THE STABILIZATION MECHANISM OF THE LIFTED JET DIFFUSION FLAME IN THE HYSTERESIS REGION

Recent experimental efforts focused on near-field coherent vortex dyna mics, and their impact on stabilization of a lifted jet diffusion flam e in the hysteresis region are reported. Simultaneous jet flow and fla me visualizations are conducted first to obtain a global feature of fl ow/flame interaction. The statistical liftoff heights are calculated b y a DIP (digital image processing) method. The gas concentration and v elocity distributions induced by the vortex evolution as well as the c orresponding flame front motion are deduced from phase-averaged measur ements of planar Mie-scattering gas concentration images, LDV and ion- signals, respectively. The planar gas concentration technique employed here extends our previous work (Chao et al. 1990, 1991 a) to include phase-averaging. Results of the experiments show that the most probabl e flame base locations in the hysteresis region are at the coherent vo rtex roll-up and pairing locations. The deeply entrained air lump caus ed by large-scale vortices during roll-up and pairing is the main obst ruction to flame propagation back to the nozzle exit and causes the hy steresis phenomenon.