Experimentación física y numérica de la hidrodinámica local de rejas de fondo transversales al flujo en captaciones de agua superficial

Abstract

In any water resources project for municipal, agricultural, industrial, energy production, etc. purposes, it is necessary to separate a wide variety of solids from the water. All types of surface water intake must have the ability to exclude debris, fish, sediment, and other materials from the water. In recent years, the need for effective exclusion of solids present in water has increased. Modern irrigation systems (eg., drip, sprinklers, valves, pumps) require more filtration than conventional flood irrigation. Sediment removal is required for small-scale water harvesting projects in remote areas, especially those located in rugged areas with difficult access that limits or prevents regular operation and maintenance activities. One of the most widely used methods for capturing surface water in mountain rivers with steep slopes and irregular channels with significant sediment transport and flood flows, are bottom bars. The bars are designed to capture as much water as possible, preventing the entry of solid material. A type of non-conventional bottom screen with bars transverse to the flow that is used with excellent results in solid-liquid separation in surface water intake is the so-called Coanda effect screen. In the present research, the local hydrodynamic behavior of bottom racks with transverse bars to the flow was studied experimentally and numerically. In the experimental phase, clean water and sediment were used. In the numerical phase, clean water was used. The most relevant characteristics of the screens studied and that differ considerably from conventional bottom racks, are the arrangement of the bars with respect to the flow direction, the longitudinal inclination and the separation between bars. The standard slot of a Coanda effect screen is 1mm, in the present study 1 and 2mm separations were used. The operation of the bars consists of a flow of water in a supercritical regime that passes over bars arranged perpendicular to the direction of the flow. The Coanda effect screen with standard triangular or wedge-type bars was used as a base. For comparative, constructive and economic feasibility purposes, two alternative bar shapes with circular and semicircular sections were evaluated. The use of the bars is oriented to the capture of surface water for consumption in rural populations of less than 1000 inhabitants of developing countries such as Ecuador, for this reason the flow rate used for all the experiments was 2.5 L/s. For the physical experimentation, a laboratory prototype was built. This consists of a prismatic rectangular channel with an adjustable slope, and the geometric characteristics of the channel are: length 2m; width 9.3 cm, angular range of the slope from 20° to 45° with respect to the horizontal. The main objective was to determine the efficiency of the racks in terms of capturing clean water, as well as the removal of sediment. The results of the physical experimentation show that the triangular section bar rack is more efficient in terms of capturing clean water, while, in terms of sediment exclusion, the circular section bar rack presents the best performance. The numerical experimentation is developed with the use of the ANSYS FLUENT code. A 2D CFD parametric study was developed to evaluate the influence on the flow of clean water captured by geometric variables such as the inclination of the screen, position of the screen along the channel (top, middle, bottom zones); bar shape and width, slot size. Total flows captured and flows for each slot of the rack were calculated. The performance of each type of rack is evaluated using relevant dimensionless parameters such as the Froude number and the Reynolds number, whose characteristic lengths for their calculation were the depth of the flow and the separation between bars, respectively. Numerical experiments show that racks with triangular and circular section bars have opposite behaviors. Triangular section bar rack is more efficient in terms of captured flow as the Froude and Reynolds numbers increase, while circular section bar rack is more efficient as the dimensionless variables decrease. Numerical experiments with equal Froude and Reynolds numbers showed greater efficiency in racks made of triangular bars over those made of circular bars of equal size and spacing. Increasing the diameter of the bars improve the efficiency of the captured flow to a level similar to that of triangular bar screens.