Acoustic resonance in heat exchanger tube bundles

Abstract

Many gas-cooled tube-and-shell heat exchangers emit an intense acoustic tone when shell side flow is brought to a certain level. The tone then persists as flow is varied. Sound levels as high as 165 dB have been measured inside heat exchangers ranging from small process units to nuclear reactors and utility power boilers. The sound is thought to be the result of periodic vortex shedding from the tubes at a frequency which coincides with the natural frequency of an acoustic mode within the heat exchanger shell. The sound within the heat exchanger shell is described by Lighthill's equation for aerodynamic sound. Sound propagation through a tube array is analogous to propagation through an array of small scatters. The speed of sound is slowed by the presence of the tubes, damping is increased, and the effective density is increased. The natural acoustic modes within the heat exchanger tube array are coupled with the modes of the entrance and exists, which are free of tubes. Thus, either numerical or matching solutions are required for the acoustic mode shapes. Tests on a variety of tube bundles in a rectangular shell are planned to measure the acoustic modes and the onset of resonance. A criterion will be established to differentiate between array geometry and damping that lead to resonance and those that are free of resonance. Models are being developed for the interaction between the coherent fluid dynamic structures and the acoustic field.