Interferometric determination of heat release rate in a pulsated flame

Introducing a new measurand, namely the rate of change of density in line-of-sight, can be of great interest for people working on thermoacoustics and combustion noise. In this work the relationship between the radiated acoustic pressure, the time rate of change of unsteady heat release and of density fluctuations in a laminar pulsated premixed flame is presented. The burner produced a ‘M’-shaped flame which is excited by harmonic modulations of the flow at two forcing frequencies f = 51 and 101 Hz. Measurements were performed by a microphone to record the sound pressure radiated by the flame in the far field, a photomultiplier to collect the light emission from which the heat release rate was estimated, and a laser vibrometer to detect the rate of change of density fluctuations within the reaction region. As the latter technique is a novel approach in combustion analysis a detailed description of the fundamentals of laser interferometric vibrometry is presented. Laser vibrometry offers the advantage to be low-demanding in terms of instrumentation and settings and to be relatively straightforward to process for a broad frequency range. The different signals recorded are correlated. An interpretation of the measurement data is given and the potential of laser vibrometry to obtain time resolved data on density and heat release rate fluctuations in the flame region is demonstrated.