Modeling of charge distributions, dielectrophoresis and charge decay in electret filter media

Electret filters find widespread application across various industries due to their exceptional filtration efficiency. These filters operate on the principle of electrostatic attraction between charged fibers and particles, electrophoresis and dielectrophoresis. However, research on electret filter media faces numerous challenges and complexities, such as charge stability over time, charge uniformity, and distribution of charges, particularly on the fibers.

In this context, numerical simulations play a crucial role in deepening the understanding of the fundamental mechanisms governing the behavior of electret filter media. They provide valuable insights into the complex interplay among electrostatic forces, airflow dynamics, and particle behavior, aspects not easily observable or measurable through experiments.

For the simulations of particulate filtration in filter media and elements, the FilterDict module of GeoDict stands out as a powerful software tool. Its capabilities to perform direct numerical simulations of electrophoresis are currently being enhanced in the framework of the ElekSim project, funded by the German Federal Ministry for Economic Affairs and Climate Action (BMWK). The overall objective of this project is to develop a new simulation environment focusing specifically on electret filters with the aim to streamline the optimization of electret filter media and to validate it against measurements. So far we have the comparison of simulation results of real filter media originating from different charging processes such as hydro charging, corona charging and triboelectric charging. The FilterDict developments include simulating diverse charge distributions in both particles and fibers, adding dielectrophoresis to existing electrophoresis capabilities, and tracking surface charge decay in electret filter media over time.

The initial verification of this simulation methodology is compared against results from Fluent® on a single fiber and simple multi-fiber models and includes a comparison of the computed filter efficiency across a given particle size distribution. Next, the new features will be validated against measurements on provided µCT scans. To investigate the effects of electrostatic charge on the filter efficiency, the filter media are additionally discharged using isopropanol to expose the pure mechanical filtration characteristics.

These insights will allow drawing new conclusions for the design of innovative next-generation electret filter media with different charge distribution methods, structural configurations, and improved charge stability.

Left: Filter media, electric field, and selected particle trajectories.

Top right: Red arrows indicate a uniform electric field. Charged particles have either a positive surface charge (Q) or negative surface charge (-Q). The force on them, F, is parallel to the electric field, with the direction depending on the charge. Uncharged particles are affected by the dielectrophoretic effect, but the forces FDEP balance out.

Bottom right: Red arrows indicate a non-uniform electric field. Uncharged particles are affected by the dielectrophoretic effect. But in this case, the forces FDEP do not balance out and even originally uncharged particles are affected by a non-uniform electric field.