Mid‑infrared laser absorption tomography for in‑situ quantitative thermochemistry measurements of hybrid rocket motors

Further development of hybrid propulsion systems requires a deeper understanding of the complex physicochemical mechanisms governing its combustion performance. A tunable diode laser absorption tomography (TDLAT) method was developed for investigating the thermochemical processes at the nozzle exit of an oxygen/Poly Methyl MethAcrylate (PMMA) hybrid rocket motor. Firing tests were conducted for different oxidizer mass fluxes ranging from 2.73 to 3.51 g/ (cm2·s). A distributed feedback (DFB) laser was tuned to cover three H2O absorption lines near 2.5 μm, using scanned-wavelength direct absorption (DA) mode with 2.0 kHz repetition rate. Under an assumption of cylindrical symmetry, a Radon transformation was applied to yield radially- and time- resolved absorption coefficient, from which the radial distribution of temperature and H2O partial pressure were reconstructed. Based on the Taylor series method (TSM), measurement uncertainty was analyzed in detail considering line-strength uncertainty, Voigt fitting residuals and Radon transformation. Finally, the radial distribution and dynamic variations of both temperature and H2O partial pressure were obtained in all firing tests, both the constructed results show measurement sensitivity to chemical kinetic progress and oxidizer mass flux changes. Our experimental results highlight the capability of TDLAT to characterize combustion processes of hybrid rocket motors.