Experimental characterization of the different nitrogen dilution effects on soot formation in ethylene diffusion flames

This paper addresses the discrimination of dilution and thermal effects on soot production when nitrogen is added to the fuel stream of a steady laminar ethylene diffusion flame. In this context, the mixture-strength flame height H-f and visible flame height H-v are unambiguously documented and exhibit opposite trends within experimental diluted range (N-2 volume fraction from 0 to 0.56). Simultaneous soot volume fraction and temperature fields are mapped for different N-2 volume fractions by the Modulated Absorption/Emission (MAE) technique. Two characteristic flame heights and the temperature at the peak soot volume fraction are shown to be crucial parameters to assess the extent of the dilution and thermal effects together with their impact on soot formation in the flame. As a result, the mixture-strength flame height is proposed to characterize the soot formation rate within soot growth region, while the visible flame height is more appropriate to describe the overall sooting propensity in the flame. The probed soot temperature is recommended as the characteristic temperature to assess N-2 dilution and thermal effects since the thermal effect is overestimated when using the adiabatic temperature. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.