Qw, cm -1 sr -1 2.5E-06 2.0E-06 1.5E-06 1.0E-06 5.0E-07 0.0E+00 0 5 10 15 20 25 30 98 r, mm 10 cm 20 cm Fig. 3.40 Radial scattering profiles at two different flame heights in the turbulent jet flame. Scattering coefficients increase by only one order of magnitude. In the ethylene flame the scattering increased generally of two or three orders of magnitude with the flame height. Therefore, also the scattering measurements show the presence of particles with very low sizes as shown in Fig.3.41 where the estimated mean sizes of the particles are shown. In the early region of the flame, the scattering signal is of the same order of magnitude of that of gas-phase compounds exceeding this value of non more that 30%. It means that the aerosol formed in this region, as shows in Fig.3.41, is constituted of particles with sizes of the order of 3 nm. At higher heights along the flame axis, the scattering signal increases above the value of the gas-phase compounds indicating the presence particles with mean sizes in the range of 6 – 10 nm.
Size, nm 10 9 8 7 6 5 4 3 2 1 0 10 cm 20 cm 0 5 10 15 20 25 30 r, mm Fig. 3.41 Radial mean particle size at two different flame heights. 5.2.2 SIZE DISTRIBUITION FUNCTIONS MEASURED BY ON-LINE SMPS Differential Mobility Analysis (DMA) measurements have been performed in the turbulent methane flame at the same locations of the optical measurements, using a TSI 3936, that allows us to obtain particle size distributions in a range 3 – 65 nm. Figure 3.42 shows the experimental configuration used for the DMA measurements. We used a 0.8 mm ID and 1.5 mm OD probe vertically positioned into the flame. Gases are sampled for effect the low pressure generated by the air flow through a calibrated throat. Therefore, by changing the air flow we were able to obtain different dilution ratios (air flow/sampling flow). In this work the measurements have been carried out using two value of dilution ratio: 1000 and 3500 in order to avoid coagulation effects. Optical results are confirmed by the size distribution functions measured by SMPS analysis in the turbulent flame. Figure 3.43 reports the size distribution functions measured on the flame axis at 10 cm. At 10 cm above the burner on the flame axis, particle diameters range from 3 nm, the limit of the detection system, up to 40 nm. The number concentration of particles with sizes below 10 nm is 99