Fluid-Elastic Instability in a Tube Array Subjected to Uniform and Jet Flow

A wind tunnel study was carried out to investigate the fluidelastic stability of a model heat exchanger tube array subjected to a uniform cross-flow of air and a concentrated jet flow of air directed down a tube lane. The latter experiments were intended to simulate the effects of a soot blower on the dynamic response of tubes which had apparently been the cause of catastrophic tube failure in a heat exchanger. The experimental results showed that the model tube array experienced fluidelastic instability when subjected to a uniform cross-flow beyond a dimensionless pitch flow velocity which was substantially above the maximum design flow velocity of the heat exchanger. These experiments established that normal operating conditions could not have been responsible for the tube failures. Additional experiments showed that a continuously translating nozzle dispensing a jet of air at the tubes caused some static deflection of the tubes but no serious vibrations were observed. However, when the nozzle was fixed at one location, whereby the jet of air issued directly down a tube lane, fluidelastic instability occurred in the first few tube rows. A simplified analysis showed that the jet could cause fluidelastic instability. It can be inferred that, for heat exchangers equipped with steam soot blowers, normal soot blower operation should not cause fluidelastic instability but that a parked soot blower can cause fatigue failure of the tubes adjacent to the impinging jet in a relatively short period of time.