Pyrene-LIF Thermometry of the Early Soot Formation Region in a Diesel Spray Flame

In order to investigate early soot formation process in diesel combustion, spectral analysis and optical thermometry of early soot formation region in a transient spray flame under diesel-like conditions (Pg\me2.8 MPa, Tg\me620-820K) was attempted via laser-induced fluorescence (LIF) from pyrene (C\d1\d6H\d1\d0) doped in the fuel. Pyrene is known to exhibit a temperature\-dependent variation of LIF spectrum; the ratio of S\d2/S\d1 fluorescence yields, from the lowest excited singlet state S\d1 and the second excited singlet state S\d2, depends on temperature. In the present study, pyrene was doped (1%wt) in a model diesel fuel (0-solvent) and the variation of LIF spectra from the pyrene in the spray flame in a rapid compression machine were examined at different ambient temperatures, ambient oxygen concentrations, measurement positions and timings after start of fuel injection. An Nd:YAG-pumped dye laser pulse at 309 nm (5mJ) was used as the excitation source and the LIF spectra were measured by a spectrometer. The pyrene LIF spectra measured in non-combusting evaporating spray in nitrogen atmosphere showed increase in the S\d2/S\d1 ratio of LIF intensity and shift of the peak wavelength of S\d1 band towards red with increasing ambient temperature. The pyrene LIF spectra measured in spray flame between start of injection and ignition showed increase in the S\d2/S\d1 ratio of LIF intensity and shift of the peak wavelength of S\d1 towards red as the time progresses after start of injection, showing that the temperature variation of the gas mixture up to ignition can be captured by this technique. The LIF spectra measured in spray flame after ignition showed increase in emission intensity in longer wavelength region due to LIF from combustion products. The pyrene LIF spectra measured in spray flame at 1.0 ms after start of injection at different ambient oxygen concentrations (5 to 21%) showed increase in the S\d2/S\d1 ratio of LIF intensity and shift of the peak wavelength of S\d1 band towards red with increasing oxygen concentration, showing that the temperature difference of the gas mixture caused by progress of reaction at different oxygen concentrations can be captured by this technique.