Effect of Vortex Formation Length on Flow-Excited Acoustic Resonance for a Single Spirally Finned Cylinder in Cross-Flow
Over the years, some effort has been expended in the improvement of heat transfer performance in tubular heat exchangers by increasing the heat transfer area of the tubes. This can be achieved by adding different types of fins to the outer surface of the tubes. However, finned tubes may lead to the generation of acute noise problems, caused by the coupling between the vortex shedding frequency and one of the acoustic natural frequencies of the duct housing the tubes. This may significantly reduce the service life of heat exchangers and may negatively impact the health of individuals working in close proximity to such acute noise intensity. Since the phenomenon is not yet fully understood, it can be dangerously unpredictable. Therefore, in this study, the flow-sound interaction mechanism of spirally finned cylinders will be investigated. Five spirally finned cylinders are investigated, which have fin densities ranging between 2 to 7 fins per inch. It has been observed that the addition of spiral fins induce an early flow separation, as compared to a bare cylinder. This causes the separated flow to be convected further downstream, and as a result of this, the vortices form further away from the cylinder’s base. As the fin spacing reduces, the vortex formation length gradually reduces due to a progressive delay in the boundary layer separation. This has shown to inherently affect the radiated sound during resonance; where vortices formed further away from the cylinder base has shown to attenuate the sound pressure during acoustic resonance excitation. A sample of the results are shown in this paper.
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