Numerical Investigation of Edge Flame Structure of Counterflow Nonpremixed Flames with Local Extinction Due to Flame Stretch

Abstract

Laminar counterflow nonpremixed flames with local extinction caused by nonuniform inflows are numerically investigated in two-dimensions in a system governed by Lewis numbers of unity and assuming a single-step, irreversible, finite-rate reaction. A numerical method based on the SIMPLE algorithm is used. Edge flame structures are investigated through the introduction of a new progress variable defined as the normalized integral of the mass fraction of a product in the mixture fraction Z space and which expresses the progress of the chemical reaction. Calculation results reveal that the edge flame structure can be classified into three regions; fully burning flame, weakened flame with a low reaction rate, and a non-reacting preheat region. Under these conditions, and in terms of the energy balance, the edge flame structure is dependent on the Z-directional heat diffusion transport, heat consumption (given by the progress variable), and heat production by chemical reaction. It is found that each type of flame structure can be well characterized by three parameters; the mixture fraction, the stoichiometric scalar dissipation rate, and the progress variable. These three parameters are identified as important factors affecting the edge flame structure of nonpremixed flames with local extinction.

Identification of Triple Flame Based on Numerical Data for Laminar Lifted Flames

“…The region without reaction upstream of the flame base is referred to as the unburned region (frozen flow structure region), followed by the transition region outside the premixed flame, in which the flame changes from the frozen structure to the fully burning diffusion flame structure. The scalar structure of the transition region can be expressed in terms of the mixture fraction and a progress variable, described in detail elsewhere (31) . The region surrounded by the premixed flames is referred to as the triple flame structure region.…”