Over-rich combustion of CH4, C2H6, and C3H8 +air premixed flames investigated by the heat flux method and kinetic modeling

An uncommon non-monotonic behavior of the temperature dependence of adiabatic laminar burning velocity has been found in over-rich methane+air flames at equivalence ratio, phi=1.4. To find out the universality and reasons of this turning point, methane, ethane and propane +air flames are studied both experimentally by the heat flux method and numerically using GRI-mech, USC-mech, UCSD-mech, FFCM mech, and Aramco mech over phi = 0.6-1.8, at unburned temperatures up to 368K, and atmospheric pressure. Results show that the over-rich phenomena stem from a unique flame structure, where, after the flame front, H2O is reduced to H-2 and C2Hx (x>1) is oxidized to CO, causing the temperature overtone (super adiabatic flame temperature), while some key reactions important for flame propagation changing their sensitivity signs. Inside the flame front, the importance of CH3 overwhelms other radicals like OH and H. By these distinguishing features, a method using temperature overtone to identify accurate turning points of over-rich regime is demonstrated. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.