微小型凹腔燃烧器中贫燃H2/空气火焰尖端分裂的形成机理研究

Flame tip opening is a classic problem in combustion field, which usually occurs in strongly stretched lean premixed flames with a sub-unity effective Lewis number. The occurrence of this phenomenon will lead to fuel leakage and decrease of the combustion efficiency. A thorough investigation has been conducted on flame tip opening in normal scale combustion, but little has been done about it in micro- and meso-scale combustion. Recently, we applied the cavity to anchoring the flame in micro channels, which demonstrated that the flame blow-off limit is greatly extended. However, for lean premixed H2/air flame, the flame tip opening phenomenon occurred at relatively high inlet velocity because its effective Lewis number is far less than unity. In this project, we are dedicated to illuminate the formation mechanisms of flame tip opening in the micro cavity-combustor based on numerical simulation and experimental investigation. Furthermore, we aim at reveal the non-monotonic effect of wall thermal conductivity, cavity location, channel gap distance, and cavity depth through a comprehensive analysis of the interactions among the Lewis number effect, flame stretching effect, heat recirculation effect and heat loss effect. This study not only can enrich the theory of micro combustion, but also can provide a guideline for optimal design and operation of the micro cavity-combustor.

火焰尖端分裂是燃烧学中的一个经典课题，常发生于预混气的有效Lewis数小于1、火焰锋面受到强烈拉伸的情况。这种现象将导致燃料泄漏和燃烧效率的降低。关于常规尺度燃烧中的火焰尖端分裂现象，文献中已经进行了充分的研究。但对微小通道中火焰尖端分裂现象的研究还非常少。本课题组近来将凹腔应用于微通道稳燃，发现火焰的吹熄极限虽然大大提高，但对于贫燃预混H2/空气火焰来说，由于其有效Lewis数远小于1，在较高的进气速度下火焰尖端发生了分裂。本项目拟采用数值模拟和实验相结合的方法，全面分析Lewis数效应、拉伸效应、热循环效应和散热损失效应之间的相互作用，以期阐明微小型凹腔燃烧器中火焰尖端分裂的形成机理，揭示壁面导热系数、凹腔位置、通道间距、凹腔深度等关键参数的非单调影响规律。课题的开展不仅能够丰富微尺度燃烧的理论体系，也为微小型凹腔燃烧器的优化设计和运行提供理论指导。

利用凹腔能显著扩大微通道燃烧器的吹熄极限，但是对于贫燃氢气/空气火焰来说，在较大的进气速度下将发生尖端分裂现象，导致燃料泄漏和燃烧效率下降。本项目主要通过数值模拟方法，系统地研究了微通道间距及凹腔几何参数、壁面材料热物性及热管理、进气温度与压力、稀释剂种类与氧气浓度等因素对微型凹腔燃烧器中贫燃氢气/空气燃烧效率的影响，结果表明：（1）微通道间距对火焰尖端分裂的影响是非单调的。（2）较低进气速度下，凹腔后壁倾角越小，燃烧效率越高；而较大进气速度下，凹腔后壁倾角越小，燃烧效率越低。（3）进气速度较低时，燃烧效率随着凹腔深度的增加而降低，而在较高速度下，燃烧效率却随着凹腔深度的增加而增加。（4）火焰分裂极限与壁面导热系数之间存在非单调的变化规律；（5）壁面发射率越大，火焰分裂极限越小。（6）在高进气速度下，双层壁面燃烧器（内层采用较高导热系数的碳化硅，外层采用较低导热系数的石英）的燃烧效率明显高于相应的单层壁面燃烧器。（7）提高燃烧器入口温度可以有效抑制火焰尖端分裂，提高燃烧效率。（8）压力从1.0 atm降到0.5 atm的过程中，燃烧效率呈现非单调的变化趋势。（9）将稀释剂由N2更换为He时，能有效地抑制火焰尖端分裂现象，提高燃烧效率。(10) 增加氧气浓度有利于火焰在较高的进气速度下仍然能保持较大的燃烧效率。. 从刘易斯数效应、拉伸效应、热循环效应以及散热损失效应等多个角度全面分析了以上规律，揭示了火焰尖端分裂的形成机理。这些研究结果能为微型凹腔燃烧器的优化设计和运行提供理论指导。