The effect of simple spiral fin around a bare tube on Karman vortex shedding was experimentally investigated. We measured the distributions of the mean velocity, the intensity of velocity fluctuation, the spectrum of velocity fluctuation and the coherence distribution of Karman vortex in the spanwise direction. And the flow around a fin tube was visualized by using smoke wire technique. As a result it was made clear that the periodic velocity fluctuation caused by Karman vortex was remarkably observed in the wake of the fin tube although the fin was mounted around a bare tube with a large pitch along the tube axis. The vortices in the near-wake of the fin tube make cells structure with the scale of the pitch length of the fins. The rotation axis of each vortex was inclined against the tube axis. The large scale vortex was formed by the coalescence of the vortex cells. Therefore, the spanwise scale of the vortex in the wake of the fin tube was larger than one pitch of the fins.

In the present paper, the effect of twist-serrated fins around a bare tube on the Aeolian tone was experimentally investigated. These fins were mounted spirally around a bare tube and had the same geometry as those actually used in boiler tubes. We measured the intensity of velocity fluctuation, spectrum of velocity fluctuation, coherence of Karman vortex in the spanwise direction, dynamic lift force, and sound pressure level of the aerodynamic noise generated from finned tubes with various fin pitches. An Aeolian tone induced by Karman vortex shedding was observed in the case of a finned tube, although the complicated fin was mounted around a bare tube. A decrease in the pitch of the fin effectively caused an increase in the equivalent diameter, which acted as the characteristic length of a cylinder with fins. The equivalent diameter depended on the Reynolds number. We modified a relation to calculate the characteristic diameter of the finned tube, which in turn was used to calculate the Strouhal number. The coherent scales in the spanwise direction for the cases with various fin pitches were slightly larger than that of a simple circular cylinder. It is known that the sound pressure level of the Aeolian tone depends on the coherent scale of the Karman vortex in the spanwise direction. However, when the pitch of the fins decreased, the peak level of the sound pressure spectrum decreased. A correlation analysis between the flow field and Aeolian tone was carried out.

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The present paper deals with acoustic resonance in tube bundles which occurred in actual boiler plants. A practical method to suppress the resonant noise is proposed based on scale model tests. In those experiments, besides the transverse modes which are relatively well-known, longitudinal modes which are known to be difficult to suppress, were realized. The relation of both kinds of acoustic modes to vortex shedding frequency was clarified. In the following step, a design method of the acoustic energy absorbers is proposed based on the feedback system stability theory. According to this theory, acoustic energy absorbers were equipped in the duct of the test model, and remarkable increase of the critical flow velocity was confirmed for both types of resonant mode. Based on these results, this method was applied to the actual boiler plants and it was confirmed that the proposed method is the effective countermeasure for both types of acoustic resonant modes.

This paper focuses on the relationship between acoustic resonance and vortex shedding from the tube banks of a boiler plant. We have built a model similar to the actual boiler plant to clarify the characteristics of acoustic resonance phenomena and vortex shedding. The model used in-line tube banks with a small tube pitch ratio. We examined the relationship between the acoustic resonance of the actual plant and that of the model, and measured the sound pressure level, acoustic pressure mode shape, spectrum of velocity fluctuation, and gap velocity. Gap velocity was defined as the mean velocity in the smallest gaps between two neighboring tubes in the transverse direction. As a result, the resonant frequencies and mode shapes of the acoustic resonances in the actual boiler plant agreed well with those in the similar model. We found many peak frequencies in the sound pressure level spectrum when acoustic resonances occurred. The typical Strouhal numbers at the onset velocity of acoustic resonances were about 0.19, 0.26 and 0.52. Periodic velocity fluctuation caused by vortex shedding was observed inside the tube banks without acoustic resonance. The Strouhal number measured for vortex shedding was 0.15. Acoustic resonances of higher-order modes were generated in this plant.