Geometric effects of racks on the performance of Bottom Intakes, using numerical model

Bottom intakes are frequently used as diversion structures for small hydro-electric power plants, agricultural and industrial uses in mountainous regions, because of their simplicity and their low costs in comparison with the other methods of river intakes. Also, Bottom intakes have the minor changes and little circumstances on river morphology, comparing with the other measures of extraction of water from rivers. The flow passing through the bottom intakes is categorized as spatially varied flow in which the discharge is decreased along the canal. Major studies on the performance of bottom intakes were conducted on experimental models. In experimental models, the study is focused on the discharge deviation due to ensemble of the racks. For determining the efficient parameters which influence the process, some extra studies on the flow characteristics are needed. The measurement of all flow characteristics could not be done in laboratory. Hence, a calibrated mathematical model permits to determine certain parameters which could not be measured in a physical model. In this study, the Flow3D software was utilized to simulate the current passing through the bottom racks in an experimental model. Firstly, the mathematical model was calibrated. The mesh size and the adequate turbulence model were investigated to assess the maximum similitude between experimental and numerical model for the discharge deviated by racks. Among 5 different turbulence models (including 1 equation model, standard k-ɛ model, k-ε RNG model, Prandtle's mixing length model and LES model), the k-ε RNG model provided more accurate results by evaluating the parameters of determination. MAE=0.18, RMSE=0.36 and R2=0.98 were obtained for k-ε RNG model. The numerical model by different mesh size (2, 4 and 8 millimeters) was built and the discharges deviated by racks in numerical model were compared with ones in experimental model. The numerical models built by 2mm and 4mm cells sizes provided the results very close to experiments, while 8mm cell size exhibited the considerable different results. Considering the time costs for numerical models, the model with 4 mm mesh size was selected as the most convenient model. For the next simulations, the meshes with 4 mm in their sizes were selected. Following the calibration of numerical model, the racks with eight different cross sectional geometries are simulated by the numerical model. The ordinary racks' geometries (oval, circular, lozenge and rectangular) and the hybrid geometries (circular and triangular, circular and rectangular) were modeled by numerical method. For each model, the diverted discharge, velocity, pressure distribution around the racks and also the velocity profile of the outlet discharge along the racks were compared with each other. Finally, according to the results, three geometry, circular, lozenges and the combination of the two later sections were compared in terms of efficiency and performance. The maximum deviated discharge was considered as the parameter for comparing the performance of each category. The maximum diverted discharge ratio belonged to triangular superimposed on semicircular, semicircular superimposed on triangular and lozenge category, with 58.25%, 56.78% and 56.68%, respectively. Although the category of triangular superimposed on semicircular category was more efficient in discharge deviation, the velocity distribution along the racks was not uniform. The pressure distribution and the differences between the maximum and minimum pressure were considered as the indicators for evaluating the performances of each category. The hybrid cross section with triangular superimposed on semicircular exhibited more uniform pressure distribution along the racks and the maximum differences between the maximum and minimum pressure occurred above and under the racks. These conditions conduct to the ability of this category in discharging the water. The results provided in this paper were applied for the clear water. We propose to evaluate the performances of racks in the presence of sediment. Especially, for certain shapes of racks, the sediment might be trapped between racks and influence the performance of trash racks. Further researches are needed for the water transporting the sediments. In this research, the hydraulic of collector was not studied. The performance of collector under different conditions subjected to the shapes of racks could be as a domain of research. The hydraulic of collector is influenced by the flow distribution along the racks. The less uniform for the water arrived to the collector, the more complexity in hydraulics of collector is expected.