2-D imaging of soot formation process in a transient spray flame by laser-induced fluorescence and incandescence techniques

Abstract

In order to investigate the soot formation process in a diesel spray flame, simultaneous imaging of soot precursor and soot particles in a transient spray flame achieved in a rapid compression machine was conducted by laser-induced fluorescence (LIF) and incandescence (LII) techniques. 3rd harmonic (355 nm) and fundamental (1 064 nm) laser pulses from single Nd:YAG laser, between which a delay of 44 ns was imposed by 13.3 m of optical path difference, were used to excite LIF due to soot precursor and LII due to soot particles separately in the spray flame. The LIF and the LII were separately imaged by two image intensified CCD cameras with identical detection wavelength of 400 nm and bandwidth of 80 nm. The LIF due to soot precursor was mainly located in the central region of the spray flame between 40 and 55 mm (270 to 370 times nozzle orifice diameter d0) downstream from the nozzle orifice. The LII due to soot particles was observed to surround the soot precursor LIF region and to extend downstream. First appearance of the LIF due to soot precursor in the spray flame preceded the appearance of the LII due to soot particles. The intensity of the LIF due to soot precursor reached its maximum immediately after rich premixed combustion. In contrast, the intensity of the LII due to soot particles increased gradually and reached its maximum after the end of injection. Measured LIF spectrum of the soot precursor in the spray flame was very broad and showed no apparent spectral structure.