The dynamics of electrically charged soot particles in a premixed ethylene flame

A tandem differential mobility analyzer (DMA) is used to measure selectively the size distributions of soot particles carrying 0, (plus_minus)1, (plus_minus)2, (plus_minus)3, and (plus_minus)4 units of electrical charge. Near soot inception the size distribution is unimodal, with a mean diameter (less than)3nm, and the particles are electrically neutral. With increasing height above the burner a bimodal size distribution evolves. The lower size mode remains electrically neutral with increasing height in the flame and retains its shape and intensity. In contrast, the upper mode includes progressively higher fractions of charged particles that reach levels of 33% positively and 33% negatively charged. At a fixed height, the size distribution narrows and shifts to a larger diameter as the number of charges per particle increases. At a fixed particle size, the number of charges obeys a Boltzmann distribution which is independent of height in the flame. These dynamics are examined in the context of a coagulation model that includes Coulomb forces and assumes small fractions of +1 and -1 particles are rapidly formed shortly after soot inception. The predictions closely mimic the observed evolution in charged particle size distributions and reproduce the experimentally observed Boltzmann charge distributions.