Acoustically assisted fibrous filtration for enhanced particle capture

Fibrous filters in air conditioning systems separate particles from the air stream by capturing them on fibers. However, particles in the 0.1-1 μm range, known as the most penetrating particle sizes (MPPS), are difficult to capture because diffusion, interception, impaction, electrostatic, and gravitational forces are not effective at moving these particles toward the fibers. In this work, we demonstrate, through experiments and scaling analysis, that an acoustic standing wave generates two force fields within a fibrous filter. Acoustic interaction forces attract particles to the fibers. Acoustic streaming forms localized mini-vortices near fiber boundaries, increasing the likelihood of particle deposition through fluid drag.Our prior simulation results also showed that acoustic interactions and streaming forces increase particle capture more than particle agglomeration. Agglomeration occurs only at very high concentrations (> 105-106 particles/cm3) for submicron aerosols. To determine capture efficiency (η), we conducted experiments in a bench-scale setup. This setup included an aerosol generation subsystem, an acoustically assisted fibrous filtration module, and sampling devices with optical and scanning mobility particle sizers. The sound pressure level inside the acoustic cavity was measured at 1533 Pa. We tested different fibrous filter combinations, varying in filter materials, fiber diameters, porosities, and loading. Here, we present the increase in η of a sintered metallic fiber with a mean fiber diameter of 6.92±2.22 μm as the filter loading increased. For a monodisperse 0.3 μm particle at a filter face velocity of 0.3 m/s,under acoustic excitation, the capture efficiency remained about 40%. Without acoustic assistance, η decreased from about 40% to 27%. These results show that adding acoustic fields to fibrous filtration can greatly enhance particle capture, particularly for the MPPS. This suggests a promising strategy to improve indoor air quality and reduce exposure to airborne contaminants.