Blade mean lines

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Blade mean lines

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► STATOR | Blade mean lines

In principle, the same features are available as for the mean lines of impellers.

For special radial diffuser blade shapes "Log. Spiral + Straight 2D" and "Circular + Free-form 2D" the mean line design is made in the frontal view. The mean lines are the inner vane sides (concave sides).

Initially the blade thickness is ignored for the mean line design (red/magenta in the sketch). The opposite side if the flow channel is generated by rotation and adding the blade thickness. The blade thickness is assumed as linear between sLE and sTE (see blade properties), if the thickness distribution was not defined yet. Otherwise the thickness distribution defined in the blade profile design is used. In the later blade profile design the thickness is added to one side of the mean line only.

Diffuser area has to be designed carefully in order to minimize losses. The quality of the diffuser design can be verified according to the following criteria (see panel Radial diffuser in Informational values area). Values outside the recommended range are displayed in red color.



Definition/ recommended range

Throat distance a3 (LE)

Throat width at inlet (leading edge)

a3 Optimum *

Optimal value: average of calculation by const. swirl and deceleration ratio

see blade properties

a3 Actual

Actual value: shortest distance from vane leading edge to neighboring vane


Outlet distance a4

Shortest distance from vane trailing edge to neighboring vane


Diffuser opening angle θ

Allowable diffusion angle

θ Maximum allowable

Max. allowable value to avoid flow separation depending on equivalent inlet radius and length


θ Actual

Actual value calculated by equivalent inlet radius, length, inlet and outlet area

Area ratio AR=A4/A3

Area or deceleration ratio

AR Optimum *

Optimal value

AR Actual

Actual value

AR < 3

Pressure recovery coeff. cp

Pressure recovery of the diffuser identified by a dimensionless coefficient

cp Ideal (loss-free) *

Pressure recovery in an ideal (loss-free) diffuser

cp Optimum *

Pressure recovery for optimal area ratio AR

cp Actual *

Pressure recovery in real diffuser (with energy losses)

based on test results; plotted in diagrams; target: cp,act = cp,opt

Diffuser efficiency ηD *

Diffuser efficiency

Inlet velocity ratio c3q/c2

Inlet deceleration ratio

c3q/c2 = 0.7...0.85 for low specific speed

Non-dimensional length L34/a3

Ratio of length to throat width

L3-4/a3 = 2.5...6

Throat aspect ratio b3/a3

Ratio of inlet width to throat width

b3/a3 = 0.8...2

Outlet rel. kin. energy *

Kinetic energy of diffuser outlet; to minimize losses in the overflow channels of multistage machines

Velocity ratio c4/c1m *

Ratio of outlet velocity to inlet velocity of downstream impeller  of multistage machines

c4/cm1 = 0.85...1.25

* for radial diffusers of pumps only