Swirling flow in a conical diffuser generated with rotor-stator interaction

Application Challenge AC6-14 © copyright ERCOFTAC 2024

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Key Fluid Physics

The main features of the flow are the on-axis recirculation region,the vortex rope and the vortex breakdown, and wakes of the blades. The separation from the blades, flow in inter-blade passages, separation in the divergent part of the draft tube and rotor-stator interaction are among other physical mechanisms which make the flow fields complicated and difficult to model.

Application Uncertainties

The complexity of the geometry, curved and bladed regions, tip-clearance and rotor-stator interaction, oscillation of the runner rotational speed which is absent in numerical simulations are some sources of uncertainties which make a high fidelity CFD model difficult to assemble.

Computational Domain and Boundary Conditions

The boundary conditions also play a prominent role to reproduce the physical mechanism of the vortex breakdown. The swirl intensity determines the occurrence of the vortex breakdown. The swirl depends on the axial and tangential velocity components, which are two dominant parameters and specify the physical mechanism of the breakdown. For swirling flows in a pipe, the former determines the radius of the vortex core and the later specifies the character of the on-axis axial velocity (jet- or wake-like). The inlet boundary condition is usually unknown at the draft tube inlet of the hydraulic turbomachines. To prevail this problem, the rotor-stator interaction, which is the interaction between the guide vane and the runner blades, is considered to retain the upstream effects on the flow in the draft tube.