CFD-Based Aerodynamic Characterization of the Air-Sampling Subsystem of a Portable Radioactive Aerosol Monitoring...

Portable radioactive aerosol monitoring systems integrate air sampling, aerosol collection, and radiation detection within a single measurement chain. In such systems, the aerodynamic conditions established during air transport through the collection filter may influence the spatial distribution of the sampled aerosol and therefore represent an important aspect of system characterization. The portable Monitoring Air pump for Radioactive Aerosol (MARE) system incorporates a high-flow air pump, an airflow monitoring unit, a pleated H13 HEPA aerosol collection filter, and a CeBr₃ scintillation detector for the continuous monitoring of airborne radioactive aerosols. In the present study, computational fluid dynamics (CFD) was employed to characterize the aerodynamic behaviour of the MARE air-sampling subsystem. Two complementary numerical models were developed using STAR-CCM+: a three-dimensional (3D) model representing the complete sampling configuration and a two-dimensional (2D) model representing a characteristic section of an individual filter pleat. The H13 HEPA filter was represented as a homogeneous porous medium using a Darcy-Forchheimer resistance formulation, thereby accounting for its macroscopic hydraulic resistance without explicitly resolving the underlying fibre-scale structure. The 3D model predicted a pressure drop of 268.7 Pa across the filter, while the corresponding 2D representative-pleat model yielded a pressure drop of 262.3 Pa. The resulting difference of approximately 2.4% indicates a consistent global hydraulic response between the two modelling approaches under the investigated operating condition. However, the 3D simulation revealed a pronounced spatial non-uniformity in the mass-flux distribution over the filter surface. The circumferential distribution was comparatively uniform at a given elevation, whereas the dominant variation occurred along the vertical direction, with higher mass flux predicted in the lower region of the filter, where the influence of the downstream suction system was strongest. At the individual-pleat scale, the mean mass flux was 1.36444 kg m^-2 s^-1. The internal region of the representative pleat exhibited mass-flux values approximately 30% above the mean, whereas the external region showed values approximately 10% below the mean, demonstrating substantial local redistribution of airflow within the pleated geometry. These results demonstrate that global pressure-drop measurements alone are insufficient to fully characterize the spatially resolved aerodynamic conditions within the sampling stage. CFD provides complementary information by resolving the local airflow distribution and identifying systematic flow non-uniformities that may be relevant to subsequent investigations of aerosol transport and deposition. The present results therefore provide an aerodynamic basis for future particle-transport and deposition studies aimed at establishing the relationship between local sampling conditions, aerosol collection, and the subsequent radiological response of the MARE system.