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

doi:10.1016/S0022-4073(03)00267-X

Journal of Quantitative Spectroscopy and Radiative Transfer

Volume 84, Issue 4, 1 April 2004, Pages 501-511

https://doi.org/10.1016/S0022-4073(03)00267-X Get rights and content

Abstract

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.

Introduction

Soot and NO_x formation, gas-phase chemistry, and radiation heat transfer are intimately coupled in flames primarily through the highly nonlinear dependence of these processes on temperature. The importance of the coupling of radiation and soot kinetics in sooting flames has been recognized and demonstrated in several studies [1], [2], [3]. Earlier numerical investigations in coflow laminar diffusion flames employed either detailed gas-phase chemistry but the simple optically thin approximation (OTA) for radiation [4], [5], [6] or very crude gas-phase chemistry and a more sophisticated treatment for radiation [1], [2]. Our recent numerical study [3] was conducted using both detailed gas-phase chemistry and non-grey radiation model in a coflow laminar ethylene diffusion flame with soot modelled using a semi-empirical model. This study found that in a moderately sooting diffusion flame both gas and soot radiation are important in regard to the visible flame height and soot volume fraction.

Mainly due to the difference in the flame structure, the amount of soot formed in laminar counterflow diffusion flames is much lower than that formed in coflow diffusion flames [7]. Detailed discussions of the effects of the diffusion flame structure on soot formation and oxidation were given in several recent studies [7], [8], [9]. Counterflow diffusion flames can be classified into two types: soot formation (SF) flames and soot formation-oxidation (SFO) flames [7], [10]. In SF flames the flame appears on the oxidizer side of the stagnation plane and the soot formed in the region between the flame and the stagnation plane is pushed away from the flame toward the stagnation plane by convection and thermophoretic effect. As a result of this specific flame structure, soot oxidation is essentially absent. In SFO flames the flame is formed on the fuel side of the stagnation plane and the soot formed on the fuel side of the flame is transported toward the stagnation plane while undergoing severe oxidation by OH and O₂. Such flames are realized by diluting the fuel stream while enriching the oxidizer stream with oxygen [7], [8], [9], [10]. Due to the absence of soot oxidation, soot formation counterflow diffusion flames provide an ideal flame configuration to validate soot surface growth sub-models. The amount of soot formed in SF flames is in general much higher than that in SFO flames and therefore a much stronger coupling between soot process and radiation is expected.

Although soot formation in counterflow diffusion flames has been extensively studied experimentally, relatively few numerical investigations incorporating a soot formation model have been reported [11], [12], [13], [14]. In these numerical studies, the effect of radiation heat loss on temperature reduction was either estimated using an empirical correlation to match the experimental temperatures obtained using a thermocouple [11], [12] or taken into account by incorporating the radiation heat loss term based on OTA into the energy equation [13], [14]. The former treatment of the effect of radiation on temperature must be considered crude for the reason that the temperatures measured by a thermocouple in diffusion flames are in general subject to relatively large errors. While the use of OTA in counterflow diffusion flames at moderate and high stretch rates is adequate, radiation absorption becomes so important at small stretch rates that OTA can cause significant errors for temperature and NO calculations as demonstrated by Wang and Niioka [15] in counterflow CH₄/air diffusion flames. It is therefore expected that radiation absorption should be considered in the prediction of soot formation in counterflow diffusion flames at low stretch. To our best knowledge, detailed numerical studies of the effects of soot and non-grey gas radiation on soot formation in counterflow diffusion flames have not been reported. Perhaps the only relevant numerical study was that conducted by Hall [16] who employed a wide-band model for gas-band radiation with radiation from soot accounted for. Although this study provided some insight into the importance of soot and gas radiation in counterflow diffusion flames, the results can only be regarded as highly qualitative since a rather artificial uniform soot layer was assumed. Moreover, the effect of radiation heat transfer on soot kinetics was not accounted for. There is therefore a need to incorporate an accurate and efficient radiation model into a flame code, such as the CHEMKIN based code used in our previous study [17], to improve the accuracy of temperature calculation, which is essential to soot and NO_x prediction.

Since the first application of the hybrid statistical narrow-band correlated-k (SNBCK) method to thermal radiation calculations by Goutière et al. [18], the efficiency of this method has been drastically improved as summarized in a recent study by Liu et al. [19]. In the present study, numerical calculations of soot formation in counterflow ethylene diffusion flames at atmospheric pressure were conducted using a CHEMKIN based code and detailed gas-phase chemistry. The soot model employed was essentially that used in our previous study [3]. Radiation was calculated using the DOM/SNBCK method. The objectives of this study are: (1) to investigate the effects of radiation and the individual influence of gas and soot radiation on soot formation in counterflow C₂H₄ SF diffusion flames, and (2) to examine the adequacy of the semi-empirical soot model we used previously [3], which was tuned for a coflow C₂H₄ flame, in the modelling of soot formation in counterflow C₂H₄ flames by comparing the numerical results against available experimental data in the literature.

Section snippets

Governing equations

Numerical calculations were carried out to model ethylene diffusion flames formed by two coaxial round jets of fuel and oxidizer streams at atmospheric pressure. Although the system is 2D (axisymmetric), the problem can be transformed into a system of ordinary differential equations (1D) valid along the stagnation-point streamline [20]. The ordinary differential equations of mass, momentum, species, and energy along with boundary conditions were given in detail in [20] and will not be repeated

Results and discussions

The computational conditions considered in this study were very close to the experimental conditions of the SF flames of Hwang and Chung [10]. In their SF flame experiments, the separation distance between the fuel (pure C₂H₄) and oxidizer (O₂ balanced by N₂) nozzles was kept at $1.42 cm$ . Both the fuel and oxidizer were supplied at room temperature. The nozzle exit velocities of both fuel and oxidizer streams were maintained at $19.5 cm / s$ . In the oxidizer the O₂ mole fraction varied from 20% to

Conclusions

Numerical study of the effects of gas and soot radiation on soot formation in counterflow ethylene diffusion flames was conducted using detailed gas-phase reaction mechanism, complex transport and thermal properties, a simplified two-equation soot model, and an accurate non-grey radiation model. Numerical results show that the soot model is capable of reproducing the experimental soot volume fractions with reasonably good agreement for different oxygen concentrations in the oxidizer. Gas

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

Abstract

Introduction

Section snippets

Governing equations

Results and discussions

Conclusions

Combust Flame

Combust Flame

JQSRT

Combust Flame

Combust Flame

Combust Flame

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Combust Flame

Proc Combust Inst

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JQSRT

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