In-situ Spectroscopic Measurement of Turbulent Flame-Front Structure

对湍流预混甲烷火焰中的OH~*、CH~*和C2~*自由基进行了局部化学发光测量，以了解火焰前锋结构和反应区化学反应的细节。为此，专门设计了卡塞格伦光学系统，它具有小至LDV测量体积的高空间分辨率。此外，为了获得高的时间分辨率，每个化学发光都用PM管以250kHz的采样率进行检测，并用LDV同时测量这三个自由基和速度，以获得火焰的前锋结构、火焰传播速度和火焰厚度。在火焰前锋处直接测量了局部Damkoler数，并展示了其细小尺度，OH*发射的概率密度函数(PDF)在对应于起皱的层流火焰或小火焰区的火焰前锋位置上显示出双峰。OH*PDF的第一个峰对应于火焰锥体外的高温区，第二个峰与主反应区的化学反应有关。褶皱层流火焰区局部OH*、CH*和C2*发射强度的时间序列信号呈现出不规则的峰值，而这三种化学发光信号却是相互对应的。这一结果表明，在起皱的层流火焰前锋上，OH-C、H-C2反应之间存在很强的联系。用CH和C2排放测量的火焰锋面厚度为0.1-0.3 mm，而用OH*排放测量的火焰锋面厚度约为1.0 mm。此外，根据层流预混火焰中的局部CH*和C2*发射强度，对湍流火焰前锋的局部反应速率和局部混合气强度进行了检测，并与层流预混火焰中这些发射的测量数据进行了比较。提出了一种通过局部化学发光测量来检测局部火焰前锋结构和火焰化学的新的光学诊断方法，并对其性能进行了验证。

Local chemiluminescence measurements of OH*, CH* and C2* radicals were carried out in turbulent premixed methane flame to make clear the details of flame-front structures and chemical reactions in the reaction zone. For this measurement, specially designed Cassegrain Optics, which has a high spatial resolution as small as an LDV measurement volume, was developed. Furthermore, to achieve a high temporal resolution, each chemiluminescence was detected by PM tube at sampling rate of 250kHz.Simultaneous measurements of these three radicals and velocity by LDV were performed to obtain flame-front structure, flame propagating speed and flame thickness. Local Damkoler number were measured directly at the flame-front and their fine scale were demonstrated.The probability density function (PDF) of OH* emission showed bi-modal peaks across the flame-front location corresponding to wrinkled laminar flame or flamelet region. The first peak of the OH* PDF was corresponding to the high temperature region outside of the flame cone and the secondary peak was associated with chemical reaction at the primary reaction zone. Time series signals of local OH*, CH* and C2* emission intensities in wrinkled laminar flame region showed irregular peaks which, however, correspond to each other in the three chemiluminescence signals. This results indicated strong linkage between OH-C H-C2 reactions at wrinkled laminar flame-front. The flame-front thickness measured by CH and C2 emissions were 0.1-0.3mm, while that was about 1.0mm by OH* emissions. Furthermore, local reaction rate and local mixture strength at turbulent flame-front are examined from local CH* and C2* emission intensities in comparison with measured data bases of these emissions in laminar premixed flame.A new optical diagnostics to examine local flame-front structure and flame chemistry by means of local chemiluminescence measurement was proposed and its performance was proven.