Characterization of detonation waves by simultaneous OH and NO planar laser-induced fluorescence

This study presents a thorough evaluation of the simultaneous planar laser-induced fluorescence (PLIF) on hydroxyl (OH) radicals and nitric oxide (NO), for characterizing hydrogen detonation fronts. The study combines experimental and numerical results to evaluate the benefits and drawbacks of different visualization strategies, namely OH-PLIF alone, NO-PLIF alone, OH-PLIF + shock det. (comparable to simultaneous OH-PLIF and schlieren), and combined OH- and NO-PLIF. Main findings are: (i) For the first time, simultaneous single-shot OH- and NO-PLIF visualizations are demonstrated on detonation waves for three mixtures with varying stability levels: Mixture (a) 2H(2)-O-2-3.76Ar, Mixture (b) 2H(2)-O-2-3.76N(2), and Mixture (c) 3H(2)-CH4-3.5O(2)-3.76Ar; (ii) the simultaneous visualization evidences the self-similarity of both techniques, with a clear interlock of OH-PLIF imaging into NO-PLIF imaging. This means that the structure of the OH-reaction zone, typically visualized from OH-PLIF, can be accurately described from NO-PLIF imaging alone, from which induction zones can also be visualized. In addition, both can clearly depict the wrinkling of the front and the presence of unburned pockets; (iii) the effectiveness of the four visualization strategies at characterizing the detonation wave of a H-2-O-2-3.76Ar mixture is determined using ZND and spectroscopic simulations. Interestingly, the induction zone (Delta(i)) is more accurately predicted by NO-PLIF (3% error) than with the combined OH + shock det. (10% error). This study highlights the potential of NO-PLIF diagnostic in characterizing detonation waves with low levels of confinement (i.e., d/lambda >>> 1) both qualitatively (e.g., reaction zone structure) and quantitatively (e.g., Delta(i) measurements) using a single diagnostic. Additionally, the simultaneous OH- and NO-PLIF diagnostic may offer significant advantages in characterizing the detonation waves in engine-relevant conditions (i.e., in complex geometries with unburned pockets or local quenching events), as combining both diagnostics enables to discriminate unburned pockets and local quenching events.