Determination of soot size and concentration in optically dense sprays by optical methods

Paper in proceeding, 2006

The understanding of the formation and oxidation of soot is of interest to become able to reduce the emission of soot particles to the atmosphere from direct injection engines. A possible path to study the formation of soot in diesel sprays is by laser-based optical methods, having the potential for high temporal and spatial resolution; moreover they can be combined to obtain information of soot at any given instant from the start of combustion. Laser Induced Incandescence (LII), Elastic Light Scattering (ELS) and Light Extinction (LE) have been widely used for soot studies [1]. Soot particle size and concentration can be measured simultaneously combining LII, ELS and LE, the combination of these three techniques has proven to be functional when used to measure soot in optically thin flames in atmospheric conditions [2] and exhaust gases [3]. Optically dense systems such as diesel spray combustion, and the diagnostic of soot in them, require special attention to the LII signal because of its strong non-linear dependence on the laser fluence. Moreover particular concern must be given to the enhanced conductive heat transfer term in the LII interpretation due to the high pressure inside the combustion chamber at which diesel engines are usually operated [4]. This paper presents a method to measure particle size and particle concentration in optically dense sprays in conditions similar to those prevailing in real direct injection engines. For this purpose a non-linear compensation method to approximate the laser intensity across optically dense sprays was developed together with a new non-continuum heat transfer model for the LII signal.