EXPERIMENTAL CHARACTERIZATION OF THE DAMPING OF FUEL-AIR RATIO FLUCTUATIONS USING TRANSFER FUNCTION ANALYSIS

摘要

This paper presents the theoretical and experimental framework used to characterize the capability of premixers used in Dry Low Emission (DLE) gas turbines to dampen fuel-to-air ratio (FAR) oscillations and thus serve as a passive control device for combustion noise. Based on a convection-diffusion volume model, transfer function analysis in the frequency-domain was used to describe the interaction between convection and turbulent diffusion mechanisms. The study showed that the best achievable damping was obtained when the ratio of convection to turbulent diffusion effects (expressed in terms of Peclet number) was unity. For this particular condition, the spreading of Residence Time Distribution (RTD) is optimal hence decreasing the coherence between incoming and outgoing perturbations. For large Peclet numbers, mixing mechanisms are not sufficient to dampen incoming FAR fluctuations and for very small Peclet numbers FAR perturbations can be communicated almost instantaneously to the premixer outlet, without attenuation.This theoretical framework provides a new and easy design tool for optimizing premixers' efficiency in attenuating FAR perturbations. The premixer length and turbulent mixing level can be chosen to maximize damping at frequencies that might trigger combustion noise. This investigation also supplies the framework to characterize experimentally the performance of premixers with a relatively simple and inexpensive setup. The effectiveness with respect to damping, of a standard mixing duct and a more recent "FAR-Damping" premixer was assessed from the magnitude of the measured premixer transfer function. The results showed an excellent ability of the FAR Damper Concept to attenuate fuel-to-air ratio perturbations and out-perform the more conventional premixer. Moreover, the excellent agreement obtained between the measurements and the fitted model validated the procedure. An additional benefit of this procedure is to provide RTDs from transfer functions measured in the frequency domain.