EXPERIMENTAL STUDIES OF PARTICLE TRANSPORT IN AIR FLOWS NEAR BLUFF BODIES, WITH AND WITHOUT ASPIRATION

摘要

This paper reviews experiments which have been carriedout to investigate several features of the transport in thevicinities of bluff bodies of particles large enough to exhibitinertial effects. The work is stimulated by its relevance to thebehavior of particulate contaminants in occupational and otherenvironmental situations. For simple bluff body flows withoutaspiration, we have concentrated on effects in the near wake.Flow visualization showed clearly the effects of ‘de-mixing’ asthe inertial parameter (St) increased, eventually to the pointwhere particles are unable to enter the near wake cavity. Studiesof the residence times of large particles injected at the base of adisc-shaped plate also showed strong dependence on St, and theresults were consistent with an explanation based on the role ofinertial forces in the macroscopic mean recirculating near wakeflow. Bluff body flows with aspiration were studied mainly fromthe point of view of aerosol sampling. New experiments weredeveloped to enable measurement of aspiration efficiency (A),reflecting the ability of particles to be transported from theexternal freestream and through the sampling orifice. Theresults for A as functions of St and entry orifice orientation withrespect to the freestream (a) showed strong trends for theforwards-facing orientations (α < 90°), but much less so forlarge orientations (α > 90°). All the trends were most markedfor large St, reflecting the importance of inertia in the aspirationprocess. Comparison of results for α = 90° and 180°respectively shows good agreement with current aspirationefficiency models.