Effect of diameter ratios on the flow-sound interaction of the straight circular finned cylinders

Abstract

Finned tubes that are widely used in heat exchangers could be susceptible to the flow-excited acoustic resonance that generates intense pressure fluctuations, which may lead to premature fatigue failure of the tubes. This thesis explores the unsteady flow development around circular finned tubes and its role in the excitation of acoustic resonance. Finned cylinders with same effective diameter (De f f ), fin pitch (p), fin thickness (t) and a range of diameter ratios between 1.25 _ Df =Dr _ 2.5 are considered. Results from the PIV measurements at Re = 20000 demonstrated that the extent of the recirculation region decreases downstream of the finned cylinders when the diameter ratio increases due to higher flow entrainment between the fins. The frequency spectra of the streamwise velocity fluctuations showed that the temporal evolution of the vortex shedding in the wake becomes more periodic when the diameter ratio of the finned cylinders (Df =Dr) increases. Moreover, the strength of the vortices increases significantly in the wake. The increase in the flow entrainment in between the fins leads to a pronounced reduction in the random velocity fluctuations in the near wake of the cylinders. The three-dimensional flow development and structural loading on the finned cylinders are studied using Large Eddy Simulation. In addition to the flow entrainment mechanism in between the fins, another entrainment mechanism in the near wake is found to be caused by the downwash flow induced by an array of edge vortices that generate at the tip of the fins. The RMS of the fluctuating lift coefficient increases significantly on the surface of the finned cylinders due to a reduction in the vortex formation length and enhanced vortex strength. Moreover, the increase in skin friction on the surface of the cylinders, as a result of the higher flow entrainment, causes a significant increase in the drag coefficient for finned cylinders with larger diameter ratios (Df =Dr ). The results from the aeroacoustic response show that the excitation of acoustic resonance in the case of finned cylinders occurs at lower flow velocities compared to that of bare cylinders. This is mainly due to the existence of more energetic vortex shedding in the wake of the finned cylinders. Moreover, the normalized acoustic pressure generated during resonance excitation increases significantly as a result of an enhanced vortex strength and a reduced vortex formation length. Phase-locked particle image velocimetry (PIV) measurements during resonance excitation reveal that the global effect of the fins on the vortex shedding affects the location of the acoustic sources and sinks in the flow field as well as the energy transfer mechanism between the flow and the sound fields.