Experimental investigation on the size-dependent maturity of soot particles in laminar premixed ethylene burner-stabilized stagnation flames

To understand soot maturation mechanism, we investigate the size-evolution of soot maturity in a series of laminar premixed burner-stabilized stagnation ethylene flames, with the maximum flame temperature Tf,max ranging from 1630 to 1869 K and the equivalence ratio φ ranging from 2.0 to 2.3. The soot particle size distribution is obtained by using the orifice sampling technique together with a scanning mobility particle sizer, while the maturity of soot is evaluated based on its oxidation characteristics and Raman spectra. The results show that at a fixed φ (say 2.2), soot particles grow bigger and become more mature concurrently in higher Tf,max flames (1809 and 1869 K). In contrast, although particle size increases obviously with time in lower Tf,max flames (1630 and 1724 K), soot maturity remains almost unchanged, indicating that particle growth is mainly controlled by particle coagulation or PAH condensation, instead of surface reactions in these flames. At a fixed Tf,max (say ∼ 1725 K), soot size growth is faster at higher φ as expected. However, when comparing soot particles of similar sizes, the particles from lower φ flames are more mature than those from higher φ flames, where the particle growth rate is too high to allow sufficient residence time for particle maturation.