Unsteady premixed flame behavior in tapered circular tubes with weak swirling flow

抄録

Lean premixed combustors have a narrow stable combustion range compared with diffusion combustors and have flashback. Due to production constraints, the combustor does not usually have a uniform cross-sectional shape. It is important to analyze the propagation behaviors of the lean premixed flame in the swirling flow in the tapered tube where the cross-sectional area changes and the acceleration or deceleration flow exists. In this study, we presented characteristics of the unsteady flame behavior of a lean premixed burner with a tapered circular tube. One of the glass tubes expanded toward the tube outlet, and the other narrowed toward the tube outlet. We analyzed characteristics of flame behaviors with relatively weak swirl flow whose geometrical swirl number in the inlet condition was from 0.24 to 0.37 by using high-speed PIV measurements in the horizontal and vertical planes. The reverse flows appeared along the tube's center axis in the tube, which expanded toward the outlet. On the other hand, the reverse flows did not appear in the tube, which narrowed toward the outlet, and the flow was accelerated. The flame propagated along the reverse flow caused in the center axis in the expanding tube. In the narrowing tube, a reverse flow field was not observed under unburned conditions, but a reverse flow region occurred upstream of the flame tip. In the narrowing tube, the flame propagation made a large difference in the flow fields compared to the unburned condition. Based on the flame propagation behaviors and the PIV results of horizontal planes, it was found that for the same swirl number at the inlet condition, the flame speed in the case of the narrowing tube at the measurement cross-section was almost twice that of the case of the expanding tube under all experimental conditions.