Fig. 2.
Particle collection on strings. (a) Diffusive capturing (with collection efficiency Ed) occurs when thermal wiggling of a particle causes it to leave the streamline around the string and deposit on the fiber. (b) Inertial impaction (with collection efficiency Ei) happens, when a particle has a large enough momentum to continue its travel towards the string instead of following the streamlines that bend around the string. If the particle velocity u is sufficiently increased (as indicated by the thick flow lines), the dominant collection mechanism becomes inertial impaction. (c) Particles leaving the streamlines due to either collection mechanism are most likely to hit the string near the edges. Collection on narrow strings is more efficient because of this ‘edge effect’.
The collection efficiency of a single filter-fiber is defined as the fraction of particles collected to the total number of particles that would have passed through the fiber if they had moved on straight lines. The total collection efficiency Ec can be defined as the sum of the collection efficiencies due to diffusion (Ed) and inertial impaction (Ei) [11]:
equation(1)
Ec=Ed+Ei=a1(dfu)−2/3+a2u/df,
where df is the fiber diameter and u the aerosol velocity. a1 and a2 are constants depending on the particle and the fluid in which it is suspended.
At high velocities, collection can be assumed to take place by inertial impaction alone, and Brownian diffusion is thus ignored. In this case, the collection efficiency is
equation(2)
particle velocity u is sufficiently increased (as indicated by the thick flow lines), the dominant collection mechanism becomes inertial impaction. (c) Particles leaving the streamlines due to either collection mechanism are most likely to hit the string near the edges. Collection on narrow strings is more efficient because of this ‘edge effect’.