Validation of surface chemistry models using low pressure stagnation-pointflames: Measurements of OH above platinum surfaces

Near-surface laser-induced fluorescence (LIF) measurements of OH are perfor med in the post-flame region of low pressure hydrogen/oxygen flames in stag nation flows above catalytic and non-catalytic surfaces. Measurements of th e normalized gradient of the OH number density near the surface are shown t o be sensitive to changes in surface reaction rates, and thus can be used a s a test of surface chemical mechanisms. In addition, such measurements do not require an absolute calibration, which increases the accuracy of the co mparison. A comparison of the observed results for varying substrate temper ature, system pressure, and flame equivalence ratio suggests that the recom bination probability of OH on platinum is overpredicted in general by curre nt surface models, and that the dependence of recombination on pressure is stronger than predicted. The qualitative trends for OH recombination with t emperature and equivalence ratio are similar to predicted trends.