Experimental Validation of a software tool to Simulate the Filtration Efficiency and Charge Decay in Electret Filters

Nonwoven filter media are commonly used to improve the indoor air quality, to reduce emissions, or to protect from airborne infections. A decisive criterion for choosing a specific filter medium is always to achieve a high filtration efficiency combined with a low pressure drop. By using electret filters, the filtration efficiency can be increased by electrostatically charging the fibers without affecting the pressure drop. However, the filters typically lose efficiency over time by discharging, especially upon loading with particles. Up to now, filter optimization has mainly been carried out experimentally, which is time-consuming and material-intensive. An alternative approach is offered by numerical simulations of the filtration process on the fiber size scale. By that, parameters can be easily varied and loading processes be accelerated. The software GeoDict was originally developed for mechanical filter media and is currently being expanded to include electrostatic effects within project ElekSim funded by the Federal Ministry for Economic Affairs and Climate Action (BMWK). The central objectives are the implementation of electrophoresis and dielectrophoresis as well as the modeling of fiber discharge.

In a first step, the above-mentioned filtration mechanisms were integrated into the software and validated using computational fluid dynamics (CFD) simulations. Subsequently, commercial filter media were investigated experimentally with regard to their filtration efficiency and compared to simulations on microtomographic (µ-CT) scans. For suitable parameters of the surface charge density and charge distribution on the fibers, there was good agreement with the experimental results. The aging behavior during loading with different aerosols is currently being investigated. The compensation of the fiber charge by oppositely charged particles as well as further discharge effects are considered in a phenomenological approach. The overall goal is a simulation tool that can predict the aging behavior when loaded with different aerosols.