Blade profiles

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Blade profiles

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► IMPELLER | Blade profiles

To create blade profiles (main and splitter) the blade thickness distribution for the hub and the shroud profile is used. By default the thickness is defined at leading edge, at trailing edge and at the control points of the blade. For the initial CFturbo-design, typical values in dependence on the impeller diameter d2 are used (see Approximation functions).

2 impeller types have special thickness distribution:

Waste water pumps have very high thickness at leading edge to avoid solid attachments. Starting from 20% of the blade length the thickness is constant up to the trailing edge.

Inducer pumps have very low thickness at leading edge to improve suction performance. The very small leading edge thickness is increasing up to 40%...80% of pitch (t=πd/nBl) to achieve constant blade thickness. The thickness distribution is asymmetric and sharpen at the suction side only.

The representation of the thickness distribution is made along the relative blade length (0 = leading edge, 1 = trailing edge).

The following properties for the profile design can be specified:

Design Mode

Linear
Linear interpolation between control points

Freeform
Bezier curves are used for the thickness distribution
LE / TE rounded: Leading and trailing edge can be rounded optionally. If a polyline was loaded this option is determined automatically and cannot be modified.

Linked to Main
Only for splitter blades: splitter profile is linked to main profile

Basic settings

Thickness definition

Thickness definition specifies the way of adding blade thickness values to both sides of the blade mean line to create the pressure and suction sides of the blade. Three types of method are supported:

Perpendicular to mean surface
After creating the mean surface from all mean lines, the thickness values are added along surface normal. Naturally, this method depends on all mean lines.

Perpendicular to mean line (recommended)
Thickness values added orthogonal to mean line inside rotational surface defined by span. Compared to method above, this definition only depends on the mean line/span itself. Therefore it provides higher stability in trimming with hub/shroud especially for highly curved blade geometry.

Tangential
This method is operating point-wise by adding the thickness values in tangential direction and is therefore the most independent method.

SS-PS-Coupling

None
No coupling between suction side and pressure side

Symmetric
Symmetric thickness distribution: control points on suction and pressure side are coupled

Constant distance
Shifting the thickness distribution to pressure/suction side whereas the distribution itself remains constant

Global point count

Global number of control points

Flexible length position

Shifting control points in horizontal direction

Hub to Shround/Tip (spanwise)

Identic profiles

All profiles have the same thickness distribution

Thickness exponent

Adjusts the morhping of hub/shroud-thickness for inner profiles. Default is linear.

Asymmetry at edges (hub)

Provides adjustments of asymmetry at leading / trailing edge relative to first inner control points respectively.

Each thickness curve has a popup menu to add/ remove polygon/ Bezier points, to load or save the curve and to reset the distribution to default.

 

 

 

For Bezier curves a Polyline to Bezier conversion is available as well as using a thickness distribution from a pre-defined profile from profile manager.

Possible warnings

Problem

Possible solutions

Pressure and suction side (...) are intersecting or swapped.

The blade sides are intersecting or they are on the opposite position. Normally this can occur only when loading profiles from file.

Check the imported profile data if
a) pressure and suction side are not intersecting
b) pressure side is always above suction side

Profile of Main/ Splitter blade exceeds its valid range.

Profile is defined for a relative blade length smaller 0% or greater 100%.

Check the imported profile data or correct the Beziér control points to lie between 0% and 100%.

Loaded profiles do not correspond to settings of design mode

Profile properties defined by context menu in the design dialog do not match Design mode settings.

May occur if thickness distribution is loaded from profile manager:

a) Check and adjust state of check box LE rounded and TE rounded

b) Apply profiles to both hub and shroud resp., or choose identical profiles

Blade thickness values don't match target thickness on LE/TE.

Current profile thickness on leading- / trailing edge deviate from the specifications of the Blade properties dialog.

Check the imported profile data if the values for leading and trailing edge match those of the Blade properties dialog.

Pressure/Suction side at Hub/Shroud:
max. thickness seems too high to get smooth surface.

The combination of of high blade thickness and high meanline curvature results in degenerated blade profiles and prevents creating smooth blade surface.

Either blade thickness at the specified profile side or meanline curvature at the specified span position has to be reduced .

 

Internal blade thickness is lower than specified in Blade properties dialog.

After changing the blade thickness on leading or trailing edge in the Blade properties dialog, the thickness of the blade at the inner control points is unaffected. It could happen that the thickness on leading and trailing edge is higher than in the middle of the blade.

Adjust the inner control points