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Gokcen, Tahir

Publications and source records attributed to Gokcen, Tahir.

35 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS2003

Iron catalyst chemistry in modeling a high-pressure carbon monoxide nanotube reactor

The high-pressure carbon monoxide (HiPco) technique for producing single-wall carbon nanotubes (SWNTs) is analyzed with the use of a chemical reaction model coupled with flow properties calculated along streamlines, calculated by the FLUENT code for pure carbon monoxide. Cold iron pentacarbonyl, diluted in CO at about 30 atmospheres, is injected into a conical mixing zone, where hot CO is also introduced via three jets at 30 degrees with respect to the axis. Hot CO decomposes the Fe(CO)5 to release atomic Fe. Then iron nucleates and forms clusters that catalyze the formation of SWNTs by a disproportionation reaction (Boudouard) of CO on Fe-containing clusters. Alternative nucleation rates are estimated from the theory of hard sphere collision dynamics with an activation energy barrier. The rate coefficient for carbon nanotube growth is estimated from activation energies in the literature. The calculated growth was found be about an order of magnitude greater than measured, regardless of the nucleation rate. A study of cluster formation in an incubation zone prior to injection into the reactor shows that direct dimer formation from Fe atoms is not as important as formation via an exchange reaction of Fe with CO in FeCO.

Evaluation Studies↗
NASA NTRS2002

Modeling of the HiPco process for carbon nanotube production. II. Reactor-scale analysis

The high-pressure carbon monoxide (HiPco) process, developed at Rice University, has been reported to produce single-walled carbon nanotubes from gas-phase reactions of iron carbonyl in carbon monoxide at high pressures (10-100 atm). Computational modeling is used here to develop an understanding of the HiPco process. A detailed kinetic model of the HiPco process that includes of the precursor, decomposition metal cluster formation and growth, and carbon nanotube growth was developed in the previous article (Part I). Decomposition of precursor molecules is necessary to initiate metal cluster formation. The metal clusters serve as catalysts for carbon nanotube growth. The diameter of metal clusters and number of atoms in these clusters are some of the essential information for predicting carbon nanotube formation and growth, which is then modeled by the Boudouard reaction with metal catalysts. Based on the detailed model simulations, a reduced kinetic model was also developed in Part I for use in reactor-scale flowfield calculations. Here this reduced kinetic model is integrated with a two-dimensional axisymmetric reactor flow model to predict reactor performance. Carbon nanotube growth is examined with respect to several process variables (peripheral jet temperature, reactor pressure, and Fe(CO)5 concentration) with the use of the axisymmetric model, and the computed results are compared with existing experimental data. The model yields most of the qualitative trends observed in the experiments and helps to understanding the fundamental processes in HiPco carbon nanotube production.

Validation Studies↗
NASA NTRS2002

Modeling of the HiPco process for carbon nanotube production. I. Chemical kinetics

A chemical kinetic model is developed to help understand and optimize the production of single-walled carbon nanotubes via the high-pressure carbon monoxide (HiPco) process, which employs iron pentacarbonyl as the catalyst precursor and carbon monoxide as the carbon feedstock. The model separates the HiPco process into three steps, precursor decomposition, catalyst growth and evaporation, and carbon nanotube production resulting from the catalyst-enhanced disproportionation of carbon monoxide, known as the Boudouard reaction: 2 CO(g)-->C(s) + CO2(g). The resulting detailed model contains 971 species and 1948 chemical reactions. A second model with a reduced reaction set containing 14 species and 22 chemical reactions is developed on the basis of the detailed model and reproduces the chemistry of the major species. Results showing the parametric dependence of temperature, total pressure, and initial precursor partial pressures are presented, with comparison between the two models. The reduced model is more amenable to coupled reacting flow-field simulations, presented in the following article.

Evaluation Studies↗
NASA NTRS2001

Iron Catalyst Chemistry in High Pressure Carbon Monoxide Nanotube Reactor

The high-pressure carbon monoxide (HiPco) technique for producing single wall carbon nanotubes (SWNT) is analyzed using a chemical reaction model coupled with properties calculated along streamlines. Streamline properties for mixing jets are calculated by the FLUENT code using the k-e turbulent model for pure carbon monixide. The HiPco process introduces cold iron pentacarbonyl diluted in CO, or alternatively nitrogen, at high pressure, ca. 30 atmospheres into a conical mixing zone. Hot CO is also introduced via three jets at angles with respect to the axis of the reactor. Hot CO decomposes the Fe(CO)5 to release atomic Fe. Cluster reaction rates are from Krestinin, et aI., based on shock tube measurements. Another model is from classical cluster theory given by Girshick's team. The calculations are performed on streamlines that assume that a cold mixture of Fe(CO)5 in CO is introduced along the reactor axis. Then iron forms clusters that catalyze the formation of SWNTs from the Boudouard reaction on Fe-containing clusters by reaction with CO. To simulate the chemical process along streamlines that were calculated by the fluid dynamics code FLUENT, a time history of temperature and dilution are determined along streamlines. Alternative catalyst injection schemes are also evaluated.

Scott, Carl D.↗
NASA NTRS1998

Computational Analysis of Shock Layer Emission Measurements in an Arc-Jet Facility

This paper reports computational analysis of radiation emission experiments in a high enthalpy arc-jet wind tunnel at NASA Ames Research Center. Recently, as part of ongoing arc-jet characterization work, spectroscopic radiation emission experiments have been conducted at the 20 MW NASA Ames arc-jet facility. The emission measurements were obtained from the arc-jet freestream and from a shock layer formed in front of flatfaced models. Analysis of these data is expected to provide valuable information about the thermodynamic state of the gas in the arc-jet freestream and in the shock layer as well as thermochemical equilibration processes behind the shock in arc-jet flows. Knowledge of the thermodynamic state of the gas in arc-jet test flows and especially within the shock layer is essential to interpret the heat transfer measurements such as in surface catalysis experiments. The present work is a continuation of previous work and focuses on analysis of the emission data obtained at relatively low-pressure conditions for which the arc-jet shock layer is expected to be in thermal and chemical nonequilibrium. Building blocks of the present computational analysis are: (1) simulation of nonequilibrium expanding flow in the converging-diverging conical nozzle and supersonic jet; (2) simulation of nonequilibrium shock layer formed in front of the flat-faced cylinder model; and (3) prediction of line-of-sight radiation from the computed flowfield. For computations of the nonequilibrium flow in the conical nozzle and shock layer, multi-temperature nonequilibrium codes with the axisymmetric formulation are used. For computations of line-of-sight radiation. a nonequilibrium radiation code (NEQAIR) is used to predict emission spectra from the computed flowfield. Computed line-of-sight averaged flow properties such as vibrational and rotational temperatures, species number densities within the shock layer will be compared with those deduced from the experimental spectra. Detailed comparisons of computational and experimental spectra will also be presented.

Gokcen, Tahir↗
NASA NTRS1995

Investigation Flow Uniformity in a Supersonic Duct with High Enthalpy Flows

Flow uniformity in a high enthalpy facility is investigated. The scramjet research facility is composed of a rectangular combustor duct connected to a 100 MW electric arc air heater. The Mach 3.3 flow is accelerated through a two-dimensional contoured nozzle. Instream measurements were made with water-cooled Pitot probes and stagnation point heat flux gages at stream enthalpy levels ranging from 4 to 7 Mj/kg. Flow surveys were made on the flow centerline and off centerline in order to measure the three dimensional uniformity of the flow in the rectangular duct. Measurements indicated that although the flow in the aspect ratio 6:1 duct was relatively uniform on the centerline, three dimensional viscous effects were apparent near the corners. Flow through the nozzle and constant area duct was modeled computationally using a two dimensional, Navier-Stokes, reacting gas code. The computations predict that the flow in the test section is in vibrational equilibrium. The computed and measured Pitot pressure and heat flux profiles are in reasonable agreement with the experimental data.

Balboni, John↗
NASA NTRS1994

Computation of axisymmetric and ionized hypersonic flows using particle and continuum methods

Comparisons between particle and continuum simulations of hypersonic near-continuum flows are presented. The particle approach employs the direct simulation Monte Carlo (DSMC) method, and the continuum approach solves the appropriate equations of fluid flow. Both simulations have thermochemistry models for air implemented including ionization. A new axisymmetric DSMC code that is efficiently vectorized is developed for this study. In this DSMC code, particular attention is paid to matching the relaxation rates employed in the continuum approach. This investigation represents a continuum of a previous study that considered thermochemical relaxation in one-dimensional shock waves of nitrogen. Comparison of the particle and continuum methods is first made for an axisymmetric blunt-body flow of air at 7 km/s. Very good agreement is obtained for the two solutions. The two techniques also compare well for a one-dimensional shock wave in air at 10 km/s. In both applications, the results are found to be sensitive to various aspects of the chemistry model employed.

Boyd, Iain D.↗
NASA NTRS1994

Computation of nonequilibrium viscous flows in arc-jet wind tunnel nozzles

The axisymmetric Navier-Stokes equations are numerically solved for air flows in converging-diverging conical nozzles. A nonequilibrium formulation with a two-temperature thermochemical model has been employed for vibrationally excited, partially dissociated and ionized air. Evaluations of the formulation is made through comparisons of the computed flow quantities with the experimental data obtained in expanding arc-jet nozzle flows. Two sets of experimental data obtained in conical nozzles with geometric exit-to-throat area ratios of 625 and 8100 cover reservoir pressures ranging from 5 to 95 atm and enthalpies from 5 to 10 MJ/kg. The computed results are in good agreement with local mass flux, Pitot pressure, and static wall pressure measurements at the nozzle exit. For low pressure reservoir conditions, comparisons of computations with the experimental data indicate that the wall catalycity is very important in predicting species mole fraction at the nozzle exit since gas phase chemistry is frozen. The current computations overpredict the atomic oxygen concentrations which in turn results in underprediction of the vibrational temperature.

Gokcen, Tahir↗
NASA NTRS1993

Computation of nonequilibrium radiating shock layers

A computational technique of coupling radiative transfer to fluid motion is developed for axisymmetric blunt body shock layer flows in a thermochemical nonequilibrium environment. The coupled formulation of radiation and flowfield leads to a governing set of integro-differential equations. This equation set is solved using a modified Gauss-Seidel line relaxation techniques which incorporates the inversion of full block matrix associated with radiative transfer using a block iteration method. The thermodynamic state of the gas is described by three temperatures: translational, rotational, and vibrational-electronic. Radiation phenomenon is assumed to be governed by the vibrational-electronic temperature. The radiative properties are described by a spectrally detailed model. The computations are presented for two cases, including the Fire II flight experiment. It is shown that the method converges and the calculated spectra qualitatively agree with the experimental data for the two test cases. The calculated total radiative flux underestimates the measured values owing to the low vibrational-electronic temperature predicted in the flowfield calculation.

Gokcen, Tahir↗
NASA NTRS1993

Computation of axisymmetric and ionized flows using particle and continuum methods

Comparisons between particle and continuum simulations of hypersonic near-continuum flows are presented. The particle approach employs the direct simulation Monte Carlo method (DSMC), and the continuum approach solves the Euler equations. Both simulations have thermochemistry models for air implemented including ionization. A new axisymmetric DSMC code which is efficiently vectorized is developed for this study. In this DSMC code, particular attention is paid to matching the relaxation rates employed in the continuum approach. This investigation represents a continuation of a previous study which considered thermochemical relaxation in one-dimensional shock waves of nitrogen. Comparison of the particle and continuum methods is first made for an axisymmetric blunt-body flow of air at 7 km/s. Very good agreement is obtained for the two solutions. The two techniques also compare well for a one-dimensional shock wave in air at 10 km/s. In both applications, the results are found to be sensitive to various aspects of the chemistry models employed.

Boyd, Iain D.↗
NASA NTRS1992

Evaluation of thermochemical models for particle and continuum simulations of hypersonic flow

Computations are presented for one-dimensional, strong shock waves that are typical of those that form in front of a reentering spacecraft. The fluid mechanics and thermochemistry are modeled using two different approaches. The first employs traditional continuum techniques in solving the Navier-Stokes equations. The second approach employs a particle simulation technique (the direct simulation Monte Carlo method, DSMC). The thermochemical models employed in these two techniques are quite different. The present investigation presents an evaluation of thermochemical models for nitrogen under hypersonic flow conditions. Four separate cases are considered that are dominated in turn by vibrational relaxation, weak dissociation, strong dissociation and weak ionization. In near-continuum, hypersonic flow, the nonequilibrium thermochemical models employed in continuum and particle simulations produce nearly identical solutions. Further, the two approaches are evaluated successfully against available experimental data for weakly and strongly dissociating flows.

Boyd, Iain D.↗
NASA NTRS1991

The coupling of radiative transfer to quasi 1-D flows with thermochemical nonequilibrium

Quasi-one-dimensional nonequilibrium nozzle flows with coupled radiative transfer are considered. The strongly coupled formulation of radiation and flowfield leads to a governing set of integro-differential equations. A fully implicit numerical method using the full matrix inversion or block iteration methods is presented to solve these equations. The nonequilibrium gas model consists of two chemical species, molecular and atomic nitrogen. The thermodynamic state of the gas is described by two temperatures, translational-rotational and vibrational, and the thermal radiation is assumed to be governed by the vibrational temperature. In radiative transfer, gases are assumed to be absorbing and emitting, and a detailed spectral dependency of the absorption coefficient is prescribed for a non-gray gas. The numerical solutions of strongly radiating nonequilibrium flows are presented for both gray and non-gray gases.

Gokcen, Tahir↗
NASA NTRS1991

Computation of thermochemical nonequilibrium flows around a simple and a double ellipse

The nonequilibrium viscous reactive flows over a simple and a double ellipse at a 30 degree angle of attack were computed. The geometry and the free stream conditions are given by INRIA/GAMNI/SMAI workshop test cases 6.2-2 and 6.2-4. The governing Navier-Stokes equations coupled with thermochemical nonequilibrium processes are solved numerically using a fully coupled, implicit, finite volume technique with a dynamically adaptive grid. The nonequilibrium gas model and the numerical method used in the calculations are briefly described.

Gokcen, Tahir↗
NASA NTRS1990

Computation of thermochemical nonequilibrium flows around a simple and a double ellipse

The nonequilibrium viscous reactive flows over a simple and a double ellipse at a 30 degree angle of attack were computed. The geometry and the free stream conditions are given by INRIA/GAMNI/SMAI workshop test cases 6.2-2 and 6.2-4. The governing Navier-Stokes equations coupled with thermochemical nonequilibrium processes are solved numerically using a fully coupled, implicit, finite volume technique with a dynamically adaptive grid. The nonequilibrium gas model and the numerical method used in the calculations are briefly described.

Gokcen, Tahir↗
NASA NTRS1989

Nonequilibrium effects for hypersonic transitional flows using continuum approach

A new thermochemical nonequilibrium formulation for hypersonic transitional flows of air is presented. Air is assumed to have five chemical species (N2, O2, NO, N, O) and three temperatures corresponding to the translational, rotational, and vibrational modes of energy. In the present study, the no-slip boundary conditions are replaced by slip boundary conditions to extend the range of the Navier-Stokes equations to high-speed low-density flows.

Maccormack, Robert W.↗
NASA NTRS1987

Computational fluid dynamics near the continuum limit

Two dimensional Navier-Stokes equations for a perfect gas are solved for hypersonic flow over a flat plate at Reynolds numbers ranging from continuum to the free molecule flow. In the transition flow regime, new slip boundary conditions are introduced, which reduce to the well-known slip conditions of Maxwell at small Knudsen numbers and yield the correct shear stress and heat transfer in the limiting case of free molecule flow. Comparison of the computed results with the existing experimental data and Monte Carlo calculations indicates that the continuum Navier-Stokes equations give surprisingly realistic results throughout the transition flow regime, when the new slip boundary conditions are used.

Maccormack, Robert W.↗