Numerical Investigation of a Coupled Blow-Off/Flashback Process in a High-Pressure Lean-Burn Combustor

Abstract

Large eddy simulation is used to investigate the flashback mechanism caused by the combustion-induced vortex breakdown (CIVB) in a high-pressure lean-burn annular combustor with lean direct injection of kerosene. A single sector of the geometry, including a central pilot flame surrounded by a main flame, is simulated at take-off conditions. A previously-developed flamelet-based approach is used to model turbulence-combustion interactions due to its relatively low cost, allowing to simulate a sufficiently long time window. In stable operations, the flame stabilises in an M-shape configuration and a periodic movement of the pilot jet, with the corresponding formation of a small recirculation bubble, is observed. Flashback is then observed, with the flame accelerating upstream towards the injector as already described in other studies. This LES, however, reveals a precursor partial blow-out of the main flame induced by a cluster of vortices appearing in the outer recirculation region. The combined effect of vortices and sudden quenching alters the mixing level close to the injector, causing first the main, then the pilot flame, to accelerate upstream and initiate the CIVB cycle before the quenched region can re-ignite. Main and pilot flames partly extinguish as they cross their respective fuel injection point, and re-ignition follows due to the remnants of the reaction in the pilot stream. The process is investigated in detail, discussing the causes of CIVB-driven flashback in realistic lean-burn systems.

Investigation of Thermoacoustic Instability for Swirling Flames in a Dual Annular Stratified Burner

“…Swirl-stabilized flames, however, present their unique instability challenges, which are hydrodynamic in nature, mainly due to features of swirling flows like vortex breakdown and precessing vortex core (PVC). These flow features are associated with unique static instability like the vortex-breakdown-induced flashback as well as thermo-acoustic instabilities [3] [4]. Scenarios where heat release rate and acoustic pressure fluctuations couple together to generate undesirable pressure waves inside the combustor are categorized as dynamic instabilities, also known as thermoacoustic instability [2].…”

Numerical Analysis of Flame Shape Impact on the Performance of Fuel Staging in a Lean-Burn Aeronautical Burner

“…Various flame shapes and their interactions with injection regimes were studied in the past, mainly for gaseous fuels in staged [8,9] or non-staged burners [10,11]. With liquid fuels, the focus was on flame transition mechanisms [12][13][14][15][16][17] or on comparing the flame shapes for different conditions [18], and not on the performance of the different flame shapes when fuel staging was applied.…”