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 (Rg ~ 20–45 nm) shaped as agglomerates with fractal dimension about 2 are observed together with a large number of smaller (Rg ~ 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 (Rg ~ 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.