THERMALLY ASSISTED FLUORESCENCE OF LASER-EXCITED OH-A (2)SIGMA(-TOOL() AS A FLAME DIAGNOSTIC)

A simple laser-induced-fluorescence measurement technique for turbulen t flame temperature and OH concentration measurement is proposed and s uccessfully tested. The main idea is that a narrow-band tunable excime r laser beam (lambda = 308 nm) is focused into a turbulent atmospheric -pressure nonpremixed flame. The OH molecule Q(1)(3) (X (II)-I-2 v '' = 0, A (2) Sigma(+)v' = 0) transition is excited. By subsequent vibrat ional energy transfer, the distribution in the molecules' upper electr onic energy level (excited CC state) is redistributed. By evaluating t he spectrum of the broadband emission from the v' = 1 --> v '' = 0 and v' = 2 --> v '' = 1 bands with a full spectral fit, we were able to e valuate the temperature and the OH molecule density. The concurrent pr ocesses of quenching, vibrational energy transfer, and rotational ener gy transfer were taken into account in the evaluation process. The res ults were compared with numerical flame calculations and revealed good agreement. One problem with this new proposed application of laser-in duced fluorescence is the self-absorption of the emitted light. This, however, is shown not to be serious, but it has to be checked carefull y. The main advantages are a simple experimental setup and procedure, high signal intensity, and a simple and straightforward data evaluatio n method. Thus this measurement technique is suitable for turbulent fl ame temperature and OH concentration measurement, and it is an alterna tive to other well-established techniques that are much more complicat ed.