Extinction of an evolving jet flame under a linearly-varying flow field

Abstract

The effects of an unsteady strain rate on the extinction of diffusion flames are studied experimentally. Experiments are carried out for an evolving jet configuration in which the flow at the tip of vortex is similar to counterflow diffusion flames. A non-reacting flow is used to characterize the flow field of the downward evolving jet flame. Rayleigh scattering images show that the fuel concentration at the jet tip is preserved in the near field, and the velocity at the nozzle exit, measured by LDV, increases linearly. Both buoyancy and curvature effects are also examined using a high-speed shadowgraph system. This transient flow field provides a linearly varying strain history when ignited by a residual flame. Twenty-two strain histories, which have been obtained for various Reynolds numbers and a constant residual temperature at the jet exit, are investigated for the occurrence of extinction of the flame tip. The experimental results show that the extinction point under a linear-varying strain rate is extended with an increase in the slope of the strain rate. An equivalent strain model under a linear-varying strain rate is introduced to complement and validate our experimental results. Although it is noted that an increase in the slope of the strain rate, which implies the flame experiences much more unsteadiness, induces a further extension of the flame extinction point in all the computational results, the initial condition also has a strong influence on the extinction point in the case of steep changes in strain rate. When the appropriate initial condition is selected, the equivalent strain model can predict the experimental extinction point successfully.