Modelling and experimental analysis of the performance of a laser Doppler vibrometer used to measure vibrations through combustive flows

The problem of measuring surface vibrations by a laser Doppler vibrometer operating with the measuring beam across a combustive flow is discussed, in order to assess the possibility of experimentally determining the dynamic behaviour of a burner under operating conditions. The instrument performance is analysed in terms of interfering and modifying inputs through the development of a model of the interferometer and by experimental validation of its predictions. Experiments are carried out on an unconfined CH₄ flame of a Bunsen burner and a metal surface under a known forced vibration. The laser vibrometer output is influenced by the presence of the flame: major effects are optical path length variations of the measuring arm of the interferometer and beam movements. All effects occur at the typical flame flickering frequency and cause a distorted vibration spectra and a reduced signal-to-noise ratio. Beam wandering and defocusing are documented by image acquisition.