On the bi-stable nature of turbulent premixed bluff-body stabilized flames at elevated pressure and near lean blow-off

Abstract

This study considers turbulent premixed bluff-body stabilized flames at elevated pressures. Specifically, the lean blow-off (LBO) limit of such flames is determined for a range of bulk velocities (5 ≤ U ≤ 50 m/s) and operating pressures up to 3 bar. Two key observations emerge from this stability assessment. The first is that considering elevated pressure leads to two stability regimes: one at atmospheric conditions and those with elevated pressure and U ≳ 20 m/s (regime-a), and another at elevated pressures with U ≲ 20 m/s (regime-b). The second observation is that within these regimes, LBO limits are insensitive to pressure. Flames in regime-a (S-flames) are found to be more stable than those in regime-b (U-flames). Advanced image-based diagnostics were employed to understand reasons for this difference in stability. Flow field measurements indicate that U-flames are associated with an outer recirculation zone (ORZ) that formed as pressure increased but receded from the burner as U surpassed ∼ 20 m/s. PLIF images of CH2O and OH demonstrated that the ORZ interacts with U-flames such that their downstream regions are prevented from collapsing to the inner recirculation zone (IRZ). Furthermore, analysis of the OH-PLIF images indicate that U-flames possess larger turbulent consumption rates, helping them form large IRZs and rendering them more susceptible to influence from the ORZ. Results of high-speed OH* imaging demonstrate that LBO events differ between U- and S-flames. Namely, while S-flames collapse to their IRZs during LBO, U-flames lift off from the burner, depleting their anchoring regions of reactions and hot products. Losing back-support in this region is what ultimately reduces the stability of U-flames. Finally, the reason U-flames lift off from the burner during LBO is elucidated by joint flow-flame measurements. Specifically, the anchoring regions of U-flames reside in regions of large axial velocity, which likely stems from their enhanced burning rates.