Effects of buoyancy on open turbulent lean premixed methane-air V-flames

Abstract

The behavior of lean premixed methane-air open V-flames of wrinkled flame fronts was studied both numerically and experimentally. In the numerical simulation, the mean flow fields of the flames were investigated when gravity was switched on and off to see the buoyancy influence on velocity and flame stretch. The results show that buoyancy accelerates the upward motion of the hot product plumes and hence changes the flow field. However, the changes are mainly concentrated in the far field area and not apparent in the area near flame brushes. The mean stretch of the flame brushes does change under the influence of gravity, but it seems that this does not influence the flame brushes strongly since the stretch rate is rather low. The experiments were done in the Bremen drop tower and the flame wrinkles were observed under both 1g0 and µg by using OH-PLIF. It can be clearly seen that the flame wrinkles were augmented under µg especially in the cases of weak turbulence. Thickness of the flame brushes was calculated according to the iso-lines of mean progress variable ē in order to see the buoyancy influence on the flame wrinkles. It was found that the impact of buoyancy is inversely related to the intensity of turbulence. The two possible mechanisms of the buoyancy influence on flame wrinkles, baroclinic mechanism and the impact of mean stretch, are discussed according to the numerical and experimental results. It seems that baroclinic mechanism is the controlling factor.