Mechanics of Fluids

632 Fluid machines
Fig. 13.31 Characteristic
curves for fixed rotational
speed N.
be used in parallel without instability. For liquids the primary function of
a pump is to increase the pressure, but fans are usually required to move
a large quantity of air or other gas without much change in pressure. The
volute is not then called upon to provide much conversion of velocity head
to static head, and for such applications an impeller with forward-facing
blades may be favoured.
13.4.3 The effects of non-uniform velocity distribution
The foregoing analysis has been based on the assumption that the velocities
at inlet and outlet of an impeller are uniform. This condition might
be approached if the volute were ideally matched to the impeller and if the
impeller had a very large number of blades. In practice, however, the number
of blades is limited, and the flow pattern in the passages between the blades
becomes distorted. One important cause of the distortion is this. The function
of the blades is to impart a whirl component of velocity to the fluid:
this requires a force in the whirl direction to be exerted on the fluid by the
blades: consequently at a given distance along a blade the pressure at the
forward side (xx in Fig. 13.32) is greater than that at the other side yy. In
other words, the individual blades act similarly to aerofoils and, as we saw
in Section 9.10.2, a circulation is necessary to produce the transverse force.
This circulation round a blade (indicated by the dotted loop in Fig. 13.32),
when superimposed on the main flow, has the effect of reducing the velocity
relative to the blade on the forward side, and increasing it on the other
side. Thus the velocity pattern indicated in the left-hand blade passage is
produced. In any diverging passage, rotating or not, there is a tendency for
flow to break away from the surface: in a pump impeller this tendency is
aggravated by the increased relative velocity at the back of the blades. Thus,
instead of following the direction of the blades, the fluid flows rather in the
direction of the dotted arrow in Fig. 13.32.
The inertia of the fluid particles also has an effect. The rotation of the
impeller requires a change of the (absolute) direction taken by the fluid
particles between two successive blades. But the particles, reluctant to do
the bidding of the impeller blades, try to lag behind the blade movement.