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

An experimental investigation of turbulent counterflow nonpremixed flames h as been undertaken in order to clarify the interaction between the properti es of the nonpremixed flames and the characteristics of the turbulent count erflow field. In particular, to distinguish between the effects of turbulen ce caused by the air and fuel streams, the turbulent characteristics of eac h flow in an opposed jet flow were controlled individually. From the visual ization by laser tomographic technique, it was found that the width of the diffusion region along the centerline regarded as a macroscopic parameter o f the local structure of nonpremixed flames was not changed by the flow tur bulence,and was determined by the mean how condition characterized by the b ulk velocity gradient, while whole diffusion regions spatially showed the t ypical wrinkled motion within the turbulent counterflowing stream. On the o ther hand, the mixture fraction fluctuations which were estimated by measur ements of the behavior of the flame and the diffusion region, depended main ly on turbulence and were not affected by the bulk velocity gradient. The m ean scalar dissipation rate chi(turb) due to the turbulence, estimated by c ombining the turbulent strain rate of the air side stream and the rms of mi xture fraction fluctuation, increased with an increase in the turbulent str ain rate of the air side stream, that is, with a decrease in the turbulent Damkohler 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 chi(total), w hich is derived from the turbulence and the mean flow velocity gradient, wa s suggested as the characteristic quantity of nonpremixed flames formed in counterflow geometry. The total scalar dissipation rate of flames at extinc tion showed almost constant value regardless of the initial turbulent condi tions. The present results agree with the laminar flamelet concept. (C) 200 0 by The Combustion Institute.