Quantification of PAH concentrations in premixed turbulent flames crossing the soot inception limit

Abstract

The current study quantifies the impact of equivalence ratio and rate of strain on the spatial distribution of major species and soot precursors in premixed turbulent ethylene flames crossing the soot inception limit. A back-to-burnt (BTB) opposed jet configuration is used to provide accurate control of the turbulent and chemical timescales. The upper nozzle features fractal grid generated turbulence and premixed ethylene/air mixtures with flames stabilised against well-defined hot combustion products emerging from the lower nozzle. The study combines simultaneous PAH/CH 2 O-PLIF and elastic light scattering (ELS) with probe sampling. Gas chromatography-mass spectrometry (GC-MS) and gas chromatography-thermal conductivity detection (GC-TCD) were used to quantify concentrations of major species and poly-aromatic hydrocarbons (PAHs). The laser based diagnostics show that the rate of strain exerts a dominant impact on the growth of soot particles with low turbulent Reynolds numbers (50 ≤ Re t ≤ 94) used to promote soot formation. The probe measurements indicate that acetylene, 1-methylnaphthalene (1-MN), m/z = 154, m/z = 276 and benzo(a)pyrene correlate with soot formation and that the equivalence ratio is the controlling PAH growth parameter. It is further shown that the spatial extent of the PAH containing reaction zone exceeds three integral length scales.

Particle size distributions in turbulent premixed ethylene flames crossing the soot inception limit

“…with flames stabilised against hot combustion products (HCPs) [37]. The HCPs were generated by stabilising nitrogen diluted hydrogen flames on a perforated plate (PP) located 100 mm upstream of the lower nozzle (LN) exit to provide a well-defined burnt gas state [38,39]. The temperature at the lower nozzle exit (T HCP = 1500 K) was measured using an R-type thermocouple [40] with combustion products generated using a nitrogen diluted stoichiometric H 2 /air flame with a mole fraction ratio x H2 /x N 2 = 0.51.…”

“…The total rate of strain (a T = a b + a t ) was estimated using the approach of Kostiuk et al [41] with a t = (ε r /ν r ) 1/2 , where ε r is the rate of dissipation in the reactants, determined using the method of George and Hussein [42] as applied by Goh et al [43], and ν r the corresponding fluid viscosity. The bulk rate of strain is The conditions correspond to recent investigation of soot compositions [38,39] and gaseous species concentrations [39]. The flame conditions were chosen to determine the evolution of PSDs spanning lightly to the heavily sooting flames at different rates of strain.…”

“…The flame conditions were chosen to determine the evolution of PSDs spanning lightly to the heavily sooting flames at different rates of strain. The impact of the variations in conditions on gaseous species concentrations and soot particle compositions was previously analysed using GC-MS (PAH sampling experiment) and laser diagnostic techniques using ELS and PAH-PLIF [38,39]. Results show that soot formation is exceptionally sensitive to the rate of strain (a T ) and, by implication, to the local flow field with soot expected to grow rapidly in regions of low rates of strain.…”

“…Gas sampling is performed via a versatile quartz glass probe featuring aerodynamic quenching and dual-port dilution at the probe tip and in the transfer-line [38,39] as shown in Fig. 2.…”

“…A pressure transducer (UNIK 5000; GE Measurement and Control) is placed downstream of the probe to control the static pressure in the sampling system. A vacuum regulator and a metering valve are used to set a pressure differential of 20 mbar between the sampling line and the flame to minimise flame disturbances and PAH losses [38,39]. This results in a constant sample…”