Enhancement of heat transfer over a cylinder by acoustic excitation

Experiments were performed to demonstrate enhancement of heat transfer around a horizontal cylinder by the presence of acoustic excitation. The horizontal cylinder was heated uniformly and placed inside a wind tunnel. The wall temperatures around the cylinder were measured and are used to determine the local heat transfer in the circumferential direction. To avoid interference with the flow, the acoustic generator, which was a loud speaker, was placed downstream of the cylinder. The frequency of the sound F_e, was set equal to the natural frequency, F_n, of the shedding vortex in the wake or a multiple of F_n. Therefore, synchronization of vortex shedding with acoustic wave can be expected. The excitation frequencies selected were F_e/F_n = 1, 2, 3, up to 8. Other frequencies at F_e/F_n = 1.5, 2.5, 3.5, up to 7.5 were also selected for comparison. During the experiments, the sound pressure varied from 0 to 100 dB and the Reynolds number varied from 2938 to 8814. The heat transfer around the cylinder was found to be significantly enhanced by the acoustic waves. More detailed measurements for the energy spectrum of the acoustic waves generated by the current speaker were made. This provides a better understanding of the physical process. Flow visualization is also performed to demonstrate synchronization of vortex shedding with acoustic excitation. The effect of sound pressure levels and Reynolds numbers on the wall heat transfer are presented and discussed.