Coupled CFD and CSM Simualtion of a Plate Fin Heat Exchanger

Abstract

Heat exchangers of any kind are exposed to thermal loading which have the potential to cause severe stresses on the equipment. The magnitude of these loads are far more pronounced when temperature differences occur. These may be during plant start-up or shut-down due to the transient nature of the temperature profiles or even imbalances that originate from improper steady-state operation. Failure of the heat exchanger may cause the complete shut-down of a plant depending on the service provided by the heat exchanger. For this reason it is of great importance that the location and magnitudes of these stresses are investigated such that both manufacturing methods and operation of the equipment can be optimised thus preventing failures. In an effort to investigate the structural integrity of the heat exchanger this thesis develops a workflow in which a computational fluid dynamics (CFD) solver is coupled with a computational structural mechanics (CSM) solver in order to identify key stress regions. As a case study to trial this method a test-rig of a plate fin heat exchanger (PFHE) used to investigate typical stresses occurring in the main heat exchanger of air separation units (ASUs) manufactured by Linde has been selected. In order to simulate two heat exchanger blocks in their entirety some additional modelling steps are included. In the CFD simulation use of porous media is made to provide a simplified representation of the fins. Similarly simple block structures are used in the CSM model whose material properties have been changed to mimic the behaviour of the fins. The thermal model gives promising results and shows good agreement to analytical solutions however experimental validation is still necessary. This work shows that the coupling of the two solvers is generally possible but can still be streamlined in some respect with data extraction from the CFD solver being relatively slow and the following mapping of the temperatures onto the CSM mesh and eventual solving of the structural mechanics left with room for improvement in terms of computational speeds. In terms of identifying key stress regions the results indicate elevated stresses occurring at the weld which connects the two heat exchanger modules. Secondarily to the large scale model a submodel was developed, depicting the perforated fin type. This task served as a proof of concept to gain a better understanding of local thermo-hydraulic phenomena especially to what extent perforation played a role in local heat transfer coefficients since such variables cannot be monitored in the large scale model. From the sub-model it was found that the perforations play a significant role in the heat transfer ability of the fin and local heat transfer coefficients in close proximity to the perforations were more than double the value of the average heat transfer coefficient observed in the bulk flow.