A Computational Fluid Dynamics (CFD) Model of the Start-up Kinetics of the Andersen Cascade Impactor (ACI)

H.K. Versteeg, P. Zhao, C. Blatchford, M. Copley, D. L. Roberts, J.P. Mitchell

Background: The dynamics of flow through the Andersen Cascade Impactor (ACI) at start-up for the performance evaluation of dry powder inhalers (DPIs) are poorly understood. Recent studies by the European Pharmaceutical Aerosols Group (EPAG) investigated the effects of reducing the sample volume on the aerodynamic particle size distributions (APSD) of aerosols generated by DPIs. When the inhaled air volume was significantly reduced below the internal volume of a Next Generation Impactor (NGI), size fractionation was incomplete, as expected, and the instrument under-reported the fine particle dose. Surprisingly, the same effect was much less severe with the ACI. The present study investigated, from a theoretical perspective, the likelihood that non-uniform particle transport is taking place in this impactor.
Materials & Methods: We report the results of a computational fluid dynamic (CFD) investigation of the transient flow through a model of the ACI to confirm the existence of flow path channelling that could explain the mass-per-stage data observed previously.
Results: The CFD results confirmed that a negative pressure wave propagates back from the vacuum source at the ACI exit through the CI. Air flow from the inhaler into the ACI is initiated when the suction wave reaches the induction port.
Conclusions: The predicted start-up kinetics compare favourably with measurements of the air flow as a function of time. Complex regimes exist in the upper stages of this CI, involving jets that provide short-circuits, and which are therefore believed to be responsible for the experimentally observed more rapid particle transport through the ACI.

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