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February 28th, 2010 at 11:02 pm

Flow-Induced Vibration mechanism

Induced vibration of any system requires the coupling of some forces moving with a flexible structure. In the case of flow induced vibration, the exciting forces shellside result from the flow of fluid and flexible structure of the beam tubes. Forces oscillate at frequencies of exciting features that increase continuously with flow rate increasing. The tubes vibrate only single frequency response called their natural frequencies. Attack occurs when the exciting frequency match the frequency response and results of vibration of the tube.

The natural frequency of tubes is primarily dependent on their geometry and material of construction. The intensity of vibration is evidenced by the amount of periodic motion, the scope of this peak to peak on the centreline of motion at-rest is called the amplitude of vibration. The energy must be available to excite the tube vibration. The vibrational energy is dissipated by internal and external damping. The exciting force could be the result of:

1. fluid dynamic mechanisms because of the parallel flow through the pipes or
2. pulsation of a compressor or pump
3. mechanical vibrations transmitted through a structure.

Unless amplified by resonance phenomena, the flow of the forces normally enountered in equipment is not sufficient to cause damage. Resonance, which can increase the deformation of the tube in magnitude, occurs when the frequency of a strong cyclical exciting matches the natural frequency of the tube

In order to predict the occurrence of flow-induced vibration, the phenomena that produces the forces exciting and dynamic response of tubes to be understood. The determination of natural frequencies of the tube is relatively straight-forward. However, the determination of the forces created by the exciting shellside flow of liquid is extremely difficult. Shellside flow in a heat exchanger follows a complex path flow. Undergoes a change of direction, acceleration and deceleration. Sometimes, the flow is not perpendicular to the tubes (crossflow), axially along the tubes (parallel flow), or anywhere in between. Phenomena include flow in crossflow vortex shedding, turbulent buffeting and fluid elastic vortex flow phenomena found in parallel flow formation includesaxial Eddy flux

Tags: Acceleration And Deceleration, Beam Tubes, Centreline, Change Of Direction, Deformation, Dynamic Mechanisms, Dynamic Response, Flexible Structure, Flow Rate, heat exchanger, Mechanical Vibrations, Natural Frequencies, Natural Frequency, Oscillate, Parallel Flow, Peak To Peak, Periodic Motion, Pulsation, Resonance Phenomena, Vibrational Energy


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