Effect of the excitation line on hydroxyl radical imaging by laser induced fluorescence in hydrogen detonations

The present study aims to evidence the effect of the excitation line on the planar laser-induced fluorescence of hydroxyl radical (OH-PLIF) imaging in H2-fueled detonation wave. We experimentally validated an updated laser-induced fluorescence (LIF) model, called KAT-LIF, which simulates spectrally-resolved fluorescence spectra, using a recently developed optical detonation duct. We numerically investigated the effects of the excitation line, the initial pressure (20–100 kPa), and the diluent (N2/Ar) on the fluorescence spectrum, the spectrally- and one-dimensionally-averaged LIF intensity, and the quantitative capabilities of the OH-LIF measurements for different 2H2-O2-3.76diluent detonable mixtures. The investigated excitation lines were (0,0)Q1(7), both (1,0)Q2(8) and (1,0)Q1(9), and (1,0)Q1(6), which all belong to the A2Σ+←X2Π transition. The main findings are the following: (i) considering the commercially available OH filters, Q2(8)+Q1(9) excitation scheme has the highest LIF intensity for all the investigated H2-fueled detonations, while Q1(7) could provide the strongest intensity with better (custom) collection optics; (ii) the maximum LIF intensity decreases with increasing pressure for all the excitation schemes; (iii) using a single point calibration, at the fluorescence peak, it is not possible to perform quantitative measurements of OH radicals for any H2-fueled detonation, using the conventional excitation schemes. Finally, we experimentally evidenced more favorable excitation schemes to obtain qualitative information far behind the front, by employing the saturated regime of fluorescence or the optically thin linear regime with appropriate laser configuration. These two excitation schemes correspond to more appropriate LIF strategies that will enable better detonation flow visualization in future studies.