Simulation of Fluidelastic Vibrations of Heat Exchanger Tubes With Loose Supports

Abstract

Fluidelastic instability is regarded as the most complex and destructive flow excitation mechanism in heat exchanger tube arrays subjected to cross fluid flow. Several attempts have been made for modelling fluidelastic instability in tube arrays in order to predict the stability threshold. However, fretting wear prediction requires a nonlinear computation of the tube dynamics in which proper modelling of the fluid forcing function is essential. In this paper, a time domain simulation of fluidelastic instability is presented for a single flexible tube in an otherwise rigid array subjected to cross fluid flow. The model is based on the unsteady flow theory proposed by Lever and Weaver [1] and Yetisir and Weaver [2]. The developed model has been implemented in INDAP (Incremental Nonlinear Dynamic Analysis Program), an in-house finite element code. Numerical investigations were performed for two linear tube-array geometries and compared with published experimental data. A reasonable agreement between the numerical simulation and the experimental results was obtained. The fluidelastic force model was also coupled with a tube/support interaction model. The developed numerical model was utilized to study a loosely-supported cantilever tube subjected to air flow. Tube-to-support clearance, random excitation level, and flow velocity were then varied. The results indicated that the loose support has a stabilizing effect on the tube response. Both rms impact force and normal work rate increased as a result of increasing the flow velocity or the support radial clearance. Contact ratio exhibited a sharp increase at a flow velocity higher than the instability threshold of the first unsupported mode. In addition, an interesting behaviour has been observed, namely the change of tube’s equilibrium position due to fluid forces. This causes a single-sided impact. At a higher turbulence level, double-sided impact conditions were dominant. The influence of these dynamic regimes on the tube/support parameters was also addressed.