Effects of hydrogen and nitrogen on soot volume fraction, primary particle diameter and temperature in laminar ethylene/air diffusion flames

This study reports the effects of hydrogen (H2) and nitrogen (N2) blended into the fuel on soot evolution and flame temperature in axisymmetric ethylene/air diffusion flames at atmospheric pressure. Two series of laminar ethylene (C2H4) diffusion flames blended with H2 and N2 were experimentally investigated, in which soot volume fraction (fv), primary particle diameter (dp) and flame temperature (T) were measured using planar laser-based techniques. These twelve flames are grouped into two sets. In the first one, H2 or N2 are added into a constant volumetric flow of C2H4 to separate their complementary effects. In the second set, the total volumetric flow rate of the mixture of C2H4/H2/N2 was kept constant for the same ratios of dilution to isolate the influence of exit velocity. For both sets, a reference flame corresponding to a mixture of C2H4/H2/N2 = 40%/40%/20%, by volume, was included to match the fuel composition of a turbulent 'target' sooting flame (termed the “Adelaide simple jet flame” in the International Sooting Flames Workshop). Planar laser-induced incandescence (LII), time-resolved LII (TiRe-LII) and two-line atomic fluorescence (TLAF) thermometry (using atomic indium as tracer) were applied to measure fv, dp and T. Radial profiles of the flame temperature were also measured using a thermocouple positioned 3 mm above the burner lip, which is just upstream from the sooting region. It is found that dilution either with H2 or N2 causes a significant reduction in fv and dp, while the influence on the peak flame temperature is weaker. Flame structure (length and spatial profiles) is also influenced by the blending with H2 and N2. This database can serve as benchmark for model development due to its systematic nature and combination of measured scalars.