Numerical investigation of the flow in a swirl generator, using OpenFOAM

Examensarbete för masterexamen
Master Thesis
Bergman, Oscar
This work presents results from OpenFOAM simulations conducted on a swirl generator designed to give similar flow conditions to those of a Francis turbine operating at partial load. Francis turbines are one of the most commonly used water turbines. In these turbines, there is however a frequent problem occuring at part load. Due to a swirling flow in the draft tube, a transient helical vortex rope builds up and creates severe pressure fluctuations in the system that increase the risk for fatique. To predict and control such flow features is therefore critical. A test rig was thus developed at the "Politehnica" University of Timisoara, Romania, to provide a detailed experimental database of such flow features. This test rig has four parts: leaning strout vanes, stay vanes, a rotating runner which is designed to have zero torque, and a convergent divergent draft tube. In this work, numerical results are compared and validated against measurements realized on the swirling flow test rig at the Polytechnica University of Timisoara in Romania. The computational mesh is created with ICEM-Hexa and the parts have been meshed separately and then merged together, using General Grid Interfaces (GGI) to couple them numerically. The finite volume method is used to solve both the unsteady and steady state Reynolds Averaged Navier Stokes equations and the standard k-epsilon model is used to close the turbulence equations. Steady-state simulations is a preliminary method, which is less time-consuming and predicts the general behavior of the flow field. It also provides good initial conditions for the unsteady simulations. For the unsteady simulations, the mesh of the rotating part of the domain is rotating and the coupling between the stationary and rotating parts is handled by a sliding GGI interface. The simulation results shows a developing vortex rope in the draft tube which gives rise to oscillations of flow properties in the whole system. The size and shape of this vortex rope, as well as the frequency of the flow property oscillations it gives rise to, is highly dependent on the rotational speed of the free runner. The results show that a rotational speed of 920 rpm on the runner, corresponds best with the measurements out of the three rotational speeds 870 rpm, 890 rpm and 920 rpm. The rotational speed of 870 rpm gives a positive moment on the runner, an rpm of 890 of almost zero moment, and a speed of 920 rpm gives a positive moment on the runner. This leads to the conclusion that the tourque in the test rig was not zero when measurements were carried out. The rotational speed of 920 rpm is however not the most corresponding rotational speed, for the results can probably give a better velocity profile if the rotational speed would be increased to maybe 930 rpm, and if LES or DES would be used for resolving the turbulence.
Strömningsmekanik , Fluid mechanics
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