Computational fluid dynamics and experimental investigation of a cascade impactor

Abstract

The cascade impactor (CI) is an air sampling device used in multiphase flow dynamics, mainly with aerosol inclusions. It is frequently used in environmental studies for the purpose of investigating air pollution in a specific area. The assessment of collected particles through the CI provides significant information related to the size range and amount of aerosols present in the air sample which may have negative effects on the health of people exposed to a polluted atmosphere. The cascade impactor is more recently used in pharmaceutical studies for the development of oral inhalation products. Retrieved data from the cascade impactor provides the aerodynamic diameter of collected particles, a very important parameter that highly affects the particles behavior during inhalation and is directly related to their deposition in the human lungs. In this thesis, the aim was to study the air-aerosols flow dynamics across the cascade impactor both experimentally and numerically. First, a Staplex cascade impactor Model 235 was installed at the University of Balamand for almost 18 days. Second, deposited particles on each stage of the impactor were collected in sample tubes. The third step was to appropriately calibrate the granulometer (HORIBA LA-950A2) using a reference material of titanium dioxide. The main purpose of the granulometer calibration was to perform a particle size analysis on the collected cascade impactor samples in order to obtain their corresponding particles size distribution function. Following the granulometric analysis, a numerical CFD model of the impactor was used to simulate the 3D, compressible and turbulent flow inside the CI. Two types of simulations were conducted: a one-phase CFD calculation of uncontaminated pure and a two-phase CFD calculation of air flow with aerosol inclusions. Numerical simulations were conducted using ANSYS-Fluent 18.0. The obtained 3D velocity streamlines suggest that running the cascade impactor at high flow rates (40 CFM) increases the reverse flow effect and generates significant recirculation vortices at the smallest fins. Finally, the important parameters and phenomena involved in the numerical and experimental results were analyzed in order to identify and discuss the source of error or bias of the expected output.