Effect of tube length on the acoustic parametric instability of downward propagating flames in tubes

Abstract

The influence of tube length on the self-excited thermoacoustic parametric instability of laminar premixed flames is investigated experimentally and analytically. Lean propane-air flames were propagated in a quarter-wavelength resonator where the tube length is systematically modulated to allow the observation of planar flames and parametrically unstable flames. Theory and experiments show that longer (shorter) tubes, resonating lower (higher) frequencies, lead to larger (smaller) length scales of sinusoidal flame wrinkling at the onset of parametric instability. The difference between the experimental and theoretical critical wavenumbers are attributed to the value of the Markstein number.