Numerical Simulation of Flow Through Settling Basin: A case study of Budhi-Ganga Hydropower Project (BHP)

An attempt has been made in this study to check the adequacy of the trap efficiency on the deposition amount, movement, and pattern of the sediment using three-dimensional Computational Fluid Dynamics (CFD) on a settling basin of Budhi-Ganga Hydropower Project (BHP). The prediction of the amount of sediment deposition, its movement, and deposition pattern is necessary during the planning, designing, and implementation phase of the hydropower projects as sediment particles larger than a certain size can cause erosion and damages to underwater machines. Four different cases with seven different particle sizes (0.8125mm, 0.4375mm, 0.2325mm, 0.1975mm, 0.15375mm, 0.10125mm and 0.075mm) has been taken during simulation. Trap efficiency for different particle sizes has been compared and trap efficiency of 99% was achieved for the particle size 0.2mm from numerical simulation which is a 13% deviation with the result obtained from physical modeling. Keywords— Settling basin, deposition pattern, trap efficiency, CFD, FLOW-3D


INTRODUCTION
The continual tectonic elevating of the major mountain ranges and consequential down cutting of the river system has caused a high natural erosion rate in the Himalayan region [1]. The withdrawal of water from sediment carrying river for the development of hydropower is a big challenge. The water withdrawn directly from such a river reduces the capacity of the conveyance system and cause erosions and damages to the underwater machines such as turbine, valves, governors, etc. This causes the operation and maintenance problem that ultimately affects the economics of the generation [2]. Thus, settling basins are designed to remove suspended sediment particles greater than a particular size that settles down and deposits sediments on the bottom and they are flushed from the settling basin by certain approaches.
The study of efficient operation of settling basin, its performance, and periodic removal of the deposited sediments by water is a challenging job. To deal with sediment related problems, the prediction of the amount of sediment deposition, its movement, and deposition pattern is necessary. These predictions during the early stage and after the implementation of the project provides the best direction for the planning, designing, or improvement of the project that ultimately optimizes the cost of the project.
The settling basin is designed to decrease the mean velocity of the flow by increasing the cross-sectional area (broadening its width and depressing the floor) that reduces the turbulence level in the water flow and permit the sediment particles to settle. The settling basin is designed in such a way that it settles the smallest diameter of grain sediments in the vertical plane at the bottom of the pool before it reaches the end of the basin. The trap efficiency of a settling basin is defined as, [3]

= −
Where, η is the trap efficiency of the settling basin Cin is the concentration of sediment at inflow Cout is the concentration of sediment at outflow The design codes for a settling basin are mostly based on empirical approaches that may not provide accurate results as there is strong impact on the water velocity distribution and the degree of turbulence in the settling basins, that is due to different geometry and inflow conditions [3,4]. Thus, to provide accurate result, certain site-specific modification is required.
The effect of the geometry is difficult to execute in 1D parameters in the analytical approaches. Owing to the complicated nature of flow dynamics of water, the functioning of headworks and settling basin is examined by Physical modeling and Numerical modeling. Physical modeling is developing a scaled physical model based on a similarity theory whereas Numerical modeling is the process of appropriately developing a mathematical model/replica for a complex physical reality that enhances engineering judgment with the power of the digital computer [5]. The physical model test is costly as well as time-consuming and on other hand Numerical modeling has certain advantages over physical modeling such as scale effect, cost, time, and interpretation. Full dynamic similitude cannot be achieved by avoiding scale effects in hydraulic models and the turbulence level in the scale model are significantly lower which culminates in the overestimation of the viscosity and the surface tension effects when simulating free surface flows over hydraulic structures using the Froude similarity [6]. Further

International Journal of Engineering Research & Technology (IJERT)
ISSN: 2278-0181 http://www.ijert.org numerical modeling can overcome various problems related to hydrodynamics and can process small details that may be difficult or be omitted in a physical model [7,8].
Hanne Novik, Abha Dudhraj, et. al. [3] used STAR CCM+ for modeling of settling basin of Lower Manang Marsyangdi Hydropower Project (144MW) and the result is compared with a physical model that has been built at the Hydro Lab Pvt. Ltd. Dhemi Harlan, Dantje K. Natakusumah, Et. al. [8] has a done settling basin modeling using FLOW-3D of Karai-7 Mini hydro power plant located in Pasir Melayu Village, Sialu Kahean Sub District, Simalungun District, North Sumatera, Indonesia. Bishwo Vijaya Shrestha [9] used SIIM 1.0 on the settling basin of Mai Khola Hydropower Project, Nepal.
Thus, attempts have been made in this study to assess the flow in the settling basin by CFD to estimate the trap efficiency of the basin. The result obtained from the proposed modeling was tested with the physical modeling result of the settling basin of Budhi-Ganga Hydropower Project (BHP)-20MW, Achham, Nepal. The performance of a settling basin is mainly governed by the geometry of the basin [10]. Further, the modified cases (i. removing divider wall in each Hooper (case I), and ii. changing transition angle to 8° (case II) and 12° (case III)) of the proposed settling basin with different size and shapes were analyzed with deposition pattern and sediment trapping efficiency.

A. Technical Data of the Project
The technical data of BHP-20MW are gained based on the project report. BHP is located in Achham district of Nepal between 29°15'30" and 29°18'35" latitude north and between 81°14'00" and 81°17'50" longitude east on the Budhi-Ganga river, a major tributary to the Seti river. The site location map of BHP is shown in Figure 1. The analysis in this study is fixated on the settling basin of BHP. The physical modeling of the settling basin has been carried out and these data are used in our computation and analysis.

1) Physical Feature
The intake is designed to withdrawal design discharge of 26 m 3 /s at the normal operating level of el. 720m (22.1 m 3 /s for power generation and 3.90 m 3 /s for flushing). The discharge from the intake is conveyed to two settling basins through two D-shaped feeder tunnels of 2.5m X 3.0m size [11]. The feeder tunnels operate under pressure flow conditions. The settling basin of a 92m long desilting chamber is proposed to remove suspended particles of 0.20mm. A basin has two chambers, each having a flushing channel with bottom width 1m and a transverse slope of 1:1 on both sides. The plan of the conveyance system and the cross-sectional view of the settling basin are shown in Figure 2 and Figure 3 respectively.

2) Inflow and Outflow Discharges
The model was run for a single settling basin with 712.0m el. at intake and a discharge of 13 m 3 /s was feed. The outflow discharge of 11.05 m 3 /s and 1.95 m 3 /s is maintained in HRT and flushing respectively [11].

B. Mathematical Formulation
The equations of motion for fluids are solved with the Volume of Fluid (VOF) method numerical techniques. The Cartesian coordinates (x, y, z) system is used to solve the differential equations that are formulated with area and volume porosity functions. This formulation, called FAVOR for Fractional Area/Volume Obstacle Representation Method is used to model complex geometric regions.

1) Governing Equation
The www.ijert.org Vol. 9 Issue 07, July-2020 equation) and the sediment scour equations [8,12]. The general mass continuity equation for incompressible fluids is: • RSOR is a mass source • ρ is the fluid density • (u, v, w) are velocity components in the coordinate directions (x, y, z) • Ax, Ay, Az is the fractional area open to flow in the x, y and z directions respectively The equations of motion for the fluid velocity components (u, v, w) in the three coordinate directions are the Navier-Stokes equations with some additional terms • VF is the fractional volume open to flow • (Gx, Gy, Gz) are body accelerations • (fx, fy, fz) are viscous accelerations • (bx, by, bz) are flow losses in porous media or across porous baffle plates • Uw = (uw, vw, ww) is the velocity of the source component, which will generally be non-zero for a mass source • Us = (us, vs, ws) is the velocity of the fluid at the surface of the source relative to the source itself.
For each species, the suspended sediment concentration is calculated by solving its own transport equation, , + ∇ • ( , , ) = ∇ • ∇ ( , ) Here • Cs,i is the suspended sediment mass concentration of species i, which is defined as the sediment mass per volume of fluid-sediment mixture; • D is the diffusivity; • us,i is the suspended sediment velocity.

C. Numerical Model 1) Model Geometry
The geometry is constructed as per drawings for the base case (proposed case) settling basin of BHP by assembling solid geometric objects as shown in Figure 4. Similarly, the geometry is constructed for the modified cases by removing the dividing wall in each Hooper (case I), and changing transition angle from 10° to 8° (case II) and 12° (case III). In the cases, the transition length for base case and case I is 27m whereas for cases II and III are 34m and 23m respectively and other dimensions are the same for all cases.

2) Model Mesh Block
The multiple mesh blocks are created to efficiently mesh the model flow domains. According to geometry 6 mesh blocks are defined. The mesh size adopted for the present study are as follows:

S.N.
Mesh Block Size (m) Remarks

3) Boundary Condition
The boundary condition associated with geometry for each mesh block are as follows:

4) Initial Condition and Physics Definition
The opening conditions in the simulation are defined in Initial conditions. In the model, it is supposed that the settling basin is initially filled with water. Areas of fluid are defined using Fluid regions. Water at 20 °C from the fluid database is used. The acceleration due to gravity is set to 9.81 m/s 2 in downward Z-direction. The Renormalization Group (RNG) model was selected for the viscosity and turbulence model as it defines accurately to low-intensity turbulence flows and flows having strong shear regions. Van Rijn equation is set for the bedload transport rate equation.

D. Physical Model
The physical hydraulic model test of BHP was conducted in Irrigation & Power Research Institute (IPRI) Amritsar, Punjab, India. The desander model was constructed to a geometrical scale of 1:12 which includes feeder tunnel, desander, and sediment flushing channel. The model was run with sediment load at intake as 2000 ppm for 1 hour (3.46 hours in prototype). The sediment sample deposited in a different section of the settling basin was collected and the efficiency of the basin was obtained. [11] III. RESULTS AND DISCUSSION The velocity vector is used to view the correctness of geometry and velocity distribution at different chainage are viewed along with the turbulent dissipation rate. The trapping efficiency for different sizes of sediment particles and sediment concentration distribution over the settling basin with respect to time is computed. The result shows that sediment concentration level is greater at entry and sediment concentration level goes decreasing on downstream similarly sediment concentration level is greater at the bottom level and goes decreasing on the upper level, this is due to the sediment settling phenomena and decrease in velocity of flow.
The horizontal velocities at the end of the transition, two sections in desander, and at the end of desander are viewed.  Six probes are defined at three different sections as shown in Figure 13 to view the change in velocity. The plot between velocity and time at those probes are shown in Figure 14, Figure  15, Figure 16 and Figure 17 for the base case, modified case I, modified case II and modified case III respectively. In all the cases it is seen that velocity converges toward fixed value with time, which reflects the stability of simulation. In all the cases, except case II, the velocity at probe 1 and probe 2 is higher as this is just after the transition length whereas in case II, the velocity in all probes is of similar magnitude.  Figure 18 to Figure 21 represents the longitudinal sectional view of settling basin with sediment net change in height for the base case, modified case I, modified case II and modified case III respectively. The sections are viewed in the middle of each hopper. It is found most of the deposition has taken place just after the end of the transition. At the end of the simulation in the right hopper the maximum deposition of sediments was found to be 0.97m, 1.06m, 0.99m, and 1.32m respectively for the base case, modified case I, modified case II and modified case III respectively. Similarly, in the left hopper the maximum deposition was 1.24m, 1.19m, 1.04m, and 1.15m respectively for the base case, modified case I, modified case II and modified case III respectively. It is supposed to settle sediment with a diameter of +0.20 mm by the settling basin of BHP. The 0.20 mm sediment particles are expected to settle with a velocity magnitude of nearly 0.2 m/s. It is found that the horizontal velocity distribution in the basin is less than 0.2 m/s. Therefore, the settling basin is operative for sediment size +0.20 mm. The trap efficiency for the range of sediment 1.00-0.25 mm, 0.25 -0.180 mm, 0.180-0.075 mm and below 0.075 mm respectively are 100%, 100%, 85.76% and 24.62% respectively. The overall trap efficiency of the settling basin obtained for about 3.5 hours (12460 sec) is about 53.12% whereas for particle above 0.20 mm is 99%. In the physical model, the trap efficiency for the range of sediment 1.00-0.25 mm, 0.25 -0.180 mm, 0.180-0.075 mm and below 0.075 mm respectively are 100%, 82.67%, 45.32%, and 23.88% respectively whereas overall trap efficiency is 34.34% and for +0.20 mm is 87.66%. The comparison of trap efficiency between numerical and physical modeling is shown in Figure 22. The trap efficiency comparison between the base case and modified cases that are obtained from numerical simulation is shown in Figure  23. It shows the overall trap efficiency of modified case II (i.e. transition angle 8°) is better than the other cases. In this study, the flow velocity, particle distribution, and concentration of the sediment particles at different time intervals are studied for BHP. These parameters obtained from numerical modeling is used to obtain the trap efficiency of the settling basin with time. The following conclusion has been drawn in this study: 1. The velocity obtained for all cases for the particles greater than +0.20 mm is around 0.2 m/s, which seems good agreement with the value obtained from the empirical result.
2. The overall trap efficiency of the base case obtained from numerical modeling is 53.12% whereas physical modeling is 34.34%. Similarly, for case I, case II, and case III, the result obtained was 51.98%, 53.97%, and 50.52% respectively.
3. Among the cases, trap efficiency for case II seems a more efficient basin whose overall trap efficiency is 53.97%.
4. In the different cases run, the case doesn't have a dividing wall and it has lower trapping efficiency compared to other cases although the deposition pattern with the base case is similar. Thus, the dividing wall plays a significant role in boosting the capacity of the settling basin.
5. It is recommended to conduct a couple of simulations with modification in the slope of the settling basin. The CFD calculation can deliver various useful information for obtaining trap efficiency and functioning of settling basin in hydropower projects.