Measurement of equivalence ratio fluctuation and its effect on heat release during unstable combustion

This paper presents the first quantitative measurements of equivalence ratio fluctuations during unstable combustion in a lean premixed model dump combustor operating on natural gas with a nominal heat release rate of 75 kW. Equivalence ratio was measured using an infrared absorption probe based on the absorption by methane of the 3.39 mum output of a He-Ne laser. In order to make quantitative equivalence ratio measurements it was necessary to independently measure the absorption coefficient over a wide range of temperatures (293-683 K) and pressures (up to 600 kPa). Using simultaneous equivalence ratio, pressure, and chemiluminescence-based heat release measurements, it was demonstrated that new insights could be obtained regarding the role of equivalence ratio fluctuations during unstable combustion. For example, the measurements indicate that the observed heat release can be primarily attributed to the measured equivalence ratio fluctuations, and they show that the equivalence ratio fluctuations exhibit significantly higher harmonics, whereas the heat release and pressure fluctuations occur predominately at the fundamental frequency of the instability.