Hydraulic Characteristics of Desander for Settling Efficiency in Design Process

Abstract

Sediment control in hydropower plants has been a major concern in the Himalayan area because of its higher sediment loads. In the case of a run-of-the-river- type dam, controlling of sediment control has been relyiedng on a sediment desilting basin or desander just after taking water from the a small reservoir. In this study, the hydraulic characteristics of an underground- type desilting basin have beenwere investigated numerically and physically. A pPhysical model has was been fabricated, and the a flow field has was been investigated vertically, horizontally, and longitudinally by using an aAcoustic Doppler Velocimetervelocimeter vertically, horizontally and longitudinally. To enhance the flow pattern at the transition area between the entrance and the basin of the desander, a divide wall and guide wall have were been applied. The dDiscrepancy ratio has was been used to check how well the numerical model estimates estimated the physical model. The A comparison between of the results of physical model and those of numerical model results shows showed good agreement, with an accuracy of 0.79- to 0.89, which value is statistically meaningful statistically. A nNew approach of for a simplified empirical method to for evaluate evaluating the settling efficiency of the a desander has was been proposed, which can could be used in under a turbulent flow condition, where the velocity profile takes is almost the same as the averaged one. The criteria for efficiency criteria ware ere defined such so that the velocity is was less than the threshold velocity for the particle movement in the x-axis direction, and the terminal fall velocity of the particle in the z-axis direction. The dDesign parameters for an underground- type desander have beenwere checked with the newly suggested efficiency for eight factors, including such asthe inlet diameter, transition length, and basin length and etc. The results showed that the basin length is was the dominant factor to for the efficiency of the basin. The turbulence energy has was been analyzed for several basin lengths to determine the entrance length of the desander. It This showed that the turbulence energy increases increased to its maximum value just after the transition area at x/H = 2.0, and then decreases decreased along the basin untill just before the outlet. An approximation is was introduced defining to determineunder the condition that whether when if the change of in the turbulence energy per grid is iswas less than 1%, %., If so, then turbulence energy had is had decayed enough to apply the classical design formula for determining the basin length. The Projecting porous plates projecting at the bottom of the desander were tested to see determine their applicability of them in reducing the basin length. It This showed that the turbulence energy reduced decreased drastically in a short distance when the projection of the porous plate is was 1.3D, which is was just over greater than the diameter of the inlet tunnel. The trajectories of a particles are were traced to check determine the effective length of the desilting basin, which can could be used as a good approach in design practice even though it cannot be considered to be a hydraulically complete solution.