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4.7 Slip
In the derivation of Euler’s pump equation it is assumed that the flow fol-
lows the blade. In reality this is, however, not the case because the flow
angle usually is smaller than the blade angle. This condition is called slip.
Nevertheless, there is close connection between the flow angle and blade
angle. An impeller has an endless number of blades which are extremely
thin, then the flow lines will have the same shape as the blades. When the
flow angle and blade angle are identical, then the flow is blade congruent,
see figure 4.15.
The flow will not follow the shape of the blades completely in a real impel-
ler with a limited number of blades with finite thickness. The tangential
velocity out of the impeller as well as the head is reduced due to this.
When designing impellers, you have to include the dierence between flow
angle and blade angle. This is done by including empirical slip factors in the
calculation of the velocity triangles, see figure 4.16. Empirical slip factors
are not further discussed in this book.
It is important to emphasize that slip is not a loss mechanism but just an
expression of the flow not following the blade.
Figure 4.15:
Blade congruent flow line: Dashed line.
Actualowline:Solidline.
Figure 4.16: Velocity triangles where ‘ indi-
cates the velocity with slip.
Pressure side
Suctionside