Fluidelastic instability in heat exchanger tube arrays and a Galerkin-free model reduction of multiphysics systems

Abstract : Heat exchangers are widely used in the power generation industries. The cross-flow type of heat exchangers are more common. The rate of heat transfer is enhanced by operating the heat exchangers at higher flow rates by means of the increased flow turbulence. Although, the high flow rate operations are favoured, there are side effects in terms of the flow-induced vibrations. In the last few decades, the topic (flow induced vibrations in heat exchanger tube bundles) is studied extensively, especially in order to understand the fluidelastic vibration. The preventive measures can be taken in other types of vibration mechanisms, such as the vortex induced vibrations and the acoustic resonance in tube bundles. The turbulence induced vibrations generally take long term to deteriorate the performance of heat exchangers, hence it involves a lesser risk of immediate damage to the heat exchangers. The failure due to the fluidelastic instability occurs suddenly and it can pose a serious risk in the plant operations. Besides the devastating nature of the fluidelastic instability, it is not well understood yet. In the first part of this thesis, the fluidelastic instability is explored by means of performing numerical simulations. The flow induced vibrations in the heat exchanger tube bundles are reviewed historically. The other mechanisms of vibrations, namely, vortex induced vibrations, turbulent buffeting and acoustic resonance in the tube arrays are briefed. In addition, the theoretical models of the fluidelastic instability are revised in order to understand the different approaches used to model the instability. Computational Fluid Dynamics (CFD) simulations are performed, first by using the Unsteady Reynolds Averaged Navier-Stokes (URANS) approach of modeling the flow turbulence, in order to verify the capability of URANS models to predict the instability thresholds dynamically. Secondly, the transient nature of fluidelastic instability is investigated by means of the Large Eddy Simulations (LES) approach of the turbulence modeling. Although the LES approach is computationally expensive in comparison with the URANS approach, the dynamic interactions between the interstitial fluid flow and a single tube from an in-line tube bundle are well captured by the LES. The post-processing of the LES results is comprised of the dynamics of fluid forces acting on a single cylinder from an array, transient surface pressure profiles on the cylinder and the interstitial velocity flow fields as a consequence of the increasing flow velocity until the onset of fluidelastic instability. A mathematical model for the fluidelastic instability is developed based on the transient interaction between the interstitial flow through an in-line cylinders array and a single cylinder form the array. Although there are significant advances in the computers today, the Direct Numerical Simulations (DNS) of large dynamic systems are infeasible. Model reduction also known as Reduced-Order Modeling (ROM) has gained an importance in almost all fields of computational sciences. In the second part of the thesis, firstly, a short introduction to the model order reduction is provided. The Proper Orthogonal Decomposition (POD) and Galerkin projections are commonly used in model reduction of the fluid systems. Almost all reduced-order models derived from the traditional POD-Galerkin ROM require the stability enablers. A novel Galerkin-free approach for model reduction of the Navier-Stokes equations is proposed in this thesis. The method uses the periodicity of the POD time coefficients and a linear interpolation technique in order to construct the off-reference reduced solutions. A test case of the flow past a cylinder at low Reynolds numbers (Re ∼ 125) is used for the demonstration of the proposed ROM. In the formulation of the proposed Galerkin-free ROM, the variables of a dynamical system are treated independently. Therefore, the method can be conveniently extended for the multi-physics dynamical systems. Lastly, the method of Galerkin-free ROM is applied to a fluid-structure interaction problem, where the moving mesh is a part of the solution state vector. A test case of the vortex induced vibration in a cylinder at Reynolds number Re = 100 and the mass ratio as the controlling parameter is considered for the demonstration.
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Submitted on : Tuesday, January 12, 2016 - 7:14:09 PM
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  • HAL Id : tel-01254523, version 2

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Vilas Shinde. Fluidelastic instability in heat exchanger tube arrays and a Galerkin-free model reduction of multiphysics systems. Engineering Sciences [physics]. Ecole Polytechnique, 2015. English. ⟨tel-01254523v2⟩

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