THE DEVELOPMENT OF HELICAL MODES IN A ROUND JET AND THE EFFECT ON THE STABILIZATION OF A LIFTED FLAME

• Main Authors: Yong-Charn Jong, 鍾永強
• Other Authors: Yei-Chin Chao
• Format: Others
• Language: zh-TW
• Published: 1995
• Online Access: http://ndltd.ncl.edu.tw/handle/91931118845544316939

博士 === 國立成功大學 === 航空太空工程學系 === 83 === The low level time harmonic excitation of the piezoelectric actuators have been successfully employed to study the development of helical modes. When forcing m=1, the flow becomes eccentric, and the flow moves radially outward or inward as the crest of pressure wave passes the shear layer. When forcing m=2, the major and minor axes of the forced jet are continuously interchanged during each cycle of the modulating wave train. In the forcing m=1 case, the streamwise vortex pairs apparently grow up in the braid region, and usually one of those is larger, which appears as the streamwise vortex tube. In the forcing m=2 case, there are two major streamwise vortices in the transverse cross section. The breaking of rotation symmetry may ultimately lead to the production of intensive (large radial outward velocity) side jets. Then at the tip of the radial ejection, it creates a pattern which is similar to the mushroom type of vortex, and the vortex entrains the external fluid. The jet flame is stabilized at locations of roll up or pairing of vortices in the hysteresis region, where the large scale coherent structures prevail. Usually, both sides of the flame base of the lifted flame are not located at the same positions, which is shifted by a wave-length of coherent vortex. The hairpins (mushroom type) in the longitudinal section are the streamwise vortices in the transverse section. Due to the entrainment increase, the streamwise vortices are flammable. If the strain rate of streamwise vortices is not large enough to extinct the flame, the streamwise vortices will serve to provide a path between the two adjacent vortices for the flame base to propagate to the next up-coming vortex.