Soot formation in diffusion flames: Flow rate, fuel species and temperature effects

Abstract

A detailed study of soot particle formation in diffusion flames has been made for a series of laminar diffusion flames using a laser scattering/extinction technique. The effects of flow rate, fuel species and temperature on the evolution of the soot particle field have been investigated. Temperature effects have been examined through nitrogen dilution of the fuel. Fluorescence measurements from gas phase species in the initial particle formation region have been used to examine the relationship between these potential soot precursors and the observed particle formation. Flow rate, fuel species and temperature variations have been observed to significantly effect the annular region of the flame, where particles first appear. Increasing the fuel flow rate results in a larger soot volume fraction while the maximum particle size observed in the flame remains nearly constant. Thus, particle number concentration appears to be the quantity most sensitive to the flow rate variation. Temperature measurements made in the flames indicate that fuel flow rate increases result in higher temperatures in the particle formation region, leading to increased soot formation. At later stages, the flames with larger soot concentrations exhibit lower temperatures which facilitates soot particle survival in the oxidation region of the flame. Measurements made in flames with different fuels but similar adiabatic flame temperatures, indicate that the temperature exerts the strongest influence on soot formation. However, distinct effects due to fuel structure still need to be considered.