Diffusional losses in particle sampling systems containing bends and elbows

Classical theoretical treatments for diffusional deposition of particles in tube flow describe the losses within straight tubes. The plumbing in many systems of practical interest, notably aerosol instruments, consists of short segments of tubing connected by elbows, bends, and other disturbances. To understand the particle losses in such systems, particle losses in tube flows containing bends and elbows has been studied for Reynolds numbers ranging from 80 to 950. Monodisperse aerosol of 5–15 nm diameter particles passed through individual bends or elbows, and through a number of bends or elbows in series. The results show that the effect of bends and elbows on particle diffusion loss is significant. For a flow configuration with four elbows in series, the penetration efficiency drops as much as 44% when compared to a straight tube with the same length. For Reynolds number smaller than 250, the enhancement of diffusion losses due to bends and elbows is sensitive to both the relative orientations of the bends and elbows and the lengths of straight tubing between them. Because of this sensitivity, direct calibration or simulation is required to assess nanoparticle penetration efficiencies for any flow system containing bends or elbows at low Reynolds number. When the Reynolds number exceeds 250, the enhancement is insensitive to the actual flow configurations. Experimental results are presented, which may be useful for design of aerosol flow systems at Reynolds number larger than 250.