Finite-rate Kinetics Research Articles

Hydrogen production from rich combustion in porous media

This paper examines rich combustion of methanol, methane, octane and automotive-grade petrol inside inert porous media in an effort to examine the suitability of the concept for hydrogen production. Species concentrations were measured and operating limits were tested of steady rich flames stabilized inside a two-layer alumina foam burner and a two-layer alumina bead burner. Using a conversion efficiency based on lower heating values, up to 56% of the methanol was converted to syngas (H 2, CO) inside the alumina foam burner and 66% inside the alumina bead burner. Using the same efficiency definition, 45% percent of the methane and 36% of the octane and petrol was converted to syngas with a significant portion of the energy remaining trapped in CH 4, C 2H 2 and C 2H 4. For methanol, the highest equivalence ratio observed for stable combustion was 6.1 inside the foam burner and 9.3 inside the bead burner which are higher than the conventional upper flammability limit (UFL) of 4.1. Methane's UFL was increased to 1.9 and, at a minimum, the conventional upper flammability limits of iso-octane and petrol were attained. A wide operating envelope was observed, which allowed for large turndown ratios up to 20:1. The composition of the products of the methanol flames examined here were close to equilibrium for relatively low equivalence ratios, while those of hydrocarbon flames differed significantly from equilibrium for all φ suggesting that finite rate kinetics are important. The high conversion efficiencies, quick startup times, compact size, and the absence of a catalyst make the present burner suitable for consideration as part of a reformer in a fuel cell powered automobile.

A three-dimensional model of steady flame spread over a thin solid in low-speed concurrent flows

Athree-dimensional model of a steady concurrent flame spread over a thin solid in a low-speed flowtunnel in microgravity has been formulated and numerically solved. The gas-phase combustion model includes the full Navier-Stokes equations for the conservation of mass, momentum, energy and species. The solid is assumed to be a thermally thin, non-charring cellulosic sheet and the solid model consists of continuity and energy equations whose solution provides boundary conditions for the gas phase. The gas-phase reaction is represented by a one-step, second-order, finite-rate Arrhenius kinetics and the solid pyrolysis is approximated by a one-step, zeroth-order decomposition obeying an Arrhenius law. Gas-phase radiation is neglected but solid radiative loss is included in the model. Selected results are presented showing detailed three-dimensional flame structures and flame spread characteristics. In a parametric study, varying the tunnel (solid) widths and the flow velocity, two important three-dimensional effects have been investigated, namely wall heat loss and oxygen side diffusion. The lateral heat loss shortens the flame and retards flame spread. On the other hand, oxygen side diffusion enhances the combustion reaction at the base region and pushes the flame base closer to the solid surface. This closer flame base increases the solid burnout rate and enhances the steady flame spread rate. In higher speed flows, three-dimensional effects are dominated by heat loss to the side-walls in the downstream portion of the flame and the flame spread rate increases with fuel width. In low-speed flows, the flames are short and close to the quenching limit. Oxygen side diffusion then becomes a dominant mechanism in the narrow three-dimensional flames. The flame spreads faster as the solid width is made narrower in this regime. Additional parametric studies include the effect of tunnelwall thermal condition and the effect of adding solid fuel sample holders.

Homogeneous kinetics and equilibrium predictions of coking propensity in the anode channels of direct oxidation solid-oxide fuel cells using dry natural gas

Abstract Direct electrochemical oxidation (DECO) solid-oxide fuel cells (SOFCs) offer the potential to generate electrical power from hydrocarbon fuels without the need for upstream fuel processing, such as reforming. However, with pure hydrocarbon fuel entering the flow channels at temperatures typically above 700 °C, fuel pyrolysis can cause molecular-weight growth and the formation of deleterious carbonaceous deposits. This paper, which develops a plug-flow model for fuel (natural gas surrogate) within the anode channels, considers the elementary gas-phase chemical kinetics of fuel pyrolysis and oxidation. It also considers the limiting case of local chemical equilibrium. Formation of cyclic hydrocarbon species is used to indicate deposit propensity. Results show that the likelihood of deposit formation depends strongly on cell temperature, current density, and residence time. Generally speaking, equilibrium favors deposit formation early in the channel whereas, owing to limited residence time, the homogeneous finite-rate kinetics predicts relatively low levels of deposit precursors. In the downstream portions, because of electrochemical oxygen flux though the electrode–electrolyte membrane, chemical equilibrium shifts strongly away from deposit formation to volatile carbon-oxygen species. However, the homogeneous finite-rate kinetics predictions show a continuing increase in coking propensity.

Simulation of swirling combustion and NO formation using a USM turbulence-chemistry model ☆

A unified second-order moment (USM) turbulence-chemistry model is used to simulate methane-air swirling combustion and NO formation for different swirl numbers. The simulation results are compared with those using the EBU–Arrhenius (E–A) combustion model, the simplified PDF model of NO formation in turbulent flows and the corresponding experimental results. The comparison indicates that the USM model is obviously better than the E–A model and the simplified PDF model. The E–A model cannot reasonably simulate the finite-rate kinetics, while the simplified PDF model, using a product of two single-variable PDF's instead of a joint PDF, remarkably under-predicts the NO reaction rate. The USM model gives the best agreement with the experimental results. Predictions show that as the swirl number increases the total NO formation at first decreases and then increases, which is in agreement with the experimental results.

Modeling Wood Degradation by the Unreacted-Core-Shrinking Approximation

The unreacted-core-shrinking approximation is used to model one-dimensional transport phenomena and finite-rate kinetics of thick wood pyrolysis. A quasi-steady-state formulation of the equations is suggested because the effects of process transients along the charred region are shown to be negligible. Temperature dynamics of the unreacted core are described with either a concentrated-parameter model or a distributed-parameter model combined with an integral solution method. Though both treatments produce qualitatively similar predictions, only in the latter case is good quantitative agreement obtained with measurements of volatile release characteristics (external radiative heat fluxes in the range 40−80 kW/m2). Depending on material properties and reaction kinetics, convective heat transfer may play an important role in the particle heating rate and conversion time. The model shows significant sensitivity to the activation energy of the pyrolysis reaction, the effective thermal conductivities of wood and char, and the wood thermal capacity.

Advanced Performance Modeling of Experimental Laser Lightcraft

A computational plasma aerodynamics model is developed to study the performance of a laser-propelled lightcraft. The computational methodology is based on a time-accurate, multi-dimensional, finite volume, chemically reacting, unstructured grid pressure-based formulation. The underlying physics are modeled using a building-block approach. The physics modeled include nonequilibrium thermodynamics, nonequilibrium air-plasma finite rate kinetics, specular ray tracing, laser beam energy absorption and refraction by plasma, nonequilibrium plasma radiation, and plasma resonance. A series of transient computations are performed at several laser pulse energy levels and the simulated physics are discussed and compared with those of tests and literatures. The computed impulses and coupling coefficients for the lightcraft compared reasonably well with those of tests conducted on a pendulum apparatus.

Kinetic and Surface Mechanisms to Growth of Hexagonal Boron Nitride

ABSTRACTA mechanism is presented for the gas-phase chemistry and surface reactions describing the growth of boron nitride films. The gas phase mechanism includes 33 species and 216 elementary reactions. Rate parameters for 117 elementary reactions were obtained from published experimental/theoretical data and those for the other 99 were determined using transition state theory. The mechanism examined here is an extension and update of a previous mechanism that contained 26 species and 67 elementary reactions. Standard reaction flux/pathway and gradient sensitivity analysis techniques are used to identify important reaction pathways. The calculations were handled through the use of the ChemKin software package. The model was applied to the growth of hexagonal boron nitride in an arcjet plasma source operating on mixture of BF3, H2 and N2. From the growth rate and experimental conditions for such reactors, this work demonstrates that species with mole fractions in the range of 1 × 10-10 − 2 × 10-4 (easily generated by gas-phase conditions) can account for the measured growth rate. A comparison of the predicted mole fractions of the gas-phase species present for the experimental residence times with those required to account for the measured film growth rates allows us to identify possible growth precursors. At the experimental settings, it is found that the residence time of the reacting species does not allow the flow to reach the chemical equilibrium, and that many radicals other than the source gases can account for the measured BN growth rate. The gas-phase is shown to be removed from thermodynamic equilibrium. For comparison, the equilibrium mole fractions were also calculated using an available equilibrium chemistry solver, STANJAN. Growth rates of 10-9 to 10-6 kg m-1 s-1 were measured for a wide range of H2, BF3 and NF3 flux. Both the finite-rate kinetics and equilibrium calculations confirm the importance of added hydrogen to facilitate boron nitride condensation from the gas phase. A simple surface mechanism with 7 steps is proposed with rate constants chosen to best fit the experimental growth kinetics. The results of the model, with surface rate coefficients determined from analogous gas-phase reactions, tuned slightly to agree with the experimental growth rates for BFx, x = 1, 2 or 3, as the rate-limiting growth precursor. It is also shown that the hydrogen atom has a great influence in the growth rate and that fluorine atom etches the hBN films.

Flame spread along horizontal solid fuel cylinders

The spread of a flame along a thermally thick horizontal solid fuel cylinder in a quiescent atmosphere is analyzed modeling the gas-phase kinetics by means of a single-step irreversible Arrhenius reaction and assuming that vaporization of the solid occurs at a constant temperature. The natural convection flow of the warm gas surrounding the flame is mostly normal to the axis of the cylinder, but a weak flow is also induced along the axis by the axial gradient of the pressure variation that gravity generates in the warm gas. This latter flow aids flame propagation in the lower part of the cylinder. The effects of finite rate kinetics and of radiative losses from the surface of the solid are studied, computing the spread rate as a function of the Damkohler number, the oxygen concentration in the atmosphere and the strength of the radiative losses for a set of material properties chosen to mimic flame spread on PMMA. The effect of a small inclination of the cylinder is also discussed, showing that a continuous transition from upward to down ward flame spread occurs when the tilt angle is small of the order of the inverse of the power 1/8 of the Grashof number based on the radius of the cylinder.

Flame spread over solid fuels in opposite natural convection

The downward spread of a flame along the surface of a thermally thick solid fuel inclined at an angle to the vertical is investigated. The flow and the heat transfer in the flame front region that controls the spread rate are described numerically using a single-step Arrhenius model for the gas-phase reaction. The effect of the finite rate kinetics is discussed in terms of a Damkohler number defined as the square of the ratio of the speed of the planar stoichiometric flame of the fuel vapor to the characteristic speed induced by buoyancy in the flame front region. The computed spread rate is lower when the burning surface faces downward than when it faces upward due to the influence of the pressure gradient parallel to the surface.

A simplified two-reaction zone model of magnesium combustion in carbon dioxide

A simplified model of magnesium particle combustion in pure carbon dioxide has been developed andutilized for prediction of particle burning times. This model includes two reaction zones—an outer zone (shell) where magnesium reacts at a transport-limited rate with carbon dioxide to form condensed magnesium oxide plus CO and an inner zone, at the particle surface, where CO reacts with liquid magnesium to form solid carbon and solid magnesium oxide, which remain with the particle. The main simplifications utilized are the neglect of reactions in the gaseous regions inside and outside the outer shell and use of average transport properties across these regions. First, two limiting cases, the first with transport-limited reaction rate at the surface and the second with zero reaction rate at the surface, were examined. Next, finite-rate kinetics of the surface reaction, with the reaction rate constant being the product of the collision rate of CO molecules with the surface and a reaction probability expressed in Arrhenius form, was examined, with the activation energy of the Mg+CO reaction being varied parametrically. Model development and resultant burning rate and burning time predictions are presented. For the two limiting cases, predicted burning times were proportional to the square of the initial particle radius, while for the finite kinetics cases, the dependency was predicted to be slightly less than second order. Comparison of predictions with the limited experimental data available indicates reasonable agreement between theory and data.

Nonlinear acoustic-pressure responses of oxygen droplet flames burning in gaseous hydrogen

A nonlinear acoustic instability of subcritical liquid-oxygen droplet flames burning in gaseous hydrogen environment are investigated numerically. Emphases are focused on the effects of finite-rate kinetics by employing a detailed hydrogen-oxygen chemistry and of the phase change of liquid oxygen. Results show that if nonlinear harmonic pressure oscillations are imposed, larger flame responses occur during the period that the pressure passes its temporal minimum, at which point flames are closer to extinction condition. Consequently, the flame response function, normalized during one cycle of pressure oscillation, increases nonlinearly with the amplitude of pressure perturbation. This nonlinear response behavior can be explained as a possible mechanism to produce the threshold phenomena for acoustic instability, often observed during rocket-engine tests.

Performance prediction for oblique detonation wave engines (odwe)

The aim of this paper is to present a mathematical model to predict the overall performance of an oblique detonation wave engine (ODWE). The model of the propulsive system accounts for the four main components of a vehicle propelled by an ODWE, the air intake, a mixing device, the ODW-based combustor and a supersonic nozzle. The combustion modeling of the ODW accounts explicitly for the finite-rate kinetics of the combustible mixture as opposed to the simpler models adopted in the previous studies which are based either on modified Rankine–Hugoniot relations with a prescribed heat addition or on the assumption of CJ equilibrium conditions behind the detonation wave. All the relevant performance parameters of the engine can be estimated together with a first guess of the geometry of the vehicle. Therefore, the model can be easily incorporated into a more global optimization procedure involving the vehicle weights and the overall mission profile.

Ignition of char-forming polymers at a critical mass flux

A simple model of ignition is discussed in the context of char-forming polymers. The model in question assumes that ignition occurs at a critical value of volatile mass flux from the degrading material. The char-forming mechanism chosen for the study is the simplest possible formulation preserving finite-rate kinetics—char and volatile products are formed together in a single step. This simple pyrolysis model has the advantage that the ultimate char yield is known a priori, and is in fact a parameter of the model. Results are presented and discussed for a number of cases, depending on char yield and char thermal properties. Complex ignition behaviour is exhibited by the model and practical consequences of the model predictions are discussed.

A Plug Flow Model for Chemical Reactions and Aerosol Nucleation and Growth in an Alkali-Containing Flue Gas

The paper presents a numerical model for the simulation of gas to particle conversion and the chemical changes during cooling of a flue gas from the combustion of fuels rich in volatile alkali species. For the homogeneous nucleation of alkali species the model uses the classical theory modified by the Tolman coefficient for the size dependent surface tension. A special adaptation of the theory to the nucleation of seeds of chloride salt from an equilibrium mixture of monomer and dimer chloride vapor molecules is also made. The growth of particles occurs by condensation and agglomeration. Different multicomponent growth models are treated. The local gas phase composition is determined from a gas phase chemical equilibrium calculation combined with finite reaction rate kinetics for slower reactions. The model is useful in the analysis of boiler operation with respect to the formation of particles, HCl, SO2, and deposits.

Candle Flames in Non-Buoyant Atmospheres

This paper addresses the behavior of a candle flame in a long-duration, quiescent microgravity environment both on the space Shuttle and the Mir Orbiting Station. On the Shuttle, the flames became dim blue after an initial transient where there was significant yellow (presumably soot) in the flame. The flame lifetimes were typically less than 60 seconds. The safety-mandated candlebox that contained the candle flame inhibited oxygen transport to the flame and thus limited the flame lifetime. The flames on the Mir were similar, except that the yellow luminosity persisted longer into the flame lifetime because of a higher initial oxygen concentration. The Mir flames bumed for as long as 45 minutes. The difference in the flame lifetime between the Shuttle and Mir flames was primarily the redesigned candlebox that did not inhibit oxygen transport to the flame. In both environments, the flame intensity and the height-to-width ratio gradually decreased as the ambient oxygen content in the sealed chamber slowly decreased. Both sets of experiments showed spontaneous, axisymmetric flame oscillations just prior to extinction. The paper also presents a numerical model of a candle flame. The formulation is two-dimensional and time-dependent in the gas phase with constant specific heats, thermal conductivity and Lewis number (although different species can have different Lewis numbers), one-step finite-rate kinetics, and gas-phase radiative losses from CO2 and H2O. The treatment of the liquid/wick phase assumes that the fuel evaporates from a constant diameter sphere connected to an inert cone. The model predicts a steady flame with a shape and size quantitatively similar to the Shuttle and Mir flames. The computation predicts that the flame size will increase slightly with increasing ambient oxygen mole fraction. The model also predicts pre-extinction flame oscillations if the rate of decrease in ambient oxygen is small enough, such as that which would occur for a flame burning in a sealed ambient.

Downward flame spread over poly(methyl)methacrylate

The thermal regime of downward flame spread over poly(methyl)methacrylate (PMMA) in an oxygen/nitrogen environment in normal gravity has been revisited experimentally, computationally, and analytically. Although spread rate data is plentiful in the literature, different fuels, usually PMMA and cellulose, have been used in thermally thick and thin limits. Moreover, most of these experiments, conducted at atmospheric oxygen levels, belong to the kinetic regime where finite-rate kinetics plays a dominant role. As a result, there remain some unanswered questions about the thermal regime, which constitutes the backbone of downward flame spread. We present spread rate data for the first time in the thermal regime where the fuel thickness is changed from the thin to the thick limit. A comprehensive computational model, verified through comparison with all relevant analytical solutions, is used to establish the transition criterion between the chemical and thermal regimes, and between the thin and thick fuel regimes. An earlier extension of the de Ris formula, capable of predicting the spread rate accurately for forced opposed flow, has been extended to downward configuration by proposing an empirically determined equivalent buoyant convection velocity. The predictions from the resulting closed form spread rate formulas, which retain the functional form of the de Ris formula, are shown to compare well with computational and experimental results, including all previously published data in the thermal regime. An alternative to traditional Damkohler number correlation, in which an equivalent oxygen level is used in conjunction with thermal-regime spread rate formulas, is proposed to capture the effect of finite-rate chemistry. A simple formula for the transition thickness between the thin and thick fuel regime is proposed as τ crit = 2 τ V f,Thin V f,Thick where the spread rates can be obtained from experiments or the proposed formulas.

Two-dimensional large eddy simulation of soot formation in the near-field of a strongly radiating nonpremixed acetylene–air turbulent jet flame

A numerical study of soot formation in the near-field of a strongly radiating, nonpremixed, acetylene–air planar jet flame is conducted using Large Eddy Simulation in two dimensions to examine coupled turbulence, soot chemistry, and radiation effects. The two-dimensional, Favre-filtered, compressible Navier-Stokes, total sensible energy and mixture fraction equations are closed using the Smagorinsky subgrid-scale (SGS) turbulence model. Major species of gas-phase combustion are obtained using a laminar flamelet model by employing experimentally obtained laminar flame state relationships for the major species mass fractions as a function of gas-phase mixture fraction. A combination of a presumed Beta filtered density function and a scale-similarity model are used to account for SGS mixture fraction and scalar dissipation fluctuations on the filtered composition and heat release rate. A soot transport and finite-rate kinetics model accounting for soot nucleation, surface growth, agglomeration, and oxidation is used. Radiation is modeled by integrating the filtered radiative transfer equation using the discrete ordinates method. Both instantaneous and time-averaged results are presented in order to highlight physical and numerical modeling issues and to examine turbulence, soot chemistry, and radiation interactions. Qualitative comparisons are made to previous numerical results and experimental data.

The transformation of chromium in a laminar premixed hydrogen-air flame

The transformation of chromium was studied numerically and experimentally in a simple laminar premixed hydrogen-air flame. Chromium was added to the flame as a vapor of chromium hexacarbonyl that provided a source of zero-oxidation-state pure elemental chromium through its rapid pyrolysis. The flame was operated over a range of equivalence ratios. The metal aerosol and vapors that were created in the flame were sampled at different heights with a dilution sampling probe. Aerosol was collected on a filter and vapors in a liquid nitrogen trap. Analysis of the samples showed an initial increase in the amount of hexavalent chromium with increasing distance from the burner, followed by a drop to about 5% far from the burner. The modeling of detailed chromium kinetics showed a similar behavior. The observations highlighted the importance of finite-rate kinetics in controlling the ultimate state of the metal oxide. Aerosol samples were analyzed with an electrical mobility analyzer and condensation nuclei counter. Mean diameters were of the order of 40 nm. The numerical model of the chromium aerosol showed qualitatively good agreement between measured and predicted aerosol size distributions.

PDF calculations of major and minor species in a turbulent piloted jet flame

The joint probability density function (JPDF) approach is used to make calculations of a piloted jet diffusion flame of CH4-air (flame L of Masri et al. [1]). The calculations include detailed chemistry, involving 16 species and 41 reactions, to account for the finite-rate kinetics. The technique of in situ adaptive tabulation (ISAT [2,3]) enables the computationally efficient inclusion of detailed chemistry in the JPDF calculations. The results of these JPDF-ISAT calculations show good agreement with the experimental data, including those for the mean and rms of minor species such as CO and H2. These results are discussed in the context with the earlier numerical studies of this flame ([4–6]). The present calculations of minor species are significantly better than those from earlier studies.

REAL GAS EFFECTS SIMULATION IN A MODEL GH2/O2 ROCKET THRUST CHAMBER

A CFD-model for a multicomponent reacting mixture of real gases at supercritical conditions is integrated into the KIVA-II code to analyze high pressure GH2/O2 ignition processes in a model single injector rocket thrust chamber. The model incorporates mixture thermodynamic anomalies accounted for in terms of the Redlich-Kwong equation for O2, while another species are treated as ideal, perfect gases, the k − ε turbulent submodel for compressible media, and equilibrium and finite-rate combustion chemistry. The detailed mechanism of H2/O2 oxidation is used to simulate ignition initiated by "hot spots". The "dense" gas formulation, in contrast to the ideal gas model, predicts mixing enhancement within the cylindrical part of the thrust chamber leading to a decrease in the oxygen jet core length, more stringent requirements for mixture flammability and reduced combustion efficiency. Using equilibrium chemistry, the model does not predict ignition and pressure rise time histories comparable to the results of calculation with finite-rate kinetics.