Particle Deposition from Natural Convection Enclosure Flow Onto Smooth Surfaces

Abstract

Deposition can be an important fate for airborne particles in indoor environments. The effects of particle size (diameters: 0.1, 0.5, 0.7, 1.3, and 2.5 μm), surface orientation, and surface-air-temperature difference (±10 K and ±1.5 K) on particle deposition velocity have been studied experimentally in a 1.22 m × 1.22 m × 1.22 m aluminum chamber. Monodispersed ammonium fluorescein particles were deposited onto the chamber surfaces under natural convection flow conditions and then extracted to determine particle deposition at seven locations along each surface. For horizontal surfaces, gravitational settling was the dominant factor for particle diameters greater than 1 μm. For vertical surfaces, several factors significantly influenced deposition. Excluding near-corner areas, the average deposition velocities on cool vertical walls varied from a maximum of 5.8 × 10⁻⁵ m s⁻¹ for 0.1 μm particles (surface-to-air temperature difference of -10 K) to a minimum of 5.3 × 10⁻⁷ m s⁻¹ for 1.3 μm particles (-1.5 K). On warm vertical walls, average deposition velocities varied from a maximum of 9.2 × 10⁻⁷ m s⁻¹ for 0.1 μm particles (+1.5 K) to below the detection limit (typically on the order of 10⁻⁸ m s⁻¹). Deposition was not uniform along the surfaces, especially on the vertical walls. Deposition in the corners varied significantly relative to deposition in the center of the walls, yet no strong, consistent pattern appears. Deposition away from the corners on the cool vertical surface was similar to model predictions for an isolated vertical flat plate. Deposition on the warm vertical surface of the enclosure tended to be much higher than model predictions, however, particularly for the larger particle sizes studied.