10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Open-source Aerosol Modelling: Electrical Charging and Transport with Applications in Low-cost Sensing
ROBERT NISHIDA, Nene Yamasaki, Adam M Boies, Simone Hochgreb, University of Cambridge
Abstract Number: 1597 Working Group: Aerosol Modeling
Abstract Electrical charging of particles and subsequent transport and electrical current measurement is used in a range of aerosol measurement devices including low-cost ultrafine particle sensors. Unipolar and bipolar diffusion chargers and photoelectric chargers yield predictable charge states necessary for correct measurement inversion to yield particle sizes and concentrations. However, each of the chargers are subject to localized flow and charging effects (de La Verpilliere, Swanson, & Boies, 2015; Nishida, Boies, & Hochgreb, 2017), which are neglected in simple zero-dimensional (0-D) charging models. In low-cost sensors, the integrated measured signal is a direct function of particle polydispersity and models often assume monodisperse particle distributions and low or high mean charge states to simplify the solution algorithm (Maisels, Jordan, & Fissan, 2002; Marra, Voetz, & Kiesling, 2010). Whereas these assumptions may be straightforward for simple systems, representing charge and diameter by a single variable may not be sufficient in other cases. Nevertheless, solving the flow and charge transfer calculations can quickly become cumbersome even for a small number of variables.
In this work, we present an open-source toolbox for generalized aerosol electrical charging and transport equations. The toolbox includes simple, 0-D models up to full, 3-D computational fluid dynamics (CFD) models. We solve the steady-state conservation equations for particle/ion charging and transport (convection, diffusion and electric field transport). The charging equations include unipolar diffusion, bipolar diffusion, and/or photoelectric charging. Sample cases are included for simple 0-D models which retain sufficient complexity in charging equations, but neglect localized geometrical effects. Source terms may be selected at run-time: for example, particle diffusion loss to walls. The solution algorithm is generalized to monodisperse or polydisperse particle distributions and up to 50+ particle charge states. The codes are written using C++ in OpenFOAM, an open-source CFD platform. By incorporating a CFD platform, the solution may be easily adapted to different flow conditions and geometries, and the code may be operated natively in parallel if needed. The computational method is verified by comparing with existing charging models and experimental data where possible (Nishida, Boies, & Hochgreb, 2018).
The modelling toolbox is used to investigate the effect of particle polydispersity in low-cost ultrafine particle sensors which charge particles with unipolar diffusion charging or photoelectric charging mechanisms. Low-cost devices are experimentally calibrated with controlled aerosol sources to provide metrics such as mean particle size and total concentration from one or more electrical current measurements. However, an aerosol with a large standard deviation in particle size will provide a significantly different signal from a monodisperse aerosol with the same mean particle size. Therefore, further understanding of the effect of polydispersity is required to improve the accuracy of low-cost sensors. We solve the conservation equations for particle/ion charging and transport (convection, diffusion and electrical transport) for a laminar, steady-state, incompressible flow. Lognormal particle size distributions are represented with upwards of 50+ coupled conservation equations for multiple size bins and charge levels. Modelling results show that the effect of polydispersity on integrated electrical current can be represented by a monodisperse distribution characterized by a surface-weighted (photoelectric charging) or length-weighted (unipolar diffusion charging) mean diameter and total concentration for a large range of particle distributions and operating conditions offering a convenient simplification to the conservation equations.
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