Abstract:
Flow induced vibrations in heat exchangers and steam generators is one of the critical problems the present industry is facing over past few decades. It is of major concern for designers, process engineers and operators, as it results in mechanical damage in the form of tube fretting wear, baffle damage, tube collision damage, tube joint leakage, tube failure due to fatigue or creep etc. Experimental investigations of cross flow induced vibrations in a tube bundle are carried out in this research work. Vibration behaviour of a single tube in a tube bundle has been examined by subjecting it to cross flow of air in a subsonic wind tunnel. The tube bundle consists of seven PVC tubes with 17.3 mm outer diameter arranged in rotated triangular configuration and has a pitch to diameter (P/D) ratio of 1.44. Tests are conducted in the wind tunnel over a range of free stream, air flow velocity from 3 to 20 m/s for which Reynolds number ranges from 0.34 × 104 ≤ Re ≤ 2.29 × 104. The target tube is instrumented with strain gauges to measure the amplitude response in flow and lateral directions for each value of the upstream velocity.
The analysis shows that amplitude of the monitored tube in the lateral (Y) direction is larger than that of the flow (X) direction due to configuration of the tube bundle. It is also observed that vibration amplitudes of the monitored tube remain small in low velocity range which occurs due to turbulence in the upstream flow, caused by surrounding tubes. Vibration amplitudes increase significantly as the flow velocity is extended ahead of a threshold velocity which indicates the onset of fluid elastic instability. The threshold velocity for fluid elastic instability to occur, also known as critical velocity is 14 m/s in the current experiment. The stability map confirms that tube bundle is unstable in the current range of velocities and fluid elastic instability is inevitable. It is established that the occurrence of fluid elastic instability can be avoided by either keeping the cross flow velocities below the critical velocity or by controlling the damping ratio of the tubes.
