58706 - Experimental Investigation of Dual-Swirl Spray Flame in a Fuel Staged Optical Model Combustor With Laser Diagnostics 

Lean premixed prevaporized combustors often feature staged combustion with a premixed main flame anchored by the nonpremixed pilot flame to obtain a wide operating range. Interaction between pilot flame and main flame is complex. The present article investigates the flame topologies and interactions in separated stratified swirl flames under various operating conditions (fuel to air ratio FAR and fuel stage ratio SR) and injector designs (main stage swirl number M and fuel injection angle JA). Experiments are carried out in the centrally staged optical model combustor at inlet pressure P₃ = 0.49-0.7 MPa and inlet temperature T₃ = 539 K. At first, the flame structures obtained from OH-PLIF are investigated and discussed for the baseline injector (M = 0.7, JA = -50^°). The V-shaped flame is stabilized in the inner shear layer (ISL) with the flame root originates from the lip for the pilot flame mode (SR = 1). Dual flame is observed in the combustor for the fuel staged combustion (SR < 1): the main flame stabilized in the outer shear layer (OSL) and the pilot flame stabilized in the inner shear layer (ISL). Increasing SR from 0.15 to 0.25, gaps between the main flame and pilot flame are decreased, indicating a stronger interaction between the two flames. Fuel distribution measured by kerosene-PLIF is analyzed with the correlation to flame structure. The existence of a good mixing of fuel and fresh air in ISL and OSL provide favorable conditions for flame front with intense chemical reaction. The flame structure for different injector geometries is then investigated. It is found that the large main stage swirl number induces a larger flame opening angle, decreasing the interaction between two flames. Fuel injected into crossflow (JA = -50^°) is found to generated a more separated flame, decreasing the flame interactions. Finally, instantaneous pilot flame structure at a lean fuel condition is obtained. Results show that flame is highly unsteady and wrinkled with local extinction, which represents the interaction between flame boundary and the shear layer strain rate.