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chapter 5 turbulent diffusion flames - FedOA

chapter 5 turbulent diffusion flames - FedOA

4.2.1 SOOT AND NOC

4.2.1 SOOT AND NOC VOLUME FRACTIONS AND MEAN DIAMETERS These results are confirmed by the profiles of light scattering reported in Fig. 3.32, in which a strong signal is measured in correspondence to the presence of soot particles, but a relevant signal is also measured in the centreline where only NOC particles are present. From the scattering data, after evaluation of gas contribution, and the total volume fraction of the particles we obtained a rough estimate of the D63 of the total particles distribution function which is reported in Fig. 3.33. The particles mean size has been evaluated using the Rayleigh equation for light scattering using the optical properties for NOC and soot reported in [75]. Close to the flame centreline where the concentration of soot is very low, the mean size of the particle is of the order of 2-3 nm whereas in the annular flame region where soot is formed the mean particle size is of the order of 10 nm. These results are in agreement with the previous findings in the ethylene flame and confirm the formation of NOC particles also in methane diffusion flames. Qw, cm -1 sr -1 1.0E-05 1.0E-06 1.0E-07 0 1 2 3 4 5 6 r, mm Fig. 3.32 Qvv by 213 nm vs. radial position. 88 z20 z25 z30 z35

Size, nm 14 12 10 8 6 4 2 0 0 1 2 3 4 5 6 r, mm 89 z20 z25 z30 Fig. 3.33 Mean particle diameters by scattering measurements vs. radial position. 4.3 COMPARISON ETHYLENE AND METHANE By comparing the volume fractions of NOC and soot for the two flames, ethylene and methane laminar diffusion flames, it is possible to observe that methane flame produces a lower amount of both NOC and soot but the reduction of soot was stronger than that of NOC. In particular, it is possible to observe that the ratio between the maximum value of soot to NOC concentration in the ethylene flame is about 1.0 while in the methane flame it is 0.2. Therefore, concluding this section on laminar diffusion flames, we can say that the proposed spectroscopic measurements, coupled with scattering and extinction measurements, have allowed the evaluation of the concentrations of soot and NOC and their average sizes in two laminar diffusion flames of ethylene and methane. NOC are immediately formed at the beginning of both flames whereas soot is preferentially formed in the ethylene flame being practically negligible in

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