Electret filters use electrostatically charged nonwoven layers to enhance particle separation efficiency. However, this positive electret effect decreases as the particle loading increases. The aim of ioniser-assisted filtration is to significantly enhance the separation efficiency of a conventional filter with low pressure loss, or to achieve the same degree of separation with reduced electrical power consumption.

The approach is based on the ionisation of the air and the subsequent electrical charging of the particles in the electric field of an ioniser. A downstream electric field induces the polarisation of the filter and the separation of the particles. This effect is switchable and persists as long as the ioniser and the electric polarisation field are in operation.

For the optimal design of the ionisation and polarisation unit, a comprehensive CFD model was developed as an extension of the open-source software OpenFOAM. The model takes into account the physical processes in detail, including ionisation, particle charging, and separation in the gas phase, as well as, bipolar charge transport within the solid filter medium. These processes are coupled via the spatially resolved calculation of the electric field.

The model is validated using a combined approach of numerical simulations and experimental investigations. For the experiments, a laboratory test rig has been setup, which allows the investigation of particle charging by ionisation and particle separation due to electrostatic effects on dielectric components. For this purpose, both fibre filter media and plastic grid structures are used.

The use of plastic grid structure keeps the numerical effort manageable and makes it possible to calculate the physical relationships at the microscale and verify them experimentally without having to perform the simulation on a real filter structure...