NUMERICAL SIMULATION OF EXTENDED SURFACE RIGID CELL FILTERS

摘要

The air flow resistance of an extended surface rigid cell filter is due to two additive contributions: the resistance of the filter media, and the resistance of the elements of the filter cartridge in which the mini-pleated panels are mounted. Filter cells of this kind available on the market can have from two to five V-shaped entrance channels. Manufacturers choose the number of Vs and the depths of the filter cell with the intention of minimizing the overall cell pressure drop. The assumptions on which these choices are based need theoretical support. The theory should be able to minimize the overall energy consumption for the entire service life of the air filter. This paper proposes a scientific approach to the problem by means of computational fluid dynamics (CFD). rnThe fluid dynamic behavior of two kinds of mini-pleated panels, which make up an extended surface rigid cell filter, was analyzed by a combined experimental and numerical approach. A empirical correlation was derived to predict the pressure drop across mini-pleated panels when exposed to the air flow perpendicular to the panel face. Rigid cell filters with 3 and 4 Vs were then built using the same mini-pleated panels used to develop the perpendicular-flow correlation. The pressure drop across the filter cells was determined both when the filters were clean and after loading them with ASHRAE standardized test dust. rnThe mini-pleated panels themselves have entrance and exit channels separated by a fibrous air filter medium, but the scale is much smaller than in the case of the overall filter cell. A finescale CFD model of mini-pleat panels was run, and the results used to characterize homogeneous porous media panels which simulate the mini-pleat panel behavior. rnCFD simulations of complete filter cells were then performed, substituting the homogeneous porous media panel model for the actual mini-pleat panels. Flow patterns were determined assuming isothermal flow, clean filter media, and turbulent flow in the V-shaped entrance and exit passages. The time-averaged 3D Navier-Stokes equations and Chen-Kim k-turbulence models were solved by using the finite volume PISO algorithm on a cartesian grid. rnNumerical simulations of multi-V cells show good agreement with the experimental measurement of air flow resistance, when a non-isotropic friction coefficient is introduced into the model. On the basis of these results a theoretical hypothesis for mini-pleated panel clogging was made. It appears that in early stages of dust loading, the different particle deposition distribution across the panels for different numbers of Vs can explain why the amount of dust per unit filtering area for a given overall cell pressure drop is higher in the case of filters with higher number of Vs.