Power Plant Engineering

GAS TURBINE POWER PLANT 291

T

φ

4 ′^4

3

Comp

Intak

e

Com

bust

ion

1

2

2 ′

Tu r b i n e

exhasut

9.8.1 Effect of Blade Friction

In a gas turbine there is always some loss of useful heat drop due to frictional resistance offered
by the nozzles and blades of gas turbine thus resulting drop in velocity. The energy so lost in friction is
converted into heat and, therefore, the gases get reheated to some extent. Therefore, the actual heat drop
is less than the adiabatic heat drop as shown in Fig. 9.26, where 1-2′ represents the adiabatic expansion
and 1-2 represents the actual expansion.

Actual heat drop = Kp(T 1 – T 2 )

Adiabatic heat drop = Kp(T 1 – T 2 ′)

Adiabatic efficiency of turbine

=

Actual heat drop

Adiabatic heat drop

=

12

12

[K (T T )]

[K (T T ) ]

−

− ′

p

p

=^12

12

(T T )

(T T )

−

− ′

For adiabatic process 1 – 2′

2

1

T

T

=

(1)/

2

1

P

P

γ− γ







In the compressor also reheating takes place, which

causes actual heat increase to be more than adiabatic heat in-

crease. The process 3-4 represents the actual compression while

3-4′ represents adiabatic compression.

Adiabatic heat drop = Kp(T′ 4 – T 3 )

Actual heat drop = Kp(T 4 – T 3 )

Adiabatic efficiency of compressor

=

3

43

K(T T)

K(T T)

pp

p

′−

′ − =

43

43

TT

TT

−

−

9.8.2 Improvement in Open Cycle

The open cycle for gas turbine is shown in Fig. 9.26. The fresh atmospheric is taken in at the

point 3 and exhaust of the gases after expansion in turbine takes place at the point 2. An improvement in

open cycle performance can he effected by the addition of a heat exchanger that raises the temperature

of the compressed air entering the turbine by lowering exhaust gas temperature that is a waste otherwise.

Less fuel is now required in the combustion chamber to attain a specified turbine inlet temperature. This

is called a regenerative cycle (Fig. 9.27).

This regenerative cycle is shown on T-φ diagram in Fig. 9.28. Where φ = entropy.

Fig. 9.26