Numerical Simulation of Tube Bundle Vibrations in Cross Flows

Abstract

In many industrial applications, mechanical structures like heat exchanger tube bundles are subjected to complex flows causing possible vibrations and damage. Part of fluid forces are coupled with tube motion and these so-called fluid-elastic forces can affect the structure dynamic behaviour generating possible instabilities and leading to short term failures through high amplitude vibrations. Most classical fluid force identification methods rely on structure response experimental measurements associated with convenient data processes. Owing to recent improvements in Computational Fluid Dynamics, numerical simulation of flow-induced vibrations is now practicable for industrial purposes. The present paper is devoted to the computation of fluid-elastic forces acting on tube bundles subjected to one-phase cross flows. What is the numerical process ? In the case where fluid-elastic effects are not significant and are restricted to added mass effects, there is no real coupling between structure and fluid motion. The structure displacement is not supposed to affect flow patterns. Thus it is possible to solve the fluid and the structure problems separately by using a fixed non-moving mesh for the fluid dynamic computation. Lift and drag forces acting on tube bundles can be computed numerically by using Large Eddy Simulation. Their spectrum and time history can be introduced as inlet conditions in the mechanical calculation providing the tube vibratory response. On the contrary when fluid-elastic effects can not be neglected, in presence tube bundles subjected to cross flows for example, a coupling between flow and structure computations is required. Such a calculation is performed in the present work. An improved numerical approach has been developed and applied to the fully numerical prediction of the dynamic behaviour of a flexible tube belonging to a fixed tube bundle subjected to cross flows. The purpose is to be able to provide a numerical estimate of the critical flow velocity for the threshold of fluidelastic instability of tube bundle without experimental investigation. The methodology consists in simulating in the same time thermohydraulics and mechanics problems by using an Arbitrary Euler Lagrange (ALE) formulation for the fluid computation. Numerical results turn out to be consistent with available experimental data obtained in the same configuration. This work is a first step in the numerical prediction of tube bundle vibrations in presence of cross flows.