FLUID AND PARTICLE DYNAMICS IN AN AEROSOL VIRTUAL IMPACTOR

The collection and characterization of chemical and biological aerosols is essential to many areas of particle research such as toxicological studies, pollutant sampling, and biohazard assessment. This work presents the simulation of a low cutpoint, high volume aerosol sampling device known as the “virtual impactor”. A steady state, three dimensional RANS type calculation is done using the FLUENTTM computational fluid dynamics code to predict the turbulent flow field inside the device. Particle collection efficiency and wall losses are then obtained by solving the particle equation of motion governed by drag for mono-dispersed samples of spherical particles in the 0.1-0.4 m diameter range. Predictions of the mean fluid velocity field with the incompressible Reynolds stress model and the compressible k-epsilon turbulence model are relied upon for conducting particle tracking calculations. FORTRAN 90 computer code is developed to solve the particle equation of motion using an implicit second order accurate time integration scheme. In addition, a multi-variate, scattered point interpolation method is implemented to obtain the fluid velocity at a position away from an Eulerian mesh point. It is found that “adaptive ” drag law models are necessary to correctly account for slip and compressibility. The results indicate the trends observed in the experiments, and a 50 % cutpoint diameter between 0.250 and 0.275 m. Recommendations for improved modeling in future work are made