Experimental and Numerical Investigation of Hydraulic Axial Thrust Fluctuation due to Draft Tube Vortex at partial load on Francis turbine

Abstract Operation range extension of Francis turbine is beneficial for effective use of hydraulic energy. However, the swirl flow in the draft tube of Francis turbine becomes large at the operating point far from the design point. As a result, the draft tube vortex occurs and this unsteady behaviour causes various problems such as pressure fluctuation, vibration and noise. Regarding the vibration, the correlation between the draft tube vortex and axial thrust fluctuation was revealed by the model test and CFD analysis in this research. In the model test, the pressure fluctuations were measured on the upper draft tube and the back chamber of runner crown. The axial thrust fluctuation of runner was measured by thrust meter on the rotational shaft. The unsteady behaviour of the draft tube vortex was captured through photos using high-speed cameras from side of upper draft tube and bottom of the bent draft tube. For the CFD simulation, the assembly of guide vane, runner, draft tube, back chamber of runner crown and the side chamber of runner band were modelled. Both study and unsteady simulation was carried out. From the CFD simulation results the pressure fluctuation at each measuring point and unsteady behaviour of draft tube vortex were obtained and compared with the model test data. Besides, the axial thrust force exerted on runner with respect to time was compared with the thrust fluctuation measured during model test. As the behaviour of draft tube vortex depends on operating point, the pressure fluctuation and thrust fluctuation were also measured from no load to full load condition during the model test. Furthermore, additional unsteady simulations were carried out at the operation point, where the increased thrust fluctuation was observed. As the result, the multiple (3~5 in number) short draft swirls formed with similar circumference as the balance holes at that flow rate. When the low-pressure regions were far from the balance holes, the pressure on the back chamber of the runner was high, and then the axial force on the runner back surface increased. On the other hand, when the low-pressure regions were closer to the balance holes, the pressure on the back chamber decreased, and then thrust on the back surface decreased. This cycle was repeated, causing the hydraulic thrust fluctuation.