Numerical study of the hydraulic effects of modifying the outlet pipe and diffuser plate in pressurized sand filters with wand-type underdrains

Abstract

The pressurized sand filters used in drip irrigation systems have three zones (the water inlet, sand bed, and water outlet) with different hydraulic behaviors. Previous studies that aimed to improve the hydraulic performance of these filters focused on the water inlet and sand bed by redesigning the diffuser plate and underdrain elements, respectively. In this study, we analyzed the consequences of modifying the water outlet of a commercial porous medium filter without changing its underdrain design. The original design consisted of a horizontal pipe connected to ten horizontal wands. Five additional designs that varied in size, orientation, and number of outlets were numerically studied in four operating modes (including two sand bed heights and two superficial velocities). Comparison with the commercial design indicated that (1) an outlet pipe with twice the cross-sectional area reduced the overall filter pressure drop by up to 12% and increased the flow uniformity within the sand bed; (2) two outlets led to symmetrical behavior for the amount of water drained per wand but did not improve the water volume drained by the central wand, which drained the least amount; and (3) a central vertical outlet of equal diameter as the commercial outlet produced a similar pressure drop but increased the flow uniformity within the porous medium. For completeness, two additional diffuser designs were also studied. In comparison with the original design, the proposed designs improved the flow uniformity and weakened the water vortex above the sand surface