TITLE.PM5

(Ann) #1
GAS POWER CYCLES 685

dharm
\M-therm\Th13-6.pm5

For the compression process 1-2, we have

T
T

p
p

(^2) rp
1
2
1
(^11) 14 1
=F 625 14
HG
I
KJ


γ− − −
γ
γ
()γ (. )
.


. = 1.688


or T 2 = 300 × 1.688 = 506.4 K

Also, ηcomp. = TT
TT

21
21


′−
or 0.8 = 506 4 300
2 300

. −
T′−


or T 2 ′ = 506 4 300
08

.
.

− + 300 = 558 K

∴ Compressor work, Wcomp. = 1 × cp × (T 2 ′ – T 1 )
= 1 × 1.005 (558 – 300) = 259.29 kJ/kg. (Ans.)
For expansion process 3-4, we have

T
T

p
p

(^3) rp
4
3
4
(^11) 14 1
=F 625 14
HG
I
KJ


γ− − −
γ
γ
()γ (. )
.


. = 1.688


or T 4 =
T 3
688

1073




    1. 688
      = = 635.66 K




Also, ηturbine =
TT
TT

34
34

−′

or 0.8 =
1073
1073 635 66

−′ 4

T
.
or T 4 ′ = 1073 – 0.8 (1073 – 635.66) = 723.13 K
∴ Turbine work, Wturbine = 1 × cp × (T 3 – T 4 ′) (neglecting fuel mass)
= 1 × 1.005 (1073 – 723.13) = 351.6 kJ/kg. (Ans.)
Net work output, Wnet = Wturbine – Wcomp. = 351.6 – 259.29 = 92.31 kJ/kg
Heat supplied, Qs = 1 × cp × (T 3 – T 2 ′)
= 1 × 1.005 × (1073 – 558) = 517.57 kJ/kg. (Ans.)

Cycle efficiency, ηcycle = W
Qs

net.
.
= 92 31
517 57

= 0.1783 or 17.83%. (Ans.)

Turbine exhaust temperature, T 4 ′ = 723.13 K or 450.13°C. (Ans.)
The T-s diagram is shown in Fig. 13.56.
Example 13.38. Find the required air-fuel ratio in a gas turbine whose turbine and com-
pressor efficiencies are 85% and 80%, respectively. Maximum cycle temperature is 875°C. The
working fluid can be taken as air (cp = 1.0 kJ/kg K, γ = 1.4) which enters the compressor at 1 bar
and 27°C. The pressure ratio is 4. The fuel used has calorific value of 42000 kJ/kg. There is a loss
of 10% of calorific value in the combustion chamber. (GATE, 1998)
Solution. Given : ηturbine = 85% ; ηcompressor = 80% ; T 3 = 273 + 875 = 1148 K ; T 1 = 27 + 273
= 300 K ; cp = 1.0 kJ/kg K ; γ = 1.4 ; p 1 = 1 bar; p 2 = 4 bar (since pressure ratio is 4) ; C = 42000 kJ/
kg K, ηcc = 90% (since loss in the combustion chamber is 10%)
For isentropic compression 1-2, we have


T
T

p
p

2
1

2
1

1
=
F
HG

I
KJ

−γ
γ
= ()

.
4.

14 1
14


= 1.486
Free download pdf