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.

Keywords

• Linear multi-burner combustor
• Lean blow-off
• Turbulent flames
• Stability limits
• CH2O-PLIF