Flow induced acoustic resonances in heat exchangers
This thesis describes an investigation into the acoustic phenomenon in in-line tubular heat exchangers subjected to cross flow. The flow through such a heat exchanger can result in the production of very high noise levels, which occur as a result of the excitation of an acoustic standing wave in the cavity between the tube rows. This acoustic vibration can occur in large and small heat exchangers alike, resulting in drastically impared performance and working life. The phenomenon associated with such vibration is poorly understood and considerable anomalies still exist in published literature. There are several theories which attempt to describe this mechanism, however, none of these can satisfactorily account for its complex nature. The objective of the present work was to carry out an investigation to assess this acoustic phenomenon. The initial stages of this work produced an experimental rig to allow the phenomenon to be fully investigated, this included the examination of the effect of row depth and bank geometry on the acoustic resonance. The next step was to determine the role played by acoustic damping. This included making measurements of the damping under flow conditions and establishing its dependancy on velocity. A method of increasing the acoustic damping of a given bank was developed and incorporated in a tube bank. The results obtained from these experiments revealed that the acoustic system behaved in a manner which was consistent with that of a self excited system. Finally an appropriate mathematical model of the system was developed. The model,, which considers an acoustic feedback effect, was found to give quite an accurate representation of the system, and has the ability to account for all the observations made in this investiagation. This, together with the experimental results, enabled a series of guidelines to be presented as a basis for the design of such a tube bank.