Uintah and Related C-SAFE Publications

2007


H.R. Zhang, E.G. Eddings, A.F. Sarofim. “Olefin Chemistry in a Premixed N-Heptane Flame,” In Energy and Fuels, Vol. 21, No. 2, pp. 677--685. 2007.
DOI: 10.1021/ef060195h

ABSTRACT

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 C3Hx and C4Hx species are well-known precursors for the formation of the first aromatic ring.



H.R. Zhang, E.G. Eddings, A.F. Sarofim, C.K. Westbrook. “Mechanism Reduction and Generation Using Analysis of Major Fuel Consumption Pathways for n-Heptane in Premixed and Diffusion Flames,” In Energy and Fuels, Vol. 21, No. 4, pp. 1967--1976. 2007.
DOI: 10.1021/ef060092z

ABSTRACT

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.



H.R. Zhang, L.K. Huynh, N. Kungwan, Z. Yang, S. Zhang. “Combustion modeling and kinetic rate calculations for a stoichiometric cyclohexane flame. 1. Major reaction pathways,” In Journal of Physical Chemistry, A, Vol. 111, No. 19, pp. 4102--4115. 2007.
DOI: 10.1021/jp068237q
PubMed ID: 17388269

ABSTRACT

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.



H.R. Zhang, Z. Yang, E.G. Eddings, A.F. Sarofim. “Pollutant Formation in Premixed and Diffusion Flames of Paraffinic Fuels Using the Reduced Utah Surrogate Mechanisms,” In American Chemical Society, Division of Fuel Chemistry, Vol. 52, No. 1, pp. 144--147. 2007.

ABSTRACT

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.



H.R. Zhang, E.G. Eddings, A.F. Sarofim. “Criteria for Selection of Components for Surrogate of Natural Gas and Transportation Fuels,” In Proceedings of the Combustion Institute, Vol. 31, No. 1, pp. 401--409. January, 2007.
DOI: 10.1016/j.proci.2006.08.001

ABSTRACT

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.


2006


J. Bigler, J. Guilkey, C. Gribble, C.D. Hansen, S.G. Parker. “A Case Study: Visualizing Material Point Method Data,” In Proceedings of Euro Vis 2006, pp. 299--306, 377. May, 2006.



W. Ciro, E.G. Eddings, A.F. Sarofim. “Experimental and Numerical Investigation of Transient Soot Buildup on a Cylindrical Container Immersed in a Jet Fuel Pool Fire,” In Combustion Science and Technology, Vol. 178, No. 12, pp. 2199--2218. 2006.
DOI: 10.1080/00102200600626108

ABSTRACT

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.



J.E. Guilkey, J.B. Hoying, J.A. Weiss. “Computational Modeling of Multicellular Constructs with the Material Point Method,” In Journal of Biomechanics, Vol. 39, No. 11, pp. 2074--2086. 2006.



Y. Guo, J.A. Nairn. “Three-Dimensional Dynamic Fracture Analysis using the Material Point Method,” In Computer Modeling in Engineering and Sciences, Vol. 1, No. 1, pp. 11--25. 2006.

ABSTRACT

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.



I. Ionescu, J.E. Guilkey, M. Berzins, R.M. Kirby, J.A. Weiss. “Simulation of Soft Tissue Failure Using the Material Point Method,” In Journal of Biomechanical Engineering, Vol. 128, No. 6, pp. 917--924. 2006.



G. Krishnamoorthy, R. Rawat, P.J. Smith. “Parallelization of the P-1 Radiation Model,” In Numerical Heat Transfer, Part B: Fundamentals, Vol. 49, No. 1, pp. 1--17. 2006.
DOI: 10.1080/10407790500344068

ABSTRACT

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.



S. Parker, K. Zhang, C. Damevski, C.R. Johnson. “Integrating Component-Based Scientific Computing Software,” In Parallel Processing for Scientific Computing, Edited by M.A. Heroux and P. Raghavan and H.D. Simon, SIAM, pp. 271--288. January, 2006.



R.P. Pawlowski, J.N. Shadid, J.P. Simonis, H.F. Walker. “Globalization Techniques for Newton--Krylov Methods and Applications to the Fully-coupled Solution of the Navier-Stokes Equations,” In SIAM Review, Vol. 48, No. 4, pp. 700--721. 2006.
DOI: 10.1137/S0036144504443511

ABSTRACT

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.



A. Santamaria, F. Mondragon, A Molina, N.D. Marsh, E.G. Eddings, A.F. Sarofim. “FT-IR and 1H-NMR Characterization of the Products of an Ethylene Inverse Diffusion Flame,” In Combustion and Flame, Vol. 146, No. 1-2, pp. 52--62. July, 2006.
DOI: 10.1016/j.combustflame.2006.04.008

ABSTRACT

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



L. Zheng, D.L. Thompson. “On the Accuracy of Force Fields for Predicting the Physical Properties of Dimethylnitramine,” In Journal of Physical Chemistry, B, Vol. 110, No. 10, pp. 16082--16088. July, 2006.

ABSTRACT

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 (Tmp) 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 Tmp = 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.


2005


C. Ayyagari, D. Bedrov, G.D. Smith. “Equilibrium Sampling of Self-Associating Polymer Solutions: A Parallel Selective Tempering Approach,” In Journal of Chemical Physics, Vol. 123, No. 12, 2005.
DOI: 10.1063/1.1979494

ABSTRACT

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.



B. Banerjee. “MPM Validation: A Myriad of Taylor Impact Tests,” C-SAFE Internal Report, No. C-SAFE-CD-IR-05-001, Department of Mechanical Engineering, University of Utah, 2005.

ABSTRACT

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.



S.G. Bardenhagen, A.D. Brydon, J.E. Guilkey. “Insight into the Physics of Foam Densification via Numerical Solution,” In Journal of Mechanics and Physics of Solids, Vol. 53, No. 3, pp. 597--617. March, 2005.
DOI: 10.1016/j.jmps.2004.09.003

ABSTRACT

Foamed materials are increasingly finding application in engineering systems on account of their unique properties. The basic mechanics which gives rise to these properties is well established, they are the result of collapsing the foam microstructure. Despite a basic understanding, the relationship between the details of foam microstructure and foam bulk response is generally unknown. With continued advances in computational power, many researchers have turned to numerical simulation to gain insight into the relationship between foam microstructure and bulk properties. However, numerical simulation of foam microscale deformation is a very challenging computational task and, to date, simulations over the full range of bulk deformations in which these materials operate have not been reported. Here a particle technique is demonstrated to be well-suited for this computational challenge, permitting simulation of the compression of foam microstructures to full densification. Computations on idealized foam microstructures are in agreement with engineering guidelines and various experimental results. Dependencies on degree of microstructure regularity and material properties are demonstrated. A surprising amount of porosity is found in fully-densified foams. The presence of residual porosity can strongly influence dynamic material response and hence needs to be accounted for in bulk (average) constitutive models of these materials.



O. Borodin, D. Bedrov, G.D. Smith, J.A. Nairn, S. Bardenhagen. “Multiscale Modeling of Viscoelastic Properties of Polymer Nanocomposites,” In Journal of Polymer Science, Vol. 43, No. 8, pp. 1005--1013. April, 2005.
DOI: 10.1002/polb.20390

ABSTRACT

A methodology for simple multiscale modeling of mechanical properties of polymer nanocomposites has been developed. This methodology consists of three steps: (1) obtaining from molecular dynamics simulations the viscoelastic properties of the bulklike polymer and approximating the position-dependent shear modulus of the interfacial polymer on the basis of the polymer-bead mean-square displacements as a function of the distance from the nanoparticle surface, (2) using bulk- and interfacial-polymer properties obtained from molecular dynamics simulations and performing stress–relaxation simulations of the nanocomposites with material-point-method simulations to extract the nanocomposite viscoelastic properties, and (3) performing direct validation of the average composite viscoelastic properties obtained from material-point-method simulations with those obtained from the molecular dynamics simulations of the nanocomposites.



J.A. Cooke, M. Bellucci, M.D. Smooke, A. Gomez, A. Violi, T. Faravelli, E. Ranzi. “Computational and Experimental Study of JP-8, a Surrogate, and its Components in Counterflow Diffusion Flames,” In Proceedings of the Combustion Institute, Vol. 30, No. 1, pp. 439--446. January, 2005.
DOI: 10.1016/j.proci.2004.08.046

ABSTRACT

Non-sooting counterflow diffusion flames have been studied both computationally and experimentally, using either JP-8, or a six-component JP-8 surrogate mixture, or its individual components. The computational study employs a counterflow diffusion flame model, the solution of which is coupled with arc length continuation to examine a wide variety of inlet conditions and to calculate extinction limits. The surrogate model includes a semi-detailed kinetic mechanism composed of 221 gaseous species participating in 5032 reactions. Experimentally, counterflow diffusion flames are established, in which multicomponent fuel vaporization is achieved through the use of an ultrasonic nebulizer that introduces small fuel droplets into a heated nitrogen stream, fostering complete vaporization without fractional distillation. Temperature profiles and extinction limits are measured in all flames and compared with predictions using the semi-detailed mechanism. These measurements show good agreement with predictions in single-component n-dodecane, methylcyclohexane, and iso-octane flames. Good agreement also exists between predicted and measured variables in flames of the surrogate, and the agreement is even better between the experimental JP-8 flames and the surrogate predictions.