Analysis of pyrolysis process in diesel-like combustion by means of laser-induced fluorescence

Inelastic light scattering, excited at lambda = 266 nm, has been measured from 300 to 600 nm in a tetradecane pulsed spray injected into a high-temperature (900 K), high-pressure (4 MPa), nearly quiescent environment. In each detection, spectra along the radial direction of the spray cross section have been simultaneously detected by means of a gated, intensified CCD camera. The inelastic light scattering has been attributed to laser-induced fluorescence (LIF) based on its spectral behavior and dependence on the laser intensity. Maxima of the fluorescence signal in the UV range occur in the central part of the test section, just after the ignition time, both in the averaged and individual measurements. These are related to formation of light, pyrolytic species and to early presence of reactive zones inside the spray core. LIF in the visible spectral region appears later and attains its maximum between 0.5 and 1 ms after the ignition, and just before the maximum of elastic scattering signal due to soot particles. The visible fluorescence is almost uniformly distributed along the spray diameter and undergoes a very peculiar nonlinear dependence on the laser fluence. This feature can be used to distinguish between LIF, laser-induced incandescence, and fluorescence from photofragmented species. LIF in the visible region is also attributed to pyrolytic species, which several papers specify to be polycyclic aromatic hydrocarbons (PAH) with several rings. Extension of this LIF signal throughout the spray cross section and its early detection indicate the need for particular care in improving the diesel combustion process, even in the very early stage after spray ignition, before massive soot formation, if organic pollutants such as PAH have to be minimized.