Cross-band infrared laser absorption of carbon monoxide for thermometry and species sensing in high-pressure rocket flows

Abstract

A novel cross-band laser absorption spectroscopy technique has been developed for quantitative measurements of gas temperature and carbon monoxide (CO) in high-pressure, high-temperature rocket combustion flows. The strategy enables a broad range of sensor operability by simultaneously probing rovibrational transitions in both the fundamental and first overtone bands of CO near $4.98\,\upmu \hbox {m}$ and $2.32\,\upmu \hbox {m}$, respectively, which sustain large differences in temperature dependence despite collisional broadening. Scanned-wavelength modulation spectroscopy methods are integrated for noise rejection in the harsh rocket operating environment. Initial experiments using the cross-band thermometry technique have been conducted on a single-element-injector rocket combustor with RP-2/GOx and ${\hbox {CH}_4}/\hbox {GOx}$ propellant combinations at pressures up to 75 bar. Measurements of the first overtone bandhead ($2.32\,\upmu \hbox {m}$) maintained adequate signal-to-noise at even higher pressures (up to 105 bar), although deviating significantly from spectral simulations. To account for collisional effects at high gas densities, empirical models for line mixing, developed via shock tube studies, were employed to enable quantitative interpretation of measured signals for temperature and CO mole fraction in the rocket combustor.