Aerosol sampling from fluctuating flows into sharp-edged tubular inlets

Aspiration from the ambient air into an inlet and subsequent particle transport through the sampling tube may lead to essential bias of the initial aerosol concentration due to inertia, gravitational settling, and several other physical mechanisms. This study deals with the aerosol sampling efficiency of sharp-edged (or thin-walled) tubular inlets under harmonically varying wind conditions. Modelling of the components of the overall sampling efficiency was performed with semi-empirical equations for the aspiration and transmission efficiencies that have recently been developed for fixed isoaxial and non-isoaxial conditions. Based on these models, we have analyzed real-life situations when the wind velocity vector is not steady, but fluctuates around predominant average values of its magnitude and orientation. Two sampling environments, horizontal aerosol flow (ambient atmosphere) and vertical aerosol flow (industrial stacks) have been considered. To determine the values of sampling efficiency components when sampling aerosols from harmonically varying wind conditions, the time-dependent function of each component has been time averaged within the period of fluctuation. It has been found that even for small fluctuations in wind direction, i.e. for quasi-isoaxial sampling, the efficiency is less than that obtained for the mean wind direction, i.e. isoaxial sampling. This difference occurs mostly because of particle impaction in the entrance region of the sampling nozzle and is more significant for larger particles.