Mechanics of Fluids

demand for electric power – for example, at night – a dual-purpose machine
driven by an electric motor pumps water to a high-level reservoir. At periods
of peak demand the machine runs as a turbine and the electric motor as an
alternator so that power is fed back to the electricity supply.) In efficiency,
however, such dual-purpose machines are somewhat inferior to those intended
only for one-way conversion of energy. We shall refer to the similarities
between turbines and pumps again but, for the sake of explicitness, we shall
fix attention first on turbines.
13.3.1 Types of turbine
As we have seen, one classification of turbines is based on the predominant
direction of the fluid flow through the runner. In addition, turbines may be
placed in one of two general categories: (a) impulse and (b) reaction. (These
names have little justification except long usage: they should be regarded
as no more than useful labels.) In both types the fluid passes through a
runner having blades. The momentum of the fluid in the tangential direction
is changed and so a tangential force on the runner is produced. The runner
therefore rotates and performs useful work, while the fluid leaves it with
reduced energy. The important feature of the impulse machine is that there
is no change of static pressure across the runner. In the reaction machine,
on the other hand, the static pressure decreases as the fluid passes through
the runner.
For any turbine the energy held by the fluid is initially in the form of
pressure. For a turbine in a hydro-electric scheme, water comes from a highlevel
reservoir: in a mountainous region several hundred metres head may
thus be available, although water turbines are in operation in other situations
where the available head is as low as three metres or less. For a steam turbine,
the pressure of the working fluid is produced by the addition of heat in a
boiler; in a gas turbine pressure is produced by the chemical reaction of fuel
and air in a combustion chamber.
The impulse turbine has one or more fixed nozzles, in each of which the
pressure is converted to the kinetic energy of an unconfined jet. The jets
of fluid then impinge on the moving blades of the runner where they lose
practically all their kinetic energy and, ideally, the velocity of the fluid at
discharge is only just sufficient to enable it to move clear of the runner. As
already mentioned, the term impulse has little justification: constant pressure
would perhaps be better. In a reaction machine the change from pressure to
kinetic energy takes place gradually as the fluid moves through the runner,
and for this gradual change of pressure to be possible the runner must be
completely enclosed and the passages in it entirely full of the working fluid.
Machines in which the fluid undergoes an appreciable change of density
involve thermodynamic principles also, but in this book we shall confine our
attention to those using constant-density fluids and operating under steady
conditions. We shall deal first with impulse turbines since they are sufficiently
different from reaction machines to justify separate consideration, and are
in many ways simpler.
Turbines 597