fluid_mechanics

674 Chapter 12 ■ Turbomachines
Rotor
Nozzle
Bucket
(a)
(b)
F I G U R E 12.22 (a) Schematic diagram of a Pelton wheel turbine,
(b) photograph of a Pelton wheel turbine. (Courtesy of Voith Hydro, York, PA.)
The two basic types
of hydraulic turbines
are impulse
and reaction.
the ratio of static pressure drop that occurs across the rotor to static pressure drop across the turbine
stage, with larger rotor pressure drop corresponding to larger reaction.2 For hydraulic impulse turbines,
the pressure drop across the rotor is zero; all of the pressure drop across the turbine stage occurs
in the nozzle row. The Pelton wheel shown in Fig. 12.22 is a classical example of an impulse
turbine. In these machines the total head of the incoming fluid 1the sum of the pressure head, velocity
head, and elevation head2 is converted into a large velocity head at the exit of the supply nozzle
1or nozzles if a multiple nozzle configuration is used2. Both the pressure drop across the bucket 1blade2
and the change in relative speed 1i.e., fluid speed relative to the moving bucket2 of the fluid across
the bucket are negligible. The space surrounding the rotor is not completely filled with fluid. It is the
impulse of the individual jets of fluid striking the buckets that generates the torque.
For reaction turbines, on the other hand, the rotor is surrounded by a casing 1or volute2, which
is completely filled with the working fluid. There is both a pressure drop and a fluid relative speed
change across the rotor. As shown for the radial-inflow turbine in Fig 12.23, guide vanes act as nozzles
to accelerate the flow and turn it in the appropriate direction as the fluid enters the rotor. Thus,
part of the pressure drop occurs across the guide vanes and part occurs across the rotor. In many respects
the operation of a reaction turbine is similar to that of a pump “flowing backward,” although
such oversimplification can be quite misleading.
Both impulse and reaction turbines can be analyzed using the moment-of-momentum principles
discussed in Section 12.3. In general, impulse turbines are high-head, low-flowrate devices,
while reaction turbines are low-head, high-flowrate devices.
12.8.1 Impulse Turbines
Although there are various types of impulse turbine designs, perhaps the easiest to understand is the
Pelton wheel 1see Fig. 12.242. Lester Pelton 11829–19082, an American mining engineer during the