Mapping the Density Fluctuations in a Pulsed Air-Methane Flame Using Laser-Vibrometry

Laser vibrometry (LV) is originally a laser-based, line-of-sight measurement technique dedicated to the analysis of surface vibrations. It was lately adapted at TU Graz for monitoring the stability of an air-methane flame (Giuliani, et al., 2006, ASME Turbo Expo, ASME Paper No. GT2006-90413). This paper reports on the mapping of density fluctuations measured with LV in a premixed air-methane flame (free jet; swirl stabilized) with a forced flow modulation (quarter-wave resonator; amplification with a siren). In order to correlate the density fluctuations with the jet aerodynamics and turbulent flame shape, stereoscopic particle image velocimetry and high-speed schlieren visualizations were used. This paper addresses issues regarding the estimate of density fluctuations, the transform from line-of-sight to local measurement with tomographic methods, and the potential of the method for detailed description of thermoacoustic couplings. One emphasized application of LV is its ability to perform precise and low-cost benchmark stability tests on a combustor during the design phase (time-resolved measurement, high frequency and phase resolution on the 5 Hz–20 kHz range with the present equipment and settings, near-constant spectral sensitivity over a large bandwidth, and no seeding required; measurement possible over the whole combustion volume).