Investigation of the Structure of a Laminar Non-Premixed Flame in an Unsteady Vortical Flow.

摘要

Although organized vortical motions have been observed in a variety of turbulent flames, there have been only a few experimental studies which have directly examined the structure of a flame imbedded in an unsteady vortical flow. In the present work, this problem is investigated using a laminar co-flowing jet flame in which a periodic vortical motion is induced by acoustic excitation of the fuel stream. A variety of optical planar imaging techniques are employed to define the instantaneous flame structure. Particle tracking is used to obtain the instantaneous two-dimensional velocity field. These measurements are combined to follow the evolution of the flame in the unsteady velocity field over one cycle of the excitation. The flame is observed to break up into a series of axisymmetric flamelets which convect and distort under the influence of buoyancy and the vortical flow. When the velocity field is viewed in a frame of reference moving with the flamelets the flow pattern is seen to consist of a pair of saddle points on either side of the flow centerline near the flame tip and a single saddle point on the centerline near the base of the flame. This latter saddle point is also evident in images of light scattered from TiO2 particles added to the fuel stream. Entrained fluid is fed into a large axisymmetric vortex surrounding the base of the flame. Because of heat release, the flame acts as an effective volume source in the flow. A high strain rate exists on the centerline at the base of the flame, and planar laser-induced fluorescence measurements of the concentration of OH show local extinction of the flame. Reprints. (jhd)