A theoretical model that determines the optimum excitation frequency for obtaining a fluorescence signal with a strong dependence on fuel mole-fraction is presented for supersonic fuel-air compressible mixing studies. The challenge associated with this is to maintain a high sensitivity to fuel mole-fraction with minimal sensitivity to temperature and pressure in a flow with large temperature variations and pressure gradients. The results of the model are applied to the mixing region behind various scramjet fuel injectors in a shock tunnel to measure fuel mole-fraction. Hydrogen fuel at a Mach number of 1.7 is injected into a mostly N2 free stream at Mach 4.8. Experimental fluorescence images are presented in streamwise and spanwise planes. Nomenclature cp Specific heat capacity at constant pressure, J.kg −1 cp,∞ Specific heat capacity of pure freestream, J.kg −1 c p,f uel Specific heat capacity of pure fuel stream, J.kg −1 f J Boltzmann fraction of the absorbing state g Spectral overlap integral, 1/cm −1 ga Absorption line shape, 1/cm −1 g l Spectral profile of laser, 1/cm −1 k Boltzmann's constant, J.kg −1 mNO Molecular mass of NO, kg mp Molecular mass of perturbing species, kg

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A theoretical model is presented for the determination of the optimum excitation frequency for the production of a fluorescence signal which is proportional to the mole-fraction of the target species while having minimal sensitivity to pressure and temperature variations. The results of the model are applied to the mixing region behind a range of scramjet fuel injectors in a shock tunnel with the aim of obtaining measurements of fuel mole-fraction. Experimental images are presented of fluorescence in streamwise and cross-planes. Analysis of the flowfields produced by a castellated and a plane-base injector shows that the castellated injector has a slight advantage over the plane-base injector, particularly in the near field.

Planar laser-induced uorescence of nitric oxide is used to measure a component of the velocity eld for the Mach 7 ow around a 30-deg half-angle, 50-mm-diam cone mounted to a long, 38-mm-diam shaft, or "sting." Transverse velocities are measured in the freestream, the shock layer, and the separated region at the junction between the cone and the sting. For most of the ow eld, the uncertainty of the measurements is between § § 50 and § § 100 m/s for velocities ranging from ¡ 300 to 1300 m/s, corresponding to a minimum uncertainty of § § 5%. The measurements are compared with the commercial computational uid dynamics (CFD) code CFD-FASTRAN TM . The agreement between the theoretical model and the experiment is reasonably good. CFD accurately predicts the size and shape of the shock layer and separated region behind the cone as well as the magnitude of the gas velocity near the reattachment shock. However, the magnitude of the velocity in the shock layer and gas expansion differ somewhat from that predicted by CFD. The discrepancies are attributed to a small systematic error associated with laser-beam attenuation and also to inexact modeling of the ow eld by CFD.

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