Study on combustion in a small-scale channel that is equivalent to reaction zone thickness

了解微通道内的燃烧特性对微尺度燃烧室的发展具有重要意义。然而，小规模阻止了实验方法。对于这个问题的一个可能的解决方案是在低压条件下使用微通道。利用该模型，可以对常压下微小管内的燃烧进行实验模拟。圆柱形石英玻璃管（内径= 2.0)采用类似于热再循环的平焰燃烧器加热。研究了压力（0.06-0.6 atm）、流速和甲烷-空气混合物组成对消光极限的影响.实验和数值模拟都观察到了低压下的温和燃烧。0.2atm下的火焰厚度比常压下的火焰厚度大10倍。从而实现了直径与火焰厚度几乎相等的极细管内燃烧。一般情况下，在正常尺度管内，火焰在熄灭极限附近具有曲率。然而，目前的火焰保持了扁平的形状，只是变厚了。实验结果表明，稀燃侧的流速上限大于浓燃侧，即可燃区并未成为关于化学计量比对称的区域。然而，数值模拟表明，可燃极限在φ=0.8附近最大，而且对于更稀薄的混合物，可燃极限的上限预计会变小.由于淬火Peclet数非常小，因此向壁的热传递相当大。而在本研究中，由于外部加热的热损失小，火焰可以稳定.在接近吹离条件下，贫侧的化学反应时间比富侧的化学反应时间快，因此火焰可以稳定到高流速。这是对可燃区实验结果的一种解释。

Understanding the combustion characteristics in a microchannel with heat recirculation is important for the development of micro scale combustors. However, small scales prevent the experimental approach. One possible solution for this problem is using of a micro channel under the low pressure conditions. By this, combustion in extremely small tube at normal pressure is expected to be simulated experimentally with meso-scall microchannels. Cylindrical quartz glass tube (i.d.=2.0) heated by flat flame burner which resembles heat recirculation was employed. Effects of pressure (0.06-0.6 atm), flow velocities, and methane-air mixture compositions at extinction limit were investigated.1. A mild combustion under low pressure was observed both experimentally and numerically. Flame thickness at 0.2atm became ten times larger than that at normal pressure. Therefore, the combustion in extremely small tube which diameter was almost the same as flame thickness was realized. In general, the flame has the curvature near the quenching limit in normal scale tube. However, the present flame retained the flat shape and only became thick.2. The experimental results showed the upper limits of flow velocity for fuel-lean side is larger than those for rich side, that is, the flammable region didn't become symmetric about stoichiometric ratio. However, the numerical simulation showed the flammable limit was largest near φ=0.8 and furthermore it is expected that upper limits for much leaner mixture become small.3. Since the quenching Peclet number is very small, heat transfer to the wall is quite large. However, in this research, the flame could be stabilized because of small heat loss by external heating.4. Close to blow off conditions at lean side, chemical reaction time is faster than that of rich side therefore the flame can be stabilized up to the high flow velocity. This is one interpretation for experimental result of flammable region.