No abstract

Numerical and experimental techniques were used to characterize aerosol penetration through bends. Agreement between numerical and physical experiments was achieved when the numerical approach was based on the use of a specially developed three-dimensional particle tracking technique. It is also demonstrated that turbulence needs to be included in a particle tracking model. The effect of flow Reynolds number upon particle deposition was examined numerically. Results show that it affects aerosol penetration somewhat; however, it does not appear sufficiently significant to warrant inclusion in any correlation model. For Stokes numbers of 0.07-0.7 and a curvature ratio of 10, the aerosol penetration does not change by more than 5% when the Reynolds number is varied from 3200 to 19 800. Physical experiments were conducted to investigate the effect of curvature ratio on aerosol penetration. The bends were constructed such that each bend had the same initial and final spatial co-ordinates, regardless of the curvature ratio. ANSI N13.1-1969 recommends that the curvature ratio should be at least 10, but the results of this study suggest that the value could be 4. When bends are fabricated from straight tubing, there is a tendency for the tubing to flatten. The effect of flattening on aerosol penetration was tested by pinching bends at the 45°location, with degrees of flattening from 0% to 50%. If the degree of flattening is less than about 25%, it does not have a substantial impact on aerosol penetration. Numerical experiments were carried out to characterize the penetration of aerosols through bends. The geometrical extent of the bends covered only the region of tubing where the radius of curvature is finite. Results were used to generate a correlation model that designers and users of aerosol transport systems can employ to predict aerosol penetration. The correlation model is valid for the range of Stokes numbers between 0.07 and 1.2, for bend angles from 45°to 180°, and for curvature ratios from 2 to 10.

Large particle sampling effectiveness of commercially available and prototype particle collectors was determined by wind tunnel testing. Included in the tests were the standard 1 CFM Andersen and a specially modified version of it which utilizes a weatherproof, directionally insensitive inlet; the standard Hi-Volume sampler; a Prototype Dichotomous sampler; and a prototype sampler which utilizes the 20 CFM Andersen with a rotating cowl inlet. The tests were performed using particles ranging in size from 5 to 50 µ , approach velocities from 5 to 15 ft/s (1.5-4.6 m/s), turbulence levels of <1 and 8%, and orienting the samplers at different directions to the flow. By use of a base condition for comparison purposes of 15 ft/s (4.6 m/s), a 15-µ aerosol, and an 8% level of turbulence intensity approaching the samplers in the wind tunnel, the following sampler effectivenesses (aerosol deposited on collection substrates of the particular sampler to that detected by an isokinetic sampling system) were noted: commerically available 1 CFM Andersen 2%; modified 1 CFM Andersen with the special inlet, 50%; standard Hi-Volume sampler 55% (wind at 45°to the ridge of the roof); Prototype Dichotomous sampler 45%; and the 20 CFM Andersen with a rotating cowl inlet 82% (tested at 5 ft/s, 1.5 m/s).

Four commercially available batch-type bioaerosol samplers, which collect time-integrated samples in liquids, were evaluated. Sampling efficiency was characterized as a function of particle size using near-monodisperse polystyrene spheres (sizes of 1-5 µm) and oleic acid droplets (3-10 µm). Results show the sampling efficiency of AGI-30 impingers range from 4-67% for particle sizes of 1 to 5.1 µm with significant variations between units; those of SKC BioSampler impingers range from 34-105% for particle sizes from 1 to 9 µm; those of a batch-type wetted wall cyclone with compensation for evaporation (BWWC-EC) range from 5 to 65% for particle sizes 1 to 10 µm; and, those of a batch-type wetted wall cyclone with no evaporation compensation (BWWC-NC) range of 55 to 88% for particle sizes of 1-8 µm. Retention efficiency was measured for 1 and 10 µm polystyrene spheres. For the AGI-30 and BWWC-EC, the retention efficiency of 1 µm particles after 1 h was less than 30%, while that of the SKC BioSampler was 59%. Due to liquid evaporation, the BWWC-NC could not be operated for 1 h. Retention efficiencies for Bacillus atrophaeus spores and Pantoea agglomerans vegetative cells were measured for the AGI-30 and the SKC BioSampler. Results for the spores were about the same as those for 1 µm non-viable polystyrene particles; however, the vegetative bacteria lose culturability and consequently show lower retention efficiencies. For the impingers, significant performance differences were observed in units delivered by vendors at different times.