4
1 Working Principles
• The outlet stator component has various functions and may get various names
depending on the function to be emphasised:
− collecting water beyond the rotor: collector
− converting a part of the kinetic energy at the rotor outlet to pressure potential
energy: pressure at rotor outlet decreases, increasing the pressure difference
over the rotor: diffuser
− exploiting the downward height, in other words guiding the water to the
downward level. This generates a pressure decrease at the rotor outlet from
the gravitational potential energy: draught tube.
The latter function is often the most important one. For example: v 2 = 5 m/s (typical): v 2
2
2 12 5
/
.
=
J/kg; 2.5 m height difference between rotor outlet and tail water:
gravitational potential energy gΔz ≈ 25 J/kg. Extraction of the energy associated to
the downward height is efficient. Kinetic energy recovery is a process involved with
high losses. So, mostly the term draught tube is used.
The turbine discussed here is called axial, since streamlines, in principle, lie on
cylinders with an axis coinciding with the centre of the machine shaft (real machines
are mostly slightly conic). The velocity vectors mainly have axial and tangential
components (radial components are very small). There are also machines with flow
occurring approximately in planes perpendicular to the shaft; in other words, the
velocity vectors mainly have radial and tangential components (the axial components are very small). These machines are called radial. Their working principle is
more complex. These machines are discussed later in this chapter. Also intermediate
forms exist, called diagonal machines or mixed-flow machines. There are still other
types such as tangential (peripheral) and diametrical (cross-flow) machines. For
the present, the fundamental discussion is limited to axial machines, as the study of
their working principle is the simplest.
1.2.2 Axial Pump
Figure 1.3 shows an axial pump for use as a submerged pump in a vertical pipe or a
pit. The working principle is similar to that of an axial turbine, but with an inverse
sense of the energy exchange. Figure 1.3 shows also a cylindrical section, drawn
with horizontal flow direction.
The following parts can be distinguished.
• Inlet: guiding the fluid towards the rotor and accelerating the fluid, as with a
turbine. There are no guide vanes. In principle, the flow direction stays axial, but
the fluid swirls somewhat due to the rotation of the rotor. In some pumps, preswirl is prevented by guide vanes in the axial direction.
• Rotor (or impeller): transferring energy to the fluid. The turning of the relative
velocity at the rotor inlet w 1 onto a more axial direction at outlet w 2 generates a
lift force in the indicated sense. The lift has a tangential component in the sense
opposite to the blade speed u. Work to be supplied to the rotor by a driving
motor corresponds with this. Energy is transferred from the rotor to the fluid. The
1 Working Principles
• The outlet stator component has various functions and may get various names
depending on the function to be emphasised:
− collecting water beyond the rotor: collector
− converting a part of the kinetic energy at the rotor outlet to pressure potential
energy: pressure at rotor outlet decreases, increasing the pressure difference
over the rotor: diffuser
− exploiting the downward height, in other words guiding the water to the
downward level. This generates a pressure decrease at the rotor outlet from
the gravitational potential energy: draught tube.
The latter function is often the most important one. For example: v 2 = 5 m/s (typical): v 2
2
2 12 5
/
.
=
J/kg; 2.5 m height difference between rotor outlet and tail water:
gravitational potential energy gΔz ≈ 25 J/kg. Extraction of the energy associated to
the downward height is efficient. Kinetic energy recovery is a process involved with
high losses. So, mostly the term draught tube is used.
The turbine discussed here is called axial, since streamlines, in principle, lie on
cylinders with an axis coinciding with the centre of the machine shaft (real machines
are mostly slightly conic). The velocity vectors mainly have axial and tangential
components (radial components are very small). There are also machines with flow
occurring approximately in planes perpendicular to the shaft; in other words, the
velocity vectors mainly have radial and tangential components (the axial components are very small). These machines are called radial. Their working principle is
more complex. These machines are discussed later in this chapter. Also intermediate
forms exist, called diagonal machines or mixed-flow machines. There are still other
types such as tangential (peripheral) and diametrical (cross-flow) machines. For
the present, the fundamental discussion is limited to axial machines, as the study of
their working principle is the simplest.
1.2.2 Axial Pump
Figure 1.3 shows an axial pump for use as a submerged pump in a vertical pipe or a
pit. The working principle is similar to that of an axial turbine, but with an inverse
sense of the energy exchange. Figure 1.3 shows also a cylindrical section, drawn
with horizontal flow direction.
The following parts can be distinguished.
• Inlet: guiding the fluid towards the rotor and accelerating the fluid, as with a
turbine. There are no guide vanes. In principle, the flow direction stays axial, but
the fluid swirls somewhat due to the rotation of the rotor. In some pumps, preswirl is prevented by guide vanes in the axial direction.
• Rotor (or impeller): transferring energy to the fluid. The turning of the relative
velocity at the rotor inlet w 1 onto a more axial direction at outlet w 2 generates a
lift force in the indicated sense. The lift has a tangential component in the sense
opposite to the blade speed u. Work to be supplied to the rotor by a driving
motor corresponds with this. Energy is transferred from the rotor to the fluid. The