Design Principles for Sensing Particle Number Concentration and Mean Particle Size with Unipolar Diffusion Charging

Diffusion charging followed by particle capture and measurement is a promising method for determining particle number concentration of automotive exhaust. In addition, the method allows for the determination of a mean particle size, which is of high relevance with respect to health impacts. We investigate three different measurement configurations based on unipolar diffusion charging and electrostatic particle capture, using an analytical non-dimensional model, as well as a transient, multi-physical 3D model. The comprehensive modeling techniques enable prediction of the transient electrical current signals induced by the motion of charged aerosols. Comparison of the multi-physical modeling approach to analytical calculations and experimental data demonstrate model validation. The understanding gained from the modeling techniques enables a study of sensor design and calibration. Particle number concentration sensing is demonstrated for both a modulated precipitation configuration and a two-stage measurement configuration. With the latter, additional information on the mean particle size is extracted.