Bed expansion at backwashing in pressurised porous media filters for drip irrigation: Numerical simulations and analytical equations

Abstract

Actions for improving the energy efficiency of pressurised porous media filters used in drip micro-irrigation require methods to properly determine the behaviour of both filtration and backwashing modes. The latter is essentially a fluidisation process. Though many authors have analysed fluidised bed columns, very few studies have dealt with the backwashing of granular media filters used in agriculture. The purpose of the present work was 1) to validate a computational fluid dynamics (CFD) model to simulate the backwashing regime, and 2) to assess the extent of analytical expressions for determining the expanded bed height. Experimental data were obtained using a laboratory filter for two types of porous media at superficial velocities <150 m h−1. The CFD model used the Eulerian-Eulerian approach, and its sensitivity was investigated in a simplified two-dimensional domain. A modified three-dimensional model effectively reproduced the bed expansion for most of the experimental conditions using large time steps (0.025 s) and few internal iterations (20). Well-known analytical equations overestimated the expanded bed height at superficial velocities <150 m h−1, but the recent RIO 1 model proposed by Kramer et al. (2020) showed very good predictive capacities. Two analytical expressions based on basic principles were developed, providing a trend which was also close to observations and a lower bound for the expanded bed height value. These conclusions were confirmed by analysing external data obtained using commercial pressurised sand filters