The effect of reservoir geometry on the critical submergence depth in hydroelectric power plants intake

The most important design index of water intakes is critical submergence depth of the intake. The depth at which the air core formed by the vortex is about to enter the intake. The emergence of a vortex and air entry into the intake increase head loss and decrease discharge coefficient. The reservoir geometric asymmetry, presence of unevenness in the bottom of the reservoir and angle of approach flow are among the factors that influence formation of the vortex and critical submergence depth. In this research, a physical model has been used to investigate the effect of reservoir geometry on the critical submergence depth. This model is designed in such a way that it can produce the strongest type of vortices with air core and with different strengths. The results showed that by creating asymmetry in the flow approaching the water intake (with side blockage in upstream), the presence of even 10% side blockage can have a great effect on the formed vortex and increase the critical submergence depth by about two times. To create uneven conditions on the reservoir bottom, blockages were created on the bottom of the reservoir upstream of the intake. The results showed that the blockage up to half of the height below the intake caused an increase of about 10–25% of the critical submergence depth, in low and high Froude numbers, respectively. However, in blockages more than half of the height below the intake, this effect increases about 60% of the critical submergence depth. In addition, the effect of the slope of the intake head wall on the order (type) of the vortex and its stability and instability was studied, and it was found that the order of the vortex decreases with the increase in the slope of the head wall toward the vertical position. Also, by increasing the slope of the head wall, the vortices form in an unstable manner. The vertical head wall can act as an anti-vortex structure and cause a reduction in critical submergence depth.