Soot formation in laminar ethane diffusion flames at pressures from 0.2 to 3.3 MPa

The effects of pressure on soot formation and the structure of the temperature field were studied in co-flow ethane-air laminar diffusion flames over the pressure range of 0.1–3.34 MPa in a high pressure combustion chamber. The selected fuel mass flow rate 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 from 1.52 to 3.34 MPa, whereas it increased considerably from atmospheric to 1.52 MPa. Flame cross-sectional area, measured at the flame height of 5 mm either bounded by maximum flame temperature or maximum soot volume fraction contours, showed an inverse dependence on pressure. Peak carbon conversion to soot, defined as the percentage of fuel’s carbon content converted to soot, showed a strong dependence on pressure at lower pressures; but this dependence grew weaker as the pressure was increased. This dependence can be expressed as a pressure scaling in the form of a power law. However, the exponent of pressure was not constant: it was about 2.2 for pressures between 0.2 and 0.51 MPa, about 1.1 for pressures between 0.51 and 1.52 MPa, and about 0.4 for pressures between 1.52 and 3.34 MPa. Averaged flame temperatures decreased with increasing pressure as a result of enhanced heat loss from the flame by soot radiation. The maximum temperature gradients increased with pressure at lower flame heights; at higher locations in the flame, after an initial increase at the lower pressure range, gradients reached a plateau at about 1.5–2.0 MPa.