Predicting air flow resistance and capture efficiency of fibrous air filter media

To more effectively control indoor concentrations of fine particles, heating, ventilating, and air conditioning (HVAC) equipment manufacturers and building engineers should consider the use of filters in HVAC systems with high enough removal efficiencies. However, all filtration technologies have energy consequences and high-efficiency filters can have significant energy penalties (or air flow reductions) associated with their use. In order to design air filters with the desired cost/benefit ratio it would be desirable to use models describing both the air flow resistance and efficiency of the filtering media used to manufacture the full scale filter elements. In fact the filter media determines the maximum efficiency of the filter element and is the major contributor to its overall air flow resistance. To simulate the capture of aerosols in fibrous air filter media it is first necessary to develop an accurate simulation of the gas flow through the media structure. The fibers forming a typical fibrous air filter medium are oriented in random directions, separated by random distances and have a range of fiber diameters. Applying mathematical models to such a chaotic geometry is impossible; all attempts at analysis of air flow patterns in filter media involve some level of geometric simplification. Early studies of this type (e. g. Kuwabara) obtained useful results with geometries simple enough to allow analytic solutions for the flow field. The availability of computational fluid dynamics (CFD) techniques now allows models with geometries much closer to real fiber structures, random in direction and spacing with any desired fiber diameter distribution. In this paper we develop a CFD calculation technique which makes only one basic simplification to the media geometry that it is modeled with sufficient accuracy by a two-dimensional pattern of circles which are randomly spaced. The computational domain figure represents a plane cut through the filter medium perpendicular to the face of the medium, i.e. in the usual direction of the air flow. Only a small lateral portion of the medium is modeled but the computational domain includes the entire thickness of the medium. Two sets of numerical simulations are considered here. The first one considering a "no-slip" boundary condition at the fiber surfaces and another one adopting "full slip". The predicted pressure drop and efficiency are presented and their validity is discusse