Flame behavior in tapered tubes with swirling flow

Abstract

Lean premixed combustion is one of the most promising techniques to reduce nitrogen oxide (NOx) emissions. However, lean premixed combustors have a narrow stable combustion range compared with diffusion combustors and have a risk of flashback. The combustor does not usually have a constant cross-sectional shape due to production restrictions. It is important to analyze the propagation behavior of the lean premixed flame in the swirl flow in the tapered glass tube where the cross-sectional shape changes and the acceleration or deceleration flow exists. In this study, we presented a visualization of the propagation behavior of lean premixed flame in two tapered tubes. One of the quartz glass tubes expanded toward the tube outlet (Tube A) and the other one narrowed toward the tube outlet (Tube B). The inclined angle of both tubes was 1.6°. We visualized the unburned flow and flame behaviors with swirl flow whose swirl vane angles were 15.0° and 22.5°. The reverse flows in the unburned condition were appeared along the center axis in tube A with 22.5° of vane angle. On the other hand, the reverse flows in the unburned condition did not appear in tube B with 22.5° of vane angle, and the flow was accelerated. The flame propagated along low-velocity flow caused in the center axis in tube A. The flashback in tube B with 22.5° of vane angle was occurred in an extremely lean condition as compared with 15.0° of vane angle. In tube B, a reverse flow field was not observed in the unburned conditions, but a reverse flow region occurred upstream of the flame tip in the swirling flow with 22.5° of vane angle.

Keywords

• lean premixed flame
• tapered tube
• swirl flow
• flame propagation
• reverse flow