Propane-air Markstein numbers from wavenumber measurements of self-excited thermoacoustic parametric instability*

Self-excited thermoacoustic parametric instabilities in downward propagating laminar premixed propane-air flames in open-closed tubes are studied. The spatial and temporal evolution of the flame in transition to parametric instability is investigated by pressure fluctuation measurements and synchonized high speed imaging. Different tube lengths are used to modulate the resonant frequency, and the parametric flame response to varying acoustic frequencies for the same mixture is studied in detail. The Markstein number, characterizing the influence on flame stretch/curvature on flame stability, is retroactively determined from wavelength measurements of cellular flames at the onset of parametric instabilities. The Markstein number is estimated by employing a one-step chemistry laminar flame model under acoustic-flow excitation whose analytical functions are reduced into a Mathieu equation. The Markstein number obtained in the present study are compared with Markstein numbers in the literature from direct, indirect and computational measurements.