Conventional analytical methods such as ¹H NMR, vapor pressure osmometry (VPO) and elemental analysis were used to characterize the soot precursor material represented by the chloroform extractable fractions of the young soot gathered at different heights of an ethylene inverse diffusion flame in terms of average structural parameters. The results indicate that the soot soluble fraction obtained at a 6 mm height has a relatively large molecular weight and has long aliphatic chains which later disappear with an increase in height above the burner base, especially in the region where the temperature is high (1200 K). This behavior is also accompanied by an increase in the aromaticity (f_a) of the samples.

The standard velocity projection scheme for the Material Point Method (MPM) and a typical form of the GIMP Method are examined. It is demonstrated that the fidelity of information transfer from a particle representation to the computational grid is strongly dependent on particle density and location. In addition, use of non-uniform grids and even non-uniform particle sizes are shown to introduce error. An enhancement to the projection operation is developed which makes use of already available velocity gradient information. This enhancement facilitates exact projection of linear functions and reduces the dependence of projection accuracy on particle location and density for non-linear functions. The efficacy of this formulation for reducing error is demonstrated in solid mechanics simulations in one and two dimensions.

The approach of mechanism generation is the accepted one of assigning generic rates to reactions in the same class. The procedure has been successfully applied to higher paraffins that include detailed sub-models of n-hexane, cyclohexane, n-heptane, n-decane, n-dodecane, and n-hexadecane and semi-detailed sub-models of iso-octane and methyl cyclohexane, in addition to reactions of aromatic formation and oxidation. Comparison between predictions and experimental data were found to be satisfactory for n-heptane, iso-octane, n-decane and gasoline premixed flames. The mechanism was also able to reproduce the measured concentrations for a n-hexadecane experiment in a jet stirred reactor. The numerical accuracy in predicting the flame structures of soot precursors, including acetylene and benzene, is one of the major foci of this study. The predicted maximum concentrations of acetylene and benzene are within 20% for most flames in this study.

Three different n-heptane mechanisms were used to simulate a fuel-rich normal heptane premixed flame in order to identify major reaction pathways for olefin formation and consumption and areas of uncertainties of these reactions. Olefins are formed mainly via β-scission and hydrogen abstraction, and smaller olefins are sometimes formed by combination of allylic radicals and H/CH₃ radicals. Olefins are consumed by direct decomposition, radical-addition, and hydrogen-abstraction reactions. Isomerization between alkyl radicals plays an important role in olefin formation and in determining olefin species distribution. Peroxy radicals contribute to the olefin formation in the low-temperature region, but further studies are needed to resolve many uncertainties. Simulation results using the Pitsch, LLNL, and Utah heptane mechanisms were compared to experimental concentration profiles of selected species, and the uncertainties in the olefin chemistry thus identified are discussed. The discrepancies in the computed concentrations of most olefin species are usually due to the combined effects of uncertainties in the kinetics of β-scission and isomerization reactions. Resolving these uncertainties in n-heptane combustion chemistry is critical for building practical mechanisms for the larger paraffins that are major components of liquid aviation and diesel transportation fuels. In addition, olefin chemistry is critical to any combustion mechanisms that focus on the soot formation, because products of olefin decomposition such as C₃H_x and C₄H_x species are well-known precursors for the formation of the first aromatic ring.

Reaction pathway analyses were conducted for three mechanisms (designated as the Pitsch, Utah, and Lawrence Livermore National Lab) for a normal heptane premixed flame (Φ = 1.9) and a normal heptane opposed diffusion flame, in order to identify the relative importance of the major fuel consumption pathways in the two flame classes. In premixed flames, hydrogen abstraction is found to be the major fuel consumption route although it is surpassed by thermal decomposition when the flame temperature exceeds 1400 - 1500 K. At the higher temperatures, however, little fuel remains in a premixed flame so that thermal decomposition provides a minor pathway for overall fuel decomposition. The principal abstractor is the hydrogen radical in all three mechanisms with the hydroxyl radical having a secondary role. In opposed diffusion flames, thermal decomposition competes with hydrogen abstraction in providing the major pathway for fuel consumption. Thermal decomposition becomes important when a large fraction of the fuel reaches the high-temperature zone in a flame. By understanding the relative importance of competing fuel consumption pathways, mechanisms can be tailored to each specific application by eliminating or lumping insignificant reactions. The results obtained in this study for n-heptane may be used to guide the reduction of existing mechanisms for a particular application or the generation of mechanisms for the combustion of larger paraffins that are major components of liquid aviation and transportation fuels.

The Utah Surrogate Mechanism was extended in order to model a stoichiometric premixed cyclohexane flame (P = 30 Torr). Generic rates were assigned to reaction classes of hydrogen abstraction, beta scission, and isomerization, and the resulting mechanism was found to be adequate in describing the combustion chemistry of cyclohexane. Satisfactory results were obtained in comparison with the experimental data of oxygen, major products and important intermediates, which include major soot precursors of C2-C5 unsaturated species. Measured concentrations of immediate products of fuel decomposition were also successfully reproduced. For example, the maximum concentrations of benzene and 1,3-butadiene, two major fuel decomposition products via competing pathways, were predicted within 10% of the measured values. Ring-opening reactions compete with those of cascading dehydrogenation for the decomposition of the conjugate cyclohexyl radical. The major ring-opening pathways produce 1-buten-4-yl radical, molecular ethylene, and 1,3-butadiene. The butadiene species is formed via beta scission after a 1-4 internal hydrogen migration of 1-hexen-6-yl radical. Cascading dehydrogenation also makes an important contribution to the fuel decomposition and provides the exclusive formation pathway of benzene. Benzene formation routes via combination of C2-C4 hydrocarbon fragments were found to be insignificant under current flame conditions, inferred by the later concentration peak of fulvene, in comparison with benzene, because the analogous species series for benzene formation via dehydrogenation was found to be precursors with regard to parent species of fulvene.

Normal heptane, isooctane and cyclohexane have been the most interested surrogate components for liquid transportation and aviation fuels, due to their roles as indicative fuels for octane number and the representative compounds for normal, iso and cyclo-paraffins. Methodologies of mechanism generation for these representative fuel fractions have been discussed in detail in literature. The basics of fuel consumption in flames have been discussed by Vovelle1, Ranzi2, Zhang3 and coworkers, among others. Ranzi et al.2 presented a lumping technique that was also discussed in detail in an earlier study3 and used for generation of reaction mechanisms that can be used to model flames of liquid fuels. The lumping approach is an effective reduction technique for models of large aliphatic fuels. Reaction pathway analysis presents another reduction technique that was used to reduce a complete kinetic set to smaller models. Doute et al.4 reduced a n-decane model by removing less important reaction routes systematically and still obtained satisfactory agreement between the experimental data and predicted results. Bollig et al.5 proposed a reduced n-heptane mechanism and modeled a diffusion flame with the emphasis on pollutant-related intermediates. The mechanism was further reduced using another technique with the assumption of partial equilibrium for intermediates. There are many important applications that need reduced kinetic mechanism, especially in those that require expensive computations but are less demanding in kinetic details. For example, only a few dozen reactions can be comfortably acquired in aerodynamic applications. In this study, the detailed Utah Surrogate Mechanisms of about 1200 reactions and 210 species3 will be reduced by a combined technique. The resultant mechanism will be used to simulate premixed and counter-flow diffusion flames of normal heptane, iso-octane and cyclo-hexane fuels. And the pollutant formation of soot precursors, e.g. benzene and acetylene, will be investigated for the three common surrogate components.

The present paper addressed the production of soot precursors, acetylene, benzene and higher aromatics, by the paraffinic (n-, iso-, and cyclo-) and aromatic components in fuels. To this end, a normal heptane mechanism compiled from sub-models in the literature was extended to large normal-, iso-, and cyclo-paraffins by assigning generic rates to reactions involving paraffins, olefins, and alkyl radicals in the same reaction class. Lumping was used to develop other semi-detailed sub-models. The resulting mechanism for components of complex fuels (named the Utah Surrogate Mechanism) includes detailed sub-models of n-butane, n-hexane, n-heptane, n-decane, n-dodecane, n-tetradecane and n-hexadecane, and semi-detailed sub-models of i-butane, i-pentane, n-pentane, 2,4-dimethyl pentane, i-octane, 2,2,3,3-tetramethyl butane, cyclohexane, methyl cyclohexane, tetralin, 2-methyl 1-butene, 3-methyl 2-pentene and aromatics. Generic rates of reaction classes were found adequate to generate reaction mechanisms of large paraffinic components. The predicted maximum concentrations of the fuel, oxidizer, and inert species, major products and important combustion intermediates, which include critical radicals and soot precursors, were in good agreement with the experimental data of three premixed flames of composite fuels under various conditions. The relative importance in benzene formation of each component in the kerosene surrogate was found to follow the trend aromatics > cyclo-paraffins > iso-paraffins > normal-paraffins. In contrast, acetylene formation is not that sensitive to the fuel chemical structure. Therefore, in formulation of surrogate fuels, attention should be focused on selecting components that will yield benzene concentrations comparable to those produced by the fuel, with the assurance that the acetylene concentration will also be well approximated.

Soot buildup and its effects on heat transfer have been investigated as part of an effort to understand the thermal response of containers of high-energy materials immersed in fires. Soot deposition rates were measured for cooled and uncooled cylindrical containers immersed in a jet fuel pool fire. The soot buildup was measured at different time intervals with a wet film gage with an uncertainty of 20%. These rates were compared with those calculated by solving the boundary layer equations along the cylinder surface including the thermophoretic transport of soot particles. Thermophoresis was the dominant soot transport mechanism controlling the deposition of soot on the container wall and gave deposition rates in good agreement with the measured values. The soot buildup was found to have an important insulating effect on the heat transfer to the container. A soot deposit thickness of 1.2 mm resulted in as much as a 35% reduction in heat flux.

This paper describes algorithms for three-dimensional dynamic stress and fracture analysis using the material point method (MPM). By allowing dual velocity fields at background grid nodes, the method provides exact numerical implementation of explicit cracks in a predominantly meshless method. Crack contact schemes were included for automatically preventing crack surfaces from interpenetration. Crack-tip parameters, dynamic J-integral vector and mode I, II, and III stress intensity factors, were calculated from the dynamic stress solution. Comparisons to finite difference method (FDM), finite element method (FEM), and boundary element method (BEM), as well as to static theories showed that MPM can efficiently and accurately solve three-dimensional dynamic fracture problems. Since the crack description is independent of the object description, MPM could be useful for simulation of three-dimensional dynamic crack propagation in arbitrary directions.

The P-1 radiation model is spatially decomposed to solve the radiative transport equation on parallel computers. Mathematical libraries developed by third parties are employed to solve the linear systems that result during the solution procedure. Multigrid preconditioning accelerated the convergence of iterative methods. The parallel performance did not depend strongly on the radiative properties of the medium or the boundary conditions. Predictions from coupling the weighted-sum-of-gray-gases model with the P-1 approximation are compared against benchmarks for model problems. The P-1 approximation resulted in only a moderate loss in accuracy while being significantly faster than the discrete ordinates method.

A Newton-Krylov method is an implementation of Newton's method in which a Krylov subspace method is used to solve approximately the linear subproblems that determine Newton steps. To enhance robustness when good initial approximate solutions are not available, these methods are usually globalized, i.e., augmented with auxiliary procedures (globalizations) that improve the likelihood of convergence from a starting point that is not near a solution. In recent years, globalized Newton-Krylov methods have been used increasingly for the fully coupled solution of large-scale problems. In this paper, we review several representative globalizations, discuss their properties, and report on a numerical study aimed at evaluating their relative merits on large-scale two- and three-dimensional problems involving the steady-state Navier–Stokes equations.

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.

Keywords: Soot, FT-IR, 1H NMR, Inverse diffusion flame

The accuracy of three force fields for predicting the physical properties of dimethylnitramine (DMNA) has been investigated by using molecular dynamics simulations. The Sorescu, Rice, and Thompson (SRT) (J. Phys. Chem. B 1997, 101, 798) rigid-molecule, flexible generalized AMBER (J. Comput. Chem. 2004, 25, 1157), and Smith et al. flexible force fields (J. Phys. Chem. B 1999, 103, 705) were tested. The density, lattice parameters, isotherm, and melting point of DMNA are calculated using classical molecular dynamics. Except for the melting point, the predictions of the three force fields are in reasonable agreement with experimental values. The calculated thermodynamic melting points (T_mp) for the SRT, AMBER, and Smith et al. force fields are 380, 360, and 260 K, respectively. The experimental value is 331 K. Modifications of the torsional barriers in the AMBER force field resulted in T_mp = 346 K, in good agreement with the experimental value of 331 K. The calculated lattice parameters and bulk modulus are also improved with the modifications of the AMBER potential. The results indicate that, although not sufficiently accurate without modifications, the general force fields such as AMBER provide the basis for developing force fields that correctly predict the physical properties of nitramines.

We present a novel simulation algorithm based on tempering a fraction of relaxation-limiting interactions to accelerate the process of obtaining uncorrelated equilibrium configurations of self-associating polymer solutions. This approach consists of tempering (turning off) the attractive interactions for a fraction of self-associating groups determined by a biasing field h. A number of independent configurations (replicas) with overlapping Hamiltonian distributions in the expanded (NVTh) ensemble with constant NVT but different biasing fields, forming a chain of Hamiltonians, were simulated in parallel with occasional attempts to exchange the replicas associated with adjacent fields. Each field had an associated distribution of tempered interactions, average fraction of tempered interactions, and structuraldecorrelation time. Tempering parameters (number of replicas, fields, and exchange frequencies) were chosen to obtain the highest efficiency in sampling equilibrium configurations of a self-association polymer solution based on short serial simulation runs and a statistical model. Depending on the strength of the relaxation-limiting interactions, system size, and thermodynamic conditions, the algorithm can be orders of magnitude more efficient than conventional canonical simulation and is superior to conventional temperature parallel tempering.

Taylor impacts tests were originally devised to determine the dynamic yield strength of materials at moderate strain rates. More recently, such tests have been used extensively to validate numerical codes for the simulation of plastic deformation. In this work, we use the material point method to simulate a number of Taylor impact tests. The goal is to par- tially validate some plasticity models used by the UINTAH multi-physics code. In addition, we would like to determine the plasticity model that is most appropriate for fire-structure interaction problems that are being simulated using UINTAH. We compare the Johnson- Cook, Steinberg-Cochran-Guinan-Lund, Zerilli-Armstrong, Mechanical Threshold Stress, and the Preston-Tonks-Wallace plasticity models. We evaluate these models for OFHC cop- per, 6061-T6 aluminum alloy, and 4340 steel alloy at various temperatures and strain rates. A number of validation metrics are presented for quantitative comparisons of numerical simulations and experimental data. It is observed that the accuracy of all the models drops when the initial conditions involve high temperatures and high impact velocities.