Stator > Blade mean lines

► 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.

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 (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