Abstract
The three-dimensional turbulent flow in a compact hydraulic machine elbow draft tube is numerically investigated for several operating conditions, covering an extended range around the best efficiency point. Comparisons with the experimental data are presented as validation. The interest is focused on the experimentally observed pressure recovery drop occurring near the best efficiency point. The flow is first analyzed locally by means of a topological analysis, then globally with an energetic approach. The study provides evidence for the role played by a Werle´–Legendre separation originating in the bend. The separation is due to the contrasting flow angles imposed by the blades, and the angle resulting from the secondary flow.
Highlights
Draft tubes are components which act to convert a maximum of dynamic pressure into static pressure
A sudden drop of the pressure recovery in the draft tube is observed near the best efficiency operating condition
The framework introduced by Tobak and Peake1͔ is adopted to describe the topological evolution of the flow in the draft tube as the flow rate is varied
Summary
Draft tubes are components which act to convert a maximum of dynamic pressure into static pressure. The extrapolation of the velocity profile from the last measurement point to the wall has a strong influence on the flow prediction in the draft tube. The pressure recovery drop takes place at the same flow rate observed experimentally, the computations overestimate the recovery factor over the whole range, with a maximal difference of 14% of the measured value. Bend curvature is forced by the secondary flow to the inner side, while the flow angle imposed by the blade geometry remains constant for all operating points The collision of these two flow directions leads to the onset on the surface of a focus in company with a saddle point leading to a global Werle –Legendre separationsee Fig. 8, showing the wall region where the onset of the critical points occurs.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
