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ACTIVE CONTROL OF FREE AND IMPINGING JETS FOR MODIFICATION OF MIXING

Jungwoo Kim
Seoul National University of Science and Technology

Jeongyoung Park
School of Mechanical and Aerospace Engineering, Seoul National University, Seoul 151-742, KOREA; Applied Technology Research Department, Hyundai Mobis

Haecheon Choi
Department of Mechanical & Aerospace Engineering Seoul National University 1, Gwanak-ro, Gwanak-gu, Seoul 08826, Korea

Abstract

The objective of the present study is to modify the mixing and heat transfer in turbulent free and impinging jets using an active control method, where the jet-exit Reynolds number is 10,000. As an active control method, timeperiodic blowing and suction is applied at the jet exit. The excitation frequency is selected to be Stθ = 0.017 corresponding to the maximum growth rate in the mixing layer, where 0 is the momentum thickness at the jet exit. First, in free jet, the modification of vortical structures due to the excitation results in turbulence suppression in a downstream location. Accordingly, mixing is reduced in a downstream location as compared with that without excitation. Secondly, in impinging jet, the effect of excitation significantly depends on the distance (H) between the jet exit and the wall. In the present study, H/D = 2, 6 and 10 are considered to investigate this effect, where D is the jet diameter. The heat transfer rate at the wall is enhanced due to the excitation in the cases of H/D = 2 and 6. On the other hand, the heat transfer rate in the case of H/D = 10 decreases as compared with that without excitation. These results are related to the changes in vortical structures by the excitation.