This article explores different equivalent diameter equations found in the literature for shedding frequency scaling and applying it to various types of finned cylinders in industrial heat exchangers. The focus is on three finned cylinder types: straight, twist-serrated, and crimped spirally finned cylinders. Within each finned cylinder category, at least three different finned cylinders are investigated. The results indicate that utilizing the appropriate equivalent diameter for vortex shedding frequency scaling collapses the data within the Strouhal bounds of a bare cylinder away from resonance excitation. However, the onset of flow-excited acoustic resonance and peak acoustic pressure in all the finned cylinder cases occur at a reduced flow velocity earlier than their equivalent diameter bare cylinder. This suggests that although utilizing the appropriate equivalent diameter can predict the shedding frequency away from resonance, it cannot be used in velocity scaling to predict the onset of acoustic resonance in finned tube bundle.
This paper presents an experimental investigation of the near-wake flow characteristics for isolated crimped spirally finned cylinders in cross-flow and its influence on the generated sound pressure during flow-excited acoustic resonance. Four crimped spirally finned cylinders are investigated, which have pitch-to-root diameter ratio (p/Dr) ranging between 0.384 ≤ p/Dr ≤ 1. A new equivalent diameter equation (Dc) has been developed to better capture the vortex shedding frequency emanating from the crimped spirally finned cylinders. The addition of crimped spiral fins reduces the coherence of the vortex shedding process as compared to that of a bare cylinder. Moreover, the addition of crimped spiral fins causes an elongation in the vortex formation region, as well as induces a larger velocity deficit in the near-wake. Reduction in the pitch-to-diameter ratio (p/Dr) leads to a progressive increase in the strength and coherence of the vortex shedding process. It also results in a gradual reduction in the vortex formation length and velocity deficit. The near-wake flow characteristics of the crimped spirally finned cylinders inherently affect the sound pressure during flow-excited acoustic resonance. Furthermore, the helical fins impose an asymmetrical inclination of the acoustic particle velocity. This hinders the flow-acoustic coupling, leading to a weakened energy transfer between the flow and sound fields. The findings of this investigation provide better understanding of the complex flow-sound interaction mechanism from crimped spirally finned cylinders in heat exchanger tube bundle.
The vortex dynamics of tandem bare and spiral finned cylinders in the cross flow are experimentally investigated at a spacing ratio of [Formula: see text]. Three different fin pitch-to-root diameter ratios ([Formula: see text]) are considered. For high [Formula: see text], the fins on the downstream cylinder inhibit the shear layer growth and flapping in the cylinders' gap. Shear layer flapping is only observed for finned cylinders with the lowest [Formula: see text]. Moreover, the gap shear layers enclose the downstream cylinder, enabling the formation of large vortex cores. The flow characteristics of finned cylinders are found to be independent of the Reynolds number with higher values of the Strouhal number due to an enlarged vortex formation length. For tandem bare cylinders, acoustic resonance excitation is triggered by two sources: (1) shear layer flapping in the cylinders' gap and (2) vortex shedding in the wake of the cylinders. All the tandem finned cylinders were able to excite acoustic resonance by the latter source. However, only tandem finned cylinders with the lowest [Formula: see text] were capable of exciting resonance by the former source. During this excitation, weaker shear layer flapping resulted in the formation of smaller vortex cores in the finned cylinders' gap. During the second resonance, single vortex pairs formed per cycle in the wake of the downstream finned cylinder, whereas two vortex pairs formed per cycle in the wake of the downstream bare cylinder. This work shows that the addition of fins changes the impinging flow mechanism and the flow topology, which cannot be captured using the equivalent diameter approach.
This paper presents an experimental investigation of the vorticity shedding and the susceptibility of acoustic resonance excitation for a square tube array with an intermediate tube spacing (i.e pitch-to-diameter ratio (P/D) of 1.73). The tube array could be rotated about an axis normal to the flow direction so that the effect of the flow approach angle could be investigated. Various Strouhal periodicities (St) were detected, and their strength, and value were dependant on the position measured within the tube bundle and the tube bundle's angular orientation. However, not all of the Strouhal periodicities measured caused self-excitation of acoustic resonance. This work illustrates the importance of considering the flow approach angle in the heat exchanger design phase to avoid the undesirable effects of acoustic resonance excitation during operation.
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