高Karlovitz数下稀燃预混火焰吹熄机制研究

This project focuses on the study of an experimental high Karlovitz number (Ka>100) piloted premixed flame, which was generated by a specially designed burner with two stratified inlets. The flowfield and flame structure will be captured by PIV and CH/OH-PLIF, separately. A series of processing methods will be applied to the data analysis. Measurements of flow fields will reveal useful information about the level and location of the shear layers. Imaging of Laser-Induced Fluorescence (LIF) of OH and CH will show the degree of flame front fragmentation increases implying the occurrence of local extinction even in regions extremely close to the pilot. The flame surface area, marked by LIF-CH is taken as an indicator of consumption rates of reactants. The flames investigated here will bridge the regions of broadened preheat thin reactions as well as the broken reaction zone in Regime diagrams. On the basis of CH layer thickness and flame quenching mechanism, modified regime diagrams will be proposed. The evolution of the flame structure, flow field, and the resulting flame strain rate are analyzed throughout extinction to distinguish the physical mechanisms of blowout. The effect of hydrodynamic instabilities and thermal-diffusive instability are also considered. A frequency analysis will be performed to characterize the dynamic changes of the instability modes during flame extinction. A criterion will be developed to consistently predict the onset of global extinction for the different inflow velocity regimes. The fundamental understandings of blow-off mechanism will help to optimize the combustor design employing lean premix mode.

本项目以高Karlovitz数（Ka>100）预混稀燃火焰为对象，应用粒子图像测速仪（PIV）、平面激光诱导荧光（PLIF）和增强型高速相机等设备测量流场参数、火焰结构及其动态特性，并自主编写后处理程序对湍流和火焰结构进行表征。通过探究强湍流下火焰化学反应区的几何特征与拓扑特性，理解火焰的局部淬熄机理，掌握小尺度涡与燃烧之间的相互作用规律，从而明确湍流燃烧模式分区图中破碎区的边界。同时，改变燃料掺混、初始温度、当量比等以控制火焰的热膨胀率、路易斯数，在考虑火焰自身不稳定性的基础上，研究火焰的吹熄极限及临吹熄极限时的拉伸率分布和火焰结构变化，进一步掌握火焰由局部淬熄向整体吹熄的转变机制。本项目的基础性研究工作为解决稀燃预混火焰的吹熄问题提供理论指导，为采用预混稀燃技术的清洁高效燃烧室的设计和优化提供可靠的实验支撑。

本项目开展了一系列基于激光诊断技术的甲烷/氢气稀燃预混火焰研究，系统探究了高Ka数下稀燃预混火焰的稳定性问题。获得主要成果有：.1）研究了火焰自身不稳定性对湍流火焰的影响规律，运用本征正交分解法对湍流火焰前锋面进行多尺度分析，发现了增加燃烧强度的最小褶皱尺度，揭示了富氢燃料因质热扩散不均匀性导致的不稳定效应，即火焰前锋面小尺度褶皱增加，波动幅度增大，湍流火焰速度增加。.2）研究了甲烷掺氢预混稀燃火焰（CH4/H2/air）稀燃吹熄不稳定性及掺氢稳焰机制。在当量比降低/湍流剪切强度逐渐增大，从而不断靠近吹熄极限过程中，对流动-火焰耦合机制进行了深入研究。研究揭示了火焰根部最先可能因过量拉伸导致局部熄火，是促进火焰整体吹熄的主要因素，内剪切层小尺度涡进入化学反应区能进一步促进局部熄火。在发生局部淬熄后，必须经过第二阶段，吹熄才会发生。研究发现剪切层涡在临吹熄时与火焰作用加剧，随着局部开口的逐渐增加，当火焰逐渐暴露在剪切层中时，火焰被大尺度的流动不稳定性（如涡脱落不稳定性、Kelvin-helmoholtz不稳定性等）主导，最终发生吹熄。.3）实现了天然气稀燃火焰从增厚火焰区到破碎区的火焰内部结构可视化。发现了湍流通过增加反应面积增强了燃烧强度，从而增加了射流火焰的稳定性；但当湍流强度超过一定值，火焰进入破碎区，火焰将出现淬熄现象。根据火焰的整体吹熄和反应区厚度的变化，在湍流预混火焰分区图上提出增厚反应区和破碎区的边界。