Self-Turbulent Flame Speeds

Abstract

This paper reports an experimental investigation of premixed propane and methaneair flames propagating freely in tubes 1.5 m long and with diameters ranging from 26 to 141 mm. The thermo-acoustic instability was eliminated by means of a novel acoustic absorber placed at the closed end of the tube. We first remark that the flame can adopt different shapes either quasi-axisymmetric and normal to the mean direction of propagation, or inclined with a larger propagation speed because of the increase in flame surface area. The

Premixed flames propagating freely in tubes

“…However it will be seen later that even slow flames obtained after axial ignition are slightly asymmetric. The measured values are in good agreement with previous measurements in the same conditions [28], except that much faster slanted flames speed were obtained here for stoichiometric propane-air flames thanks to the new type of igniter. It is worth noticing that previous measurements were done by igniting the flame with a lighter without any control of the initial condition.…”

“…In order to compare these results to numerical models, we have to evaluate the cut-off wavelength in each configuration. According to analytical calculations of the stability of premixed planar flames including expansion effects, gravity and preferential diffusion [1, 28,31], and for downward propagating flames above the threshold of cellular instability, there is a band of unstable wavelengths limited by two neutral wave numbers:…”

“…Flames in tubes with smaller diameter would be affected by heatlosses while tubes with larger diameter can lead to secondary instabilities on the flame [28]. The equivalence ratio, φ, of the premixed gas is controlled via a PC-interface connected to mass-flow regulators.…”

“…The acoustic damper at the bottom part of the burner consists of a small annular slit, of height h and length l, that dissipates acoustic energy by terminating the tube with a real (resistive) acoustic impedance equal to the characteristic acoustic impedance of free air (see ref. 28 for details). A lightweight refractory fabric is placed over the open end of the burner to prevent mixing with ambient air while permitting the exhaust of the premixed or burnt gas.…”

“…The movement of a flame element will then only results from its own propagation at the normal flame speed U L and from its advection at the local gas velocity produced by hydrodynamical effects induced by the whole flame shape. A recent experiment showed that it was possible to observe an almost stationary downwards propagation of premixed methane-and propane-air flames in tubes of various diameters, thanks to a new type of acoustic damper that eliminates the thermo-acoustic instability which otherwise would produce large acceleration effects on the flame and change drastically the flame propagation [28]. The attention was paid to the velocity of propagation of axis-symmetrical flames to be compared to self-similar relations describing free flame propagation in tubes [26][27].…”