Combustion of Lean Methane–air Flames in Mesoscale Reactor with Opposite Gas Flows

In this work, we study a countercurrent premixed rich-lean coaxial reactor to realize an overall lean combustion of hydrocarbon fuels. Flame topology and stability of the axially symmetric reaction fronts of methane-air mixtures are investigated both numerically and experimentally. These studies show that depending on the equivalence ratio of the mixture fed into the inner tube, playing the role of supporting a pilot burner, there can occur two or three-layered stable flame structures. It is demonstrated that flames can be stabilized over a wide range of mixture compositions and flow rates. In particular, it is possible to stabilize methane-air flames with the overall equivalence ratio below the lean flammability limit for this mixture due to the existence of the weak supporting flame and combined effect of the flame stretch and heat recirculation realized in the examined configuration. The diverse unsteady combustion modes, including flame oscillations and flame periodic extinction and ignition, are also found. The prospects of further investigations are discussed.本研究针对逆流预混浓-稀同轴反应器展开探究，旨在实现烃类燃料的整体稀燃。本工作通过数值模拟与实验手段，对甲烷-空气混合物的轴对称反应前沿的火焰拓扑结构与稳定性开展了系统研究。研究结果表明，基于充当引燃辅助燃烧器（pilot burner）的内管通入混合气的当量比（equivalence ratio）差异，体系可形成双层或三层稳定火焰结构。研究证实，火焰可在宽泛的混合气组分与流速范围内实现稳定。具体而言，由于存在弱引燃火焰，且本研究所采用的构型可实现火焰拉伸（flame stretch）与热回流（heat recirculation）的协同作用，因此即便整体当量比低于该混合气的稀燃极限（lean flammability limit），仍可稳定甲烷-空气火焰。此外还观测到多种非稳态燃烧模态，包括火焰振荡、周期性熄火与复燃现象。最后对后续研究的发展前景进行了探讨。