Measurement of OH Density and Gas Temperature in Incipient Spark-Ignited Hydrogen-Air Flame

For investigating the spark ignition process of hydrogen-air mixture, density of OH radicals and gas temperature are measured in the incipient spark-ignited hydrogen-air flame using laser-induced predissociation fluorescence (LIPF). The spark discharge occurs between a 2-mm gap with approximately 10 ns pulse duration. Hydrogen-air mixture is ignited by spark discharge of E = 1.2 ~ 1.4E_min, where E is the spark energy and i?min is the minimum ignition energy. In hydrogen(50%)-air mixture ignited with E = 1.35E_min, OH density decreases after the spark discharge from 2 times 10¹⁶ cm^-3 (t = 0 mus) to 2 times 10¹⁵ cm^-3 (t = 100 mus), where t is the postdischarge time. On the other hand, the gas temperature is 900 K at t = 30 mus and 1400 K at t = 200 mus. Hydrogen(15%)- air mixture ignited with E = 1.40E_min also shows the similar tendency to the hydrogen(50%)-air mixture. In stoichiometric (hydrogen(30%)-air) mixture (E = 1.25Em{n), OH density is approximately constant at 2 times 10¹⁶ cm^-3 for 150 mus after the spark discharge, and the gas temperature increases from 1100 K (t = 0 mus) to 1800 K (t = 150 mus). They are higher than those of other mixtures (15% and 50% hydrogen). It indicates that the reaction in the stoichiometric flame proceeds faster than other flame.