Soot concentration and primary particle size in swirl-stabilized non-premixed turbulent flames of ethylene and air

Turbulent non-premixed swirl-stabilized flames were investigated experimentally in a gas turbine model combustor with optical access. Velocity and soot concentration fields were measured for three levels of air flow for a fixed flow rate of ethylene as the fuel. Stereoscopic particle image velocimetry was used to get the three-dimensional velocity field data within the combustor. The time-averaged soot volume fractions and primary soot particles sizes were obtained using the laser induced incandescence technique. These two measurements were conducted separately, but under the identical experimental flow conditions. The velocity measurements showed a region of high velocity flow, sandwiched between inner and outer recirculation zones. The boundaries of the recirculation zones in all three cases displayed very high turbulence intensities. Most of the soot was found to be within the inner recirculation zone, and the regions having maximum time-averaged soot concentrations grew radially outward with axial height. The soot concentrations showed a strong dependence on air flow rate; a small increase in air flow rate caused a significant reduction in soot concentrations. The primary soot particle diameters inferred from laser induced incandescence measurements covered the range from 30 to 50 nm.