Temperature measurements using UV-induced vibrational hydrogen Raman bandshape spectroscopy

Non-intrusive optical techniques have the potential to provide highly accurate quantitative spatial and temporal information in high-pressure, rocket engine-like test articles. Single laser-pulse UV Raman spectroscopic methods can be used in these devices for combustion analysis to aid in the development of advanced rocket engine propellant injectors. Traditional Raman techniques assume an ideal gas law analysis with an independent mechanical pressure measurement to relate measured species number densities to species concentrations and temperature. Alternatively since a molecule's spectral signature is a function of temperature, temperatures can be determined directly by best-fit matching experimental Raman spectra with theoretical spectra. Development of this UV Raman bandshape spectroscopy technique can allow direct temperature measurements without the need for an independent calibration of the Raman system or pressure measurement, which is advantageous in non-uniform flows where the local pressure may vary spatially or not be accurately known. Thus, the impetus of this work is to apply the UV Raman bandshape technique to the H2 Stokes vibrational Q -branch Raman spectrum to use as a thermometer for H2-fueled combustion in rocket engine-like test articles.; A detailed simulation program, including molecular kinetic phenomena, has been developed for the H2 Stokes vibrational Q -branch Raman spectrum. Raman derived temperatures are determined from parameter estimation methods based on signal-to-noise (S/N) weighted non-linear least-squares fitting of the experimental Raman spectra with theoretical spectra. Uncertainties in the Raman-derived measurements are then estimated from the parameter error estimates (goodness-of-fit criteria) and Monte-Carlo simulations based on the experimentally determined data error structure with variances estimated from S/N levels for the measured signal intensities. To demonstrate the technique, Raman temperature measurements are performed in heated dilute-H2/N2 mixtures and steady laminar rich H2-air flames at atmospheric pressure. The Raman temperatures are compared to thermocouple measured temperatures and calculated adiabatic flame temperatures based on measured mass-flow rates. Time-averaged and single-pulse Raman measured temperatures are obtained with uncertainties of ∼5, 2, and 8% for the corresponding temperatures of 295, 1050, and 2190K. Finally, an attempt is made to apply the Raman bandshape technique during hot-fire testing of a high-pressure (∼1800psi) rocket engine-like test article consisting of a multi-element liquid-H2/liquid-O2 injector operating at a low oxidizer/fuel mixture ratio (∼0.66) to simulate an advanced rocket engine preburner.