Influence of turbulence on aerosol sampling efficiency

Abstract

The effect of turbulence as the measurement of overall sampling efficiency of an aerosol sampling inlet has been studied because it may be an important factor leading to discrepancies in the results between different investigators. The characteristic wind turbulence parameters, scale and intensity, may be very different between ambient and wind tunnel measurements, or between different wind tunnels. In this study, the influence of turbulence on overall aerosol sampling efficiencies has been examined by varying tunnel wind conditions through the use of grids, a mixing fan and a null condition with no mechanical manipulation. Turbulence intensity was varied between 1.4 and 7.5%, and the turbulence scale ranged from 0.5 cm to the order of 10 cm. The inlets used are sharp-edged, thin-walled brass tubes with diameters ranging from 0.32 to 1.03cm. The results demonstrate that turbulence does influence overall sampling efficiency. The difference between overall sampling efficiencies among the various wind conditions appears to be due to the combined effect of turbulent scale and intensity interacting with the size of the inlet diameter. At low Stokes numbers, Stk < 1.0, the spread of transmission efficiency for a given Stokes number is about 30 %. As the Stokes number increases, Stk > 1.0, the difference between transmission efficiencies for a given Stokes number increases to as much as 100%. The probable reason for the increase in transmission efficiency range is the increase of the particle relaxation time, compared to the time scale for significant changes in the gas velocity. For Stk < 1.0 and turbulence intensity of ⩽ 7.5 %, turbulence produces a spread in overall sampling efficiencies of less than 15% for the inlet diameters studied.