An in situ combustion carbon deposit diagnostic instrument based on Raman microscopy

Carbon deposit of the aero-engine combustor, resulting from incomplete combustion and fuel pyrolysis, can cause nozzle blockage, fuel consumption increase, power decrease, and even flight unsafety. In this work, an in situ combustion carbon deposit diagnostic instrument is developed to reveal the crystalline structure and the changes under real combustion conditions. The instrument integrates the in situ microscopic Raman technique and the combustion system. The burner is characterized by a sloping tip, making it possible to observe the coke from the side view. The burner is installed to the optical positioning stage by a specially made adapter so that the relative location is fixed and it is possible to observe the carbon deposit from the ignition. The carbon deposit of acetylene/air diffusion jet flame was studied. A 50× objective lens was used to collect the Raman scattering signal of carbon deposits continuously 30 s after ignition. A five-band model was used to fit the Raman spectra. The time-resolved information was calculated, including the normalized total area, area proportion, peak ratio, and crystalline size. The results show that the carbon deposit of acetylene flames with different velocities presents different tendencies of formation and degree of graphitization, which is attributed to the influence of temperature and flow. The performance of this system is evaluated quantitatively. The signal-to-noise ratio of Raman spectra of carbon deposits ranges from 6.4 to 28.9. This work provides an in situ method to analyze the dynamic change of carbon deposit on the burner, and further work is needed to reveal the mechanism.