Hongsheng Guo

The chemical effects of carbon dioxide as an additive in an ethylene diffusion flame: implications for soot and NOx formation

Abstract A numerical study of the chemical effects of carbon dioxide addition on both the fuel side and the oxidizer side of a diffusion flame was conducted in an attempt to explore the chemistry mechanism of the experimentally observed soot suppression by carbon dioxide addition. The laminar ethylene diffusion flame established in a counterflow configuration was considered by using detailed chemistry and transport properties. A novel strategy was developed that is able to isolate the chemical effects of a species added on either the fuel or the oxidizer side. Numerical results show that carbon dioxide, added either on the fuel side or the oxidizer side, indeed participates in chemical reactions. The specific aspects of the chemical effects of carbon dioxide addition that have direct implications for chemical suppression of soot formation are reduced concentration of acetylene and flame temperature and conversion of carbon dioxide by hydrogen atom to hydroxyl which prompts oxidation of soot precursors in the soot formation region. Reactions CO 2 +H → CO+OH and CO 2 +CH → HCO+CO were found to be responsible for the chemical effects of carbon dioxide addition. The chemical effects of CO 2 addition on the fuel side are small but becomes significant when introduced on the oxidizer side. The chemical effects of CO 2 addition were also found to suppress NO x formation.

The chemical effect of CO2 replacement of N2 in air on the burning velocity of CH4 and H2 premixed flames

When CO2 is added to air or is used to replace N2 in air, it is anticipated that the burning velocity of fresh fuel mixtures may be affected through the following three mechanisms: i) the variation of the transport and thermal properties of the mixture, ii) the possible direct chemical effect of CO2, and iii) the enhanced radiation transfer by CO2. Experimental measurements of the burning velocity of CH4/O2/ CO2 mixtures at various equivalence ratios and pressures have been conducted by Zhu et al. [1] using double flames in the counterflow configuration. The radiative effect of CO2 on the burning velocity of CH4/O2/N2/CO2 mixtures has been recently studied by Ju et al. [2] and Ruan et al. [3]. Moreover, it has also been pointed out in several studies that CO2 is not inert but directly participates in chemical reactions primarily through CO OH 7 CO2 H [4-6]. The objective of this study is to numerically investigate the chemical effects of CO2 replacement of N2 in air on the burning velocity of lean to stoichiometric CH4/O2/N2/ CO2 and H2/O2/N2/CO2 mixtures at 1 atm.

Effects of Gas and Soot Radiation on Soot Formation in a Coflow Laminar Ethylene Diffusion Flame

Abstract A computational study of soot formation in an undilute axisymmetric laminar ethylene-air coflow jet diffusion flame at atmospheric pressure was conducted using a detailed gas-phase reaction mechanism and complex thermal and transport properties. A simple two-equation soot model was employed to predict soot formation, growth, and oxidation with interactions between the soot chemistry and the gas-phase chemistry taken into account. Both the optically thin model and the discrete-ordinates method coupled with a statistical narrow-band correlated-K based wide band model for radiative properties of CO, CO2, H2O, and soot were employed in the calculation of radiation heat transfer to evaluate the adequacy of using the optically thin model. Several calculations were performed with and without radiative transfer of radiating gases and/or soot to investigate their respective effects on the computed soot field and flame structure. Radiative heat transfer by both radiating gases and soot were found to be important in this relatively heavily sooting flame studied. Results of the optically thin radiation model are in good agreement with those obtained using the wide band model except for the flame temperature near the flame tip.

Numerical modelling of soot formation and oxidation in laminar coflow non-smoking and smoking ethylene diffusion flames

A numerical study of soot formation and oxidation in axisymmetric laminar coflow non-smoking and smoking ethylene diffusion flames was conducted using detailed gas-phase chemistry and complex thermal and transport properties. A modified two-equation soot model was employed to describe soot nucleation, growth and oxidation. Interaction between the gas-phase chemistry and soot chemistry was taken into account. Radiation heat transfer by both soot and radiating gases was calculated using the discrete-ordinates method coupled with a statistical narrow-band correlated-k based band model, and was used to evaluate the simple optically thin approximation. The governing equations in fully elliptic form were solved. The current models in the literature describing soot oxidation by O2 and OH have to be modified in order to predict the smoking flame. The modified soot oxidation model has only moderate effects on the calculation of the non-smoking flame, but dramatically affects the soot oxidation near the flame tip in the smoking flame. Numerical results of temperature, soot volume fraction and primary soot particle size and number density were compared with experimental data in the literature. Relatively good agreement was found between the prediction and the experimental data. The optically thin approximation radiation model significantly underpredicts temperatures in the upper portion of both flames, seriously affecting the soot prediction.

The flame preheating effect on numerical modelling of soot formation in a two-dimensional laminar ethylene–air diffusion flame

Numerical modelling of soot formation is conducted for an axisymmetric, laminar, coflow diffusion ethylene–air flame by two different methods to investigate the effect of flame preheating. The first method cannot account for the preheating effect, while the second one can. A detailed gas-phase reaction mechanism and complex thermal and transport properties are used. The fully coupled elliptic equations are solved. A simple two-equation soot model is used to model the soot process coupled with detailed gas-phase chemistry. The numerical results are compared with experimental data and indicate that the flame preheating effect has a significant influence on the prediction of soot yields. Both methods give reasonable flame temperature and soot volume fraction distributions. However, quantitatively the second method results in improved flame temperatures and soot volume fractions, especially in the region near the fuel inlet, although the maximum flame temperatures from both methods are slightly lower than that from the experiment.

Implementation of an advanced fixed sectional aerosol dynamics model with soot aggregate formation in a laminar methane/air coflow diffusion flame

An advanced fixed sectional aerosol dynamics model describing the evolution of soot particles under simultaneous nucleation, coagulation, surface growth and oxidation processes is successfully implemented to model soot formation in a two-dimensional laminar axisymmetric coflow methane/air diffusion flame. This fixed sectional model takes into account soot aggregate formation and is able to provide soot aggregate and primary particle size distributions. Soot nucleation, surface growth and oxidation steps are based on the model of Fairweather et al. Soot equations are solved simultaneously to ensure convergence. The numerically calculated flame temperature, species concentrations and soot volume fraction are in good agreement with the experimental data in the literature. The structures of soot aggregates are determined by the nucleation, coagulation, surface growth and oxidation processes. The result of the soot aggregate size distribution function shows that the aggregate number density is dominated by small aggregates while the aggregate mass density is generally dominated by aggregates of intermediate size. Parallel computation with the domain decomposition method is employed to speed up the calculation. Three different domain decomposition schemes are discussed and compared. Using 12 processors, a speed-up of almost 10 is achieved which makes it feasible to model soot formation in laminar coflow diffusion flames with detailed chemistry and detailed aerosol dynamics.

A Numerical Study on the Influence of CO2 Addition on Soot Formation in an Ethylene/Air Diffusion Flame

Earlier studies have confirmed that the addition of CO2 to a diffusion flame suppresses the formation of soot. However, a consensus has not been reached on the fundamental mechanisms of the suppression. In this paper, the influence of CO2 addition on soot formation in an ethylene/air diffusion flame is investigated by numerical simulation, with focus on the fundamental mechanism of the suppression effect on soot formation. A special strategy is employed to separate the chemical effect from the thermal and dilution effects. The simulation results confirm that the addition of CO2 suppresses soot formation through not only the thermal and dilution effects, but also the chemical effect. The chemical effect of CO2 addition is primarily caused by the reduced concentration of radical H due to the reaction CO + OH ⇐ CO2 + H, which suppresses the soot inception and surface growth rate. The chemical effect of CO2 addition has negligible effect on soot oxidation process.

A Numerical Investigation of Thermal Diffusion Influence on Soot Formation in Ethylene/Air Diffusion Flames

Thermal diffusion, caused by temperature gradients, tends to draw lighter molecules to warmer regions and to drive heavier molecules to cooler regions of a mixture. The influence of thermal diffusion on soot formation in coflow laminar ethylene/air diffusion flames is numerically investigated in this paper. Detailed reaction mechanisms and complex thermal and transport properties are employed. The fully elliptic governing equations are solved. Radiation heat transfer from the flames is calculated by the discrete-ordinates method coupled to an SNBCK-based wide band model. A simplified two-equation soot model is used. The interactions between soot and gas-phase chemistry are taken into account. The thermal diffusion velocities are calculated according to the thermal diffusion coefficients evaluated based on multicomponent properties. The results show that thermal diffusion does affect soot formation in ethylene/air diffusion flames. Although the effect on soot formation in pure ethylene/air flame is not significant, the influence is enhanced if lighter species, such as helium, are added to the fuel or the air stream. The peak integrated soot volume fraction doubles if thermal diffusion is not taken into account in the simulation of the flame with helium addition to the air stream.

Soot and NO formation in counterflow ethylene/oxygen/nitrogen diffusion flames

Formation of soot and NO in counterflow ethylene/oxygen/nitrogen diffusion flames was numerically investigated. Detailed chemistry and complex thermal and transport properties were used. A simplified two-equation soot model was adopted. The results indicate that NO emission has negligible influence on soot formation. However, soot formation affects the emission of NO through the radiation induced thermal effect and the reaction induced chemical effect. When the oxygen index of the oxidant stream is lower, the relative influence of chemical reaction caused by soot on NO emission is more important, while the relative influence of the radiation induced thermal effect becomes more important for the flame with a higher oxygen index in the oxidant stream.

Effects of gas and soot radiation on soot formation in counterflow ethylene diffusion flames

Numerical study of soot formation in counterflow ethylene diffusion flames at atmospheric pressure was conducted using detailed chemistry and complex thermal and transport properties. Soot kinetics was modelled using a semi-empirical two-equation model. Radiation heat transfer was calculated using the discrete-ordinates method coupled with an accurate band model. The calculated soot volume fractions are in reasonably good agreement with the experimental results in the literature. The individual effects of gas and soot radiation on soot formation were also investigated.