微小型凹腔燃烧器内甲烷/空气预混火焰特性

Combustion characteristics of premixed CH_4/air flame in mesoscale channel with cavities

对有凹腔的微细通道内甲烷/空气的预混燃烧进行了实验研究,并与无凹腔的情况进行了比较。结果表明,无凹腔时,只出现了稳定或振荡的倾斜火焰;有凹腔时,在很宽的速度范围内火焰均能被有效地稳定,当进气速度接近吹出极限时,火焰锋面发生弯曲和脉动。当量比为0.8、0.9和1.0时有凹腔的微细通道的吹出极限分别为0.8、1.35和1.75 m·s-1,是对应进气条件下燃烧速度的几倍,这表明凹腔具有很强的稳燃能力。数值模拟结果表明,凹腔的斜壁与下游的水平壁面之间的转折点存在很大的速度梯度和剪应力,导致了火焰在高速下被拉断而吹出。总之,有凹腔的微细通道内火焰的稳定性主要由反应区和流场之间的相互作用决定。

Premixed CH4/air combustion in a mesoscale channel with cavities were experimentally investigated and compared with that without cavity. The experimental results demonstrate that no symmetric stable flame is observed in the channel without cavities and fame is prone to inclining and pulsating. In contrast, flame can be effectively anchored by the recirculation zone and low velocity zone in the channel with cavities. When the inlet velocity is close to blowout limit, curved fluctuating flames occurs. The blowout limit of the channel with cavities is 0.8, 1.35 and 1.75 m ~ s-1 for the equivalence ratio of 0.8, 0.9 and 1.0 respectively, which is several times larger than the corresponding burning velocity of incoming CH4/air mixture. These indicate that the cavities have a strong ability to anchor fame. Numerical simulation is performed to help analyzing the flame blowout mechanism, and reasonable accuracy of the numerical model adopted is confirmed. Results reveal that a large shear stress exists at the transition point between the ramped cavity wall and downstream inner wall, making fame split at high inlet velocity, and it is difficult to stabilize the flame in a straight channel. Once the second part of the reaction zone is separated from the one in the cavity, it is prone to blowout. In summary, flame behavior in themesoscale channel with cavities strongly depends on the interactions between the reaction zone and flow field.