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

Abstract

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.

The impact of detailed multicomponent transport and thermal diffusion effects on soot formation in ethylene/air flames

“…Higher order transport effects have been studied in coflow diffusion flames experimentally in [21,22] by adding inert diluents of varying diffusivities and numerically in [23,24]. In [23] it is shown that with a simplified two-equation soot model, thermal diffusion affects soot formation in an ethylene/air flame when helium is added to the fuel stream.…”

Synthesis of stable anisotropic carbon particle aggregates covered by surface nano-graphitic sheets

“…3), soot in flames [29,32], laboratorial pyrolytic carbon particles [28] or DLCA [46]: many of which are around 1.7-1.9. These take the form of their morphology through inherent interaction such as the thermal diffusion effect [47], inherent Coulombic interaction [23,27], or other dispersion forces. Therefore, the resulting value of 1.65 might be resulted from anisotropic aggregation through electric migration mediated by the external columbic force in the electric field.…”

The effect of preferential diffusion on soot formation in a laminar ethylene/air diffusion flame

“…It has been shown by Dworkin et al [11] that compared to the mixture-average method, the inclusion of the complex multicomponent transport property calculation method for gas species has only a minor effect on soot formation. Our previous study [10] indicated that thermal diffusion of gas species has negligible effect on soot formation in an ethylene/air diffusion flame. Therefore, in order to save simulation time, preferential diffusion was taken into account by calculating transport properties of gas species using the mixture-average method given by Kee et al [21] in the first simulation (SIM1), with thermal diffusion of H and H 2 being included while that of other species being neglected.…”

“…The effect of transport property calculation method on soot formation in laminar flames has also been investigated previously. The studies of [9,10] suggested that thermal diffusion of gas species had negligible effect on soot formation in an ethylene/air diffusion flame, but it became significant when some light components, such as hydrogen or helium, were introduced to fuel or air stream. Dworkin et al [11] showed that compared to the approximate mixture-average method, the inclusion of the complex multicomponent transport property calculation method had only a minor effect on soot formation in laminar ethylene/air diffusion flames.…”

“…Dworkin et al [11] showed that compared to the approximate mixture-average method, the inclusion of the complex multicomponent transport property calculation method had only a minor effect on soot formation in laminar ethylene/air diffusion flames. It has been clear from the previous studies that as long as non-uniform Lewis number, which is defined as the ratio of the mixture heat conduction rate to species mass diffusion rate, is assumed and the difference in diffusion rates of different species is considered, the transport property calculation method based on most existing schemes in literature has only a minor impact on soot formation for most hydrocarbon flames but significantly affects results when there are some light components (such as hydrogen or helium) in fuel or oxidant stream [9][10][11]. However, few studies have investigated the influence of complete neglect of the difference in species diffusion coefficient, i.e.…”