1. CHAPTER FOUR TURBINES 4-1 Brief Description
A course in Turbomachinery…………………………………..….…Lecturer: Dr.Naseer Al-Janabi
CHAPTER FOUR
TURBINES
4-1 Brief Description
Turbines have been used for centuries to convert freely available
mechanical energy from rivers and wind into useful mechanical work,
usually through a rotating shaft. Whereas the rotating part of a pump is
called the impeller, the rotating part of a hydroturbine is called the
runner. When the working fluid is water, the turbomachines are called
hydraulic turbines or hydroturbines.
When the working fluid is air, and energy is extracted from the wind,
the machine is properly called a wind turbine. The word windmill should
technically be applied only when the mechanical energy output is used to
grind grain, as in ancient times. However, most people use the word
windmill to describe any wind turbine, whether used to grind grain, pump
water, or generate electricity. In coal or nuclear power plants, the working
fluid is usually steam; hence, the turbomachines that convert energy from
the steam into mechanical energy of a rotating shaft are called steam
turbines. A more generic name for turbines that employ a compressible
gas as the working fluid is gas turbine. (The turbine in a modern
commercial jet engine is a type of gas turbine).
IMPULSE TURBINES
In an impulse turbine, the fluid is sent through a nozzle so that most
of its available mechanical energy is converted into kinetic energy. The high
speed jet then impinges on bucket-shaped vanes that transfer energy to the
turbine shaft, as sketched in Fig The modern and most efficient type of
impulse turbine was invented by Lester A. Pelton (1829–1908) in 1878,

2. and the rotating wheel is now called a Pelton wheel in his honour. The
A course in Turbomachinery…………………………………..….…Lecturer: Dr.Naseer Al-Janabi
and the rotating wheel is now called a Pelton wheel in his honour. The
buckets of a Pelton wheel are designed so as to split the flow in half, and
turn the flow nearly 180° around (with respect to a frame of reference
moving with the bucket), as illustrated in Fig.4.1.
Fig.4.1 Impulse turbine: (a) side view of wheel and jet; (b) top view of
bucket; (c) typical velocity diagram.

3. P = pQu(vj - u)(1 - cosp) (4.2)
A course in Turbomachinery…………………………………..….…Lecturer: Dr.Naseer Al-Janabi
The mass flow would be m = pQ = pAj Vj .
P = pQ(u1vt1 - u2vt2) = pQ{uvj - u[u + (vj - u)cosp]} (4.1) Or
P = pQu(vj - u)(1 - cosp) (4.2)
Finally, we see from Power Eq. that the shaft power output P is zero
if u= 0 (wheel not turning at all). P is also zero if u= Vj (bucket moving at
the jet speed). Somewhere in between these two extremes lies the
optimum wheel speed. By setting the derivative of Eq.4.2 with respect to u
to zero, we find that this occurs when u= Vj /2 (bucket moving at half the
jet speed, as shown in Fig.4.2 below.
Fig.4.2 1 2
For a perfect nozzle, the entire available head would be converted to jet
velocity:
Vj = #2$% (4.3).
Actually, since there are 2 to 8 percent nozzle losses, a velocity coefficient
Cv is used: u* = Vj

4. Maximum efficiency occurs at - = 1 C � 0.47 .
A course in Turbomachinery…………………………………..….…Lecturer: Dr.Naseer Al-Janabi
Vj = Cv J2gH � Cv � 0.98. u
J2gH
Maximum efficiency occurs at - = 1 C �
Fig.4.3 below plotted for an ideal turbine (β= 180°, Cv =1.0) and for typical
working conditions (β= 160°, Cv = 0.94). The latter case predicts ηmax =85
percent at - = 0.47, but the actual data for a 24-in Pelton wheel test are
somewhat less efficient due to windage, mechanical friction, back
splashing, and non-uniform bucket flow. For this test ηmax =80 percent, and,
generally speaking, an impulse turbine is not quite as efficient as the
Francis or propeller turbines.
Fig. 4.3
- =
= peripheral velocity factor. 2 v

5. of turbine. Take velocity coefficient (Cv = 0.985).
A course in Turbomachinery…………………………………..….…Lecturer: Dr.Naseer Al-Janabi
EX: The mean bucket speed of a Peloton wheel is 15 m/s. The rate of flow of
water supplied by the jet under a head of 42 m is 1 m3/s. If the jet is
deflected by the buckets at an angle of 165˚. Find the power and efficiency
of turbine. Take velocity coefficient (Cv = 0.985).