Lean blow-off investigation in a linear multi-burner combustor operated in premixed and non-premixed modes

Abstract

The flame behaviour and the lean blow-off (LBO) mechanism of a multi-burner linear arrangement of five identical burners each fitted with a central bluff body and a swirler up- stream, operated in either premixed or non-premixed mode, was studied experimentally. The flame stability was assessed over a wide range of velocities. High-speed (5 kHz) OH∗ chemiluminescence and low-speed (10 Hz) CH2O-PLIF were employed. As the velocity in- creased, the premixed flame exhibited a shape change with significant CH2O build up in the region between adjacent burners, suggesting the presence of localised quenching where the flames interact. Close to LBO, the flame showed a pattern of lift-off and reattachment over individual bluff bodies, with movement of OH∗ and CH2O pockets across the burners. The non-premixed flame was shorter and showed less visible interaction between adjacent burners. When the fuel velocity was reduced, the flame showed intermittent blow-off/reignition prior to the complete extinction. The flame shapes and stability regions were compiled in a regime diagram and compared to a full annular combustor that used the same individual burners. Finally, the LBO conditions were correlated through a Damkh¨oler number (Da) based on laminar flame speed and compared to the annular combustor and a scaled-up single burner. In the premixed flame, the linear multi-burner LBO conditions agreed with those in the annular geometry when expressed in terms of Da, but the single burner did not. For the non-premixed flame, the correlation proved less successful. The results reported here improve the understanding of the LBO process in practical systems and can assist the development of turbulent flame models.