An Experimental Study on Combustion in Mesoscale Coaxial Swirling Burner Arrays

ABSTRACT

An experimental study of combustion in diffusion and coaxial mesoscale burner arrays was conducted. To that end, different linear and planar arrangements were tested, and blow-off limits, flame geometry, and oscillations parameters were determined. The new results of the experiments showed that the coaxial burner arrays, in general, are characterized by leaner values of the blow-off limit compared to premixed flames. Moreover, the large planar arrangement (square 5 × 5) provides the global blow-off equivalence ratio value of about 0.1 at low thermal power operating modes (less than 2 kW). All arrays of coaxial jets feature flame separation into small coaxial flamelets. However, linear coaxial arrangements provide complete flame separation, whereas jets of a planar array are not fully separated and merged in pairs. At the equivalence ratio value less than 0.45, separated flames start oscillating strongly until the blow-off occurs, that is unacceptable in terms of combustion efficiency, pollutant emission, and related parameters. Therefore, the most favorable operating modes correspond to the equivalence ratio ranging from 0.45 to 0.6.

KEYWORDS:

• Coaxial jet
• diffusion combustion
• swirling flow
• multiple flames
• jet arrangement

Nomenclature

AFRstoich	=	stoichiometric air-to-fuel ratio, [-];
CFD	=	Computational Fluid Dynamics;
Din	=	outer diameter of inner tube, [m];
din	=	inner diameter of inner tube, [m];
dout	=	inner diameter of outer tube, [m];
f	=	flame oscillations frequency, [Hz];
L	=	flame length, [m];
LCV	=	fuel low calorific value, [MJ/kg];
Mair	=	air mass flow rate, [kg/s];
Mfuel	=	fuel mass flow rate, [kg/s];
NOx	=	nitrogen oxides;
PIV	=	Particle Image velocimetry;
p	=	pitch distance, [m];
Reair	=	the Reynolds number of air jet, [-];
Refuel	=	the Reynolds number of fuel jet, [-];
S	=	swirl number, [-];
Sh	=	the Strouhal number, [-];
TP	=	thermal power, [W];
Vair	=	mass flow averaged velocity of air jet, [m/s];
Vfuel	=	mass flow averaged velocity of fuel jet, [m/s];
Greek:	=
νair	=	air kinematic viscosity, [m2/s];
νfuel	=	fuel kinematic viscosity, [m2/s];
ϕ	=	fuel-air mixture equivalence ratio, [-].

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was financially supported by RSF (Russian Science Foundation) grant No. 22-29-20220