Numerical Simulation of Flow and Electric Fields in an Electrical Mobility Spectrometer
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An electrical mobility spectrometer (EMS) is used to classify airborne, electrically charged particles in nanometer ranges. It is capable of measuring the electrical mobility of the particles ranging from 10 – 700 nanometers, under the influence of an electric field. The EMS design can be described as an assembly of two concentrically cylindrical electrodes with an air gap between the walls. In the EMS, air and aerosol flows enter from one end, pass through the annular and exit the other end. Electric field is applied between the inner and outer electrodes. Particles having specific mobility are collected on a designated electrode ring where electrical signals are measured to obtain size distributions. Flow condition and electric field pattern are important factors influencing accurate particle size distribution measurements. In this study, a computational model of the instrument was developed to predict the behavior of the flow and electric fields under various design parameters, including ratios of sheath air and aerosol flow rates, Reynolds numbers, electrode ring width, ring separation and arrangement and type of flow guide materials. The incompressible Navier-Stokes equation and the Maxwell’s equation are numerically calculated for the flow and the electric fields, respectively, with a commercial computational fluid dynamic software package, CFDRC™. The software was based on finite volume method. It was found that the numerical simulation results exhibited a qualitatively well-agreed trend with the published results in the literature. Prediction of flow and electric field conditions was particularly useful in the instrument design. A prototype of the particle size spectrometer is planned to be built and tested, based on the results of this model.
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