The influence of burner geometry and flow rates on the stability and symmetry of swirl-stabilized nonpremixed flames

Abstract

The flame characteristics of a swirl-stabilized, nonpremixed natural gas burner have been established as a function of equivalence ratio and swirl number for three quarls and three fuel nozzle arrangements over a heat-release range of about 5–85 kW and combustion-air Reynolds numbers of 29,000 and 45,000, corresponding to bulk air velocities of 8.5 and 13.2 m/s. The quarl half-angles were 20° and 25°, the ratios of length to the throat diameter were L D = 1 and 1.9, and the maximum swirl number was 1.8. The lean stability limits were narrower for the higher bulk air velocity and the 20°, L D = 1 quarl supported the leanest flames; the poorest stablility was associated with the 25°, L D = 1.9 quarl. The stability limits generally reduced as the swirl number increased above 0.2, although for the higher bulk air velocity and for the L D = 1 quarls there was a local “bulge” towards lean equivalence ratios at swirl numbers between about 0.9 and 1.6. Flame lengths decreased with increasing swirl and bulk air velocity but the recirculation zone length was largely unaffected by changes in these two quantities. Axial fuel injection through a centrally located nozzle resulted in symmetric flows only for fuel-lean flames as swirl numbers below 0.6 and for near stoichiometric flames at swirl numbers above unity. The asymmetric was not due to asymmetric inlet velocity profiles. Angled annular or radial injection of the natural gas extended the range of operation for which symmetrical flames could be obtained, without greatly changing the flame lengths or stability characteristics.