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MIXING ENHANCEMENT OF TURBULENT COAXIAL JETS BY INFLOW PULSATION

Seong Jae Jang
Department of Mechanical Engineering, KAIST 373-1 Guseong-dong, Yuseong-gu, Daejeon, 305-701, Korea

Hyung Jin Sung
Department of Mechanical Engineering, KAIST 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Korea

Abstract

The objective of the present study was to investigate the effects of inflow pulsation on the flow characteristics and the mixing properties of a turbulent confined coaxial jet flows. Large eddy simulations were performed at Re = 9000, based on the bulk velocity and outer radius of annular jet. The mean velocity ratio of central to annular jets was 0.6. The pulsation at the inflow jets was generated by varying the flow rate. First, the inflow pulsation was applied at frequencies in the range 0.1 < St < 0.9 while other parameters are fixed. The pulsation frequency responses were scrutinized by examining the phase- or time-averaged turbulent statistics. The pulsation frequencies of St = 0.180 and 0.327 gave the greatest enhancement in mixing and reduction of the reattachment length, respectively. Then, the effects of the phase difference between two inflow jets at the two optimal frequencies were investigated. The inner and outer Kelvin-Helmholtz vortices were investigated in detail to observe the effects of the coherent structures on the mixing enhancement and the reduction of the reattachment length. The optimal conditions of the phase difference in the mixing enhancement and the reduction of the reattachment length were obtained when the strength of the outer vortices was strong. Then, if the strength of the inner vortices is weakened by the phase difference, the reattachment length is minimized, and if the strength of the inner vortices is strong, the mixing is enhanced.