Effects of Inert Fuel Diluents on Thermoacoustic Instabilities in Gas Turbine Combustion

Abstract

The effects of inert diluents in the fuel mixture of a model swirl stabilized gas turbine combustor, under thermoacoustically unstable limit cycle operation, were studied experimentally. The measurements included Particle Image Velocimetry (PIV), high speed CH * chemiluminescent imaging and dynamic pressure signals. The paper focuses on the dynamic phenomenon of the period doubling bifurcation, which came about when the equivalence ratio (φ) of undiluted flames was enriched from 0.55 to 0.60 under constant Reynolds number (Re). The bifurcation featured an emergence of an aerodynamically related timescale in addition to the fundamental timescale which was induced from an unstable acoustic eigenmode. The aerodynamic timescale is introduced by azimuthal convection of a high heat release rate region, and is linked in the literature with a precessing motion of the recirculation zone. Prior to the bifurcation, the flame anchored between the wall and the outer shear layers of the recirculation zone assuming a V-shape. The dynamics were attracted to a Period-1 limit cycle. Post the bifurcation the flame expanded in the inner shear layers of the recirculation zone assuming an M-shape, while the dynamics were attracted to a Period-2 limit cycle. The latter operational condition was subject to nitrogen dilution in order to parametrically increase the molar fraction of inert diluent in the fuel stream. It was found that on increasing the diluent molar fraction the amplitude and the frequency of the limit cycle fundamental acoustic mode decreased. Also, inert dilution suppressed the aerodynamic timescale and the flame assumed a V-shape again. A mechanism to interpret this mechanism is suggested. Increasing the diluent molar fraction of the fuel makes the flame susceptible to quenching because the extinction strain rate of the mixture decreased. Thus, the flow imposed strain rates quenched the flame when it attempted to anchor on the inner shear layers of the vortex breakdown induced recirculation zone. The inability of the flame to anchor on the shear layers suppressed the aerodynamic mode. The paper argues that the existence of inert diluents in the fuel can significantly alter the dynamic state of the combustor, since the anchoring locations of the flame greatly depend on the composition-sensitive extinction strain rate of the mixture.

Effects of oxy jet in cross-flow on the combustion instability and NOx emissions in lean premixed flame

“…The soot formation, quenching distances and minimum ignition energies can be optimized with different dilutions [33][34]. The anchoring zones of the flame greatly depend on the composition-sensitive extinction strain rate of the mixture [35]. At the same time, the injection medium can significantly alter the dynamic characteristics of the lean premixed flame.…”