Passive noise control technique for suppressing acoustic resonance excitation of spirally finned cylinders in cross-flow

Abstract

This paper presents an innovative passive noise control technique for suppressing flow-excited acoustic resonance of spirally finned cylinders in cross-flow. This was achieved by maintaining the number of fins, but varying the fin density along the finned cylinder’s span to create a non-uniform finned cylinder. Two non-uniform finned cylinders were investigated with pitch ratios of p(mid)/p(side) = 0.5 and 1.7. To better understand the influence of the non-uniform finned cylinders on the wake structure, bare cylinders with the same effective diameter were investigated. In the non-uniform finned cylinder case, due to the variation in the fin density, the effective diameter along its span changes. Thus, the effective diameter of a non-uniform finned cylinder happens to be a dual diameter cylinder. Three dual diameter cylinders were investigated with diameter ratios ranging between 0.5 ≤ D(mid)/D(side) ≤ 2. These diameter ratios closely correspond to the pitch ratio (p(mid)/p(side)) and effective diameter ratio (Dc(mid)/Dc(side)) of the non-uniform finned cylinders investigated. The results show that the non-uniform finned cylinderscause spatial variations in the vortex shedding process due to dissimilarities in the flow behavior at different fin density regions. Temporal variations were also revealed due to changes in the vortex shedding frequency at different spanwise regions. This resulted in a significant decrease in the spanwise flow coherence and vortex shedding strength. The spatio-temporal variations in the shed vortices at different spanwise regions observed by the non-uniform finned cylinders were found to be analogous to dual diameter cylinders with diameter ratios equivalent to the pitch ratios of the non-uniform finned cylinder. As a result, substantial attenuation in the acoustic pressure during flow-excited acoustic resonance was achieved by the non-uniform finned cylinders. The results presented in this paper show that non-uniform finned cylinders are a viable noise control technique for suppressing flow-excited acoustic resonance in tube bundles of heat exchangers.