<< Click to Display Table of Contents >> Blade setup 

► Impeller  Blade properties
On page Blade setup basic blade properties are defined.
3D types: blade is curved in 3D
Freeform 3D 

Helical 3D 

2D (axial) types: blade is curved in 2D when looking in axial direction
Freeform 2D (axial) 
Circular 2D (axial) 
Straight 2D (axial) 


2D (radial) types: blade is curved in 2D when looking in radial direction
Freeform 2D (radial) 
Straight 2D (radial) 
The initial blade shape depends on the machine type and can be customized in the Impeller preferences.
PUMP 

Centrifugal & Mixedflow 
Freeform 3D 
└ Waste water pump 
Freeform 2D (axial) 
Axial 
Freeform 3D 
└ Inducer 
Helical 3D 
FAN 

Centrifugal & Mixedflow 
Circular 2D (axial) 
└ Squirrel cage 
Circular 2D (axial) 
Axial 
Freeform 3D 
COMPRESSOR 

Centrifugal & Mixedflow 
Ruled surface 3D 
TURBINE 

Radial & Mixedflow 

Axial 
Freeform 3D 
Only the Freeform 3D blade shape provides complete flexibility, all other types result in limitations in blade angle specification and mean line design.
In case of Ruled surface 3D blade shape and linked splitter blades the linkage can be specified in more detail. See Ruled Surface blade.

Blade shape 
Impeller type 
Meridional shape 
Splitter blades 
3D 
Freeform 3D 
(no limitations) 

Ruled surface 3D 

Radial elements 3D 

Helical 3D 
axial impellers only 

not available 

2D (axial) 
Freeform 2D (axial) 
centrifugal & mixedflow impellers only 
available only if the meridional direction is mainly radial: hub must overlap shroud in zdirection about 50% or more hub must not have axial parts within blade area 

Circular 2D (axial) 

Straight 2D (axial) 

2D (radial) 
Freeform 2D (radial) 
axial impellers only 
available only if the projection of the shroud mean line in radial direction (relating to leading edge point) hits the hub surface hub must not have radial parts within blade area 
not available 
Straight 2D (radial) 
Blade thickness can be important for the blade angle calculation due to the blockage effect and flow acceleration.
By different thickness on hub and shroud side a tapering to the blade tip can be designed. Initial thickness values are based on empirical functions.
2 impeller types have special thickness requirements:
•Waste water pumps have very high thickness values at leading edge to avoid solid attachments (10% of d2 for 1 blade, 5% of d2 for more blades). The rest of the blade has smaller thickness of 30% relative to the max. thickness at leading edge.
•Inducer pumps have very low thickness values at leading edge to improve suction performance: 6%...10% of normal blade thickness.
Blade thickness mode
In general, it's a controversial issue to consider blade blockage effect for blade angle calculation or not and it in which way. Exactly at blade edge the thickness is 0 due to the rounding of the blade edge. Immediately after the blade leading edge (or before the blade trailing edge) the blade is blocking the flow in a certain manner. This blockage is dependent on the blade thickness, the blade angle and the blade angle distribution and which is hence a rather complex with respect to the blade geometry. One can consider the blockage either by the following thickness modes:
•Tangential: the blade thickness is projected tangentially σ = s / sin(βBl).
•Orthogonal: the blade thickness is not projected at all σ = s.
•None: the blade thickness is not considered σ = 0.
These options will have an influence on the calculation of the meridional velocity component cm and therefore on the blade angle calculation when pressing button Calculate βB or if the checkbox Automatic is selected. Beyond it, it will influence the meridional flow calculation too.
Pump, Fan, Compressor 
Turbine 

from ratio Q for shockless inflow / Q for max. efficiency 
fully automatic by theory of WIESNER adapted by Aungier 

or directly by incidence angle i (RQ=100% or i=0° for shockless inflow) 
or directly by incidence angle i (i=0° for shockless inflow) 

or from ratio of incidence angle i / blade angle βB 
iRel = i / βB 

For inducer pumps there is an additional check if the incidence is > 1° even for high flow rates (overload) to prevent pressure side cavitation.
Squirrel cage fans have high incidence typically resulting in blade inlet angles β1B≈ 80°.
[ Pump, Fan, Compressor impellers only ]
You have to use one of the following slip models:
Slip model theory 
Hints 
closed empirical model, 

closed empirical model 

input of coefficient a 

Userdefined 
manual selection of angular deviation ß2Bß2 resp. velocity ratio cu2/cu2,∞ 
specific slip models for specific impeller types 
Using the button Show calculation details provides specific information about the βB2 calculation.
Problem 
Possible solutions 

Average blade blockage factor of ... is larger than warning level of ... at leading/ trailing edge. 

Blade thickness s is blocking a significant part of the flow passage u = πd/ number of blades. The blockage factor is calculated as F = s / u. The average blade blockage is calculated with hub and shroud value. 
Reduce number of blades and/or blade thickness. 
Blade blockage factor of ... is outside the valid range of 0...1 at leading/ trailing edge at hub/ shroud. 

Blade thickness is blocking the flow passage completely at the specified position. 
Reduce number of blades and/or blade thickness. 
Blade number different than initially defined. 

Number of blades differs from the number that was initially selected in Main dimensions used for empirical correlations to calculate the main dimensions. This can result in inconsistent impeller design. 
It makes no sense to use other number of blades for main dimension calculation and blade design itself. Before modifying the number of blades here one should adapt the number in Main dimensions, update the empirical parameters and the main dimension. 
Mean lines (except hub) may be extrapolated. ("Freeform 2D" blade shape only) 

The hub is the master mean line for "Freeform 2D" blade shape. For this blade shape the geometry of all other mean lines is designed automatically in such way that it is exactly overlapping the hub mean line if viewing in zdirection. The resulting blade shape is twodimensional. If the other curves have points with higher radius at trailing edge/ lower radius at leading edge than the last/ first hub point (sloping meridional edge), then these curves have to be extrapolated. 
Use axis parallel (const. radius) or slightly sloping meridional leading/ trailing edge. Leading edge: The shroud point should have higher or equal radius than the hub point. Trailing edge: The shroud point should have lower or equal radius than the hub point. 
Blade shape [Radial Elements 3D]: requires the maximum Zextension 

The hub is the master mean line for "Radial elements 3D" blade shape. The geometry of all other mean lines is designed automatically in such way that it forms a blade consisting of radial fibers. The resulting blade shape is threedimensional. If the other curves have points with lower zvalues at leading edge/ higher zvalue at trailing edge than the first/last hub point, these curves have to be extrapolated. In this case the blade would have a bad quality in the extrapolated region. 
Use radial (const. axial position) or sloping meridional leading/ trailing edge. Leading edge: The shroud leading edge should have a higher or equal axial position compared to the hub. Trailing edge: The shroud trailing edge should have a lower or equal axial position compared to the hub. 
"Ruled surface" blades may export low quality surfaces when using two mean lines only. 

Impeller with splitter blades can have wavy blade surface if only 2 blade profile sections are used. 
Increase the number of blade profile sections (page "Blade angles"). 
"Straight 2D (axial)" blades not possible for selected 

The hub mean line is the master mean line. All other mean lines are adapted automatically in order to overlap the hub mean line if viewing in zdirection. If the other mean lines are extended they will be extrapolated automatically. For specific combinations of meridional leading edge and blade angles βB1 an extrapolation is impossible. 
Leading edge: The point on shroud should be moved to a higher radius. βB1: Blade angle should be increased. 
"Circular + Freeform 2D (axial)" blades not possible for selected distance and angle combination. 

Construction of circular arc is not possible for given parameters. Therefor calculation of blade is blocked 
Modify α3 or a3 for this blade shape. For further information see blade properties for stators. 
Extrapolation of "Circular + Freeform 2D (axial)" blades not possible for secondary spans. 

The minimal inner radius for the secondary spans is limited by the circular arc (design curve) defined by α3 and a3. 
Try to reduce effect of extrapolation by adjusting meridional Leading edge or change parameters defining the circular arc (design curve) of this blade shape. See blade properties for stators. 
"Straight 2D (axial)" blades not possible for selected 

The blade angle is too small or too large  therefore designing a "Straight 2D" blade shape is impossible. 
Trailing edge: The edge should be moved to a higher radius. αLE/βLE: Blade angle should be increased. 
"Straight 2D (radial)" blades not possible for selected Blade angles not within a valid range. 

Projection of the designed mean line onto the other spans fails for this blade shape. 
Blade angle should be specified within the recommended range. 