J.-Y. Chen

A General Procedure for Constructing Reduced Reaction Mechanisms with Given Independent Relations

Abstract Simplification of a detailed reaction mechanism is possible if there exist independent, approximate relations which permit calculation of concentrations of some species in terms of other known concentrations. Given these independent relations, a general procedure has been developed for combining a set of elementary chemical reactions into a reduced mechanism.

Pdf Modeling of Turbulent Nonpremixed Methane Jet Flames

Abstract Anexpanded model of turbulent nonpremixed combustion is herein presented. In the model, the scalar mixing and reactions are described by a probability density function (pdf) submodel capable or handling five scalars, while the turbulent velocity field is described by a second-order moment closure. Two plausible chemical reaction models are considered: a five-scalar, four-step, reduced reaction mechanism and a four-scalar constrainted equilibrium model. Detailed comparisons of model predictions with laser Raman experimental data provide a valuable evaluation of the models ability in predicting nonequilibrium chemistry in turbulent nonpremixed flames. Overall, the model fails to predict greater departure from chemical equilibrium as mixing rates are increased. Interestingly, this failure is not due to the chemical model, both of which perform satisfactorily. Instead, the failure to predict greater departure from chemical equilibrium is a subtle artifact of the current Monte Carlo simulation of tur...

Chemical closure model for fractal flamelets

Abstract A chemical closure model for premixed turbulent flames is proposed and tested by analysis and numerical computation for flames with vanishingly small density change. The model is based on the assumption that reaction zones can be modeled as thin sheets—flamelets—and that the geometry of these sheets can be represented by fractal surfaces. The model expression for mean fuel consumption rate is 〈ω〉=C R ρ〈δY f u L 〉 f (lƒ/η) D−2l F −1 〈C〉(1−〈c〉) with ƒ given by ƒ=[1−(1−A t −1 4 R 1 −3 4 ) exp (−A t 1 4 R 1 −1 4 u′/〈ul〉 f )] and l η =A t 1 4 R 1 3 4 where D is the fractal dimension of the flamelet surface and is the new parameter introduced by the fractal geometry assumption. This model is tested in simplified analyses of normal and oblique flames with good results. The oblique flame analysis provides new insight into the definition of the turbulent burning velocity. Numerical computations are performed with a conditioned second-order closure scheme, and the chemical closure model performance is found to be good. Computed results with a gradient transport model for species diffusion show that turbulent fluxes are significantly under predicted in comparison with the second-order closure results.

A model for soot formation in a laminar diffusion flame

A simple model has been developed for the prediction of soot volume fractions in a laminar diffusion flame. Measurements and computations of a counterflow flame have been used to evaluate the correlation between soot surface growth rates and the mixture fraction or fuel atom mass fraction. An average particle number density was used to permit the determination of the aerosol surface area. Equations for the momentum, mixture fraction, and soot volume fraction were solved numerically for an axisymmetric laminar diffusion flame. Good agreement was obtained with the measurements for two different experimental conditions.

PDF modeling and analysis of thermal NO formation in turbulent nonpremixed hydrogen-air jet flames☆

Abstract A numerical model is developed based on the probability density function (pdf) approach to study the thermal NO formation in turbulent nonpremixed hydrogen jet flames. In particular, the effects of nonequilibrium chemistry and radiation heat loss on the thermal NO formation are examined. The numerical results indicate that when the NO x emission index is scaled with a properly defined flame residence time, it shows the negative one-half power dependence on the Damkohler number as observed by Chen and Driscoll [Twenty-Third Symposium (International) on Combustion]. An analysis is performed suggesting that the observed power dependence is due to the nonequilibrium chemistry effect, and it depends on the self-similar laws for jets. Comparisons of numerical results obtained with and without radiation heat loss show a significant impact on the thermal NO formation for jet flames with radiant franctions greater than 5%.

Stochastic Modeling of Partially Stirred Reactors

Abstract Modeling of Partially-Stirred-Reactors (PaSR) by stochastic simulation is carried out for investigating the influence of unmixedness on thermo-chemical properties. First, the joint scalar probability density function (pdf) of the PaSR is derived using a control volume analysis to provide theory for development of stochastic simulation procedures. Second, through numerical exploration and analysis, an improved stochastic algorithm is developed to eliminate the dependence of solution on time step. The limitations on age distribution from the stochastic simulation are identified. For nonpremixed combustion with either the modified Curls mixing model or the Linear-Mean-Square-Estimation (LMSE) mixing model, analytic expressions are derived for the unmixedness in terms of residence time and mixing frequency. Numerical simulations have been performed revealing that the unmixed nature has profound influence on ignition delay and NO formation in PaSR with hydrogen combustion. The performance of recently...

Chemical models for pdf modeling of hydrogenair nonpremixed turbulent flames

Abstract Development of simplified chemical models is necessary for probability density function (pdf) modeling of turbulent flames with multiple species. Two promising approaches, the reduced mechanism method and the constrained equilibrium method, have been extensively studied to develop six plausible chemical models for nonpremixed combustion of hydrogen with air. Examination of the thermodynamic properties predicted by these chemical models in the allowable domains reveals that in regions where temperatures are above 1400 K, all six models yield similar results. In regions near the pure mixing limits, the partial equilibrium assumption for bimolecular reactions breaks down and yields unrealistic results. Reasonable agreement between the measured temperatures and those from Monte Carlo simulations with all six chemical models has been obtained for a nonpremixed turbulent jet flame at low mixing rates. As mixing rate increases, flame blowout has been predicted only by a chemical model that treats the hydrogen atom H by its kinetics rate equation, which is based on the reduced mechanism method. The predicted jet velocity at flame blowout is much higher than that observed in experiments. One of the implications from the present results is the need for improved submodels for mixing processes.

Second-order conditional modeling of turbulent nonpremixed flames with a compositep PDF

Abstract A second-order conditional model for turbulent nonpremixed flames with a composite probability density function (pdf) is presented. The composite pdf represents three parts of the scalar field: a fully turbulent part, a surrounding nonturbulent part, and a superlayer part. The conditional equations are derived using Favre averaging (density weighting) with the interfacial interaction terms appearing in explicitly separate groups. These equations are closed by a second-order scheme and models for the interfacial terms. Nitric oxide formation is computed by a model including the effects of intermittency and nonequilibrium oxygen-atom cocentration. The performance of the proposed model is evaluated by comparisons of model predictions with three different sets of experimental data for turbulent nonreacting and reacting jets. The calculated results are also compared with those obtained without the consideration of the superlayer and with those predicted by a unconditional model using different pdf shapes. Although the uncertainties of the experimental data prevent us from drawing solid conclusions from these comparisons, it is shown that the model predictions can be improved by incorporating intermittency and a composite pdf. The improvements are most significant for the scalar field. The nitric oxide calculations indicate that the effect of the superlayer can be significant, suggesting the use of the proposed model in this area.

Reduced Reaction Mechanisms for Methanol-Air Diffusion Flames

Abstract Reduced reaction mechanisms for methanol-air diffusion fiames are developed by systematic reduction of a skeletal mechanism. By examining the magnitude of the net production rates for the intermediate species, it is shown that for stagnation point diffusion flames the steady state assumption is reasonable for CH2OH, HCO, O, OH, H02, and H2O2, but not for CH2O. By eliminating these species from the skeletal mechanism, a five-step reduced mechanism is derived CH3OH + 2H = CH2O + 2H2, (R1) CH2O = CO + H2, (R2) CO + H2O = CO2 + H2,(R3) 2H2 + O, + M = 2H2O + M, (R4)O2 + 3H2 = 2H + 2H2O. (R5) To explore the impact of making the steady state assumption for CH2O, this five-step mechanism is further reduced to four steps by eliminating CH2O from the mechanism. Comparisons of calculated results for the stagnation point diffusion flame show a reasonable agreement among the different mechanisms. However, the four-step mechanism yields peak CH2O concentrations about six limes those predicted by the skeletal m...

PDF modeling of chemical nonequilibrium effects in turbulent nonpremixed hydrocarbon flames

An approach for modeling nonequilibrium chemistry in turbulent hydrocarbon nonpremixed flames with a four-scalar constrained equilibrium chemistry submodel is developed. The evolution of these scalars is simulated by the Monte Carlo technique with turbulence scales obtained from a second-order moment closure scheme for the velocity field. The calculated results for a turbulent propane-air jet flame compare favorably with available experimental data. The results indicate a significant departure from chemical equilibrium, but local flame extinction is not observed even at locations near the nozzle exit.