Soot formation in annular non-premixed laminar flames of methane-air at pressures of 0.1 to 4.0 MPa

Laminar non-premixed methane-air flames were studied over the pressure range of 0.5 to 4.0 MPa using a new high pressure combustion chamber. Flame characterization showed very good flame stability over the range of pressures, with flame tip rms flicker of less than 1% in flame height. At all pressures, soot is completely oxidized within the visible flames and the maximum optical thickness observed was 0.51. Spectral soot emission (SSE) and line-of-sight attenuation (LOSA) measurements provided radially resolved soot volume fraction and soot temperature at pressures from 0.5 to 4.0 MPa. Such measurements provide an improved understanding of the influence of pressure on soot formation and have not been obtained previously in laminar non-premixed flames for pressures above 0.4 MPa. The total uncertainty of the LOSA soot volume fraction measurements is estimated to be 20--30% (95% confidence interval). The total uncertainty of the SSE soot temperature measurements is estimated to be 3.5% and the total uncertainty of the SSE soot volume fraction measurements is estimated to be 35--40%. SSE and LOSA measurements of soot concentration typically agree to within 30% and both methods exhibit similar trends in the spatial distribution of soot concentration. Maximum soot concentration variation with pressure is described by a power law, where the exponent on pressure is about 2 for the range of pressures between 0.5 MPa and 2.0 MPa, and about 1.2 for pressures of 2.0 MPa to 4.0 MPa. The pressure dependence of peak carbon conversion to soot is also well described by a power-law relationship, where the pressure exponent is unity for pressures between 0.5 MPa and 2.0 MPa and 0.1 for 2.0 MPa to 4.0 MPa. The pressure dependence of sooting propensity diminished at pressures above 2.0 MPa. Soot concentration measured in this work, when transformed to line-integrated values, are consistent with measurements of line-integrated soot volume fraction available in the literature. Soot temperature measurements indicate that the overall, temperature at a given height above the nozzle exit of the burner decrease with increasing pressure; however, the differences across pressures diminish with increasing height in the flame. Low down in the flame, temperatures are about 150 K lower at pressures of 4.0 MPa than those at 0.5 MFa. In the upper half of the flame the differences reduce to 50 K.