Runner for francis type hydraulic turbine

Abstract

There is disclosed a runner for a Francis-type hydraulic turbine where each runner blade has a outlet edge comprising a deflected edge portion. The deflected edge portion is located adjacent to the junction or attachment point of the outlet edge to the band of the runner to define a stress reducing tip extension portion of the blade adjacent the band. This tip extension portion reduces stress concentration adjacent where the outlet edge joins the runner band.

Description

BACKGROUND OF THE INVENTION [0001] The present invention relates to hydraulic machines and in particular to improvements in runners for Francis-type machines. [0002] Conventional Francis-type runners are composed of a crown with a concentric band or ring and an array of blades extending between and interconnecting the crown with the ring. The runner is rotated about the axis of the crown by the action of water passing through a plurality of flow paths formed between each pair of adjacent blades in the array of blades. Each of the blades has a flow dividing edge which defines the line of demarkation between a suction surface leading in the direction of rotation and a trailing pressure surface trailing in the direction of rotation. This edge, at the inlet of the blades, is normally oriented to slope from the band toward the crown to advance in the direction of rotation of the runner. [0003] More recently, WO 98 / 05863 published Feb. 12, 1998 to Billdal et al discloses a runner for a Fr...

AI Technical Summary

Benefits of technology

[0006] The tip extension portion reduces both static and dynamic stress concentrations by having the shape of the blade at the outlet edge adjacent the band as a tip extension portion. This tip extension portion for the blade is less costly to manufacture than trying to build the material at the corner or thicken weld fillets since this tip extension portion adds minimal blade volume and results in significant stress reduction. Since the tip extension portion is built into the blade at the hydraulic design stage the weld fillet size remains small, there is no negative impact on hydraulic performance or cavitation behavior. It is further envisaged that existing runner blades can have their shape altered at the junction portion between the blade outlet edge portion and the band to provide the tip extension portion provided the outlet edge is continuous between the crown and the band.

Problems solved by technology

This phenomenon may result in blade erosion and cracking adjacent the band.

However, thickening of the blade adjacent the junction with the band must be substantial in size to relieve the stress concentration and this results in a more costly solution.

Increasing the weld fillet size also adds cost to the manufacture of the runner and can cause problems with distortion of the runner due to the increase in weld material deposited.

Both these traditional solutions typically add uneven surface contours to the blade that result in negative impacts on hydraulic performance in both efficiency and cavitation behavior of the runner.

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