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

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 (f(v)) profiles were measured using laser-induced incandescence (LII), while spatial distribution of the CH* and C-2* 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 (T-s) was evaluated through fitting the spectrally resolved soot luminosity, while flame gaseous temperature (T-g) 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*, C-2*, soot and temperatures, as a function of the height above the burner, were well resolved. It was found that CH* and C-2* chemiluminescence overlap in space. The thickness of CH* layer is larger than that of C-2*, and it peaks at slightly different location, CH* appears approximately 1 mm before C-2*. The distance between the two peaks decreases linearly with the increase in pressure. The lowest value of the initial soot volume fraction (f(v)) was 0.19 ppb, measured at pressure 27 kPa. It was found that f(v) scales with the pressure following a power function of the form f(v) alpha Pr-n, 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, T-inception, of 1465 +/- 66 K. It was found that the T-inception is lower than the maximum flame temperature, T-max, by similar to 45 degrees and appeared similar to 1 mm further than the location of T-max. Using the Lagrangian quantity df(v)/dt and f(v), it was possible to reveal the soot growth rate, k(SG). At pressure of 27 kPa, the value of k(SG) was evaluated as 20 s(-1).

Automated summary

This study experimentally investigates the structure of laminar premixed ethylene/air flat flames at low pressure. The aim is to understand the conditions and precursors affecting soot particle inception. Laser-induced incandescence and emission spectroscopy techniques were used to measure the profiles of soot volume fraction, CH* and C-2* radicals, and temperatures. The study reveals the spatial distribution of these variables as a function of height above the burner and their relationship with pressure.