Forced response on swirl stabilized flames in hydrogen enriched gas turbines

The forced response of swirl-stabilized lean-premixed turbulent flames to inlet velocity and/or equivalence ratio oscillations in hydrogen enriched laboratory-scale gas turbine combustors was experimentally investigated. To characterize the response of premixed and partially premixed flames, several measurement techniques were used. Inlet velocity and equivalence ratio fluctuations were measured by the two-microphone method and the infrared absorption technique, respectively. Global heat release oscillations were determined by measuring naturally occurring flame chemiluminescence emissions using photomultiplier tubes. Steady-state flame structure and phase-resolved flame dynamics were characterized by CH* chemiluminescence imaging using an intensified CCD camera.;Results show that the response of a premixed flame to inlet velocity oscillations is strongly dependent on the effective flame length. This extends previous experimental and analytical results from laminar to turbulent premixed flames. The nonlinear response of the flame is manifested by shear layer rollup and unsteady flame liftoff, depending on the amplitude of perturbations. The response of hydrogen-enriched flames is linear, irrespective of vortex shedding. The forced response of a flame to equivalence ratio oscillations alone was achieved by using a fuel modulation method. When the flame is perturbed by mixture ratio modulations, it shows both linear and nonlinear responses. The nonlinearity is associated with the intrinsically nonlinear dependence of burning velocity and equivalence ratio upon the heat release. Finally, the response of a partially premixed flame to perturbations of both inlet velocity and equivalence ratio was studied. Unsteady local extinction events were identified as a mechanism of nonlinearity, which results from flame stretching effects. In particular, unsteady local extinction is more obvious in the nonlinear dynamics of hydrogen enriched partially premixed flames. The phase difference between inlet velocity and equivalence ratio nonuniformities is a key parameter controlling the linear and nonlinear response of a partially premixed flame. Results also show that in the linear regime, the response of a partially premixed flame can be reconstructed using the responses of a flame to velocity and equivalence ratio, using a vector summation rule.