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.