Computation of Critical Submergence for Dual Intakes in Open Channel Flows using CFD

Air entrainment into the water intakes due to insufficient cover of water poses problem to the hydraulic machinery. Critical submergence is a cover of water at which air entrainment starts into the intakes. This paper deals with computation of critical submergence at in-line dual intakes in channel flow. Vortex formation at multiple intakes is more complex than that occur in single intake. A three-dimensional multi-phase steady state volume of fluid (VOF) model was developed to compute the critical submergence and air-water interaction at the surface interface of in-line placed dual square intakes in open channel flows. Reynolds-averaged Navier–Stokes (RANS) equation with standard k- ω turbulence model (Wilcox, 2006) was used to discretize the fluid flow inside the test domain. Air entraining vortex at critical condition was identified using phase volume fraction studies and surface streamlines. Multiphase CFD study helped to understand the flow structure and turbulence characteristics of the vortex flow at the vicinity of intakes. Effects of mutual spacing between the intakes were also analyzed. Cross and separation of flow generated in the presence of intakes was found to affect the critical submergence significantly. Surface reverse flow and flow stagnation between the intakes were observed from velocity field studies. Critical condition arrived first in the downstream intake for same discharges in both the intakes. Mutual interference of the intakes diminishes once the centre to centre spacing of the intakes is equal to or higher than 2.5 times of the intake size. The second intake downstream of the first one reached the critical condition usually when its intake Froude number is greater than or equal to that of the first intake. CFD results were validated with the observed one obtained through experimentation at Hydraulic Engineering Laboratory of IIT Roorkee, India.