Akio Kitajima

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 χturb 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 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 χtotal, 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.

Experimental investigation of the flame structure and extinction of turbulent counterflow non-premixed flames

Extinction conditions and flame structures for methane-air turbulent non-premixed flames are investigated experimentally for a counterflow nozzle-type burner system. Extinction limits are measured by varying fuel concentrations diluted by nitrogen, mean, flow velocities of both burners, and turbulent characteristics generated by perforated plates. In particular, the flow turbulence of each burner is controlled individually. The extinction limits for the same fuel concentration can be expressed by approximately the same bulk velocity gradient. At the condition of lean fuel concentration or high intensity of flow turbulence, the flame strength is weak. It is shown that the flame strength is influenced by the turbulence of the air stream rather than that of the fuel stream. The flame shape was observed by a laser tomographic technique under various conditions. Because the methane-air counterflow non-premixed flames in the present study are formed in the air stream, the diffusion region can be visualized as the vanishing strip of seeding particles between near flames and the stagnation plane. In the case of laminar flames, there is no difference of mean flame locations, or the width of the diffusion region, for a certain range of fuel concentration. In the case of turbulent non-premixed flames, it is also observed that relative flame locations in the diffusion region are almost the same, and the width of the diffusion region is not changed for various turbulent flow conditions. It is shown that the diffusion region containing turbulent flames keeps the structure, as laminar flamelets.

Study on Ignition-Like Behavior Induced by Interaction of Curved Non-Premixed (Diffusion) Flames

An abrupt temperature increase induced by the interaction of “curved” diffusion flames is studied experimentally. The main purpose is to reproduce and measure the numerically predicted unsteady behavior of interacting flames experimentally. A pair of curved non-premixed (i.e., diffusion) flames, which was made by four slot burners (two combined counter flow burners), is utilized. Curved flames are either formed over the fuel stream (normal diffusion flame) or oxidizer stream (inverse diffusion flame) depending on the condition, and their interaction is demonstrated. The characteristics of the ignition-like behavior, namely, a time-sequential change in sound and luminescence from the image generated by the flame fluctuation, are acquired by a microphone and a high-speed camera, respectively. From the periodic unsteady behavior of interacting flames, the ignition-like behavior is successfully confirmed in the experiment. The observed change of frequency of the ignition-like behavior against the bulk velocity gradient agrees with that predicted previously in the numerical analysis.

対向流CH4-N2/O2-N2非予混合火炎の消炎に対する巨視的な火炎構造の変化と流体変動の影響

Extinction limits of counterflow CH4-N2/O2-N2 non-premixed flames were investigated experimentally by manipulating the global flame structure, that is flame location in counterflow, by varying stoichiometric mixture fraction : Zst, and under fixed stoichiometric adiabatic flame temperature by keeping molar stoichiometry : Mst constant. Furthermore, the effects of two fluid dynamical fluctuations that were oscillated flow at frequency : 20 Hz and perforated plate turbulent flow on the extinction characteristics of the counterflow flames were also examined. As a result, the extinction limits of the laminar flame increase with increase in Zst. Amount of oxygen in oxidizer flow increases with Zst and oxygen promotes chain-branching reaction : O2+H→OH+O. Therefore the laminar flames stability becomes higher with increase in Zst under the constant adiabatic flame temperature. For the oscillatory flow, the extinction character indicates the similar tendency to the laminar flame, that is, the oscillated flame shows quasi-steady extinction phenomena and the extinction limits always depend on the flow condition of oxidizer even if the flame is located in the fuel side. On the other hands, the perforated plate turbulent flame stability always depends on both the flame location and the streamside with turbulence. The extinction limits of the counterflow turbulent flame are more influenced by the turbulent fluctuation of the flow in which the flame is located than that of the flow without the flame. It is thought to be that the difference for the extinction character between the oscillatory flame and the turbulent flame is due to unsteadiness of the fluctuation in the flow.

Turbulent effects on the flame behavior of turbulent counterflow nonpremixed flames

The flow field of the turbulent counterflowing stream with and without nonpremixed flames, and the behavior of diffusion region of turbulent nonpremixed flames have been investigated experimentally. The turbulent characteristics of each side flow in a counterflowing stream would be regarded as keeping their initial turbulent characteristics from one burner exit to near the stagnation region. And the mutual interferences of turbulence of both flows are slight in the present nonreactive turbulent counterflow field. The trends of velocity fields at every turbulent condition agree well with the flow field of nonreactive turbulent counterflowing stream and it is considered that the mutual interferences of both air side stream turbulence and fuel side stream turbulence seem to be slight in a turbulent counterflowing stream with nonpremixed flames. The behavior of diffusion region including the nonpremixed flame is mostly affected by the turbulence of the air stream. And it indicate that the presence of a certain close relation between the macroscopic behavior of diffusion region caused by the turbulent effect and extinction phenomena of turbulent nonpremixed flames formed in a counterflowing stream.