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

Abstract

Six coflowing laminar, partially premixed methane/air flames, varying in primary equivalence ratio from ∞ (nonpremixed) to 2.464, have been studied both computationally and experimentally to determine the fundamental effects of partial premixing. Computationally, the local rectangular refinement solution–adaptive gridding method incorporates a damped modified Newton’s method to solve the system of coupled nonlinear elliptic partial differential equations for each flame. The model includes a C2 chemical mechanism, multicomponent transport, and an optically thin radiation submodel. Experimentally, both probe and optical diagnostic methods are used to measure the temperature and species concentrations along each flame’s centerline. Most experimentally measured trends are well predicted by the computational model. Because partial premixing decreases the flame height when the fuel flowrate is held constant, computational and experimental centerline profiles have been plotted against nondimensional axial position to reveal additional effects of partial premixing. Heat release profiles, as well as those of several species, indicate that the majority of the partially premixed flames contain two flame fronts: an inner premixed front whose strength grows with decreasing primary equivalence ratio; and an outer nonpremixed front. As the amount of partial premixing increases, computational results predict a continual reduction in the amount of flow radially inward; the resulting decrease in radial transport is responsible for various effects observed both computationally and experimentally, including a cooling of the gases near the burner surface. At the same time, radiative losses decrease with increasing amounts of premixing, resulting in higher flame temperatures.