Views
5 years ago

chapter 5 turbulent diffusion flames - FedOA

chapter 5 turbulent diffusion flames - FedOA

photofragmentation

photofragmentation signal (at 512 nm). Moreover both signals are faster than LII signal characterized from a duration of hundreds of nanoseconds as reported by Vander Wal [57]. Normalized intensity LII/C 2 ratio 1 0.8 0.6 0.4 0.2 0 0.18 0.16 0.14 0.12 0.1 0.08 0.06 0.04 0.02 0 0 0.5 1 1.5 2 2.5 Laser Power, mJ 64 Particles Temperature LII / C 2 Swan Band emission 4100 4000 3900 3800 3700 3600 3500 Fig. 3.7 Particles Temperature and LII/C2 ratio as function of the laser power. LIF at 300 nm C 2 emission at 512 nm 0 5 10 15 20 25 30 35 40 Time, ns Difference of normalized intensity signals Fig. 3.8 Temporal evolutions for 300 nm e 512 nm at 3.5 mm HAB, C/O= 0.77, (a). Temporal evolution of the difference between normalized peak emission at 512 and normalized peak emission 300 nm for different C/O ratios at 3.5 mm HAB, (b). 0.8 0.4 0 -0.4 -0.8 C/O=0.71 C/O=0.77 C/O=0.80 C/O=0.85 C/O=0.92 T, K 0 10 20 30 40 Time, ns

The different temporal decay of the emission processes allows us to distinguish between LIF and LII by fixing the delay time at 8 ns after the laser pulse (acquisition duration of 20 ns) and 40 ns (acquisition duration of 100 ns) respectively. Furthermore, the choice to collect the ultraviolet fluorescence signals at 300 nm is due to the fact that at 300 nm the fluorescence showed very broad and very close to the spectra maximum. The LII signals, instead, have been collected at 500 nm, in visible spectra region, according to that reported in literature [50, 51]. 3.1.3 SIGNALS CALIBRATION To allow quantitative information about particles concentrations from in-situ optical measurements, LIF and LII collected in the described procedures need to be calibrated. The calibration procedures have been based on the use of extinction measurements that consent to estimate the volume fraction of both NOC and soot particles once the optical properties of the two classis are know [75]. Calibration measurements were performed in a premixed ethylene/air flame with C/O=0.85 and a cold gas velocity of 10 cm/s. LIF and LII signals were then compared with ultraviolet and visible light extinction coefficients measured at the same flame locations. Figure 3.9 reports the LIF signals collected at 300 nm and the NOC’s volume fraction, by ultraviolet extinction measurements, as function of HAB, whereas in Fig. 3.10 are reported the LII signals collected at 500 nm and soot volume fraction, by visible extinction measurements. These correlations allow as to calibrate both LIF and LII in order to obtain separate information on the two particles classis by the followed proportionality: LIF ∝ fvNOC LII ∝ fvSoot 65

DNS of Turbulent Nonpremixed Ethylene Flames
Heat release rate measurement in turbulent flames
A STATISTICAL THEORY ON THE TURBULENT DIFFUSION OF ...