Distribution of aerosols in turbulent airflows.

To investigate the dispersion of aerosols in an airflow, a test facility was build at the HermannRietschel- Institute. The distribution of aerosols across the cross section of the test duct was measured at several distances from the emission source. The turbulence intensity of the airflow in the duct was varied by installing different turbulence generating grids (I% to 20%) at the top of the test duct. Far enough downstream the source, the concentration profiles have a Gaussian distribution. Analogous to the potential core length in a velocity profile of a round free jet, the core length of the aerosol concentration can be defined as the distance from the emission source to the cross section where the Gaussian distribution is fully developed. Up to this distance the maximum concentration in the centre of the jet remains constant, and beyond the core length it starts to decrease with increasing distance. The core length of the particle concentration is shorter than the potential core length. The expansion of the jet depends on the Reynolds number of the jet. With low Re the core length increases due to a laminar initial flow at the nozzle. The mixing with the surrounding air in the channel is less compared to turbulent flows. With increasing distance from the source the airflow becomes turbulent. · The measurements show that the dispersion of the aerosol jet occurs faster when the turbulence intensity of the airflow increases. This results in an increase of the standard deviation of the profile.