Combustion and Flame

Published by Elsevier
Online ISSN: 0010-2180
Publications
Article
The pyrolytic and oxidative behaviour of the biofuel 2,5-dimethylfuran (25DMF) has been studied in a range of experimental facilities in order to investigate the relatively unexplored combustion chemistry of the title species and to provide combustor relevant experimental data. The pyrolysis of 25DMF has been re-investigated in a shock tube using the single-pulse method for mixtures of 3% 25DMF in argon, at temperatures from 1200 to 1350 K, pressures from 2 to 2.5 atm and residence times of approximately 2 ms.
 
Article
The identity of radical species associated with particulate formed from the oxidative pyrolysis of 1-methylnaphthalene (1-MN) was investigated using low temperature matrix isolation electron paramagnetic resonance spectroscopy (LTMI-EPR), a specialized technique that provided a method of sampling and analysis of the gas-phase paramagnetic components. A superimposed EPR signal was identified to be a mixture of organic radicals (carbon and oxygen-centered) and soot. The carbon-centered radicals were identified as a mixture of the resonance-stabilized indenyl, cyclopentadienyl, and naphthalene 1-methylene radicals through the theoretical simulation of the radical's hyperfine structure. Formation of these radical species was promoted by the addition of Fe(III)2O3 nanoparticles. Enhanced formation of resonance stabilized radicals from the addition of Fe(III)2O3 nanoparticles can account for the observed increased sooting tendency associated with Fe(III)2O3 nanoparticle addition.
 
Article
The detailed chemical structures of three low-pressure (35 Torr) premixed laminar furan/oxygen/argon flames with equivalence ratios of 1.4, 1.8 and 2.2 have been investigated by using tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry. About 40 combustion species including hydrocarbons and oxygenated intermediates have been identified by measurements of photoionization efficiency spectra. Mole fraction profiles of the flame species including reactants, intermediates and products have been determined by scanning burner position with some selected photon energies near ionization thresholds. Flame temperatures have been measured by a Pt-6%Rh/Pt-30%Rh thermocouple. A new mechanism involving 206 species and 1368 reactions has been proposed whose predictions are in reasonable agreement with measured species profiles for the three investigated flames. Rate-of-production and sensitivity analyses have been performed to track the key reaction paths governing furan consumption for different equivalence ratios. Both experimental and modeling results indicate that few aromatics could be formed in these flames. Furthermore, the current model has been validated against previous pyrolysis results of the literature obtained behind shock waves and the agreement is reasonable as well.
 
Article
The experimental study of the oxidation of cyclohexane has been performed in a jet-stirred reactor at temperatures ranging from 500 to 1100 K (low- and intermediate temperature zones including the negative temperature-coefficient area), at a residence time of 2 s and for dilute mixtures with equivalence ratios of 0.5, 1, and 2. Experiments were carried out at quasi-atmospheric pressure (1.07 bar). The fuel and reaction product mole fractions were measured using online gas chromatography. A total of 34 reaction products have been detected and quantified in this study. Typical reaction products formed in the low-temperature oxidation of cyclohexane include cyclic ethers (1,2-epoxycyclohexane and 1,4-epoxycyclohexane), 5-hexenal (formed from the rapid decomposition of 1,3-epoxycyclohexane), cyclohexanone, and cyclohexene, as well as benzene and phenol. Cyclohexane displays high low-temperature reactivity with well-marked negative temperature-coefficient (NTC) behavior at equivalence ratios 0.5 and 1. The fuel-rich system (ϕ = 2) is much less reactive in the same region and exhibits no NTC. To the best of our knowledge, this is the first jet-stirred reactor study to report NTC in cyclohexane oxidation. Laminar burning velocities were also measured by the heated burner method at initial gas temperatures of 298, 358, and 398 K and at 1 atm. The laminar burning velocity values peak at ϕ = 1.1 and are measured as 40 and 63.1 cm/s for Ti = 298 and 398 K, respectively. An updated detailed chemical kinetic model including low-temperature pathways was used to simulate the present (jet-stirred reactor and laminar burning velocity) and literature experimental (laminar burning velocity, rapid compression machine, and shock tube ignition delay times) data. Reasonable agreement is observed with most of the products observed in our reactor, as well as the literature experimental data considered in this paper.
 
Article
The low-temperature oxidation of n-heptane, one of the reference species for the octane rating of gasoline, was investigated using a jet-stirred reactor and two methods of analysis: gas chromatography and synchrotron vacuum ultra-violet photo-ionization mass spectrometry (SVUV-PIMS) with direct sampling through a molecular jet. The second method allowed the identification of products, such as molecules with hydroperoxy functions, which are not stable enough to be detected using gas chromatography. Mole fractions of the reactants and reaction products were measured as a function of temperature (500-1100K), at a residence time of 2s, at a pressure of 800 torr (1.06 bar) and at stoichiometric conditions. The fuel was diluted in an inert gas (fuel inlet mole fraction of 0.005). Attention was paid to the formation of reaction products involved in the low temperature oxidation of n-heptane, such as olefins, cyclic ethers, aldehydes, ketones, species with two carbonyl groups (diones) and ketohydroperoxides. Diones and ketohydroperoxides are important intermediates in the low temperature oxidation of n-alkanes but their formation have rarely been reported. Significant amounts of organic acids (acetic and propanoic acids) were also observed at low temperature. The comparison of experimental data and profiles computed using an automatically generated detailed kinetic model is overall satisfactory. A route for the formation of acetic and propanoic acids was proposed. Quantum calculations were performed to refine the consumption routes of ketohydroperoxides towards diones.
 
Article
The experimental study of the oxidation of a blend containing n-decane and a large unsaturated ester, methyl oleate, was performed in a jet-stirred reactor over a wide range of temperature covering both low and high temperature regions (550-1100 K), at a residence time of 1.5 s, at quasi atmospheric pressure with high dilution in helium (n-decane and methyl oleate inlet mole fractions of 1.48 × 10(-3) and 5.2 × 10(-4)) and under stoichiometric conditions. The formation of numerous reaction products was observed. At low and intermediate temperatures, the oxidation of the blend led to the formation of species containing oxygen atoms like cyclic ethers, aldehydes and ketones deriving from n-decane and methyl oleate. At higher temperature, these species were not formed anymore and the presence of unsaturated species was observed. Because of the presence of the double bond in the middle of the alkyl chain of methyl oleate, the formation of some specific products was observed. These species are dienes and esters with two double bonds produced from the decomposition paths of methyl oleate and some species obtained from the addition of H-atoms, OH and HO2 radicals to the double bond. Experimental results were compared with former results of the oxidation of a blend of n-decane and methyl palmitate performed under similar conditions. This comparison allowed highlighting the similarities and the differences in the reactivity and in the distribution of the reaction products for the oxidation of large saturated and unsaturated esters.
 
Article
The flame and soot structure and the soot surface growth and oxidation properties of round laminar jet diffusion flames were studied experimentally at pressures of 0.1–1.0 atm. Measurements were made along the axes of flames fueled with acetylene–nitrogen mixtures and burning in coflowing air with the reactants at normal temperature (300 K). The following properties were measured as a function of distance from the burner exit: soot concentrations by deconvoluted laser extinction, soot temperatures by deconvoluted multiline emission, soot structure by thermophoretic sampling and analysis using Transmission Electron Microscopy (TEM), concentrations of major stable gas species (N2, Ar, H2O, H2, O2, CO, CO2, CH4, C2H2, C2H4, C2H6, C3H6, C3H8, and C6H6) by isokinetic sampling and gas chromatography, concentrations of some radial species (H, OH, O) by deconvoluted Li/LiOH atomic absorption, and flow velocities by laser velocimetry. These measurements yielded local soot surface growth and oxidation rates, as well as local flame properties that are thought to affect these rates. Present measurements of soot surface growth rates (corrected for soot surface oxidation) in laminar diffusion flames at various subatmospheric pressures were consistent with earlier measurements of soot surface growth rates in laminar premixed and diffusion flames involving a variety of hydrocarbons at atmospheric pressure; in addition, rates from all the available flames were in good agreement with each other and with existing Hydrogen-Abstraction/Carbon-Addition (HACA) soot surface growth mechanisms in the literature for values of steric factors in these mechanisms on the order of unity, as expected. Similarly, present measurements of early soot surface oxidation rates (corrected for soot surface growth and prior to consumption of 70% of the maximum mass of the primary soot particles) for fuel-rich and near-stoichiometric conditions in laminar diffusion flames at pressures of 0.1–1.0 atm were consistent with earlier measurements of rates of early soot surface oxidation in laminar premixed and diffusion flames involving a variety of hydrocarbons at atmospheric pressure; in addition, rates from all available flames could be explained by reaction with OH, as proposed by K.G. Neoh et al. (in: Particulate Carbon, Plenum, New York, 1980, p. 261), having a collision efficiency of 0.13 and supplemented to only a minor degree by direct soot surface oxidation by O2.
 
Article
An experimental study is reported on the physical characterization of the structure of ethanol/argon/oxygen coflow laminar spray diffusion flames in the pressure range 0.1–0.9 MPa. Diagnostic techniques include phase Doppler anemometry to measure the droplet size distribution and the axial and radial velocity components of the droplets. The gas-phase velocity is determined using measurements from the smallest (low Stokes number) droplets and is corrected for thermophoretic effects. Temperature information is obtained using thin-film pyrometry combined with an infrared camera. All flames present a cold inner core, in which little or no vaporization takes place, surrounded by an envelope flame buried in a thermal boundary layer, where most of the droplet evaporation occurs. The thickness of this thermal boundary layer scales with the inverse of the Peclet number. Especially near the base of the flame, photographic evidence of streaks, which in some case even reveal the presence of soot, suggests that some droplets survive the common envelope flame and burn isolated on the oxidizer side in a mixed regime of internal/external group combustion. The reconstruction of the entire droplet vaporization history confirms this evidence quantitatively. A criterion for droplet survival beyond the envelope flame based on the critical value of a suitably defined vaporization Damköhler number is proposed. The scaling and self-similar behavior of the investigated flames suggest that a mixed regime is established, with a momentum-controlled cold core and a buoyancy-controlled high-temperature boundary layer, the thickness of which varies significantly with pressure, as expected from Peclet number scaling. The growth of this layer and the thickness of the vaporization region are reduced at pressures above atmospheric because of density effects on thermal diffusivity. Some aspects of the design of the combustion chamber and of the atomizer system are discussed in detail since they are critical to the suppression of instabilities and to the establishment of a well-defined high-pressure quasi-steady laminar environment.
 
Article
Results of finite-difference, time-dependent numerical studies of the near field of subsonic, reactive square jets were presented. The simulations model space/time-developing compressible (subsonic) jets, using species- and temperature-dependent diffusive transport, and finite-rate chemistry appropriate for H2 combustion. Comparative measurements of entrainment for square jets were obtained based on evaluations of streamwise mass-flux to obtain an assessment on how the jet development is affected by chemical exothermicity and density differences between the jet and the surroundings. Depending on initial conditions (i.e., on the chemical exothermicity level implied by the initial reactant concentration), chemical energy release and expansion effects can be significant in determining reduced entrainment and initial jet growth relative to corresponding nonreactive jets. The instantaneous product formation rates are closely correlated with the local entrainment rates controlled by the vorticity bearing fluid. Instantaneous entrainment rates—based on the rate of increase of mass flux of rotational fluid—were found to be significant in the regions of roll-up and initial self-deformation of vortex rings, and then farther downstream, in the vortex merging region, where fluid and momentum transport between the jet and its surroundings are considerably enhanced by the presence of hairpin vortices aligned with the corners. Analysis of the combustion dynamics in terms of scalar mixing fraction diagnostics previously used in laboratory reactive turbulent jet experiments, was shown to be also potentially useful in characterizing their transitional regime by bringing out the relation between product formation rates and underlying fluid dynamical events.
 
Article
The oxidation and auto-ignition of cyclohexane, cyclohexene, and cyclohexa-1,3-diene have been studied by rapid compression between 600 K to 900 K and 0.7 MPa to 1.4 MPa to identify the low-temperature pathways leading to benzene from cyclohexane. Auto-ignition delay times were measured and concentration-time profiles of the C6 intermediate products of oxidation were measured during the auto-ignition delays. Cyclohexane showed two-stage ignition at low temperatures, but single-stage ignition at higher temperatures, and a well-marked negative-temperature coefficient. By contrast there was neither a cool flame, nor a negative-temperature coefficient for cyclohexa-1,3-diene. Cyclohexene behaved in an intermediate way without a cool flame, but with a narrow, not very marked negative-temperature coefficient. The identified C6 products belong to three families: the bicyclic epoxides and cyclic ketones, the unsaturated aliphatic aldehydes, and the conjugated alkenes, which are always the major products. The formation of C6 products from cyclohexane is explained by the classical scheme for low-temperature oxidation, taking into account addition of O2 to cyclohexyl radicals and the various isomerizations of the resulting peroxy radicals. Most of the C6 products from cyclohexene are predicted by the same scheme, beginning with the formation of the allylic cyclohexenyl radical. However, addition of HO2 to the double bond has to be included to predict the formation of 1,2-epoxycyclohexane. For cyclohexa-1,3-diene, the classical scheme is not valid: the C6 oxygenated products are only formed by addition of HO2 to the double bond. For all three hydrocarbons, the pathways to benzene are those leading to conjugated alkenes, and they are always more efficient than those producing oxygenated products, either by adding HO2 to double bonds, or by addition of O2 to the initial cyclic radical.
 
Article
In this work, we have developed a detailed chemical kinetic model and reacting flow simulation for the hexadiene-doped 2-d methane diffusion flames studied experimentally by McEnally and Pfefferle. The GRI-Mech 2.11 methane oxidation and Lawrence Livermore butane oxidation mechanisms were used as the base mechanism to which hexadiene chemistry generated by Reaction Mechanism Generator (RMG) was added. Some important chemically activated pathways leading to aromatic species formation, including the reactions on C5H7, C6H10, C6H9, C6H7, C8H8 and C8H9 potential energy surfaces, are examined in great detail using quantum chemistry (CBS-QB3) and master equation analysis as implemented in Variflex.An efficient program to solve the doped methane diffusion flame was developed. The solver uses the method of lines to solve the species mass balance equation arising in the diffusion flame. It assumes that the temperature and velocity profiles of the doped flame are the same as those of the undoped flame.The mole fractions of various species as predicted by our model are compared to the experimentally measured mole fractions. The agreement between theory and experiments is quite good for most molecules. The added hexadiene dopants to the flame decompose to produce significant amount of cyclopentadienyl radical, which combines with methyl radical to produce benzene. We also show that styrene is formed primarily by recombination of cyclopentadienyl and propargyl radicals, a pathway which to our knowledge, has not been included in prior flame simulations.
 
Article
Soot formation in argon-diluted mixtures of acetylene, allene, and 1,3-butadiene was studied behind reflected shock waves by monitoring attenuation of a laser beam in both the visible (632.8 nm) and the infrared (3.39 μm) regions of the spectrum. Experiments were conducted for temperatures in the range 1500–3100K, reflected shock pressures in the range 0.3–7.0 bar, and total carbon atom concentrations in the range (2–20) × 1017 atoms/cm3. During the pyrolysis of individual hydrocarbons, a bell-shaped dependence of soot yield on temperature, similar to that previously reported for toluene, was observed for all three compounds. For acetylene, a decrease in total pressure shifted the soot bell to higher temperatures with a significant increase in the maximum soot yield. The analysis of a computer simulation for acetylene pyrolysis indicated that the reactions involving C2H3, C4H3, and C4H4 may be those which lead to the formation of aromatic structures. The experimental results also show that soot is formed much faster and in much larger quantities from allene than from 1,3-butadiene. A conceptual model which explains the observed phenomena is suggested.
 
Article
The kinetics of the thermal decomposition of RDX and the formation of three nitroso intermediates have been studied in hexaproteo (h6) benzene; hexadeutero (d6) benzene at 180°C and 200°C; and, as a melt at 210°C in sealed capillaries. Decomposition rates were determined by vapor-phase chromatographic analysia of the residues with time for unreacted RDX and the nitroso intermediates formed. RDX exhibited first order decomposition kinetics in benzene solutions, but showed zero order kinetics in the melt at 210°C. A kinetic isotope effect was observed in d6-benzene, and together with the nitroso intermediates formation, indicated the following consecutive concurrent sequence as one decomposition pathway: RO3 → RO2 → RO → R → products, where RO3 is RDX; and RO2, RO, and R are the 1-nitroso-3,5-dinitro-, 1,3-dinitroso-5-nitro-, and the 1,3,5-trinitroso-1,3,5-triazacyclohexanes, respectively. Rate constants obtained for decomposition of RO3 and RO2 are discussed in terms of the nitroso intermediates formed.
 
Article
The effect of products and 1,3,5 trinitrobenzene on the thermal decomposition of RDX in static system at 195°C has been studied. Formaldehyde, hydroxymethyl formamide, methylene diformamide, and 1,3,5, trinitrobenzene are all shown to enhance the reaction rate. Different initial steps in the mechanisms are indicated for the gaseous and solution phase decompositions.
 
Article
In this work a simplified but effective closure for the numerical simulation of turbulent premixed flames is developed, being applicable for pressures up to 1.0 MPa. Here the reaction source term of the reaction progress variable is modeled with an algebraic relation for the flame-wrinkling ratio . The closure is based on a three-parameter description, including an explicit pressure term. In order to determine the fit parameters, an extensive numerical optimization study is performed, where the calculated and measured flame angles of 101 different Bunsen-type flames are compared for operating pressures between 0.1 and 1.0 MPa. Experimental data on 20-mm Bunsen-type flames for lean methane/air, ethylene/air, and propane/air mixtures for different flow and turbulence inlet conditions were provided by the group of Kobayashi (Japan). For each fuel the three parameters of the algebraic relation are varied such that a minimum least-square deviation between computed and measured flame cone angles is achieved. It is found that for all the fuels investigated this relation collapses to a unique equation, in which the Lewis number of the fuel/air mixture is included. It has been proposed recently that differential molecular transport effects, as described by the Lewis number, may be related not only to laminar flame instability effects at low turbulence but also to visible effects at higher degrees of turbulence. This fits to our finding of the explicit Lewis number dependency in the source term. It is possible to substantiate the applicability of this algebraic closure for methane/air flames for higher pressures up to 3 MPa in reasonable agreement with experimental data, as described in Appendix A.
 
Article
Values of laminar burning velocity, ul, and the associated strain rate Markstein number, Masr, of H2–air mixtures have been obtained from measurements of flame speeds in a spherical explosion bomb with central ignition. Pressures ranged from 0.1 to 1.0 MPa, with values of equivalence ratio between 0.3 and 1.0. Many of the flames soon became unstable, with an accelerating flame speed, due to Darrieus–Landau and thermodiffusive instabilities. This effect increased with pressure. The flame wrinkling arising from the instabilities enhanced the flame speed. A method is described for allowing for this effect, based on measurements of the flame radii at which the instabilities increased the flame speed. This enabled ul and Masr to be obtained, devoid of the effects of instabilities. With increasing pressure, the time interval between the end of the ignition spark and the onset of flame instability, during which stable stretched flame propagation occurred, became increasingly small and very high camera speeds were necessary for accurate measurement. Eventually this time interval became so short that first Masr and then ul could not be measured. Such flame instabilities throw into question the utility of ul for high pressure, very unstable, flames. The measured values of ul are compared with those predicted by detailed chemical kinetic models of one-dimensional flames.
 
Article
Benzene depletion in a laminar premixed flat stoichiometric low-pressure methane/air/benzene (1.5%) flame was investigated using a recently developed kinetic model that has been tested for low-pressure combustion of acetylene, ethylene, and benzene. Experimental flame structures of two stoichiometric methane/air flames (v =34.2 cm s−1, 5.33 kPa) with and without the addition of 1.5% of benzene were measured previously by means of molecular beam sampling using mass spectrometry as well as gas chromatography coupled to mass spectrometry as analytic tools. Model computations were conducted using the Premix code within the Chemkin software package. Experimental temperature profiles were used as input. The analysis of rates of production of selected species allowed for the identification of major formation and depletion pathways. The predictive capacility of the model was assessed in both flames. Good to excellent agreements between predictions and measured mole fraction profiles were obtained for reactants, intermediates, and products such as methane, O2, methyl, H, OH, CO, CO2. and H2O. Benzene depletion and the formation and consumption of intermediates such as cyclopentadiene were predicted correctly. According to the model, methyl is exclusively formed by hydrogen abstraction from methane and subsequently oxidized by reaction with O to formyl, (1) CH3 + O ⇄ HCO + H2, and formaldehyde, (2) CH3 + O ⇄ CH2O + H, the latter channel being the dominant one. Benzene consumption occurred mainly by hydrogen abstraction with OH and H as reactant but also the contribution of its oxidation by O to phenoxy was significant. Phenol and phenoxy chemistries are strongly coupled, unimolecular decay of phenoxy to cyclopentadienyl radicals and CO is the dominant consumption route. Small PAH are predicted to be formed in the reaction zone, followed by complete depletion in the postflame region. The pressure dependence of the dominant dimethylether formation (32) CH3 + CH3O ⇄ CH3OCH3 was found to be significant and assessed by means of Quantum Rice-Ramsperger-Kassel (QRRK) analysis.
 
Article
IR spectra of 3,3,3-trinitrobutyric acid, 1, 3,3-dinitrovaleric acid, 2, bis(2,2,2-trinitroethyl)carbonate, 3, and bis(2,2-dinitropropyl)carbonate, 4, were analyzed from 163K to the decomposition temperature. Below the decomposition temperature most of the structural alterations involve the COOH group. Once decomposition begins, the spectral changes also involve the CNO2 groups. The backbone of the gem-dinitroalkyl compounds degrades more readily than the gem-trinitro compounds. Concentration-time profiles of the gas products from high-rate thermolysis (>75K s−1) show that NO2 is the predominant initial decomposition product while CO2 is initially important only from 2. NO is a major initial product of the gem-dinitro series but not the gem-trinitro series. Through the duration of observation, the concentration of the carbon oxidation products increases while [NO2] decreases, suggesting that NO2 is the principal oxidizing agent. The decomposition-to-deflagration transition occurs in the gem-trinitro compounds but not the gem-dinitro compounds under these experimental conditions.
 
Article
Yields of small char particles, formed from larger particles by attrition and fragmentation during combustion, were measured at temperatures characteristic of fluidized bed combustors. The small particles were generated by burning char in a furnace on a vibrating screen. Particles passing through 1.3 × 1.3-mm openings were collected outside the furnace and weighed after complete destruction of the parent particles. The use of the screen isolates the process of fragment formation from the process of elutriation, and allows greater control of collision energy and frequency than is possible in a fluidized bed. Pittsburgh Seam char initially 5 mm in diameter was burned at temperatures of 900, 1000, and 1100 K in the presence of 2, 5, 10, and 21 mol% oxygen. By varying the vibration frequency and energy of collisions, the yield of fines was varied from 0 to 40 wt% of the fixed carbon in a batch feed. The yield of fines decreased with increasing temperature, decreased with increasing oxygen, and increased with simultaneous increases in the energy and frequency of collisions. The results were consistent with a model for parallel attrition and fragmentation, in which attrition is viewed as purely mechanical, proportional to collision frequency, and proceeding at a rate independent of combustion. Fragmentation was seen as occurring at a critical value of the char porosity, at a rate controlled by the removal of char by combustion. The critical porosity depends upon collision energy, rather than collision frequency. The model reproduced important features in the dependence of the fine particle yield on vibration frequency, collision energy, oxygen concentration, and temperature, in both diffusion and chemical rate-controlled combustion regimes, under conditions at which attrition made a greater contribution to small particles than fragmentation, and vice versa.
 
Article
Absolute rate coefficients for the reaction of NCO radicals with methane (k1), ethane (k2), and propane (k3) were measured as a function of temperature in a heatable quartz reactor by means of the laser photolysis/laser-induced fluorescence (LP/LIF) pump-probe technique. NCO radicals were produced by the fast precursor reaction NH(a1Δ) +HNCO → NH2 + NCO, following the 193-nm photolysis of isocyanic acid. The measured rate coefficients can be described by the following expressions: (1)k1(512<T<1113K)=1012.99±0.12×exp⁡(−34.0±1.8kJmol−1/RT)cm3mol−1s−1, (2)k1(296<T<922K)=108.21×(T/298K)(6.89±0.02)×exp⁡(12.2±0.5kJmol−1/RT)cm3mol−1s−1, (3)k3(300<T<849K)=1011.49×(T/298K)(2.15±0.02)×exp⁡(−1.8±0.4kJmol−1/RT)cm3mol−1s−1A comparison with the corresponding reactions of CN, Cl, and OH radicals with alkanes suggests that all these title reactions also proceed predominantly via a hydrogen atom abstraction mechanism to form HNCO.
 
Article
A unified model with a single set of kinetic parameters has been proposed for modeling the autoignition delay times of a wide range of alkanes using detailed kinetic mechanisms automatically generated by software EXGAS. The validations were based on recent data of the literature obtained in shock tubes and in rapid compression machines. The compounds studied were n-butane, n-pentane, iso-pentane, neo-pentane, 2-methylpentane, n-heptane, iso-octane, n-decane, and mixtures of n-heptane and iso-octane. Investigated conditions are the following: temperatures range from 600 to 1200 K, including the negative temperature coefficient (NTC) region, pressures range from 1 to 50 bar, and equivalence ratios range from 0.5 to 2. Tests were also performed for results obtained in a jet-stirred reactor and a sensitivity analysis was performed in the case of n-heptane. Simulations using generated mechanisms for the nine isomers of heptane were performed and showed that the variations of autoignition delay times follows globally well that of octane numbers at 650 K in a rapid compression machine at 4 bar. The influence of pressure (from 3 to 200 bar) was theoretically investigated.
 
Article
The rapid thermal decomposition of five substituted benzofuroxans and a 3,4-dimethylfuroxan was examined by rapid-scan FTIR spectroscopy as a function of heating rate and pressure. The product distribution and parent molecular structure proved difficult to correlate in the benzofuroxan series, but it is distinctly different from the alkylfuroxan. Rapid heating cleaves 3,4-dimethylfuroxan into two equivalents of acetonitrile N-oxide, whereas only small molecule fragments are produced by benzofuroxans.
 
Article
A simple first-principles model of counter current heat-recirculating combustors is developed, including the effects of heat transfer from the product gas stream to the reactant stream, heat loss to ambient, and heat conduction in the streamwise direction through the dividing wall (and heat transfer surface) between the reactant and product streams. It is shown that streamwise conduction through the wall has a major effect on the operating limits of the combustor, especially at small dimensionless mass fluxes (M) or Reynolds numbers that would be characteristic of microscale devices. In particular, if this conduction is neglected, there is no small-M extinction limit because smaller M leads to larger heat recirculation and longer residence times that overcome heat loss if M is sufficiently small. In contrast, even a small effect of conduction along this surface leads to significantly higher minimum M. Comparison is made with an alternative configuration of a flame stabilized at the exit of a tube, where heat recirculation occurs via conduction through tube wall; it is found that the counter-current exchanger configuration provides superior performance under similar operating conditions. Implications for microscale combustion are discussed.
 
Article
During the heterogeneous oxidation of metallic particles in a gaseous oxidant, a film of oxide usually separates the metal and oxidant, with the reaction rate being governed to a great extent by the protective properties of the film. A theoretical and experimental study of the influence of the failure of the protective oxide film on the metal's heterogeneous ignition has been performed. The possible reasons for the fracture are analyzed; also the critical conditions of the oxide film are estimated. Experiments conducted with aluminum and titanium powders have shown that the fracture of the protective oxide film results in a significant increase in the metal's reactivity. The assumptions of a theoretical analysis are confirmed experimentally.
 
Article
Extensive results from axisymmetric convergent-nozzle and straight-tube opposed jet burners (OJBs) characterized strain-induced extinction of unanchored (free-floating), laminar H2/N2-air flames. Parameters included (a) plug-flow and parabolic input velocity profiles, (b) jet exit diameters ranging 2.7 to 7.2 mm for nozzles and 1.8 to 10 mm for tubes, (c) various relative jet gaps, and (d) 14 to 100% H2 in the fuel jet. Extinction, a sudden rupture (blowoff) of the mostly-airside disk flame, occurred as fuel and air flows were slowly increased and a critical radial strain rate was exceeded. The disk flame was restored at much lower flows, unique to H2 systems. Focusing schlieren, thermocouple, and airside LDV (and PIV) data confirmed the one-dimensional (I-D) character of nozzle-OJB flow fields; axial widths of velocity- and thermal-layers varied as (input strain rate)−1/2 for both nozzles and tubes. The global approximation of a I-D applied stress rate (ASR), using average air jet velocity divided by exit diameter, enabled high quality correlations of extinction data with varied H2 concentrations, for both nozzles and tubes. Pre-extinction ASRs for nozzles agreed closely with LDV-measured centerline input strain rates; for tubes, however, an empirical factor of 3 produced close agreement. For methane-air extinction, nozzle-OJB ASRs agreed within 4% of independent nozzle and Tsuji burner results. For extinction of 100% H2-air, an ASR of 5670 1/s compared with 7350, 8140, and 8060 from independent 1-D numerical evaluations using potential-flow inputs; for 50 to 14% H2 inputs, agreement was much closer. The nozzle-ASR/tube-ASR ratio for extinction was ≥3 for <50% H2 inputs, 2.74 ± 0.03 for 50 to 100% H2 inputs, and 2.83 for methane-air. Because these ratios exceeded 2.0, which “accounted” for centerline velocity inputs from parabolic profiles, an additional 3/2 radial strain component was apparent and was supported by the axial velocity gradient measurements.
 
Article
The effect of fuel hydrocarbon structure on soot emissions was studied using a carbon-14 isotope tracer technique. A diesel engine or a laminar wick diffusion flame generated radioactive soot from a #2 diesel fuel containing 14Chydrocarbons. For the same diesel fuel, the individual soot yields of several carbon atom types were determined. For the diesel engine, the soot yield from aromatic carbons was about a factor of 1.5 greater than that from nonaromatic carbons. For the wick flame, the soot yield from aromatic carbons was about a factor of 2.0 greater than that from nonaromatic carbons. Interestingly, these differences in soot yield between aromatic and nonaromatic carbons within the same fuel are too small to explain the differences in soot yield between aromatic and nonaromatic fuels. This is consistent with the hypothesis that aromatic carbons increase soot emissions by greatly increasing the number of nascent particles while contributing only slightly more than other carbons to the final soot mass. It also shows that variations in fuel hydrocarbon structure have a minimal effect on the soot particle growth process, which produces most of the soot mass. The wick flame was set to give a soot yield that was 20 times greater than that of the diesel engine; yet, the carbon-14 concentrations in the soot were similar. This is expected if the formation of soot growth species from the original fuel molecules is independent of the amount of soot ultimately formed from the growth species. Also, the soot yield of a given carbon atom type was unaffected by the molecular weight of the hydrocarbon molecule that contained it. So for #2 diesel fuel, soot is produced equally from all points along the fuel's distillation curve (molecular weight range) even though heavier fuels make more soot.
 
Article
Rapid-scan infrared spectroscopy (10 scans s−1) was used to characterize the gas products in real time that were generated by fast pyrolysis (50–250K s−1) of nine molecular and homopolymeric nitrate esters. On the basis of concentration versus pressure plots of the initial gas products, these compounds cluster into three categories: (1) those for which the side-chain nitroxy products dominant, (2) those for which nitroxy and framework or backbone decomposition products compete, and (3) those for which backbone decomposition products are most prevalant. These classes qualitatively correlate with the carbon, nitrogen, and oxygen balance in the nitrate ester and may be related to the extent of decomposition reactions in the condensed phase. The products liberated by two copolymers of nitroxymethyl- and azidomethyloxetanes closely resemble the sum of the products from the constituent homopolymers.
 
Article
The oxidation of NH3 under fuel-rich conditions and moderate temperatures has been studied in terms of a chemical kinetic model over a wide range of conditions, based on the measurements of Hasegawa and Sato. Their experiments covered the fuels hydrogen (0 to 80 vol%), carbon monoxide (0 to 95 vol%), and methane (0 to 1.5 vol%), stoichiometries ranging from slightly lean to very fuel rich, temperatures from 300 to 1330 K, and NO levels from 0 to 2500 ppm. A detailed reaction mechanism has been established, based on earlier work on ammonia oxidation in flames and on selective noncatalytic reduction of NO by NH3. The kinetic model reproduces the experimental trends qualitatively over the full range of conditions covered, and often the predictions are in quantitative agreement with the observations. Using reaction path analysis and sensitivity studies, the major reaction paths have been identified. The comparatively low temperatures in the present study, as well as the presence of NO, promote the reaction path NH3→NH2→N2 (directly or via NNH), rather than the sequence NH3→NH2→NH→N important in flames. The major conversion of fuel-N species to N2 occurs by reaction of amine radicals with NO, in particular NH2+NO. In the presence of CH4, NO is partly converted to cyanides by reaction with CH3. The mechanism is recommended for modeling the reduction of NO by primary measures in the combustion of biomass, since it has been validated under conditions resembling the conversion of early nitrogenous volatile species in a staged combustion process. It is also appropriate for studies of NO formation in the combustion of gas from gasifying coal.
 
Article
The rates of combustion of four size-graded fractions of semi-anthracite (78, 49, 22, and 6 μm) have been measured in the temperature range 1400–2200°K and at oxygen partial pressures of 0.1 and 0.2 atm. The rates were usually much less than the limiting rate of oxygen diffusion to the particles. For the 78-, 49-, and 22-μm fractions the chemical reaction rate coefficients [g/(cm2 sec atm. O2], calculated from the measured rates corrected for external diffusion resistance, were independent of particle size, and of burn off up to 99% burn off. The particles burned with decreasing size and density. The temperature coefficient of the reaction corresponded to an apparent activation energy of about 20 keal/mole. These factors indicate that the reaction rate is limited by the combined effects of pore diffusion and chemical reaction on the pore walls. The chemical reaction rate coefficients were independent of oxygen partial pressure at 0.1 and 0.2 atm. The absolute values of the rates and other results agree well with those of previous workers for anthracites and semi-anthracites.The values of the chemical rate coefficient for the 6-μm fraction were lower than for the larger fractions, such behaviour indicating that this fraction burned with little restriction by pore diffusion.The results are compared with other workers' data for the combustion rates of anthracites and semi-anthracites.
 
Article
The ignition and combustion of boron particles is investigated at high pressures and temperatures produced by the combustion (unsteady fast deflagration) of nitrogen-diluted hydrogen/oxygen mixtures. A novel device is described to inject particles after sufficient delay for combustion and gas mixture transients to equilibrate, permitting particle ignition at high temperature and nearly constant pressure conditions. Particle injection is calibrated at high pressure with aluminum particles showing agreement with independent determinations. Using this technique, the ignition and combustion times of ∼24 micron crystalline boron particles are measured over a range of pressures (30–150 atm), temperatures (2440, 2630, 2830 K), and excess O2 concentrations (5, 11, 20%). Although boron has been observed elsewhere to exhibit a two-stage ignition process at lower pressures and temperatures, only a single stage is observed here. Boron particle ignition delays are reduced with increased pressure, decreased particle size, and increased temperature. Combustion times drop significantly between 2440 and 2600 K, decreasing by a factor of at least two. Two proposed ignition-enhancing agents, CO2 and HF, show no signs of accelerating ignition and 5% HF actually increases ignition delays. Measured ignition delays are compared to predictions from two ignition models, one incorporating a convective heating model, the other detailed surface chemistry, showing generally good agreement for ignition delays except for an underprediction of the measured decrease in particle ignition delays with increasing pressure. This study demonstrates that boron particle lifetimes at elevated pressures are sufficiently short to make particles smaller than 20 μm suitable for high-speed air-breathing propulsion applications.
 
Article
A model is presented to predict nonadiabatic combustion of syngas under gas turbine conditions. Mixing, combustion, and heat loss are described with four independent scalar variables. These are the mixture fraction, an enthalpy variable and two reaction progress variables for combustion of hydrogen and carbon monoxide. In the combustion model, turbulence is taken into account by weighting with an assumed shape probability density function. The model is used to calculate a 16-kW flame in an air cooled combustion chamber. The fuel consists of 40% CO, 40% H2, and 20% N2 resulting in a calorific value of 11.9 MJ/kg. The calculated CO, CO2, O2, and NO concentrations are compared with suction probe measurements at several locations in the combustion chamber. The nonadiabatic calculations and the measurements show good agreement. Adiabatic calculations show a significant overprediction of NO. It is concluded that the nonadiabatic model is necessary and suitable to calculate NO formation in flames with heat loss.
 
Article
The paper reviews experimental and theoretical work in the last 20 years on shock waves in gases containing either particles or droplets. Although there have been a number of books and reviews published of late on shock-wave techniques, they have been of a general nature. The present paper assumes some familiarity with the generalities and reviews in depth two-phase systems, with particular reference to those in which the solid-liquid phase evaporates and chemically reacts.
 
Article
Knowledge of the chemical structure of young soot and its precursors is very useful in the understanding of the paths leading to soot particle inception. This paper presents analyses of the chemical functional groups, based on FT-IR and 1H NMR spectroscopy of the products obtained in an ethylene inverse diffusion flame. The trends in the data indicate that the soluble fraction of the soot becomes progressively more aromatic and less aliphatic as the height above the burner increases. Results from 1H NMR spectra of the chloroform-soluble soot samples taken at different heights above the burner corroborate the infrared results based on proton chemical shifts (Ha, Hα, Hβ, and Hγ). The results indicate that the aliphatic β and γ hydrogens suffered the most drastic reduction, while the aromatic character increased considerably with height, particularly in the first half of the flame.
 
Article
The oxidation of propanal has been studied below the ignition region. The nature and relative concentration of products depend on both the temperature and the fuel-oxygen ratio due to the alternative reactions for the propionyl radical: The products are described in terms of the subsequent reactions of peroxypropionyl and ethyl radicals, which, in competition, lead to the formation of peroxypropionic acid and ethyl hydroperoxide. By using propanal-2,2-d2, the mechanism of several reactions are elucidated; in particular, the formation of acetaldehyde.
 
Article
The homogeneous, gas-phase formation of polychlorinated dibenzo-p-dioxins (PCDD) and polybrominated dibenzo-p-dioxins (PBDD) has been observed from the high-temperature thermal decomposition of 2,4,6-trichlorophenol (2,4,6-TCP) and 2,4,6-tribromophenol (2,4,6-TBP), respectively. Experiments were conducted in a 1.0-cm-i.d. flow reactor over a temperature range of 300°–800°C with reactant concentrations of ∼ 3.0 × 10−7 mol/L in a reaction atmosphere of dry air. The 1,3,6,8- and 1,3,7,9-tetra chlorinated isomers were the dominant PCDDs observed from the thermal oxidation of 2,4,6-TCP with maximum yields of 0.05% each. The corresponding tetrabrominated isomers were observed from the thermal oxidation of 2,4,6-TBP; however, the maximum yields were approximately 500 times higher. The observed PCDD/PBDD yields and the temperature of their formation can be readily accounted for using a modified form of the original gas-phase formation model of Shaub and Tsang, if the activation energy for the formation of diphenyl ether by displacement of from halophenol by phenoxy is decreased from 26 to 19.5 and 8.8 kcal/mol, for the chlorinated and brominated systems, respectively. This suggests that gas-phase formation reactions make a significant contribution to observed dioxin and furan yields in full-scale incinerator.
 
Article
Rapid-scan infrared spectra with a temporal resolution of 0.2 s of the first observed gas products, several mm from the surface of 34 fast heated (100–130K s−1) energetic materials, were compared over a pressure range of 1–1000 psi Ar. The first observed gas products were strongly dependent on the pressure for many of the compounds and exhibited three distinct zones of products. This behavior is consistent with the model for HMX combustion. A second category of compounds, which was further subdivided three ways, contains compounds that lack the three zone behavior and were much less pressure dependent. Most of the patterns are best explained by the relative importance of heterogeneous gas-phase/condensed-phase reactions. For strongly pressure dependent compounds these heterophase reactions become progressively more important with increasing pressure.
 
Article
We have investigated the use of a tunable laser system that operates in the ultraviolet (UV) to ignite premixed reactive gaseous flows of and at atmospheric pressure. The amount of incident laser energy (ILE) required to ignite the premixed flows as a function of laser excitation wavelength shows two distinct minima. The spectral position of these minima correspond exactly to the location of the resonance, two-photon excitation wavelengths of atomic hydrogen and deuterium at 243.07 and 243.00 nm, respectively. The relative spacing between these minima at the energy level of the 1S–2S two-photon excited transition is 22 cm−1, which is in excellent agreement with the known value for HD deuterium isotope shift (22.4 cm−1). We believe that this is both the first report of a sensitive wavelength dependence on the laser energy required to ignite these mixtures through resonant multiphoton excitation of H and D atoms (produced from H2 and D2 photolysis) and the first report of a deuterium isotope-wavelength-effect in laser ignition. Measurement of the ILE required for ignition versus equivalence ratio (Φ) shows that the most efficient ignition occurred with ∼ 0.55 mJ ILE at Φ = 0.7 in the fuel-lean region. Strong experimental evidence is given that shows that ignition occurs through the efficient resonant formation of a well-controlled, laser-produced microplasma. An estimate of the laser power dependence for the photolysis, excitation, and ionization processes responsible for microplasma initiation was determined in a molecular-beam time-of-flight mass spectrometer. Threshold for microplasma formation (70 torr) was determined in a variable pressure flow cell. These new experimental results indicate that resonance enhancement in the formation of a microplasma is a well-controlled ignition method that appears to alleviate the problems associated with the sharp thresholds encountered in the well-known, nonresonant laser-produced spark (gas breakdown) process.
 
Propellant data using NTO, Cu(NTO) 2 , Fe(NTO) 3 , CC, and AIO as ballistic modifiers in HTPB-AP composite solid propellants
Non-isothermal TG thermograms (under static air atmosphere) of non-aluminized HTPB-AP propellants.
Non-isothermal TG thermograms (under static air atmosphere) of aluminized HTPB-AP propellants.
Non-isothermal TG thermograms (under static air atmosphere) of AP and AP plus additives (2% by weight).
Plot of ln t id vs. 1/T for non-aluminized HTPB-AP propellants. 
Article
The effect of 5-nitro-2,4-dihydro-3H-1,2,4-triazole-3-one (NTO) and two of its transition metal salts, namely Cu(NTO)2 and Fe(NTO)3, during the combustion of composite solid propellants (CSPs) of hydroxyl-terminated polybutadiene (HTPB) and ammonium perchlorate (AP) has been studied. The activities of Cu(NTO)2 and Fe(NTO)3 have been compared with those of CuO and Fe2O3 at their equivalent metal concentrations. The processing parameters and mechanical properties of the propellants were also evaluated using Cu(NTO)2 and Fe(NTO)3 as additives, and a comparison has been made with that of copper chromite (CC) and active iron oxide (AIO) at pilot plant scale. The safety aspects of using these energetic salts as burning-rate modifiers have been studied in terms of the impact-sensitivity of the modified propellants. An attempt has been made to evaluate experimentally the condensed phase activity of these additives during the slow thermolysis of propellants, as well as AP. Rapid thermolysis of the propellants and AP has been studied using measurements of ignition delay.
 
Article
By using a relative rate technique, the rate constant for the gas phase reaction of OH radicals with diacetylene, a reaction considered to be of importance in fuel-rich acetylene oxidation, has been determined at 297 ± 2K and atmospheric presure. On the basis of a rate constant for the reaction of OH radicals with n-butane of 2.58 × 10−12 cm3 molecule−1 s−1, a rate constant for the reaction of OH radicals with diacetylene of (1.62 ± 0.06) × 10−11 cm3 molecule−1 s−1 was obtained, where the error limit does not take into account the uncertainties in the rate constant for the reaction of OH radicals with n-butane.
 
Article
Nitric oxide is observed to inhibit the rate of soot oxidation by oxygen atoms at 298K. Small amounts of added NO reduce the rates of production of CO2 and CO by up to 35%. We show experimentally that NO is not reducing the gas phase O atom concentration. Thermal desorption mass spectrometry shows a small adsorption of NO on the soot; this NO adsorption corresponds to 1.5% of the carbon atoms on the surface of the individual soot spheres. This inhibition is interpreted in terms of a relatively small number of reactive sites on the soot at which soot gasification occurs and which are effectively blocked by NO. When considered together with our previously reported work on oxidation of soot by oxygen atoms at 298K, these results allow a partial mechanism to be formulated for this soot oxidation process.
 
Article
A fully coupled 2D fluid–solid direct numerical simulation (DNS) approach is used to simulate co-flow flame spread over poly(methyl methacrylate) (PMMA) at different angles of inclination. Comparison of simulations and experimental measurements are conducted over a range of flame spread rates. Results show that the heat flux to the preheating region varies considerably in time — contradicting often employed assumptions used in established flame spread theories. Accounting for the time dependent behavior is essential in accurate predictions of flame spread, however, a universal characterization in terms of easily defined parameters is not found. Alternatively, a reaction progress variable based embedded flame model is developed using mixture fraction, total enthalpy and surface temperature. State maps of the gas-phase properties and surface heat flux are constructed and stored in pre-computed lookup tables. The resulting model provides a computationally efficient and a local formulation to determine the flame heat flux to the surface resulting in excellent agreement to DNS and experiments for predictions of flame spread rate and position of the pyrolysis front.
 
Article
Laser-induced fluorescence (LIF) has been developed for visualization of fuel distribution fields in an operating spark-ignition (SI) engine. Since the standard research fuel iso-octane, does not yield a useful LIF signal a fluorescent additive was used. None of the commonly used seeds were found adequate. A seed not commonly used in this context, 3-pentanone, C2H5COC2H5, was chosen due to favorable vaporization characteristics and fluorescent properties. Results from preparatory investigations in the actual engine environment are presented and related laboratory data are discussed. The two-dimensional LIF technique was applied to a spark-ignition engine and the fuel distribution at the ignition time was recorded. The resulting images were processed and converted into fuel/air equivalence ratio using an in situ calibration technique. The processed fuel distribution maps presented a noise level of ∼ 10% and a systematic error not exceeding 0.03 fuel/air equivalence units. An increased combustion variability was observed when changing from a homogeneous to an inhomogeneous fuel/air mixture. Correlations of image data to the combustion development indicated that the increased cyclic variability could be largely explained by variations in the mean fuel concentration around the spark gap. The initial flame development therefore seems to be controlled by the average amount of fuel near the spark gap, whereas the actual distribution of the fuel within this volume is of less importance.
 
Article
A kinetic investigation is presented of the reactions of ground state atomic carbon, , with the molecules methyl chloride, methyl bromide, methyl iodide, methyl fluoride, dichloromethane, chloroform and carbon tetrachloride. Atomic carbon was generated by the repetitive pulsed irradiation (λ > ca. 160 nm) of C3O2 in the presence of excess helium buffer gas and the added reactant gases in a slow flow system, kinetically equivalent to a static system. C(23PJ) was then monitored photoelectrically by time-resolved atomic resonance absorption in the vacuum ultraviolet (λ = 166 nm, 33PJ ← 23PJ) with direct computer interfacing for data capture and analysis. The following absolute second-order rate constants for the reactions of C(23PJ) with the above collision partners are reported (errors 2σ): View Within ArticleThese yield, to the best of our knowledge, the first reported body of absolute rate data for reactions of ground state carbon with these reactants. Although a large body of absolute rate data for atomic silicon in its ground state has now been determined directly in recent years by time-resolved atomic resonance absorption spectroscopy in the ultraviolet itself with a wide range of collision partners, there are no analogous data for these particular reactants with which the data for C(23PJ) may be compared. Estimates of the small energy barriers consistent with the rate data for C(23PJ) + CH3Cl, Br, I coupled with the bond dissociation energies of these polyatomic reactants in particular confirms the values of the bond dissociation energies for CCl, Br, I derived from mechanistic interpretation on C2 Swann band emission from alkali metal-organic halide diffusion flames.
 
Article
Collisional lifetimes of OH (2Σ+, ν′ = 0) radicals excited by tunable laser pulses have been measured in three different low pressure C3H8  O2. Profiles of the quenching rate in the reaction and burnt gas zones are given. The pressure dependence has also been studied, as well as the effects of rotational redistribution. It is concluded that a mean constant quenching rate can be used in a given flame.
 
Article
In order to explore the characteristics of turbulence and turbulent premixed flames in a high-pressure environment, a nozzle-type burner with a turbulence generator was installed in a high-pressure chamber. Turbulence measurements and combustion experiments were conducted with the chamber pressure up to 3.0 MPa. Methane-air mixtures were used for the combustion experiments and confirmed that the turbulent premixed flames were successfully stabilized. Flame observations were made using instantaneous Schlieren photographs and high-speed laser tomography. Turbulence measurements were conducted using a hot-wire anemometer installed in the high-pressure chamber. It was found that the scales of turbulence generated by perforated plates at elevated pressure are smaller than those at atmospheric pressure. From flame observations, the following features of the flames at elevated pressure were found: (1) wrinkled structures of the flames become very fine and complex, and the cusps become sharp as pressure rises;(2) the flamelet breaks at many points of the flames and the scales of broken flamelets become small; (3) small-scale parts of the flame front convex to the unburned mixture frequently occur and move quickly to the unburned side. The effects of ambient pressure on turbulence characteristics and possible mechanisms which produce the wrinkled structure of the fine scales and generate flame front disturbances in the high-pressure environment are discussed.
 
Article
High-heating-rate (≤100°C/s) thermolysis studies at various pressures (1–1000 psi) are described for NH4NO3(AN), [NH3CH2CH2NH3](NO3)2(EDDN), [NH2NH3](NO3)(HN), AN/EDDN, and HAN/HN (HAN = hydroxylammonium nitrate) by using the rapid-scan FTIR/thermal profiling technique. For all of the solid materials, melting is detected in advance of decomposition. HNO3 is the first gas decomposition product detected, and undoubtedly is formed by endothermic proton transfer. Pressures above atmospheric are required to produce exothermic events, probably because the gas decomposition products are able to build around the condensed phase to the critical concentration needed for ignition. HN3 is a significant decomposition product of HN and mixtures containign HN, which may contribute to the high impact sensitivity of the material. For all of the nitrate salts and mixtures investigated, AN is found to be a recombination product during thermal decomposition. This fact may contribute to the similarity of the burning rates of aluminized propellants made from these materials.
 
Article
Chemical features of the mechanism of the apparent transition from decomposition to deflagration frequently observed in compounds containing the trinitromethyl group have been elucidated by using rapid-scan FTIR spectroscopy with temperature profiling as a function of pressure. The compounds studied were [(NO2)3CCH2O]xCH44−x, x = 2–4 (TEFO, TNEOF, TNEOC, respectively). TEFO, TNEOF, and TNEOC were studied in the most detail. They melt 40°C or more below the decomposition point when rapidly heated (70°–200°C/s). The first detected thermolysis step is homolysis of a few CCO2 bonds per molecule liberating NO2(g). The temperature at which this occurs at 15 psi Ar is TEFO (145°C) < TNEOF (180°C) < TNEOC (200°C), which is consistent with CC bond scission also occuring in the condensed phase. The resultant residue exotherms in a narrow temperature range (230°–250°C) irrespective of the static pressure (15–1000 psi Ar) or the orginal sample. This indicates that the ignition-like step involves the residue alone in the condensed phase without participation of the initially released NO2(g).
 
Non-isothermal TG and DTA phenomenological data of propellants
Non-isothermal TG and DTA Phenomenological data on AN and AN Additives
shows that all the salts enhance the rate of burning of HTPB-AN propellants consider- ably. The order of efficiency for the tested transition metal salts of NTO is as follows:
Article
Transition metal [Mn(II), Fe(II), Fe(III), Co.(II), Ni(II), Cu(II) and Zn(II)] salts of 5-nitro-2,4-dihydro-3H-1,2,4-triaole-3-one (NTO) have been incorporated as ballistic modifiers in composite solid probellants (CSPs) of hydroxyl terminated polybutadiene (HTPB) – ammonium nitrate (AN). The steady burning rate of the propellants was considerably increased by the additives, Zn(NTO)2 being the most efficient one. The condensed phase thermolysis of such propellants was studied using non-isothermal TGA and DTA and the effect of additives is evident from these studies also. Also, the effect of these additives on the non-isothermal heating of AN from room temperature to ∼300°C was studied using TGA and DTA. The additives lowers the decomposition temperature of AN considerably. DTA studies show that the additives alter the phase transition temperatures of AN. Heats of combustion of the modified propellants were found to be higher than for the control one.
 
Article
A three-dimensional reacting flow modeling approach is presented for diesel engine studies that can be used for predictions of trends in soot emissions for a wide range of operating conditions. The modeling framework employs skeletal chemistry for n-heptane for ignition and combustion, and links acetylene chemistry to the soot nucleation process. The soot model is based on integration and modification of existing submodels for soot nucleation, agglomeration, oxidation, and surface growth. With the optimized modeling parameters, the simulations agree well with results of high-pressure shock tube studies of rich n-heptane mixtures, reproducing the trends for soot mass over a range of temperature and pressure conditions (T=1550–2050 K, P=20, 40, and 80 MPa). Engine simulation results for soot mass are in excellent agreement with diesel engine smoke number measurements over a range of injection timings (−11° ATDC–2.4° ATDC) and two exhaust gas recirculation levels (16 and 26–27%). The model results demonstrate that correct description of the soot formation, as well as the soot transport processes, is critical for achieving reliable predictive capabilities in engine simulations.
 
Article
Chemiluminescence from BaBr(A2Π1/2,3/2, B2Σ+-X2Σ+) following the exothermic Br-abstraction reactions of Ba[6s5d(3DJ)], 1.151 eV above the 6s2(1S0) ground state, with CF3Br and CF2Br2 is investigated in the time domain. The Ba(3DJ) is produced following the initial pulsed dye-laser excitation of atomic barium via the allowed transition at λ = 553.5 nm {Ba[6s6p(1P1)] ← Ba[6s2(1S0)]} in excess helium buffer gas at 900 K. This optically metastable state is generated subsequently in the long-time domain by a combination of radiative and collisional processes. It is then monitored by the spectroscopic atomic emission marker transition at λ = 791.1 nm {Ba[6s6p(3P1 → Ba[6s2(1S0)]} where Ba(3P1) results from collisional activation of Ba(3DJ). Molecular chemiluminescence, which is weak, is monitored via the long wavelength transitions: BaBr(A2Π1/2 → X2Σ+, λ = 1002 nm, Δv = 0), BaBr(A2Π3/2 → X2Σ+, λ = 943 nm, Δv = 0) and BaBr(B2Σ+ → X2Σ+, λ = 883 nm, Δv = 0). The equality in the first-order decay coefficients for the atomic and molecular profiles indicates that the three molecular states BaBr(A2Π1/2, A3/2, B2Σ+) are generated directly on collision of Ba(3DJ) with CF3Br and CF2Br2. A kinetic analysis employing both the integrated intensities of the atomic emission and these weak long wavelength molecular emissions, coupled with optical sensitivity calibrations, yields molecular electronic branching ratios in the BaBr(A2Π1/2,3/2, B2Σ+) states from the two reactants. These are found to be as follows: CF3Br: BaBr(A2Π1/2) 11.8 ± 4.3%, BaBr(A2Π3/2) 3.8 ± 1.4%, BaBr(B2Σ+) 1.5 ± 0.8%; CF2Br2: BaBr(A2Π1/2) 11.1 ± 4.7%, BaBr(A2Π3/2) 6.0 ± 2.6%, BaBr(B2Σ+) 1.4 ± 0.5%. These results, including the elucidation of the fundamental collisional processes leading to the chemiluminescence, may be contrasted with the observation of such emission, where feasible, in low pressure flames. Normally, this has involved recording the overall emission intensity from specific molecular states of BaBr on reaction of Ba with Br2. The logarithmic variation of these branching ratios with the energies of the states is essentially Boltzmann in character in both cases as found hitherto for reactions of Ba(3DJ) with other halogenated targets with which the present results are compared. The branching ratios observed here yield effective temperatures of ca. 940 and 1080 K for the collision of Ba(3DJ) with CF3Br and CF2Br2, close to the ambient temperature of the measurements. This is consistent with the absence of selection rules for the yields of these excited molecular states on collision and reflecting the role of late barriers in the potential surfaces involved. For the reaction of Ba(3DJ) + CF2Br2, the production of the BaBr(C2Π1/2) is close to thermoneutral; however, the C-X chemiluminescence was not observed, presumably on account of the low branching ratios into this higher-lying molecular state.
 
Top-cited authors
H.J. Curran
  • National University of Ireland, Galway
C.K. Westbrook
  • Lawrence Livermore National Laboratory
Thierry Poinsot
  • Institut de Mécanique des Fluides de Toulouse
Derek Bradley
  • University of Leeds
Chih-Jen Sung
  • University of Connecticut