OH rotational temperature and number density measurements in high-pressure laminar flames using double phase-conjugate four-wave mixing

OH number densities and rotational temperatures up to 9 bar have been measured using double-phase-conjugate four-wave mixing (DPCFWM) in flat laminar premixed methane/air flames. By phase conjugating the backward pump to the forward pump with a phase-conjugate mirror conventional degenerate four-wave mixing (DFWM) becomes DPCFWM and quantitative measurements of OH radicals and OH rotational temperatures in flames at high pressure are possible. Our results show that conventional DFWM with a standard mirror is a low-biased measurement at high pressure primarly due to a fluctuating interaction volume which results from large fluctuating density gradients at the edge of the flame and from flame movement. A comparison is made between a laser-saturated fluorescence technique, conventional DFWM, and DPCFWM at 1, 5 and 9 bar.