Understanding the transient flow behavior of abbreviated impactors for testing of dry-powder inhalers

We present the results of a computer model of the transient flow behavior of abbreviated impactors (AIM) during testing of dry-powder inhalers. The principles of the model were established in a cross-industry study of full-resolution Next Generation Impactors (NGI) and Andersen impactors (ACI). Here, we apply the model to abbreviated impactors that were examined experimentally in a preliminary study of the reduced NGI (rNGI), of the Fast Screening Impactor (FSI), and of the Fast Screening Andersen (FSA) impactor, reported in a companion abstract at this conference (Mitchell et al.). The flow rate rise times of the FSI and FSA predicted by the model were significantly shorter than those of the rNGI and full-resolution impactors NGI and ACI, as expected, and in agreement with experimental results. The correlation between the system volume and flow rate rise time of AIM impactors was good, which suggests that the rise time is mainly associated with evacuation of air out of the impactor system to reduce the pressure by 4 kPa, which is the surrogate DPI resistance. The final nozzle stages and the MOC in the rNGI have high resistance but have only a modest effect because of the small volume between these components and the vacuum outlet. Some quantitative differences between model predictions and experimental results were found, particularly with the rNGI where the experimental results are sparse. The cause of these differences is at present unknown, and further experimental work is needed to develop a fuller understanding of these AIM systems.