Charge characteristics of sub-10 nm soot particles in premixed ethylene flames

In this work, the charge characteristics of sub-10 nm soot particles were studied in burner-stabilized-stagnation premixed ethylene flames, at an equivalent ratio of 2.0 and over the calculated maximum flame temperature (Tmax) range of 1665-1933 K, using micro-orifice probe sampling in tandem with neutralizer, half-mini differential mobility analyzer (Half-mini DMA) and electrometer. To obtain the charge fraction, the charged and total particle size distributions (PSDs) were measured with the neutralizer off and on, separately. Our results showed that both negatively charged particles (NCP) and positively charged particles (PCP) grew to bigger particles with the increase of height above burner (Hp). However, compared to PCP, NCP were relatively smaller in size, but higher in concentration, possibly because negative ions such as electrons diffused faster to coagulate with soot. Moreover, charge fraction decreased along Hp and newly nucleated soot particles were all neutral at higher Hp, which could be attributed to the higher temperature near the flame front, where the concentrations of ions were higher. Another interesting finding was the increase in charge fraction by two orders of magnitude (from 0.1% to 10%) as Tmax increased from 1665 K to 1933 K. All of these suggested the existence of particle-ions interaction close to the flame front, especially at high temperatures.