Two-Dimensional Direct Quenching Measurement of OH in a Cross Section of a Bunsen Flame

In flame analysis, the laser-induced fluorescence (LIF) method is becoming popular for detecting intermediate species and measuring temperature. However, when a typical tunable laser with a pulse duration of around 10 ns is used, the pressure of the target to be measured is often too high to obtain a fluorescent signal having a linear relationship to molecular concentration, due to collisional quenching by which relaxation without fluorescence occurs. In a reactive field such as a flame, especially when two-dimensional measurement is desired, the problem of quenching is more serious because the quenching time constant differs at each measuring point. Accordingly, an effective two-dimensional quenching measurement method is desirable. For this purpose, we attemped to determine the quenching time constant distribution, using an image intensifier and a XeCl excimer-laser-pumped dye laser with a doubling crystal, which is the typical and widely used tunable laser, at the pressure at which the LIF signal is significantly affected by collisional quenching. In this way, we succeeded in determining the quenching time-constant distribution in the cross section of a premixed Bunsen flame at 1/2 atmospheric pressure. In addition, this experiment is the first measurement of two-dimensional fluorescence decay time on the order of a few nanoseconds.