Comparison of Hydroxyl Concentration Profiles using Five Laser-Induced Fluorescence Methods in a Lean Subatmospheric-Pressure H₂/O₂/Ar Flame

Abstract

We have assessed the relative consistency among laser-based fluorescence methods for kinetic studies in flames by comparing the OH concentration profiles determined by five such methods in a lean H₂/O₂/Ar flame at 72Torr. For all the methods, a single rovibronic transition is excited within the A ²Σ⁺−X² Π system of OH. Relative OH concentrations are determined by monitoring the resulting fluorescence using: (1) single-photon excitation within the (0,0) band followed by broadband detection of the (0,0) rovibronic manifold; (2) single-photon excitation within the (0,0) band followed by detection of the (0,1) manifold; (3) single-photon excitation within the (1,0) band followed by broadband detection of the (1,1) rovibronic manifold; (4) two-photon excitation within the (0,0) band followed by detection of the (0,0) manifold; and (5) saturated single-photon excitation within the (0,0) band followed by narrowband detection of one rovibronic transition in the (0,0) manifold. In each case, good agreement is obtained between the relative fluorescence profiles and the absolute OH concentration profile determined by integration of the one-dimensional flame equations. Under our conditions, broadband detection within the (1,1) manifold provides the best compromise between high fluorescence signal and low background, thus giving the most favorable detection limit. The conformity among the five diagnostic methods suggests that the most stringent criterion for presuming agreement between predicted and experimental concentration profiles should be ∼ 15% for radical flame species.