Finite element modelling of high performance aeronautical filters
The primary aim of this study is to develop and validate a computer model for the design of pleated cartridge filters. Mathematical modelling of incompressible creeping flow and low permeability Darcy flow are well established and a number of reliable schemes for the simulation of these regimes are available in the literature. In the context of the present studies a computer model for modelling of combined Stokes/Darcy flows encountered in pleated cartridge filters has been developed. A two-dimensional simulation code that can model fluid hydrodynamics in permeable media as in dead-end pleated cartridge filters has been developed in FORTRAN. The developed code named as ACFAMP- Aircraft Cartridge Filter Analysis Modelling Program has been used to evaluate the performance and design of pleated cartridge filter elements. The governing equations of the flow model can be well represented by the Continuity, Stokes and Darcy equations respectively. Viscosity variations and non-Newtonian effects have been introduced to model the behaviour of shear-thickening fluids used in aircraft hydraulic circuits. The governing equations of the flow model are solved by using a weighted residuals finite element method. Two separate modelling approaches have been adopted in the model development. The first approach is based on modelling of flow through porous media by employing a slightly compressible form of continuity equation utilising nine-noded Lagrangian elements for domain discretisation in conjunction with a U-V -P finite element scheme. The second strategy is based on modelling of coupled Free/Porous regimes making the model industrially complaint. This scheme utilises the stabilised C0 continuous Taylor-Hood element, which generates stable solutions for incompressible flows without any problems of numerical locking. Initial numerical experiments were based on simple geometries and the complexity of the geometries was finally extended to model the pleated cartridge filters. A permeability model that interprets medium compression effects and which is based on data obtained from flat sheets of the filter material used in the fabrication of the filter cartridge is incorporated within the main model. During pleating, bending and creasing the medium deforms and leads to a loss of permeability and/or effective filtering area results. The combined effects of compression and reduction in filtration area cause deviations from Darcy's law. The effects of medium compression, pleat deformation and pleat crowding have also been analysed. The present work is directed towards the prediction of accurate values of compression factors and percentage losses in the filtration area, which assist in designing pleated cartridge filter elements. The simulated results have been compared against the experimental data for purposes of validation.