A coupled CFD and DEM study to evaluate a trash-boom debris retentency

Debris in rivers represents a massive problem for dams, especially hydroelectric plants. Debris can cause an obstruction of the intake and the penstock structures and increase the turbine maintenance frequency due to the possibility of machinery damage. This study focuses on Edgard de Souza dam conditions, located at Tietê River, one of the most polluted in Brazil, responsible for flood control in the region. Numeric models allow the design of several trash boom systems models by evaluating its efficiency to retain and direct most of that debris to a structure that removes it from water. It can also optimize the system structure to reduce hydrodynamic forces and cost without building scaled models for each system design. In this study, numerical models were used to define the flow field, via CFD, and a DEM model coupled to calculate the interactions between the particles and the fluid medium, particles and trash boom system and particles themselves, to accurately estimate debris trajectory. The trajectory calculated allow the analysis of the system efficiency by retaining and directing debris. The CFD problem involves a multiphase model for air and water, incorporating an open channel model to simulate the interaction of phases with the trash boom structure. These structures are usually long - in this case, 120 mmeters. For simplification purposes, a 1 meter long section is assumed to fit the computational costs by assuming a periodic condition at the lateral boundaries that can emulate part of an extensive trash boom line. By coupling the software Ansys Fluent, used for CFD, and ESSS Rocky, used for DEM (Discrete Element Method), in a 1-way simulation, it is possible to model a passive collector's operation under control flow conditions. Some different cases are set up to evaluate the effect of debris density and size, flow velocity, trash boom geometry, and depth on debris behavior and trash boom efficiency. The results are compared with data from scale-model experiments conducted in the towing tank at Instituto de Pesquisas Tecnológicas do Estado de São Paulo (IPT). Moreover, the results represent part of a removal debris project in the region and will be used in the design and dimensioning of another area of a complex project.