Impeller > Mean line design > Blade properties

► IMPELLER | Blade properties

Definition of blade properties is made in three steps:

Absolute and relative flow

Absolute velocity
Relative velocity
Rotational speed

Fundamental kinematic equation
of Turbomachinery

Velocity triangles

Radial impeller	Axial impeller

Significant cross sections

Index	Position
0	just before	leading edge
1	just after	leading edge
2	just before	trailing edge
3	just after	trailing edge

Axial impeller

Radial impeller
Pump, Fan, Compressor

Radial rotor
Gas/Hydro turbine

Specification of number of blades

Usual number of blades are:

Pump	3 ... 7 Wastewater: 1 ... 3 Barske (low nq): 12 ... 24 Inducer: 1 ... 3
Fan	6 ... 10 Squirrel cage: 30 ... 60
Compressor	Depending on blade exit angle ß2: •12 for ß2≈30° •16 for ß2≈45°...60° •20 for ß2≈70°...90°
Radial-inflow gas turbine	12 ... 20
Axial gas turbine	30 .. 70 (100)
Axial compressor	20 .. 40
Francis turbine	6 .. 16

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 gas 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.

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.