Flow characteristics and acoustic resonance excitation of finned cylinders in cross-flow

Abstract

The flow characteristics and acoustic resonance excitation of bare and finned cylinders of different arrangements in cross-flow were experimentally investigated. In the design phase of finned tube heat exchangers, finned cylinders are treated as bare cylinders with a diameter equivalent to their flow blockage. This technique is used in conjunction with empirical data obtained from bare cylinder measurements to estimate the vibration/acoustic excitation parameters such as the Strouhal number and critical flow velocity at which resonance is expected to materialize. However, detailed particle image velocimetry (PIV) measurements revealed that the equivalent diameter approach does not consider the intrinsic changes in the flow characteristics caused by the addition of fins. Dynamic lift force and aeroacoustic response measurements revealed that these changes affected the finned cylinder’s susceptibility to acoustic resonance excitation, different than its equivalent diameter bare cylinder. These variations were amplified when finned cylinders were placed in a tandem arrangement, causing significant changes in the impinging flow mechanism and topology. This resulted in quantitative differences in the excitation parameters between the finned and bare cylinders. These findings ultimately show the need for empirical finned cylinder data in order to reliably estimate excitation parameters in the design phase of heat exchangers. Another simplification made in the design phase of heat exchangers is that the flow is assumed to approach the tube bundle at a zero angle of attack. However, this is not the case in industrial applications. Strouhal periodicities measured using tubes instrumented with pressure taps at different locations within a square tube bundle showed strong dependence on the flow approach angle. This greatly influenced the aeroacoustic response. The viability of non-uniformly distributing the fins along their span to suppress acoustic resonance in the tandem arrangement was studied. Tandem non-uniform finned cylinders reduced the vortex shedding periodicity compared to uniform-finned cylinders with the same number of fins. This led to weaker acoustic resonance associated with the vortex shedding process. However, further work is required to optimize the distribution of the non-uniform fins to control the shear layer instability in the cylinders’ gap, which is the dominant excitation source for pre-coincidence acoustic resonance.