Ethylene/air laminar diffusion flames were studied at sub-atmospheric pressures to simulate a non-buoyant environment and at super-atmospheric pressures for comparison, at fuel flow rates of 0.48 mg/s and 1.16 mg/s. Flame properties including flame geometry, soot formation and temperature field of the flames were studied. Overall, luminous flame height decreased with decreasing pressure to the point of visible luminosity disappearance, resulting in blue flames at a near vacuum. Flame width increased with decreasing pressure until the flame was almost spherical. Soot formation was also found to decrease with decreasing pressure and existed at very negligible concentrations in a near vacuum. Subatmospheric peak soot volume fractions ranged from about 0.1 ppm to 0.93 ppm at 0.48 mg/s, whereas at 1.16 mg/s, peak soot volume fractions were substantially higher. At subatmospheric pressures, higher fuel flow rates produced flames with higher soot concentrations. Soot production was restricted to an annular region with this annular region shifting closer to the flame centerline with increasing height from the burner. At locations about halfway between the burner rim and the flame tip, soot volume fraction decreased with increasing radial distance from the flame centerline. These results are consistent for both high and low pressure flames. The annular soot formation region was also located further from the flame centerline for the higher flow rate flames because of the difference in luminous flame shape. At 0.48 mg/s, the fraction of carbon in the fuel converted into soot was between 0.1 % and 1.2 % in the sub-atmospheric pressure range, from 0.2 atm to 1 atm.
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