Low-pressure ethylene/air laminar premixed flames: characterisations and soot diagnostics

Abstract

In this work, the structure of laminar premixed ethylene/air flat flames at low pressure are studied experimentally. The aim of the work is to exploit the spatial expansion of laminar 1D flames and advanced laser techniques to better understand the conditions and precursors affecting soot particles inception. Soot volume fraction (fv) profiles were measured using laser-induced incandescence (LII), while spatial distribution of the CH* and C2* radicals was measured using spatially resolved emission spectroscopy. Spatially resolved laser-induced fluorescence has been used to record emission from 2–3 rings, 3–4 rings and > 5 rings. The temperature of soot particles (Ts) was evaluated through fitting the spectrally resolved soot luminosity, while flame gaseous temperature (Tg) was measured using a fine thermocouple. The laminar flow velocity was modeled and used to evaluate the reaction time at each HAB. Taking advantage of the expanded flame structure at low pressure, the profiles of CH*, C2*, soot and temperatures, as a function of the height above the burner, were well resolved. It was found that CH* and C2* chemiluminescence overlap in space. The thickness of CH* layer is larger than that of C2*, and it peaks at slightly different location, CH* appears approximately 1 mm before C2*. The distance between the two peaks decreases linearly with the increase in pressure. The lowest value of the initial soot volume fraction (fv) was 0.19 ppb, measured at pressure 27 kPa. It was found that fv scales with the pressure following a power function of the form fv α Prn, where n is 2.15 ± 0.7. It was observed that, in all the flames investigated, the initial soot particles first appear at a common critical inception temperature, Tinception, of 1465 ± 66 K. It was found that the Tinception is lower than the maximum flame temperature, Tmax, by ~ 45° and appeared ~ 1 mm further than the location of Tmax. Using the Lagrangian quantity dfv/dt and fv, it was possible to reveal the soot growth rate, kSG. At pressure of 27 kPa, the value of kSG was evaluated as 20 s−1.