Synchrotron radiation studies of additives in combustion I: Water

Small Angle X-ray Scattering (SAXS) measurements have been performed on a partially pre-mixed ethylene-air flame with and without the introduction of a water aerosol for the same flame height. Our measurements are sensitive to particles with radii in the range of 6-78 nm. It is found that water addition leads to soot suppression below the detection limit at low heights along the flame centerline (<11 mm) and to an increased sooting tendency higher up in the flame. There, larger primary particles (R_g∼20-45 nm) shaped as agglomerates with fractal dimension about 2 are observed together with a large number of smaller (R_g ∼ 10 nm) smooth particles, which we identify with sub-primary units observed in previous work on pure diffusion flames. In the case of the un-doped partially pre-mixed flame, however, only relatively (R_g ∼ 20-35 nm) large spherical particles (Porod’s exponent about 4) are observed. An intriguing result is that water addition induces the enhanced formation of sub-primaries, with these having more than 2 orders of magnitude higher number concentration with respect to the spherical particles in the water free flame.