Mixing characteristics and structure of a turbulent jet diffusion flame stabilized on a bluff-body

摘要

Flow dynamics, scalar mixing, and pollutant formation in a turbulent jet diffusion flame stabilized on a bluff-body are investigated using large-eddy simulation. The density weighted filtered equations for the flow and mixing fields are solved using dynamic models for the subfilter quantities. Subfilter combustion processes are modeled by the conditional filtering method. An integrated formulation that considers only axial variation of conditionally filtered quantities is presented. Results show that vortex shedding from the coflow stream and its interaction with the high speed main jet play an important role in the generation of high dissipation layers in the intense mixing region. The mechanisms that generate the high dissipation layers in the intense mixing region are identified. The relatively uniform composition of combustion products in the recirculation zone helps the flame stabilization by maintaining low scalar dissipation rate and high temperature in the vicinity of the stoichiometric surfaces. The present integrated formulation is shown to reproduce these characteristics of the mixing field and to predict the flame structure and NO formation well. The weighted integral formulation of the conditional velocity allows the entrainment of the combustion products in the intense mixing region into the recirculation zone. The proper prediction of low conditional scalar dissipation in the recirculation zone is shown to be crucial for accurately describing the stabilization process. The decrease of NO at the end of the recirculation zone is reproduced due to the well-predicted mixing characteristics.