A comprehensive examination of the structure and extinction of turbulent nonpremixed flames formed in a counterflow

Abstract

An experimental investigation of turbulent counterflow nonpremixed flames has been undertaken in order to clarify the interaction between the properties of the nonpremixed flames and the characteristics of the turbulent counterflow field. In particular, to distinguish between the effects of turbulence caused by the air and fuel streams, the turbulent characteristics of each flow in an opposed jet flow were controlled individually. From the visualization by laser tomographic technique, it was found that the width of the diffusion region along the centerline regarded as a macroscopic parameter of the local structure of nonpremixed flames was not changed by the flow turbulence, and was determined by the mean flow condition characterized by the bulk velocity gradient, while whole diffusion regions spatially showed the typical wrinkled motion within the turbulent counterflowing stream. On the other hand, the mixture fraction fluctuations which were estimated by measurements of the behavior of the flame and the diffusion region, depended mainly on turbulence and were not affected by the bulk velocity gradient. The mean scalar dissipation rate χ turb due to the turbulence, estimated by combining the turbulent strain rate of the air side stream and the rms of mixture fraction fluctuation, increased with an increase in the turbulent strain rate of the air side stream, that is, with a decrease in the turbulent Damköhler number, Da. However, it is known that in a counterflow field the strain caused by the mean flow is also effective for properties such as the transport phenomena. Then, the total scalar dissipation rate χ total, which is derived from the turbulence and the mean flow velocity gradient, was suggested as the characteristic quantity of nonpremixed flames formed in counterflow geometry. The total scalar dissipation rate of flames at extinction showed almost constant value regardless of the initial turbulent conditions. The present results agree with the laminar flamelet concept.