Rotational hydrogen thermometry by hybrid fs/ps coherent anti-Stokes Raman scattering in the plume of a burning metalized propellant

We employed a hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) instrument to probe rotational temperatures of molecular hydrogen in the multiphase reaction zone of an aluminized ammonium perchlorate (AP) propellant flame. Significant concentrations of hydrogen, present in the plume due to the decomposition of the propellant binder material and subsequent reactions with AP oxidizers, allowed for single-shot thermometry at the laser repetition rate of 1 kHz. A time-asymmetric picosecond probe pulse time-gated the impulsively generated Raman coherence at a delay of 2.66 ps from the pump and Stokes pulses, before any appreciable coherence dephasing occurred in the atmospheric pressure flames and mitigating uncertainties in the Raman transition frequencies and dephasing processes. Measurements in near-adiabatic H₂-air flames at equivalence ratios of ϕ = 1–1.8 demonstrated measurement accuracy to near 5% of equilibrium predictions with a precision approaching 3% for high signal-to-noise ratio spectra. Introduction of a time-delayed probe pulse provided Raman-resonant spectra from the plumes of burning propellants with ample signal above broadband background emission, which were fit to libraries of synthetic spectra to infer the gas rotational temperature 0–15 mm from the burning surface. The mean fitted temperature of 2494 K from three propellant burns compares favorably to other measurements of gas and particle temperatures in similar propellant studies.