Soot in diesel fuel jets: effects of ambient temperature, ambient density, and injection pressure

Abstract

Measurements of soot distributions in fuel jets injected into high-temperature, high-pressure diesel-like operating conditions were made in an optically accessible constant-volume combustion vessel. A laser-extinction technique was used to make quantitative measurements of path-length-averaged soot volume fraction. Flame luminosity and planar laser-induced incandescence imaging were used to visualize the sooting region of the fuel jet. Flame lift-off lengths were also measured and used in the interpretation and analysis of the soot measurements. Fuel was injected with a common-rail diesel fuel injector equipped with a single 100-μm-diameter orifice. The fuel used was #2 diesel fuel. The matrix of experimental conditions included ambient gas temperatures from 850 to 1300 K, ambient gas densities from 7.3 to 30.0 kg/m 3, and injection pressures from 43 to 184 MPa. The results show that peak soot level in a fuel jet increases with increasing ambient gas temperature, with the increase scaling linearly with temperature. However, near the tip of the flame, the soot levels decrease with increasing ambient temperature, indicating significantly higher soot oxidation rates in the flame-tip region at higher temperatures. The results also show that the peak soot level in a fuel jet increases with increasing ambient gas density and decreasing injection pressure. The increase with increasing ambient density is nonlinear with respect to density. The increase with decreasing injection pressure is linear with decreasing injection velocity (or the square root of the pressure drop across the injector orifice). Overall, the trends observed in diesel fuel jet soot closely correlate with the cross-sectional average equivalence ratio at the lift-off length, with soot levels decreasing as the equivalence ratio decreases (i.e., as more air entrainment and mixing of fuel and air occur upstream of the lift-off length).