Transfer function measurements in a model combustor: Application to adaptive instability control

Due to its good performance in terms of low pollutant emissions, the lean premixed, prevaporized combustor design has received a special interest in the field of gas turbines. This article reports an experimental study of a laboratory-scale, premixed, prevaporized, swirl-stabilized combustor operated with preheated air and fed with liquid heptane as fuel. Under certain operating conditions, it exhibits well-established combustion oscillations at frequencies near 400 Hz. To understand and control these instabilities, it is useful to determine the burner response to external modulations. A novel actuator relying on secondary air modulation was designed and integrated in the injector to allow measurements of the combined transfer function of the burner and actuation system. Two different techniques were used for this determination: direct measurement between command and pressure on photo-multiplier signals based on spectral and cross-spectral densities, or identification through a finite impulse response numerical filter. The two techniques are in good agreement. This identification is then successfully applied to active control of combustion instabilities. The mean pressure oscillation amplitude is reduced by about 50% (from 650 Pa to 400 Pa, the noise level being 130 Pa). This modest reduction is due to the fact that the actuator operates at a frequency that is slightly greater than its cutoff frequency and that its effectiveness is therefore degraded. This highlights the difficulties associated with the bandwidth limitations of practical actuators.