Ultraviolet absorption and fluorescence measurements in turbulent spray flames

Studies of elemental and organic carbon formation/oxidation in diffusion or rich premixed combustion systems focus mostly on soot particles and polycyclic aromatic hydrocarbons. In recent years, a new class of carbon compounds, in the form of molecular clusters (nanoparticles) of organic composition has been inferred, detected and modeled within and at the exhausts of a variety of combustion systems. This new category of pollutants seems to be related to the formation processes of ultrafine particles found at the exhaust of practical combustion systems, which are considered dangerous to human health. Heavy molecular weights and extremely small sizes of nanoparticles prevent their detection by ordinary techinques (e.g., GC-MS), particularly in combustion conditions. The paper reports on optical investigations based on ultraviolet (UV) extinction spectroscopy and laser-induced fluorescence (LIF) of turbulent flames and exhausts of a spray burner, involving processes by far more complex that gaseous laminar flames. The light source for extinction was the plasma emission resulting from the optical breakdown of a Q-switched Nd:YAG laser pulse, which featured intense, fast and bright broadband UV radiation. Extinction spectra in the UV band range 200-500 nm and LIF UV-visible spectra, excited at lambda = 266 nm, were measured and analyzed at different heights above the nozzle, which allowed us to infer the formation/oxidation of species strongly absorbing in the far UV region and fluorescing at longer wavelengths. Resulting data were correlated with the special extinction of air-diluted exhausts.