Planar OH density and apparent temperature measurements in a supersonic combusting flow

Distributions of the absolute density of OH and of the population densities of five OH rotational states were obtained in a supersonic H-2-air combustion tunnel using planar laser-induced fluorescence. A tunable KrF excimer laser was used to excite, nonsimultaneously, the ive separate ro-vibronic lines in the (3-0) vibrational band of OH. Measurements were calibrated against measurements in an atmospheric air furnace, where known quantities of OH are formed by the thermal dissociation of H2O molecules. Despite flame intermittency, a sum of each of these nonsimultaneous measurements represents the time-averaged population density of the probed state. Present results show that these time-averaged measurements of the populations of the probed states still follow a Boltzmann distribution. Therefore, measurements of the average population of any pair of states can provide the two independent average properties required to define the average thermodynamic state of OH.