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chapter 5 turbulent diffusion flames - FedOA

chapter 5 turbulent diffusion flames - FedOA

Two vertical

Two vertical turbulent non-premixed flames burning ethylene and methane have been obtained by flowing respectively methane from a 2.5 mm and ethylene from a 3 mm internal diameter nozzle into ambient air. 36

2.3 OPTICAL MEASUREMENTS In this paragraph a short introduction to the used optical techniques, for combustion generated particles characterization, will be given. More in general, in the last years optical and spectroscopic measurements have been shown to be powerful tools for in-situ measurements of combustion by-products such as: OH, CH and other radicals, NOx, PAH, NOC and soot. Optical diagnostics have the merit to be highly non intrusive and non invasive, if correctly applied, and able to furnish qualitative and quantitative information about the flame structure, species formed and flame pollutant emission. On the other hand the use of these techniques needs great attention about the exact signals interpretation and about the optical properties of the species investigated. 2.3.1 LASER INDUCED INCANDESCENCE As well reported in the recently review paper of Schulz et al. [50], and previously by Santoro and Shaddix [51], this technique has proven to be a powerful tool for particle concentration and primary particle size measurements in combustion, both in laboratory flames and in practical combustion devices. Laser-Induced Incandescence (LII) consists in heating particles, to a temperature well above the flame temperature, by means of laser source and in measuring the correspondent, black body, radiative emission. Moreover, it has been demonstrated that while the intensity of the LII signal, “Prompt of LII”, is correlated with the volume fraction of the particles in the detection region, the decay rate of the LII signal, “Time Resolved of LII (TR-LII)”, is mainly a function of the primary particle size. The first steps towards the development of this technique for soot characterization in flame were carried out by Eckbreth [52] (1977) and Melton [53] (1983). Subsequently these early, but fundamentals, works, a great number of publications appeared in literature showing the potentialities of the LII as tool for soot characterization in many 37

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