Effect of Diluent Addition on Combustion Characteristics of Methane/Nitrous Oxide Inverse Tri-Coflow Diffusion Flames

ABSTRACT

The purpose of this study is to examine the dilution effect on combustion characteristics of methane/nitrous oxide inverse tri-coflow diffusion flame through the addition of various dilution gases. Two diluents, namely carbon dioxide and argon, were used. To scrutinize the overall combustion characteristics, the effects of dilution on flame appearance and pollution emissions were calculated numerically and observed experimentally. In terms of flame appearance, with increasing the central oxidizer flow rate, the flame became an inner-oxidizer-injected flame structure and then converted to a double flame structure. When 40% Ar and 20% CO₂ were added to the oxidizer individually, a liftoff edge flame was formed at high central oxidizer flow rate. This theoretical prediction provided a fast and preliminary explanation of the various flame structures and concurred with the trend of IDF formation observed in the experiment. Regarding the flue gas emission, the addition of Ar in CH₄/N₂O diffusion flame can effectively reduce the yield of CO, whereas the addition of CO₂ leads to an increase of CO concentration in the flue gas. Simulation results indicated that for Ar addition, the production of NO_x is dominated by the inert effect at dilution levels lower than 40%, whereas the thermal/diffusion effect dominates at dilution levels higher than 40%. CO formation is mainly dominated by the thermal/diffusion effect. In CO₂ dilution, NOx formation is dominated by the inert and chemical effects at dilution levels lower than 40%. However, at dilution levels higher than 40%, the chemical effect is more dominant. Thus, CO formation is dominated mainly by the chemical effect.

KEYWORDS:

• Nitrous oxide
• inverse diffusion flame
• dilution
• argon
• carbon dioxide

Acknowledgments

The authors thank the financial support from the Ministry of Science and Technology, Taiwan, under the grant numbers, MOST 108-2628-E-006-008-MY3 and MOST 109-2221-E-006-037-MY3. The authors wish to thank Dr. Guan-Bang Chen for numerical assistance. Computer time and numerical packages provided by the National Center for High-Performance Computing, Taiwan (NCHC Taiwan), are gratefully acknowledged.

Nomenclature

${\mathrm{A}}_{0,\mathrm{Q}},{\mathrm{A}}_{\mathrm{n},\mathrm{Q}}$	=	The coefficient of the flame shape general solution
${\mathrm{B}}_{0,\mathrm{Q}},{\mathrm{B}}_{\mathrm{n},\mathrm{Q}}$	=	The coefficient of the flame shape general solution
C	=	Ratio of radius
${\mathrm{C}}_{\mathrm{p}}$	=	Specific heat capacity
${\mathrm{J}}_0,{\mathrm{J}}_1$	=	The Bessel function of the first kind of order 1 and 2
m	=	Mass
M	=	Molecular weight
N	=	Mole
$\mathrm{P}\mathrm{e}$	=	Peclet number, $\mathrm{P}\mathrm{e}\equiv \rho {\mathrm{u}}_{\mathrm{r}\mathrm{e}\mathrm{f}}{\mathrm{r}}_{\mathrm{s}}/(\lambda /{\mathrm{C}}_{\mathrm{p}})$
R	=	Velocity ratio, $\mathrm{R}={\mathrm{V}}_{\mathrm{o}\mathrm{x}\mathrm{i}\mathrm{d}\mathrm{i}\mathrm{z}\mathrm{e}\mathrm{r}}/{\mathrm{V}}_{\mathrm{f}\mathrm{u}\mathrm{e}\mathrm{l}}={\mathrm{V}}_1/{\mathrm{V}}_2$
r	=	Radial coordinate
${\mathrm{r}}_{\mathrm{s}}$	=	The distance between central axis and outer rim
u	=	Axial velocity
Y	=	Mass fraction
$\tilde{\mathrm{Y}}$	=	Dimensionless mass fraction
Greek	=
$\lambda$	=	Thermal diffusivity
${\upsilon }^{\prime }$	=	The coefficient of reactant
${\upsilon }^{{ }^{\prime \prime }}$	=	The coefficient of product
$\mathrm{\Omega }$	=	The diluent concentration, $\mathrm{\Omega }=\left(\frac{{\mathrm{X}}_{\mathrm{D}\mathrm{i}\mathrm{l}\mathrm{u}\mathrm{e}\mathrm{n}\mathrm{t}}}{{\mathrm{X}}_{{\mathrm{N}}_2\mathrm{O}}+{\mathrm{X}}_{\mathrm{D}\mathrm{i}\mathrm{l}\mathrm{u}\mathrm{e}\mathrm{n}\mathrm{t}}}\right)\mathrm{n}100\mathrm{\%}$
${\mathrm{\Phi }}_{}$	=	The equivalence ratio
${\mathrm{\Phi }}_{\mathrm{l}\mathrm{o}\mathrm{c}\mathrm{a}\mathrm{l}}$	=	The local equivalence ratio
${\mathrm{\Phi }}_{\mathrm{n}}$	=	The eigenvalue of ${\mathrm{J}}_1\left({\mathrm{\Phi }}_{\mathrm{n}}\right)=0$
$\eta$	=	Dimensionless axial coordinate
$\xi$	=	Dimensionless radial coordinate
$\left({\xi }_{\mathrm{f}},{\eta }_{\mathrm{f}}\right)$	=	Flame coordinate
Subscript	=
i	=	The ith ring of multiport burner
j	=	Spicies, j=F(fuel) or j=O(oxidizer)
n	=	The terms of series

Additional information

Funding

This work was supported by the Ministry of Science and Technology, Taiwan [MOST 108-2628-E-006-008-MY3,MOST 109-2221-E-006-037-MY3].