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., ASME Turbo-Expo 2006, GT 2006-90413). This paper reports on the mapping of density fluctuations measured with LV in a premixed air-methane flame (free-jet; swirl-stabilised) 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 PIV and high-speed schlieren visualisations were used. The 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 emphasised 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–20kHz range with the present equipment and settings; near-constant spectral sensitivity over a large bandwidth; no seeding required; measurement possible over the whole combustion volume).