Experimental analyses of the transition to secondary thermoacoustic instabilities of H2-CH4-air premixed flames in slender tubes.

Thermoacoustic coupling between a premixed flame and the tube that encloses it causes the reactive front to switch from a steadily propagating state into an oscillatory one. For certain parametric sets (tube length, mixture equivalence ratio, gas-wall temperature) the primary, low-amplitude, oscillations transform into secondary oscillations, which come with corrugated flames and higher amplitude acoustic waves.

An experimental set-up has been constructed, consisting in an open-closed round tube of D = 15.25 mm of extendable length L = 0 – 1.8m . The tube is filled with a selected mixture of air, methane and hydrogen which is ignited at the open end. Simultaneous measurements of pressure at various locations, along with high-speed videos of the flame, provide information on the interaction between the detected flame surface and pressure waves.

The transitional mechanism is observed to depend on the radial coordinate. To this end, theoretical analyses of a tube ideal acoustics with realistic non-homogeneous temperature distributions are performed. They predict axisymmetric acoustic velocity modes of much higher amplitude around the axis and, also, vorticity modes in the vicinity of the flame, which are compared against the effect of non-ideal pulsating flow profiles with preliminary PIV measurements to dilucidate the origin of the instability.