STUDY OF FLAME STABILITY IN A STEP SWIRL COMBUSTOR

A prime requirement in the design of a modern gas turbine combustor is good combustion stability, especially near lean blowout (LBO), to ens ure an adequate stability margin. For an aeroengine, combustor blow-of f limits are encountered during low engine speeds at high altitudes ov er a range of flight Mach numbers. For an industrial combustor, requir ements of ultralow NOx emissions coupled with high combustion efficien cy demand operation at or close to LBO. In this investigation, a step swirl combustor (SSC) was designed to reproduce the swirling pow patte rn present in the vicinity of the fuel injector located in the primary zone of a gas turbine combustor. Different flame shapes, structure, a nd location were observed and detailed experimental measurements and n umerical computations were performed. It was found that certain combin ations of outer and inner swirling airflows produce multiple attached flames, a flame with a single attached structure just above the fuel i njection tube, and finally for higher inner swirl velocity, the flame lifts from the fuel tube and is stabilized by the inner recirculation zone. The observed difference in LBO between co- and counterswirl conf igurations is primarily a function of how the flame stabilizes, i.e., attached versus lifted. A turbulent combustion model correctly predict s the attached flame location(s), development of inner recirculation z one, a dimple-shaped fame structure, the flame lift-off height and rad ial profiles of mean temperature, axial velocity, and tangential veloc ity at different axial locations. Finally, the significance and applic ations of anchored and lifted flames to combustor stability and LBO in practical gas turbine combustors are discussed.