Carbonaceous deposits in a fuel-film cooled rocket combustor: Spectroscopy

Carbonaceous deposits formed in fuel-film cooling act as a thermally insulating barrier, but the mechanisms for their formation at the high enthalpies and pressures of rocket engines are not well studied. Various spectroscopic techniques were used to investigate the microscopic chemical and physical structure of carbonaceous deposits formed in a small-scale H2O2-kerosene rocket combustor. Where possible, spectra were measured as a function of axial distance from the fuel-film injector to identify changes in deposit structure as the fuel-film boundary layer vaporized and mixed with the hot core flow. Two carbon layers were found, a tenacious dense layer beneath a porous soot layer. Gas chromatography mass spectrometry was used to determine condensable polycyclic aromatic hydrocarbon (PAH) concentrations in the dense and soot deposits. Raman spectroscopy measured planar coherence length and other indicators of structural order while x-ray photoelectron spectroscopy determined carbon hybridization, surface oxidation, and deposit impurities. All three of these spectroscopy techniques showed independent signs of the dense layer forming via heterogeneous condensation of heavy PAHs, whereas the soot layer deposited via thermophoresis.