2-DIMENSIONAL IMAGING OF OH RADICALS IN SPARK-IGNITION KERNELS BY LASER-INDUCED FLUORESCENCE

Two-dimensional laser-induced fluorescence imaging of OH radicals has been used to observe the temporal and spatial development of the OH di stribution during the ignition of a lean methane-air mixture (phi = 0. 56) ignited by an electrical spark. A frequency-doubled dye laser, whi ch is pumped by a XeCl excimer laser, has been used to excite fluoresc ence from OH radicals. A series of OH concentration maps from a cross- section through the ignition kernel during the ignition process have b een obtained. It can be seen that the gas flow induced by the initial shock wave from the breakdown phases has a significant influence on th e development of the ignition kernel. The ignition kernel is produced by a narrow cyclindrical plasma channel with high temperature, then ex pands to an ellipsoid during the ignition. At about 1 ms after the ons et of the spark the ignition kernel expands from the ellipsoid to a to roidal shape. A flame front emerges from the ignition kernel at about 10 ms.