Measurement of Flame Transfer Functions in Swirl-Stabilized, Lean-Premixed Combustion

Abstract

TO MEET increasingly stringent emission standards for nitric oxides, modern gas turbine designs use lean-premixed combustion. While meeting these new environmental standards, leanpremixedcombustion systems introduce some substantial operability concerns with increased susceptibility to blowout, flashback, and instabilities. Significant effort is required to overcome these design challenges to allow turbines to operate in an efficient and environmentally friendlymanner. This brief communication provides results ofongoingexperimentalmeasurementsof lean-premixedcombustion flamedynamics, necessary to furtherpredictivecapabilities ofmodels for combustion instabilities.Measurementsweremadeof linearflame transfer functions for both velocity and equivalence ratio oscillations. The flame transfer functions showed that the flame behaves as a lowpassfilter for both types of excitation, but some important differences in thegainandcutofffrequencyoccurred.Althoughthegainandcutoff frequency both increased with equivalence ratio for velocity perturbations, they were observed to have no change with operating equivalence ratio for the case of equivalence ratio oscillations. The types of combustion instabilitiesmost commonly encountered in premixedcombustion systemswerefirst characterized byRayleigh [1]. In this type of instability, a feedback loop is formed between the fluctuations in heat release rate (HRR) of the flame and the combustor/flow train acoustics [2,3]. Under certain operating conditions, the coupled system can become unstable, resulting in high-amplitude pressure fluctuations that can be detrimental to combustor hardware as well as efficiency. The specific coupling mechanisms by which these instabilities may arise are a significant area of research and readers are directed to the literature for a more significant discussion of the phenomenon [3,4]. For the purposes of this study, two possible mechanisms were considered [5,6]: coupling through velocity and equivalence ratio oscillations, as depicted in Fig. 1. Velocity (mass flow) coupling occurs when the acoustics directly cause a fluctuating mass flow upstream of the flame. The mechanism for equivalence ratio oscillations is known to be related to the injector design [7]. When considering a system level model of instabilities, knowledge of the flame and acoustic transfer functions is necessary to yield an understanding of the occurrence of instabilities [3]. Making useful predictions of instabilities using a closed-loop model, like the one described here, ultimately relies on component models to predict the individual transfer function blocks, of which the flame is the most difficult to characterize. The flame transfer function (FTF) represents the dynamics of the flame response to a perturbation as a function of frequency: