Premixed methane-air flame under alternate gravity

Abstract

The study is devoted to the flames instabilities that occur during the transition from the normal gravity to zero one. The experiments were carried out at the Bremen Drop tower. A stoichiometric methane-air flame was investigated during the first second after the capsule release. High amplitude oscillations with periodic flame flashback inside the burner and detaching of the hot spots during the transition period were found. The similarity of oscillation spectra for stoichiometric flames with different flow rates was observed. The coincidence of the first and second fundamental harmonics of the oscillations frequencies for all the studied flows indicates instability mechanism independence from the Reynolds number for the investigated range of Re: 600–2000 (laminar and weak turbulent regimes). It was found that the oscillations logarithmic decrements for all flow Reynolds numbers under investigation have a similar value. Based on a linear approximation, the hydrodynamic instability of a flame was considered and it was shown that the transition from conditions of normal terrestrial gravity to microgravity leads to the flame instability region extension which was confirmed experimentally. A following instability occurrence model was proposed. The buoyant motion of Kelvin-Helmholtz vortices arising at the air/combustion products interface slows down due to gravity decrease. Therefore the vortices deform not only the flame front tip but side part as well, leading to flame front deformations and constrictions, forming separating hot spots.