Study of Heat Transfer and Flow-Induced Vibrations of Circular Cylinders in Cross Flow

Abstract

This work investigates the phenomenon of flow-induced vibrations (FIV) experienced by bluff bodies subjected to fluid flow. FIV arises from periodic flow separation and vortex shedding, resulting in dynamic forces that induce motion or vibration in the body. This dynamic interaction becomes crucial in industrial systems, such as heat exchanger tubes, marine risers, and offshore structures. Notably, FIV poses a significant challenge in the design of modern shell-and-tube heat exchangers, being a major contributor to tube failures, leading to fatigue damage and potential catastrophic consequences. The primary objective of this research is to comprehensively understand the intricate dynamics of the interactive vibration system involving two circular cylinders and its impact on thermal performance considering one- and two-degrees of freedom motion which is often overlooked in FIV studies. Both high-fidelity numerical simulations and experimental investigations are employed to provide a holistic perspective. A parametric study is conducted to examine the effects of cylinder arrangement: spacing ratio and stagger angle, and diameter ratio on the FIV and heat transfer behavior of dual cylinders. Four spacing ratios (L/D) are selected (1.5, 2.5, 4, and 6) to encompass the major flow regimes of extended body, reattachment, and co-shedding, where L represents the distance between the cylinders, and D denotes the diameter of each cylinder. Stagger angles of 0°, 30°, 45°, 60°, and 90° are selected to represent the tandem, staggered, and side-by-side arrangements. The reduced velocity is systematically varied from 0 to 14 to encompass the lock-in region, where maximum vibration amplitude occurs. The parameters evaluated include lift and drag coefficients, Strouhal number, vortex shedding patterns, vibration frequency and amplitude, and Nusselt number. Additionally, this thesis explores the effectiveness of integrating a slit into the cylinder as a passive flow control technique to suppress FIV. The results demonstrated that spacing ratio, stagger angle, and diameter ratio significantly influence flow dynamics, cylinder vibration response, lock-in region, and heat transfer characteristics. Specifically, the lock-in region was widest in the tandem arrangement within the co-shedding regime, and the downstream cylinder experienced varying drag, either negative or positive, depending on the spacing ratio. Additionally, it was found that vortex shedding and cylinder vibrations enhance heat transfer, increasing the Nusselt number. Transitioning from a tandem to a staggered arrangement with a 60° angle resulted in a 9.4% increase in average Nusselt number for the downstream cylinder, while also reducing its transverse vibration by 75.3% at a reduced velocity of 8. Secondary lock-in and third harmonic excitation were observed in several cases, influenced by variations in diameter- and spacing- ratios. Considering the effectiveness of a parallel slit in the cylinder to suppress FIV, a 20% slit significantly reduced vibration amplitude by 34.4% compared to the solid cylinder. Increasing the slit size to 30% effectively eliminated vortex shedding and vibrations. The thesis concludes with recommendations for future work and provides valuable insights for engineers to mitigate FIVrelated structural risks, thereby enhancing equipment design and structural integrity.

Date of Award	7 May 2024
Original language	American English
Supervisor	MD FAZLULKARIM (Supervisor)