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

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Summary

Introduction

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.

Conclusion
Findings
Aref ϭ Reference section area C ϭ Velocity field

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