The Khimti Hydropower Plant is located in western Nepal. An externsive study of the hydraulic performance of the settling basins was carried out by Buthwal Power Company at the hydraulic laboratory of the Institute of Engineering, Tribhuvan University, Kathmandu, in cooperation with NTNU. The tests include velocity measurements in the sand trap. The current project will investigate the flow field in the sand traps using CFD models, and compare the results with the measurements from the physical model.
Khimti river at the intake site
Physical model of the intake, showing the desilting basins
Physical model of the desilting basin at the laboratory in Kathmandu
The sand trap had two basins. The right basin had a flat bottom, using conventional flusing by lowering the water level. The left basin had a hopper-type bed, for use of continous flushing. Only the left basin was investigated in detail in the numerical model.
The water level in the left basin could vary. A situation with high water level was modelled. The entrance of the sand trap was pressurized, and the free water surface was located 1.2 meters above the top of the pressurized section.
The numerical study computed the velocities in the sand trap. The laboratory measurements gave a relatively uniform profile in the sand trap. Varying configurations of parameters for the numerical model gave more or less uniform profile. The best fit was obtained using the Power-Law Scheme, and a roughness coefficient equivalent to a Strickler value of 90. These results are presented below.
Plan view of bed shear stress at the entrance region
The entrance region is slightly skewed compared with the direction of the main sand trap. A divide wall is constructed at the middle of the entrance region, shown in black on the figures. Note the colour legends are distorted in the entrance figures:
Legend for the colours of the bed shear stress
Plan view of velocity close to the bed at the entrance region
Scale for the velocity in the entrance region
Three-dimensional view of the bed of the sand trap, with the entrance at the lower right corner of the image. The colours show the vertical level.
The figure shows the hopper-type bed level of the main sand trap. The entrance region has a flat bed. The figures below show the sand trap from the same angle.
A number of cross-sections are shown with filled colours, denoting the horizontal velocity. The grid of the bed is also given, in blue.
Enlargement of the entrance region of the image above
The pressurized part of the sand trap, between the roof and the free surface, is blocked out. It is shown with black in the figures above.
The bed level grid is given in a 3D view, together with a longitudinal profile in the middle of the sand trap, slightly to the left of the divide wall. The longitudinal profile is filled with colour, denoting the horizontal velocity.
The region with both the pressurized and free surface is given below, in longitudinal profiles. A recirculation zone is present, just after the free surface section starts.
Longitudinal profile at the central part of the left basin. The colours show the horizontal velocity
Longitudinal profile at the central part of the left basin. The colours show the vertical velocity
Longitudinal profile close to the left side, where the colours show the horizontal velocity
Longitudinal profile close to the left side, where the colours show the vertical velocity
A number of cross-sections are given below, where the colours show the horizontal water velocity.
Cross-section no. 39
Cross-section 39 is inside the pressurized region. Note the recirculation zones at the bed corners.
Cross-section no. 39
Cross-section 39 is the second cross-section in the free surface region. The green colours close to the water surface shows the recirculation zone. There is also recirculation at the bed corners.
Cross-section no. 41
Cross-section no. 58
Cross-sections 41 and 58 shows relatively non-uniform flow in the entrance region.
Cross-section no. 61
Cross-section no. 83
Cross-section no. 61 and 83 are in the sand trap itself. There are 134 sections in the grid, so these sections are at the upstream part. A non-uniform profile is computed, with higher velocities at the left side (right in the figures, as they are seen upstream). This corresponds with measurements in the physical model.
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This project was partly carried out by Mr. Pravin Raj Aryal as a part of his MSc thesis. The laboratory data was measured by Hydro Lab Pvt. Ltd., Kathmandu, Nepal.