Numerical and experimental study of the leakage flow in guide vanes with different hydrofoils

Abstract

Clearance gaps between guide vanes and cover plates of Francis turbines tend to increase in size due to simultaneous effect of secondary flow and erosion in sediment affected hydropower plants. The pressure difference between the two sides of the guide vane induces leakage flow through the gap. This flow enters into the suction side with high acceleration, disturbing the primary flow and causing more erosion and losses in downstream turbine components. A cascade rig containing a single guide vane passage has been built to study the effect of the clearance gap using pressure sensors and PIV (Particle Image Velocimetry) technique. This study focuses on developing a numerical model of the test rig, validating the results with experiments and investigating the behavior of leakage flow numerically. It was observed from both CFD and experiment that the leakage flow forms a passage vortex, which shifts away from the wall while travelling downstream. The streamlines contributing to the formation of this vortex have been discussed. Furthermore, the reference guide vane with symmetrical hydrofoil has been compared with four cambered profiles, in terms of the guide vane loading and the consequent effect on the leakage flow. A dimensionless term called Leakage Flow Factor (Lff) has been introduced to compare the performances of hydrofoils. It is shown that the leakage flow and its effect on increasing losses and erosion can be minimized by changing the pressure distribution over the guide vane. Highlights Use of the computational tool (CFD) to predict the flow phenomena in hydropower application. Validation of the computationally obtained pressure with pressure sensors and velocity with PIV experiment. Use of the validated numerical model to compare the guide vane profiles for erosion-friendly design of the turbines.

Numerical Investigation of the Effects of Leakage Flow From Guide Vanes of Francis Turbines using Alternative Clearance Gap Method

“…However, the vortex seems to dissociate gradually while it travels from NACA 4412 GV profile with all clearance gap. The flow field depicts similar findings in the past experiment with single GV test rig in the case of NACA 0012 hydrofoil (Thapa, Dahlhaug, and Thapa, 2017 a, b).…”

“…This equation gives good agreement between masstransfer simulations with experimental data (Chitrakar, Thapa, Dahlhaug, and Neopane, 2017). The sensitivity study of the turbulence models SST, BSL and Omega RS suggests SST model be suitable for the current case too (Chitrakar, Thapa, Dahlhaug, and Neopane, 2016).…”

“…The behavior of fluid flow from GV is highly unpredictable, due to wakes through trailing edge and cross leakage flow through the clearance gap (Eide and Brekke, 2002, Eide, 2004, Brekke, 1988. Experimental investigations on flow behavior in GV were carried out in past to predict the effect of outflow from GV trailing edge towards runner inlet (Thapa, Dahlhaug, and Thapa, 2018;Dahlhaug 2017, Chitrakar, Thapa, Dahlhaug and. This results in the highest wear in the GV region due to accelerated flow through GV surface and leakage flow .…”

“…The plot of normalized total pressure contour (CTP) was observed in the case of NACA 0012, NACA 2412 and NACA 4412 hydrofoils. The normalized pressure is the ratio of total pressure to the average total pressure at the inlet (Chitrakar, Thapa, Dahlhaug, & Neopane, 2017). This gives loss of pressure energy of fluid while travelling from leading edge to trailing edge, such that value below 1 corresponds to losses.…”

A Review on Sediment Erosion Challenges in Hydraulic Turbines

“…Similarly, for the design of the guide vanes, it is found that the change in the guide vane's hydrofoil profiles could minimize the erosion in runner and guide vanes at some operating conditions [24]. The hydrofoils are usually standard profiles such as NACA.…”