Analysis of head losses in a bulb turbine draft tube by means of unsteady numerical simulations

The draft tube of a hydraulic turbine is the turbine element located downstream of the runner. It has a divergent shape in order to convert the residual kinetic energy leaving the runner into pressure and thus increase the specific energy transfer to the runner. The performances of low head bulb turbines are highly influenced by the head losses in the draft tube. The prediction of these head losses in a design process is thereby a major issue. The numerical prediction of the head losses in the draft tube is a real challenge because the flow in the draft tube is highly unsteady with high Reynolds numbers, a swirl and an adverse pressure gradient.

These characteristics render conventional industrial approaches not appropriate for head losses prediction. The objective of this work is twofold:

(i) to improve the numerical prediction of the turbulent flow in the draft tube by using URANS and LES unsteady approaches and paying special attention to the description of the inlet boundary conditions of the draft tube and
(ii) to conduct a detailed analysis of the energy transfers in the draft tube in order to better understand the origin of the head losses.

An unsteady inlet boundary condition for the simulations reproducing the flow field at the runner outlet is developed. Numerical results are compared to experimental measurements in order to evaluate the predictive capacity of each turbulence modelling approach (URANS and LES). This validation step highlights the importance of defining properly the three velocity components at the draft tube inlet. Thanks to a detailed analysis of the mean kinetic energy balance in the draft tube, the hydrodynamic phenomena responsible for head losses are identified. The head losses prediction differences between URANS and LES are thus analyzed in detail and possible improvements for the head losses prediction are identified. Finally, this analysis enables to understand the head losses evolution observed between several operating points of the turbine.