Effect of gas additives on soot formation in flat laminar premixed ethylene/air flames under low pressure

Abstract

This study reports on the effect of the additive gases, namely N2, CO2, and Ar, on the soot formation in laminar premixed C2H4/air flames at low pressure of 40 kPa. The flames blended with N2, CO2, or Ar were experimentally investigated, where the emission spectra of CH* and C2*, qualitative concentration of PAHs, and soot volume fraction (fv) were measured using laser and optical diagnostic methods. The flame temperature was measured using a thermocouple. The results reveal that additive gases significantly influence flame height above the burner and fuel combustion as well as reduce soot formation and flame temperature. With respect to N2 and CO2, Ar proves the most effective in reducing soot volume fraction, achieving a 100-fold reduction compared to the reference flame. Moreover, the additive gases were found to delay the ignition, leading to a 5 mm downstream shift in soot inception. Despite the difference in the properties of the three gas additives, it was found the first incepted soot particles were detected at a common temperature, Tinception, of 1515 ± 70 K. Three regimes related to the soot appearance rate were identified, which are the fast-increasing range from the soot inception location, the plateau region, and the decreasing range to the flame front of 20 mm. According to the Lagrangian time-derivative of soot volume fraction (dfv/dt) as a function of fv, the constant of surface growth (kSG) was determined to be 170 s−1 in the flame with additives of CO2 and Ar, measured at 40 kPa. The identified turning point between the plateau region and the negative soot increase regions can be used as an indicator of the transition from higher to lower soot formation rates in soot modeling.