This document discusses the 3D design of Francis turbine blades. It describes how the median surface forms the foundation of the blade geometry. Thicknesses are applied perpendicular to this surface to define the pressure and suction sides. Hydraulic surfaces are then connected between the sides to define the inlet and outlet edges. Round fillets are added where the blades connect to the crown and band. Openings between blades are measured to check for hydraulic imbalance before welding.
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Introduction
Fabrication process
The crown, the band and the blades are machined to final shape before they are welded
together within the crown and the band. As a supplemental operation the blades will be
polished to obtain the required surface finish before welding.
In this document, our objective is to design the geometry of the blade. The finished
results can be used in all steps of the production: engineering design, supply, cost
estimate, machining, welding design.
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MEDIAN SURFACE
• The surface is the foundation of
the geometry. Since everything
that follows increased in
complexity at every step, the
best way is to have it as simple
as possible. If necessary, a linear
surface with varying angle can
be added at the inlet and at the
outlet of the blade for controlling
the shape.
Front view Side view
Bottom view Isometric view
Crown
Band
Suction
side
Pressure
side
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BLADE THICKNESSES
• The thicknesses are applied to the
vertical distance from the transparent
median surface at a local angle of 90
degrees and proportionally applied on the
length in the flow direction. On the picture
aside the blue and green curves will be
the guiding rails of the pressure and
suction side.
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HYDRAULIC SURFACES
• The white curves are connecting both
sides of the blade. They will be used as
sections to define the inlet edge surface.
Similarly the pressure and suction sides
will be defined with the blue and green
arrays curves.
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GEOMETRIC DETAILS
• The hydraulic profile is a
surface design. Once
converted to solid, round filets
are made to connect to the
crown and the band. On the
outlet edge, we have added a
round corner on the suction
side. This outlet shape
minimizes the vibration of the
runner due to the fluctuating
vortex generated by the
edges.
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GEOMETRIC DETAILS
• The blades and their groove will be
numerically machined. The grooves will be
designed in a way to equilibrate the
amount of deposited metal on each side.
The bending action of the welding
operation is then reduced. Depending on
the geometry of the runner the center of
the welds may not coincide with the center
of the blade as we can see on the crown
side. The green surface is the median of
the blade and the center of the weld joint is
beside.
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OPENNINGS
• The measurement is the
smallest distance
between the outlet edge
of the pressure side and
the suction side of the
adjacent blade. This
verification on all the
blades is made prior to
the welding operations.
It is a way of checking
the hydraulic unbalance
of the water flow leaving
the runner.
