Blade properties

<< Click to Display Table of Contents >>

Navigation:  Impeller > Mean line design >

Blade properties

Previous pageReturn to chapter overviewNext page

► IMPELLER | Blade properties

Definition of blade properties is made in two steps:

(1) Blade setup

(2) Blade angles

Specification of number of blades and number of spans

Usual number of blades are:

Pump

3 ... 7
Wastewater: 1 ... 3

Inducer: 1 ... 3

Fan

6 ... 10
Squirrel cage: 30 ... 60

Compressor

Depending on blade exit angle ß2:

12 for ß230°

16 for ß245°...60°

20 for ß270°...90°

Radial turbine

12 ... 20

Axial turbine

30 .. 70 (100)

Many blades - causing low blade loading - are related to higher friction losses. By choosing of fewer blades - leading to a higher blade loading - the hydraulic losses may rise due to increased secondary flow and stronger deviation between blade and flow direction.

The recommended number of blades according to Pfleiderer is displayed as a hint at the information image [ for centrifugal & mixed-flow pumps, fans, compressors only ]:

with kz = 6.5 ... 8.0 for compressors, else 5.0 ... 6.5.

The recommended number of blades using the Zweifel work coefficient is displayed as a hint at the information image [ for axial turbines only ]:

with Δz the axial chord length and dav the average impeller diameter.

The Zweifel work coefficient is in the range of ψ = 0.75..1.15 and is specified in the approximation functions.

 

The span positions are illustrated as meridional lines in the Meridian diagram in the information area. By default the meridional lines are equally spaced between hub and shroud.

Splitter linked to Main blade

If the impeller has splitter blades then the shape of the splitter can be linked to the main blade optionally. If linked the splitter blades are truncated main blades. Otherwise the splitter blade can be designed completely independent.

Information

In the right panel some information are displayed which result from calculated or determined values:

(1) Velocity triangles

The velocity triangles of inflow and outflow are displayed.

Continuous lines represent flow velocities on hub (blue) and shroud (green).

Velocities directly before and behind blade area are displayed by dashed lines to show the influence of blockage in the flow domain.

Furthermore the blade angles are displayed by thick lines in order to see the incidence angle on the leading edge and the flow deviation caused by slip velocity on trailing edge.

(2) Values

Numerical values of velocity components and flow angles are displayed in a table. A short description is at mouse cursor too:

z

Axial position

d

Diameter

αF

Angle of absolute flow to circumferential direction

βF

Angle of relative flow to circumferential direction

u

Circumferential velocity

cm

Meridional velocity  (cm=wm)

cu

Circumferential component of absolute velocity

cr

Radial component of absolute velocity

cax

Axial component of absolute velocity

c

Absolute velocity

wu

Circumferential component of relative velocity: wu+cu = u

w

Relative velocity

τ

Obstruction by blades (see below)

i

Incidence angle: i = β1B - β1

δ

Deviation angle: δ = β2B - β2

w2/w1

Deceleration ratio of relative velocity

c2/c1

Absolute velocity ratio

ΔαF

Abs. deflection angle: ΔαF= αF2 - αF1

ΔβF

Rel. deflection angle: ΔβF= βF2 - βF1

φ=ΔβB

Blade camber angle: φ=ΔβB= βB2 - βB1

γ

Slip coefficient

Δ(cu·r)

Swirl difference

T

Torque

H/Δpt

Head/Pressure difference (total-total)

(3) Default ßB, mean line design only

Default blade angles for the optimal Free-form 3D blade shape is displayed compared to the currently specified/ calculated angles. Deviations from default values are marked in red color. Default blade angles are calculated based on
  - Shockless inflow for ßB1 at blade leading edge
  - Euler equation for ßB2 at blade trailing edge


For some simplified blade shapes the blade angles of some sections result from the mean line design - see Blade angles/ "Auto".

If the mean line design already exists in the component then these dependent angles are calculated automatically for information, otherwise the table cells remain empty.

(4) Meridian

The Meridian with the locations of the spans is displayed in this diagram.

Radial element blades

For Radial element blades the number of spans is fixed to 11. Furthermore a Distribution exponent can be specified. This exponent has influence on the distribution of spans and herewith especially on the shape of the leading edge (turbine). For highly spatial curved blades the continuity of the blade surface can be influenced by this parameter.

Impact:

Distribution exponent = 1: spans uniformly distributed (default)

Distribution exponent < 1: spans concentrated towards shroud

Distribution exponent > 1: spans concentrated towards hub