Effects of Spark Component on Free Radical Formation in Spark Ignition Process of Combustible Mxture.

对火花点火初期OH自由基的生成进行了实验研究。目的是阐明火花放电对OH自由基初始形成的影响，以及OH自由基在燃烧化学反应中的启动作用。用双单色仪和带通滤光片的超高速摄像机观察了OH自由基自辐射(306 Nm)的行为。用超高速相机进行纹影照相，观察了火焰核心的生长过程。使用丙烷-空气预混气体作为可燃气体：丙烷的浓度在所有试验中为4.7vol%。利用电容火花点燃预混料。电极由不锈钢制成，直径为0.5 mm。火花间隙调整为0.8 mm。放电能量在15~80mJ之间变化，放电后在1.2~40mus范围内成功观察到OH自由基的行为。OH自由基浓度在放电后即刻较高，从10mus开始逐渐下降。随着火花能量的增加，火花点火初期生成的羟基自由基量增加，火核生长速度加快。研究发现，当火花能量较大时，初始火核中几乎没有OH自由基存在，且存在于初始火核边缘附近。从火核的纹影照片中观察到，随着火花能量的增加，火核中心的高温区保持较长的时间。高温区的寿命是火花持续时间的两倍以上。结果表明，这一高温区加速了高温区与丙烷-空气预混物交界处的OH自由基的形成。

Study on OH radical formation at the beginning of spark ignition has been performed experimentally. The purpose is to declare the effect of spark discharge on the initial formation of OH radical and the role of OH radical in the initiation of combustion chemical reaction. A double monochromator and an ultra-high-speed camera with a band pass filter were employed to observe the behavior of OH radical by its self-emission (306nm). The growth of a flame kernel was observed by schlieren photography with the ultra-high-speed camara. Propane-air premixture was used as a combustible gas : The concentration of propane was 4.7 vol% for all tests. Capacitance spark was employed to ignite premixtures. Electrodes were made of stainless steel and their diameter were 0.5 mm. Spark gap was adjusted to be 0.8 mm. The energy of spark was varied from 15 to 80 mJ.The behavior of OH radical was observed successfully from 1.2 to 40 mus after a spark discharge. The concentration of OH radical is high just after a spark discharge and decreases gradually from 10 mus. As spark energy increases, amount of OH radical formed at the beginning of spark ignition increases and growth rate of a flame kernel increases. It was found that, when the spark energy is large, there is little OH radical in the initial flame kernel and it exists near the edge of the initial flame kernel. From schlieren photographs of flame kernels, it was observed that high temperature region in the center of the flame kernel remains longer as spark energy increases. The life time of the high temperature region is longer than double the spark duration. It is suggested that this high temperature region accelerates the formation of OH radical at the boundary between the high temperature region and the propane-air premixture.