Evaluation of Metal Oxide Whiskers Grown on Screens for Use as Aerosol Filtration Media

Metal oxide whiskers grown on commercially available screens were evaluated as new filter media. The screens were woven from phosphor bronze wire of diameter D wire = 114 μm in a square mesh with fraction open area ϵ = 0.303. Whiskers' diameters are remarkably uniform for the chosen growth conditions, averaging about 0.6 μm. Whisker lengths are much more variable, and whiskers over 20 μm long can be grown. Pressure drop Δp N and aerosol penetration P N through TV identical screens in series were measured. N varied from 1 to 10. The aerosols were dioctylphthalate droplets of diameter D P equal to 1.0, 2.0, or 4.6 μm. The face velocity U was varied from 0.15 to 5 m/s. Capture efficiency was expressed in terms of a single-wire capture efficiency η computed as follows: η = (1 − P 1/N N )/(1 − ϵ). For a given screen, the single-wire efficiency was found to be an increasing function of both particle size and face velocity, suggesting that capture is dominated by impaction. For each screen, η was successfully correlated with a Stokes number Stk = ρ P D 2 P C P U/18μD wire . However, the un-oxidized screens also required accounting for the variation of the Reynolds number Re = ρD wire U/μ. The influence of whiskers on η increases as the average whisker length increases or as Stk decreases, and increases for η of 10 to 50-fold, relative to unoxidized screens, have been measured. To compare the advantage of lower penetration with the disadvantage of higher pressure drop, a figure of merit Nη/Δp N was computed. For D P = 1.0 μm, whiskers are advantageous relative to unoxidized screens over the entire range of velocity; for D P = 2.0 μm, whiskers are advantageous for U < 2 m/s; and for D P = 4.6 μm, whiskers are advantageous for U < 0.5 m/s.