Molecular tagging velocimetry of NH fluorescence in a high-enthalpy rarefied gas flow

In this paper, a new type of molecular tagging velocimetry based on NH fluorescence was developed and validated for the velocity measurements of a high-enthalpy rarefied gas wind tunnel where the maximum flow velocity exceeds 6 km/ s near the nozzle exit at 0.2 Pa. The feasibility of this technique using the short-lived NH fluorescence was demonstrated in the hypersonic rarefied gas flow with yielding velocity profiles at multiple downstream locations from the nozzle exit. The total uncertainty of the measured flow velocities was estimated to be less than 6% when the flow velocity is above 2000 m/ s. For the new tagging technique, only a single laser and a single time-gated camera are required for velocity measurement, due to the existence of NH radicals in the arc-discharged-N2 mixed with a small amount of H-2. Therefore, the NH-MTV not only shows great promise for tagging in high-enthalpy rarefied gas of nitrogen or air flows without seeding, but also possesses high practicability and facility for velocity measurement.