Study of the Combined Effect of Ammonia Addition and Air Coflow Velocity on a Non-premixed Methane Jet Flame Stabilization

ABSTRACT

Ammonia is a promising carbon-free fuel that can be used for CO₂ reduction. However, ammonia industrial use presents challenges, including flame stabilization. In this study, a non-premixed methane-ammonia jet flame in an air coflow was considered to observe the effects of ammonia addition on conventional fuels flame stabilization. First, the effects of the introduction of ammonia on the stabilization regimes of the methane jet flame were studied. Then the coupled effects of the variation in ammonia concentration and air coflow velocity on flame stabilization were investigated. In the present jet configuration, a sharp reduction of the stabilization domain was observed with ammonia addition: the liftoff and re-attachment velocities were obtained for mixtures of up to 50% of ammonia in the fuel, a ratio above which the flame could not be stabilized. When increasing the coflow velocity, a sudden drop in the re-attachment velocity occurred for methane/ammonia flames. This re-attachment drop was associated with an increase in the height of the lifted flame when the jet velocity decreases before re-attachment, for large enough coflow velocity and ammonia concentration. A critical height above which the lifted flames all present the same ascending behavior could be defined and characterizes this peculiar phenomenon.

Keywords:

• ammonia-methane mixture
• non-premixed jet flame
• stabilization regimes
• hysteresis

Nomenclature

CL,	=	liftoff coefficient (ULO/SL0)
CA,	=	reattachment coefficient (UA/SL0)
Di,	=	jet burner inner diameter
Do,	=	jet burner outer diameter
e,	=	burner rim thickness
E,	=	ammonia mixing ratio
HL,	=	lifted flame height, i.e. distance between the burner rim and the lifted flame base
HT,	=	jet breakpoint height
HTc,	=	critical jet breakpoint height corresponding to a change in the jet breakpoint evolution with the jet Reynolds number in laminar to turbulent region.
L,	=	burner pipe length
LHVi,	=	lower heating value of species i
Qd,	=	diluent flow rate
Qf,	=	total fuel mixture (fuel + diluent) flow rate
ReJ,	=	jet Reynolds number
ReJc,	=	critical jet Reynolds number corresponding to the change in the jet breakpoint evolution with the jet Reynolds number in the laminar to turbulent region.
SL0,	=	laminar burning velocity at ϕ = 1.0
Uco,	=	air coflow velocity
UJ,	=	jet velocity
ULO,	=	jet velocity at liftoff
UA,	=	jet velocity at re-attachment
Xi,	=	mole fraction of species i

Acknowledgments

This work was carried out under the Collaborative Research Project of the Institute of Fluid Science, Tohoku University, and in the framework of the Japan-France International Associated Laboratory (LIA) ELyT Global.