Pressure Dependence of Soot Formation in Diffusion Flames

Abstract

The effects of pressure on soot formation and the structure of the temperature field were studied in co-flow methane-air laminar diffusion flames over a wide pressure range, from atmospheric to 6 MPa (60 atm) in a high-pressure combustion chamber. The selected fuel mass flow rates provided diffusion flames in which the soot was completely oxidized within the visible flame envelope and the flame was stable at all pressures considered. The spatially resolved soot volume fraction and soot temperature were measured by spectral soot emission as a function of pressure. The visible (luminous) flame height remained almost unchanged, about 9 mm, from 1 to 10 MPa, whereas it increased considerably from atmospheric to 1 MPa. Between 1 MPa and 6 MPa, the cross-sectional area of the flame (measured from the radius defined by either the maximum soot or maximum temperature annuli) showed an inverse dependence on pressure. Peak soot concentrations showed a strong dependence on pressure from 1 MPa to 4 MPa; however this dependence got relatively weaker between 4 MPa and 6 MPa.