Measuring and processing in-cylinder measurements of NO and OH obtained by laser-induced fluorescence in a diesel rapid compression machine
Demory, R., Crua, C., Gold, M.R. and Heikal, M.R. (2006) Measuring and processing in-cylinder measurements of NO and OH obtained by laser-induced fluorescence in a diesel rapid compression machine In: 13th International Symposium on Applications of Laser Techniques to Fluid Mechanics, 26-29 June 2006, Lisbon, Portugal.
Official URL: http://in3.dem.ist.utl.pt/lxlaser2006/
In-cylinder concentrations of nitric oxide (NO) and hydroxyl radicals (OH) were measured qualitatively by laser-induced fluorescence (LIF) in a rapid compression machine at conditions representative of a modern diesel passenger vehicle. In order to take into account the effects of collisional quenching and population (Boltzmann) distribution on the NO-LIF measurements, a phenomenological model was developed, based on high speed videos of the flame, the analysis of the heat release rate, and the flame structure information drawn from OH-LIF. The fluorescence signal was also corrected for broadband interferences and laser pulse energy variations. The formation of NO started with the onset of the diffusion flame, no fluorescence being recorded during the pre-mixed phase. The peak signal was recorded after the end of heat release, 1.5 to 2 ms after the peak mixing-controlled heat release rate. Whereas conditions on the lean side of the diffusion flame are ideal for the formation of NO, only a small portion of the NO fluorescence originated from this region. The model showed that the fluorescence signal level is hindered during the combustion by high levels of collisional quenching and, on a smaller scale, broader population distributions. To estimate the location and extent of the diffusion flame, the NO results were correlated with planar LIF measurements of OH. The measured OH distributions compared favourably with results from simulations using the KIVA 3V release 2 code. The OH radicals are shown to be mainly present in the mixing controlled phase, distributed in a thin layer around the vapour fuel in the jet, within the diffusion flame location. OH radicals could be seen as early as 0.4 ms before the pre-mixed heat-release spike and until the end of apparent heat release. In the conditions studied, the diffusion flame therefore spans most of the combustion process, starting very soon after auto-ignition. Finally, high-speed videos of the flame luminosity were used to monitor the evolution of the flame and form the basis of corrections to the NO-LIF distributions. The first visible signs of combustion (carbon oxidation) matched the time and the location of the onset of the diffusion flame.
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