Numerical studies on the structure of two-dimensional H2/air premixed jet flame

Abstract

The burning characteristics of a premixed, H2/air Bunsen-type flame are investigated using a time-dependent, axisymmetric numerical model with variable transport properties and a detailed-chemical-kinetics mechanism. The temperature, species concentration, and velocity fields are investigated under fuel-lean, stoichiometric, and fuel-rich conditions. The calculations show that under fuel-lean conditions the flame exhibits the “tip-opening” phenomenon, while under fuel-rich condition the tip of the flame burns intensely. These results are in agreement with the experimental findings of Mizomoto et al. who have suggested that the tip-opening phenomenon results from the nonunity Lewis number. To further investigate the impact of local Lewis number on the premixed-flame structure, numerical experiments are performed by modifying the local Lewis numbers of the individual species. Results for the fuel-lean condition confirm that the local Lewis number is responsible for the tip-opening phenomenon. Indeed, when the local Lewis number is set equal to 2, the burning pattern of the fuel-lean premixed flame resembles that of a fuel-rich flame with a closed tip. The spatial distributions of NO in the fuel-lean, stoichiometric, and fuel-rich flames are also examined. Under the fuel-lean and stoichiometric conditions, the NO is formed along the high-temperature cone of the flame, as expected. In the fuel-rich case, a dual flame structure is observed. The NO production occurs primarily in the secondary “diffusion” flame which is established at the interface of the excess fuel and ambient oxygen. Buoyancy-induced toroidal vortices are found to form in the vertically mounted premixed flames. However, the dynamic characteristics of a premixed flame, in contrast to those of a jet diffusion flame, are observed to be dependent on the inlet velocity profile of the fuel jet.