Fluid-elastic instability in tube arrays subjected to air-water and steam-water cross-flow

Fluid-elastic instability and flow induced vibrations in heat exchanger tubes have led to numerous accidents and economic losses in the past. Efforts have been made to systematically study the cause of these vibrations and develop remedial design criteria for avoidance of these instabilities. Forty years of research have led to the development of reliable design criteria based on the results of single phase flow. Experiments with two-phase flow have been lagging and the data in two-phase flow experiments show significant scatter. Moreover, majority of the experiments in two-phase flow have been carried out with air-water as the fluid whereas the true representation of the flow situation in a heat exchanger would be steam-water flow. In this research, experiments were systematically carried out with air-water and steam-water flow. A normal square tube array of pitch-to-diameter ratio of 1.4 was used in the experiments. The tubes were suspended from piano wires and strain gauges were used to measure the vibrations. Tubes made of aluminum, stainless steel and brass were systematically tested by maintaining approximately the same stiffness in the tube systems. The parameters measured at the onset of instability were the critical fluid velocity, the frequency of vibration, the damping ratio and the void fraction. Instability was clearly seen in single phase flow and the critical velocity was found to be proportional to tube mass. Instability in two-phase flow was not as distinct as in single phase flow but could be detected after post-processing the experimental data.; The present study shows that fully flexible arrays become unstable at a lower flow velocity when compared to a single flexible tube surrounded by rigid ones. The study also concludes that tubes are more stable in steam-water flow as compared to air-water flow. This is a consequence of lower drag forces encountered in steam-water flow. Nucleate boiling on the tube surface is found to have a stabilizing effect on fluid-elastic instability.