Finite Element Simulation of Aerosol Particle Trajectories in a Cantilever-Enhanced Photoacoustic Spectrometer for Characterization of Inertial Deposition Loss

Abstract

The cantilever-enhanced photoacoustic spectrometer is a sensitive instrument developed originally for trace gas measurements and has lately been successfully applied for measuring light-absorbing particles, such as aerosols. The finite inertia of aerosol particles can cause the particles to be deposited on the walls in the spectrometer’s flow channels, which creates a source of uncertainty for the measurement process. In this study, we characterized this inertial deposition in the spectrometer using finite element-based modeling. First, computational fluid dynamics was used to calculate the distribution of airflow within a 3D model of the spectrometer’s flow channels. Then, the trajectories of aerosol particles were computed to evaluate the inertial deposition losses. The modeling method was validated by computing inertial deposition for two known cases of laminar flow, namely particles flowing through a pipe with a 90-degree bend and a pipe with an abrupt contraction. The particle transmission of the photoacoustic spectrometer was experimentally measured. Differences and similarities between measured and modeled results are discussed. The modeled inertial deposition losses ranged from approximately 5% to 70% for particle diameters between 50 and 500 nm. This modeling approach provides valuable insight into the influence of particle size and flow rate on the inertial deposition and also pinpoints the physical location of the loss within the spectrometer, which is valuable for improving the measurement process.