Large eddy simulation of fuel sensitivity in a realistic spray combustor II. Lean blowout analysis

Abstract

Large eddy simulation (LES) is conducted for lean blowout (LBO) prediction in the Referee rig combustor, developed within the national jet fuel combustion program (NJFCP). Starting from the stable flame solution at a stable equivalence ratio (ER) (discussed elsewhere [1]), multiple step-by-step and direct reductions in the fuel flow rate are conducted. Each reduced ER is simulated until either the flame stabilizes to a new burning state or completely blows out, which is then defined as the predicted LBO ER. In the NJFCP, two fuels are considered: a baseline fuel (A2), and an alternate fuel (C1). The predicted LBO points are within 13% and 15% of the experimental values for A2 and C1, respectively, and C1 blows out at an ER that is 5.7% higher than that of A2, consistent with experimental observations. The heat release rate (HRR) efficiency gradually reduces with ER before it suddenly drops to zero at the LBO point, whereas, the evaporation efficiency stays the same, suggesting a chemistry-dominant blow-out for this configuration. Burning mode analysis shows that the gradual reduction in HRR is correlated with the non-premixed part of the flame, whereas the premixed anchoring stays the same, until it finally extinguishes as pockets from the cold recirculation zone hit it over time. Pressure signatures show activated acoustic modes during the stable burning, but a highly irregular dynamics is observed as the LBO is approached. Fuel-sensitive differences through burning mode analysis and pressure signatures are apparent even at stable burning conditions, that point towards an eventual fuel-sensitive LBO for this configuration.