite effective, providing extended protection periods against this CWA simulant. A key part of the research effort focused on improving the preparation of the catalytically active membranes to prepare membrane bundles with reproducible transport characteristics and reactivity. A key challenge for the large-scale production of such devices, furthermore, remains the development of a simple non-destructive test that assures that the produced parts are appropriate for the proposed application and that they continue to remain active during their shelf-life prior to their use. As part of this effort a simple propane-in-air light-off test was studied, which was shown capable to track the reactivity and performance of the MFTCMR. [ABSTRACT FROM AUTHOR]

The hydrogen oxidation chemistry constitutes the foundation of the kinetics of all carbon- and hydrogen-containing fuels. The validation of rate constants of hydrogen-related reactions can be complicated by uncertainties associated with experimental data caused by the high reactivity and diffusivity of hydrogen. In the present investigation accurate experimental data on flame propagation and extinction were determined for premixed and non-premixed hydrogen flames at pressures between p = 1 and 7 atm. The experiments were designed to sensitize the three-body H + O 2 + M → HO 2 + M reaction, whose rate is subject to notable uncertainty. This was achieved by increasing the pressure and by adding to the reactants H 2 O and CO 2 whose collision efficiencies are high compared to other species. In the present study, directly measured flame properties were compared against computed ones, in order to eliminate uncertainties associated with extrapolations, as is the case for laminar flame speeds. The measured extinction strain rates exhibit both a positive and negative dependence on pressure with and without weighting with the density, and this non-monotonic behavior is caused by the competition between the H + O 2 → O + OH and H + O 2 + M → HO 2 + M reactions as well as HO 2 kinetic pathways as pressure increases. The various kinetic models considered in this investigation did not reproduce equally well the non-premixed flame extinction data with added H 2 O. On the other hand, the predicted extinction strain rates were consistent between the various models in the case of added CO 2 . Finally, it was shown that the formulation of binary diffusion coefficient pairs including H–N 2 and H 2 –N 2 has a first order effect on the prediction of extinction strain rates of non-premixed H 2 flames. [ABSTRACT FROM AUTHOR]

The influence of correlated scalar fluctuations on time-averaged laser absorption tomography measurements of temperature and species in a piloted turbulent premixed flame was examined using a coupled spectroscopic and fluid-dynamic analysis. To understand bias associated with turbulence, spatio-temporally resolved temperature and species mole fraction profiles predicted by large eddy simulations (LES) were used to synthetically generate time-resolved line-of-sight absorption measurements at short time scales (microsecond) to reflect the unsteady nature of a canonical jet burner across various transverse measurement planes. Inversion methods were employed on the time-averaged line-of-sight data to produce radially-resolved temperature and mole fraction profiles, analogous to those produced by laser absorption tomography performed on a time-averaged axisymmetric flowfield. It is shown that bias in the measurements compared to true time-averaged scalar fields is a function primarily of temperature dependence in absorptivity and non-zero correlation between temperature and species concentration scalars. A first-order correction to tomography measurements is proposed to account for the bias based on estimated correlations and the known spectroscopic parameters of the probed absorption transitions. [ABSTRACT FROM AUTHOR]

Abstract: The combustion characteristics of promising alternative fuels have been studied extensively in the recent years. Nevertheless, the pyrolysis and oxidation kinetics for many oxygenated fuels are not well characterized compared to those of hydrocarbons. In the present investigation, the first chemical kinetic study of a long-chain linear symmetric ether, di-n-butyl ether (DBE), is presented and a detailed reaction model is developed. DBE has been identified recently as a candidate biofuel produced from lignocellulosic biomass. The model includes both high temperature and low temperature reaction pathways with reaction rates generated using appropriate rate rules. In addition, experimental studies on fundamental combustion characteristics, such as ignition delay times and laminar flame speeds have been performed. A laminar flow reactor was used to determine the ignition delay times of lean and stoichiometric DBE/air mixtures. The laminar flame speeds of DBE/air mixtures were measured in the stagnation flame configuration for a wide rage of equivalence ratios at atmospheric pressure and an unburned reactant temperature of 373K. All experimental data were modeled using the present kinetic model. The agreement between measured and computed results is satisfactory, and the model was used to elucidate the oxidation pathways of DBE. The dissociation of keto-hydroperoxides, leading to radical chain branching was found to dominate the ignition of DBE in the low temperature regime. The results of the present numerical and experimental study of the oxidation of di-n-butyl ether provide a good basis for further investigation of long chain linear and branched ethers. [Copyright &y& Elsevier]

Abstract: The oxidation characteristics of several small methyl and ethyl esters with carbon number less than six were investigated in laminar flames. The kinetics of such fuels are subsets of those of larger alkyl esters that are constituents of practical biodiesel fuels. A total of seven fuels, namely methyl formate, methyl acetate, methyl propionate, methyl butanoate, ethyl formate, ethyl acetate, and ethyl propionate were considered. Experiments were conducted at atmospheric pressure, elevated reactant temperatures, and over a wide range of equivalence ratios. Laminar flame speeds were determined in the counterflow configuration in which flow velocities were measured using particle image velocimetry. Several detailed kinetic models were tested against the experimental data, and insight was provided into the high-temperature combustion kinetics of the aforementioned fuels. Based on comparisons between experimental and computed results it became apparent that the chemistry of alkyl-ester combustion chemistry is evolving and much needs to be done in order to derive improved rate constants for a wide range of elementary steps. [Copyright &y& Elsevier]

Abstract: Biofuels are considered as potentially attractive alternative fuels that can reduce greenhouse gas and pollutant emissions. iso-Pentanol is one of several next-generation biofuels that can be used as an alternative fuel in combustion engines. In the present study, new experimental data for iso-pentanol in shock tube, rapid compression machine, jet stirred reactor, and counterflow diffusion flame are presented. Shock tube ignition delay times were measured for iso-pentanol/air mixtures at three equivalence ratios, temperatures ranging from 819 to 1252K, and at nominal pressures near 40 and 60bar. Jet stirred reactor experiments are reported at 5atm and five equivalence ratios. Rapid compression machine ignition delay data was obtained near 40bar, for three equivalence ratios, and temperatures below 800K. Laminar flame speed data and non-premixed extinction strain rates were obtained using the counterflow configuration. A detailed chemical kinetic model for iso-pentanol oxidation was developed including high- and low-temperature chemistry for a better understanding of the combustion characteristics of higher alcohols. First, bond dissociation energies were calculated using ab initio methods, and the proposed rate constants were based on a previously presented model for butanol isomers and n-pentanol. The model was validated against new and existing experimental data at pressures of 1–60atm, temperatures of 650–1500K, equivalence ratios of 0.25–4.0, and covering both premixed and non-premixed environments. The method of direct relation graph (DRG) with expert knowledge (DRGX) was employed to eliminate unimportant species and reactions in the detailed mechanism, and the resulting skeletal mechanism was used to predict non-premixed flames. In addition, reaction path and temperature A-factor sensitivity analyses were conducted for identifying key reactions at various combustion conditions. [Copyright &y& Elsevier]

Abstract: Understanding the kinetics of large molecular weight aldehydes is essential in the context of both conventional and alternative fuels. For example, they are key intermediates formed during the low-temperature oxidation of hydrocarbons as well as during the high-temperature oxidation of oxygenated fuels such as alcohols. In this study, an experimental and kinetic modeling investigation of n-butanal (n-butyraldehyde) and iso-butanal (iso-butyraldehyde or 2-methylpropanal) oxidation kinetics was performed. Experiments were performed in a jet stirred reactor and in counterflow flames over a wide range of equivalence ratios, temperatures, and pressures. The jet stirred reactor was utilized to observe the evolution of stable intermediates and products for the oxidation of n- and iso-butanal at elevated pressures and low to intermediate temperatures. The counterflow configuration was utilized for the determination of laminar flame speeds. A detailed chemical kinetic interpretative model was developed and validated consisting of 244 species and 1198 reactions derived from a previous study of the oxidation of propanal (propionaldehyde). Extensive reaction pathway and sensitivity analysis was performed to provide detailed insight into the mechanisms governing low-, intermediate-, and high-temperature reactivity. The simulation results using the present model are in good agreement with the experimental laminar flame speeds and well within a factor of two of the speciation data obtained in the jet stirred reactor. [Copyright &y& Elsevier]