Experimental investigation for fluid elastic instability in normal triangular finned tube arrays subjected to water cross flow

Abstract

The failure due to flow induced vibrations in heat exchangers occurs when shell side cross flow velocity exceeds critical velocity at instability. This ultimately results in excessive vibrations and failure of the tubes because of fretting wear. It is necessary to know the critical velocity of heat exchanger tube arrays to avoid the risk of failure as a result of flow induced vibrations. This is possible by designing the shell and tube heat exchangers so that they reach instability at higher velocities. The study aims at conducting an experimental program to determine critical velocity at instability for three normal triangular tube bundles having pitch ratio of 2.1. Plain tube configuration, a finned tube configuration having fin density of 4 fpi and a finned tube configuration having fin density of 10 fpi are tested in the water. The vibration characteristics of normal triangular tube bund are compared with available results for other tube array patterns, to compare their instability thresholds. The design changes made in the outer box are also assessed to find if fluid elastic instability is reached at lower flow rates. The research outcomes indicate that design changes in the setup have resulted in reaching clear instability for all the normal triangular tube bundles. The instability thresholds for plain bundles when compared against finned bundles show that instability is reached at higher velocity for finned bundles. The comparison of vibration characteristics of different tube array patterns indicate that instability thresholds for normal triangular tube array configuration are delayed than other tube array configurations including normal square, rotated square and parallel triangular.