Numerical predictions were made of aerosol penetration through a model transport system. A physical model of the system was constructed and tested in an aerosol wind tunnel to obtain comparative data. The system was 26.6 mm in diameter and consisted of an inlet and three straight sections (oriented horizontally, vertically, and at 45°). Particle sizes covered a range in which losses were primarily caused by inertial and gravitational effects [3-25Atm aerodynamic equivalent diameter (AED)]. Tests were conducted at two flow rates (70 and 130 L/min) and two inlet orientations (parallel and perpendicular to the free stream). Wind speed was 3 m/s for all test cases. The cut points for aerosol penetration through the experimental model vis-a-vis the numerical results are as follows: At a flow rate of 70 L/min with the inlet at 0°, the experimentally observed cut point was 16.2 µ AED while the numerically predicted value was 18.2 µ AED. At 130 L/min and 0°, the experimental cut point was 12.8 µ AED as compared with a numerically value of 13.7 µ AED. At 70 L/min and a 90°inlet orientation, the experimental and numerical cutpoints were 11.2 and 11.6 µ AED, respectively; and, at 130 L/min and 90°, the experimental cut point was 12.0 µ AED while the numerically calculated value was 11.1 µ AED. Slopes of the experimental penetration curves are somewhat steeper than the numerically predicted counterparts.
Deposition of aerosol particles on the inner walls of sampling probes is of concern in many aerosol sampling applications. Only inertial and gravitational effects have been considered in previous studies of the aerosol deposition; however, the lift force on particles is also of significance. In this investigation, experiments have been conducted to construct a database for aerosol deposition in Willeke-type sampling probes. An empirical correlation has been made between wall losses and the depositional forces of drag, gravity, inertia, and particle lift through the use of dimensionless parameters. Inclusion of the lift effect in this correlation not only helps to better predict particle behavior in the sampling inlet, but it also provides a basis for understanding of the intrinsic deposition phenomenon. The correlation has a geometric standard deviation of 1.13 and a 0.93 correlation coefficient relative to the experimental data.
Because some designers of aerosol transport systems use the assumption that aerosol penetration through a system is maximized if the flow Reynolds number is 2,800, we have conducted tests to determine if such an assumption is appropriate. Although we do not believe that optimal performance of an aerosol sample transport system can be presented solely in terms of the Reynolds number, we have presented our results in terms of that parameter to compare our work with the results of an earlier study. Two types of experiments were performed. First, the penetration of liquid aerosol particles through horizontal tubes was experimentally investigated for a range of design and operational conditions. For a particle size of 10 microm aerodynamic diameter, the maximum penetration through a 6.7 mm diameter tube was associated with a Reynolds number of approximately 2,000; the maximum penetration through a tube of 15.9 mm occurred at a Reynolds number of about 3,000; and the maximum penetration through a 26.7 mm diameter tube occurred at about 4,000. It was also experimentally demonstrated that for a fixed flow rate through a horizontal tube, there is an optimum tube diameter for which the aerosol penetration is a maximum. An early study dealing with aerosol particle penetration through a 16.8 mm inside diameter loop of tubing (two vertical tubes, two horizontal tubes and three 90 degrees bends) suggested there was a fixed Reynolds number for optimal aerosol penetration independent of particle size. Those experiments were repeated here and the agreement with those tests is excellent; namely, the maximum penetration through a loop of 15.9 mm diameter tube occurs at a Reynolds number of approximately 2,800, independent of particle size. However, when the tube diameter of the transport system layout was changed to 26.7 mm, the Reynolds number associated with maximum penetration varied for different particle sizes, occurring at Reynolds numbers of approximately 5,600 for 8 microm AD particles, 3,800 for 10 microm particles and 3,000 for 15 microm particles.
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