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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 + Freeform 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.
Name 
Description 
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 (lossfree) * 
Pressure recovery in an ideal (lossfree) 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 
Nondimensional length L34/a3 
Ratio of length to throat width 
L34/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