Experimental study of the effects of oxygen enrichment on the stability of a low-swirl biogas non-premixed flame

• Oxygen enrichment on the stability of a low-swirl non-premixed biogas flame was experimentally investigated. • Oxygen enrichment was found to significantly expand the flammability limits of biogas flame. • The effect of oxygen is more pronounced at flame lean conditions and less significant at the upper limits. • The blowout mechanism is controlled by the fuel jet in the upper limit, while it is governed by the swirling flow in the lower limit. This experimental study investigates the impact of oxygen enrichment on the stability of a low-swirl biogas non-premixed flame. A synthetic biogas mixture (CH 4 /CO 2 :60/40 by volume) was injected through a central nozzle, while a coaxial airflow, which passes through a low‐swirl generator with a swirl number (S) of 0.39, was enriched with up to 24 % oxygen. The biogas flame is then ignited and stabilized at the burner exit. High-speed imaging was used to investigate the biogas flame stability limits and a two-dimensional Particle Image Velocimetry (PIV) was used to document the reacting flow fields especially near the upper (UBL) and lower blowout (LBL) limits. The results indicate that while oxygen enrichment has a limited effect on expanding the upper blowout limit, it significantly extends the lower blowout limit, particularly at low fuel flow rates, emphasizing its stronger stabilizing role under flame lean conditions. The UBL appears to be mostly controlled by the momentum of the central fuel jet. It is seen that increase in the centerline jet velocity pushes the lifted flame farther downstream and when severely stretched due to elongated vortical structures, the flame becomes instable and susceptible to blowout. Conversely, at the LBL, the blowout is governed by the strength of the swirling co-flow, as increase in co-flow makes the flame leaner and more strained which eventually causes blowout.