CFD Simulations of Flow and Pressure Distribution in a Coarse Industrial Self Cleaning Filter

Sammanfattning: The intention of this master thesis is to model the internal flow of an Alfa Laval course Filter with 20 cm inlet and normal outlet (ALF20), which is an automatic self cleaning filter aimed to rinse the cooling water upstream the inlet of a compact heat exchanger. Along the automatic cleaning process the ALF20 turn into a flushing and a back flushing operation mode. The ALF20 consists of a filter body, a diverter valve and a filter basket. Three connection tubes where the inlet, the outlet and the flushing outlet pressure were measured were connected and used in the measurement. Two filter baskets were available with different hole diameters and hole area percentage. Experimental measurements of pressure difference and temperature are done for normal, flushing and back flushing operation modes in the Process Laboratory at Alfa Laval AB in Lund. The measurement considers flow in empty the ALF20 filter house, the empty ALF20 with diverter valve and the ALF20 in full configuration with filter baskets 1 or 2 attached. Alternative cylinders aimed to simulate fouling are attached in further measurements. The computational fluid dynamics calculation model has been developed and checked while comparing computational results with the measurement tests of the corresponding test cases. A literature survey is presented covering related topics in basic fluid dynamics, empirical data regarding flow through perforation holes, turbulence and Computational Fluid Dynamics. Since the radial forces on the filter basket may result in failure, these forces will be evaluated in terms of direction and magnitude at the filter basket surface. The internal flow geometries of measured ALF20 (CAD drawing received from Alafa Laval A/S. Soeborg) cases are created and meshed for use in the commercial Computational Fluid Dynamics (CFD) software FLUENT. Evaluations are done to determine a grid independent mesh, most suitable turbulence model and the most appropriate inlet velocity boundary condition. Since, the filter baskets is made of a perforated plate consisting of a large number of holes which not can be represented in the model in detail, a porous medium model is used to represent the perforated areas. Pressure loss coefficients are taken from the literature and are modified according to local kinetic energy and pressure differences over the filter basket surface. Pressure differences and velocities are taken from preliminary CFD simulation using non modified pressure loss coefficient. All computational results are post processed in FLUENT and presented in the form of velocity path lines, pressure contours and tables. Comparisons with the measured experimental pressure drop are done for all cases and can be found in the result tables. Since the deviation from measured results is moderate, the computational model seems to agree well with the real flow situation. Radial forces pointing from the outside of the filter basket towards the center seems to occur in the back flushing mode in the rear end of the filter basket.