Degree of mixing downstream of rectangular bends and design of an inlet for ambient aerosol



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Texas A&M University


Tests were conducted to characterize mixing in a square and a rectangular duct with respect to suitability for single point sampling of contaminants. Several configurations, such as a straight duct with unidirectional flow at the entrance section and straight ducts preceded by mixing elements (a 90?? mitred bend, double 90?? bends in S- and U-type configurations) were tested. For a straight duct of square cross section, the COV of tracer gas concentration at 19 duct diameters downstream of the gas release location is 143% (Center release). COVs of velocity and tracer gas concentration downstream of each mixing element in square duct setups were verified throughout this study. In the case of a rectangular duct with a 3:1 (width to height) aspect ratio, COVs of velocity and tracer gas concentration only downstream of a 90?? mitred bend were verified.
Tests were conducted to develop improved inlets for a Battelle bioaerosol sampling system. New inlets have been developed called the All Weather Inlets (AWI), which are designed to prevent entry of precipitation while maintaining aerosol penetration. The AWI has two inlets - one that samples at a flow rate of 780 L/min and the other one that is operated at a flow rate of 90 L/min. The initial version of the AWI-780 L/min unit featured an internal cone, which was removed because the penetration of the AWI-780 without the bottom chamber was higher than that of the Battelle inlet ?? 81% with the cone while 86% without the cone for around 9.5 ??m AD at 2 km/h. The best bug-screen configuration was verified and a cutpoint management process was performed. The inlets were tested with different wind speeds from 2 to 24 km/h to verify the wind sensitivity of those inlets.