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

Six coflowing laminar ethylene/air flames, varying in primary equivalence r atio from infinity (nonpremixed) to 3, have been studied both computational ly and experimentally to determine the fundamental effects of partial premi xing. Computationally, the local rectangular refinement solution-adaptive g ridding method is applied to a vorticity-velocity formulation of the conser vation equations; a damped modified Newton's method is used to solve the sy stem 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 thi n radiation submodel. Experimentally, temperatures are measured with thermo couples, and major species and C1 to C12 hydrocarbons with mass spectrometr y. Good agreement is observed for temperature, major species, and several m inor species. Heat release profiles, as well as those of several species, i ndicate that the partially premixed flames contain both an inner premixed a nd an outer nonpremixed flame front. As the primary equivalence ratio decre ases, an increasing delay (to higher nondimensional axial positions) in the development of temperature and species profiles along the flame centerline s 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 hydr ocarbons 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.