Underwater modal analysis of heat exchanger tubing using pressure transducers

Abstract

In order to perform an accurate flow-induced vibration analysis of heat exchanger tubing, natural frequencies, mode shapes, and damping values must be known. This study concerns experimental determination of the above parameters for titanium tubing in an underwater environment. Being a lightweight material with high stiffness, it was anticipated that high natural frequencies with small damping values would be encountered which could be erroneously altered by the mounting and cabling of traditional instrumentation such as accelerometers. A 1-g microaccelerometer was used to perform a preliminary modal survey to determine system natural frequencies, mode shapes, and separation of tube and fixture modes. It was found that mounted instrumentation did indeed cause mass loading problems resulting in erroneous natural frequencies and damping values. Noncontacting pressure transducers were used to confirm the identical tube mode shapes and to then determine true tube natural frequencies and damping values using a variety of excitation techniques. The study concludes by evaluating the most applicable techniques to be used in order to obtain valid data.