In-situ Spectroscopic Measurement of Turbulent Flame-Front Structure

在紊流预混甲烷火焰中进行了OH*、CH*和C2*自由基的局部化学发光测量，以明确火焰前结构和反应区化学反应的细节。为此，开发了专门设计的Cassegrain Optics，它具有小到LDV测量体积的高空间分辨率。此外，为了获得高时间分辨率，每个化学发光都被PM管检测，采样率为250kHz。利用LDV同时测量了这三种自由基和速度，得到了火焰前缘结构、火焰传播速度和火焰厚度。在火焰前直接测量了局部Damkoler数，证明了它们的精细尺度。OH*发射的概率密度函数（PDF）在火焰前位置呈现双峰峰，对应于褶皱层流火焰或小火焰区域。OH* PDF的第一个峰对应于火焰锥外的高温区，第二个峰对应于主反应区的化学反应。褶皱层流火焰区域局部OH*、CH*和C2*发射强度的时间序列信号呈不规则峰状，但在三种化学发光信号中却相互对应。结果表明在起皱层流火焰前OH-C - H-C2反应之间有很强的联系。CH和C2发射测得的火焰前厚度为0.1 ~ 0.3mm， OH*发射测得的火焰前厚度约为1.0mm。此外，从局部CH*和C2*发射强度与层流预混火焰中这些发射的实测数据进行比较，研究了湍流火焰前的局部反应速率和局部混合强度。提出了一种利用局部化学发光测量来检测局部火焰前缘结构和火焰化学性质的新型光学诊断方法，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.