Computational and experimental study of axisymmetric coflow partially premixed ethylene/air flames

Six coflowing laminar ethylene/air flames, varying in primary equivalence ratio from infinity (nonpremixed) to 3, have been studied both computationally and experimentally to determine the fundamental effects of partial premixing. Computationally, the local rectangular refinement solution-adaptive gridding method is applied to a vorticity-velocity formulation of the conservation equations; a damped modified Newton's method is used to solve the system of coupled nonlinear elliptic partial differential equations for each flame. The numerical model includes a 50-species chemical kinetic mechanism with C1 to C6 hydrocarbons, multicomponent transport, and an optically thin radiation submodel. Experimentally, temperatures are measured with thermocouples, and major species and C1 to C12 hydrocarbons with mass spectrometry. Good agreement is observed for temperature, major species, and several minor species. Heat release profiles, as well as those of several species, indicate that the partially premixed flames contain both an inner premixed and an outer nonpremixed flame front. As the primary equivalence ratio decreases, an increasing delay (to higher nondimensional axial positions) in the development of temperature and species profiles along the flame centerlines is observed both computationally and experimentally; this is explained by a computed decrease in the amount of flow radially inward. The nonpremixed flame contains the highest concentrations of acetylene and several C4 hydrocarbons but the lowest concentrations of methane, formaldehyde, and C3H4, indicating that partial premixing shifts the pyrolysis mechanism toward odd-carbon species. (C) 2001 by The Combustion Institute.