Thermally assisted fluorescence of laser-excited OH A^2Σ^+ as a flame diagnostic tool

Abstract

A simple laser-induced-fluorescence measurement technique for turbulent flame temperature and OH concentration measurement is proposed and successfully tested. The main idea is that a narrow-band tunable excimer laser beam (λ = 308 nm) is focused into a turbulent atmospheric-pressure nonpremixed flame. The OH molecule Q(1)(3) (X(2)II υ″ = 0, A(2)Σ+υ″ = 0) transition is excited. By subsequent vibrational energy transfer, the distribution in the molecules' upper electronic energy level (excited Σ(+) state) is redistributed. By evaluating the spectrum of the broadband emission from the υ' = 1 ? υ″ = 0 and υ' = 2 ? υ″ = 1 bands with a full spectral fit, we were able to evaluate the temperature and the OH molecule density. The concurrent processes of quenching, vibrational energy transfer, and rotational energy transfer were taken into account in the evaluation process. The results were compared with numerical flame calculations and revealed good agreement. One problem with this new proposed application of laser-induced fluorescence is the self-absorption of the emitted light. This, however, is shown not to be serious, but it has to be checked carefully. The main advantages are a simple experimental setup and procedure, high signal intensity, and a simple and straightforward data evaluation method. Thus this measurement technique is suitable for turbulent flame temperature and OH concentration measurement, and it is an alternative to other well-established techniques that are much more complicated.