Flame propagation enhancement by plasma excitation of oxygen. Part II: Effects of O2(a1Δg)

The isolated effect of O-2(a(1)Delta(g)) on the propagation of C2H4 lifted flames was studied at reduced pressures (3.61 kPa and 6.73 kPa). The O-2(a(1)Delta(g)) was produced in a microwave discharge plasma and was isolated from O and O-3 by NO addition to the plasma afterglow in a flow residence time on the order of 1 s. The concentrations of O-2(a(1)Delta(g)) and O-3 were measured quantitatively through absorption by sensitive off-axis integrated-cavity-output spectroscopy and one-pass line-of-sight absorption, respectively. Under these conditions, it was found that O-2(a(1)Delta(g)) enhanced the propagation speed of C2H4 lifted flames. Comparison with the results of enhancement by O-3 found in part I of this investigation provided an estimation of 2-3% of flame speed enhancement for 5500 ppm of O-2(a(1)Delta(g)) addition from the plasma. Numerical simulation results using the current kinetic model of O-2(a(1)Delta(g)) over-predicts the flame propagation enhancement found in the experiments. However, the inclusion of collisional quenching rate estimations of O-2(a(1)Delta(g)) by C2H4 mitigated the over-prediction. The present isolated experimental results of the enhancement of a hydrocarbon fueled flame by O-2(a(1)Delta(g)) along with kinetic modeling results suggest that further studies of CnHn + O-2(a(1)Delta(g)) collisional and reactive quenching are required in order to correctly predict combustion enhancement by O-2(a(1)Delta(g)). The present experimental results will have a direct impact on the development of elementary reaction rates with O-2(a(1)Delta(g)) at flame conditions to establish detailed plasma-flame kinetic mechanisms. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.