Structure and blow-off mechanism of rod-stabilized premixed flame

Abstract

In order to determine the key feature in the blow-off and to reexamine the combustion mechanism, an experimental investigation was performed on propaneair flames stabilized by cylindrical rod bluff-bodies when the number and diameter of rods were changed under a constant blockage ratio of 0.4. Measurements were made of pressure gradient fluctuations along the rod surface and the rms values of ion current on the wake axis. The development of a flame generated at the attachment point was also pursued by using high-speed schlieren photography and ionization probes. The results showed that the instantaneous flame structure is composed of a series of small scale eddy-flames and large scale lump-flames containing a small definite number of eddy-flames. The attached flame causes a locally intense fluctuation of the surface pressure gradient at the attachment point probably due to flame-flow interaction. The lump-flames originate in the strongly stretched eddy-flames because of the maximum velocity gradient across a boundary layer at that point, and the surface pressure gradient fluctuation governs the process of lump-flame formation. It is also shown that the recirculation zone length varies with the range of L R d = 1.5–6.0 , which agrees well with that of the lump-flame formation point Lcd = 1.4–5.8. From these results a local extinction of the excessively stretched weak eddy-flames at the end of the recirculation zone was found to trigger the blow-off of the rod-stabilized flame. This enabled a qualitative modeling of the mechanisms of the lump-flame formation and flame stabilization behind the rods.