Pressure drop in fibrous filter – Representative domain size and effect of fibre orientation

Abstract

Clean air or gas is critical e.g. in many industrial or health applications. Various filtration processes are employed to ensure that liquid droplets and/or solid particles, i.e. so–called aerosols, are removed efficiently and at low costs from the respective gas streams. Fibrous filters are often used in this context, because of their low cost, their high capture efficiency, and their low pressure drop. Hence, the performance of fibrous filter materials is judged based on their pressure drop and capture efficiency. A generalized model to describe and predict these two parameters would be helpful in developing optimal filter material.In the current work, numerical investigations using the commercial CFD tool ANSYS CFX are engaged to predict the pressure drop caused by multiple randomly–oriented fibres in a 3D domain. The fibres are randomly generated until the desired filter solidity is met. Hereby, the generation algorithm ensures that no fibres intersect. The implementation is based on a fictitious–domain approach, which has the great advantage to perfectly decouple the mesh from the fibre generation, as fibres are implemented as volumes of infinitely–high flow resistance in the fixed mesh. Hence, there is no need for a case–specific and body–fitted mesh. The effect of the domain size is particularly investigated and we find that a domain size of more than 45 times the fibre diameter in all three directions, proves to give domain–size–independent results for the pressure drop. The boundary conditions are also carefully analyzed, and we find that particularly the symmetry planes parallel to the mean flow direction are not appropriate as the representative domain is cut out of the filter layer. Here, the introduction of a pseudo–plane proves to minimize non–physical flow fields. Moreover, using scanning microscopy, a sample fibrous filter is analysed to characterize the (random) fibre orientation. It is found, that within the filter plane an evenly–distributed fibre orientation angle α can be observed, while out of the filter plane the orientation angle β appears rather limited. This limitation is also implemented into the model. Finally, several simulations are performed for various operating conditions to infer an empirical correlation to predict the pressure drop. This correlation is validated using experimental data.