Chapter 4 Gas Turbine

AE 2031 APPLIED THERMODYNAMICS S. Y. B. Tech.
GAS TURBINE
Prof. Aniket Suryawanshi
Asst. Prof. Automobile Engg. Dept.
R. I. T. Rajaramnagar

OUTLINE OF CHAPTER
Working Principle and Applications of gas turbine.
Types of Gas turbine.
Modified Bryton cycle: Regeneration
Modified Bryton cycle: Reheat
Modified Bryton cycle: Intercooling.
Numerical on gas turbine work ratio and efficiency.
Gas turbine irreversibility's and losses.

The simple gas turbine power plant mainly consist of a gas turbine coupled to a axial air
compressor and combustion chamber which is placed between the compressor and turbine in
the fuel circuit.
The gas turbine use gas as working medium by which heat energy is converted into
mechanical work or thrust. Gas is produced in the engine by the combustion of the fuel in the
combustion chamber.
Gas turbine is a rotary internal combustion engine.
Hot gases coming out from Combustion Chamber discharges over the blades of turbine
wheel. It causes the spinning action of the turbine blade about the shaft.
GAS TURBINE POWER PLANT - INTRODUCTION

Atmospheric air is compressed to high pressure and temperature in the axial compressor.
This high pressure and temperature air is passed through nozzle in the CC, where fuel is
injected in the form of spray and combustion takes place. Resulting combustion products
enters in the expansion zone where it expands through a turbine to produce shaft work.
The exhaust gases leaving the turbine are finally discharged into the atmosphere.
Most of the power plant uses it to the run the auxiliaries devices of the plant such as cooling
fan, water pumps and the generator itself also.
GAS TURBINE POWER PLANTS – WORKING PRINCIPLE

GAS TURBINE POWER PLANTS – ADVANTAGES
Compared to Steam-Turbine and Diesel Propulsion Systems, Gas Turbine offers :
• Greater Power for a given size and weight,
• High Reliability,
• Long Life,
• More Convenient Operation.
• Engine Start-up Time reduced from 4 hrs to less than 2 min…!!
• Less area requires for storage of fuel.
• Less maintenance cost,
• Simple construction, no need of boiler, condenser as in the other cases.
• Kerosene, Paraffin, benzene and powdered coal like cheaper fuels used.
• Less requirement of water, so can be installed at water scarcity area.
• Less pollution.
• Easy handling.

GAS TURBINE POWER PLANTS – DISADVANTAGES
• Manufacturing a turbine blades is much difficult and costly.
• For same power o/p gas turbine produces 5 times more exhaust gases
than I. C. engine.
• Gas Turbine blades require special cooling system.
• Lower thermal efficiency as 15% to 20% in gas turbine as 25% to 30%
in I. C. engine. (66% of the power developed is used to drive the
compressor. Therefore the gas turbine unit has a low thermal efficiency.)
• High frequency noise from the compressor is objectionable.
• Requires Special metals and alloys to cast parts of the gas turbine. ( The
running speed of gas turbine is in the range of (40,000 to 100,000 rpm)
and the operating temperature is as high as 1100 – 12600C.)

Two Major Application Areas :
1. Aircraft Propulsion
2. Electric Power Generation.
Electric Power GenerationAircraft Propulsion
GAS TURBINE POWER PLANTS – APPLICATIONS

GAS TURBINE POWER PLANTS – APPLICATIONS

GAS TURBINE POWER PLANTS – AEROSPACE APPLICATIONS

• Air at ambient conditions is drawn into the compressor, where its temperature and
pressure are raised.
• The high pressure air proceeds into the combustion chamber, where the fuel is
burned at constant pressure.
• The high-temperature gases then enter the turbine where they expand to atmospheric
pressure while producing power output.
Some of the output power is used to drive
the compressor.
The exhaust gases leaving the turbine are
thrown out (not re-circulated), causing the
cycle to be classified as an open cycle.
BRAYTON CYCLE: OPEN CYCLE FOR GAS-TURBINE ENGINES
Gas turbines usually operate on an open cycle.

• The compression and expansion
processes remain the same, but the
combustion process is replaced by a
constant-pressure heat addition
process from an external source.
• The exhaust process is replaced by a
constant-pressure heat rejection
process to the ambient air.
BRAYTON CYCLE: CLOSED CYCLE FOR GAS-TURBINE ENGINES
The open gas-turbine cycle can be modelled as a closed cycle, using the air-standard
assumptions.

The ideal cycle that the working fluid undergoes in
the closed loop is the Brayton cycle. It is made up
of four internally reversible processes:
1-2 Isentropic compression;
2-3 Constant-pressure heat addition;
3-4 Isentropic expansion;
4-1 Constant-pressure heat rejection.
The T-s and P-v diagrams of an ideal Brayton cycle.
Note: All four processes of the Brayton cycle are
executed in steady-flow devices thus, they should be
analyzed as steady-flow processes.
BRAYTON CYCLE: IDEAL CYCLE FOR GAS-TURBINE ENGINES

The energy balance for a steady-flow process can
be expressed, on a unit–mass basis, as
The heat transfers to and from the working fluid
are:
The thermal efficiency of the ideal Brayton cycle,
is the pressure ratio.where
BRAYTON CYCLE: IDEAL CYCLE THERMAL EFFICIENCY
Processes 1-2 and 3-4 are Isentropic,
P2 = P3 and P4 = P1.
1 1
2 2 3 3
1 1 4 4
T P P T
T P P T
 
 
 
   
        
   
, 1
1
1th Brayton
pr


  
 
 
 
  

14
The combustion process is replaced by a
heat-addition process in ideal cycles.
Air-standard assumptions:
1.The working fluid is air, which continuously
circulates in a closed loop and always behaves
as an ideal gas.
2.All the processes that make up the cycle are
internally reversible.
3.The combustion process is replaced by a
heat-addition process from an external source.
4.The exhaust process is replaced by a heat-
rejection process that restores the working
fluid to its initial state.
Cold-air-standard assumptions: When the working fluid is considered to be
air with constant specific heats at room temperature (25°C).
Air-standard cycle: A cycle for which the air-standard assumptions are
applicable.
AIR-STANDARD ASSUMPTIONS

Inlet conditions to a Brayton cycle are 1 bar and 300 K. The cycle pressure ratio is 6.5.
The temperature at the inlet to the turbine is 1500 K. Calculate the performance
parameters of the cycle.
Process 1→2 :  
1
1.4 1
2 2 1.4
2
1 1
(300 ) 6.5
512.132
T P
T K
T P
K



 
   
 

rp =6.5
300 K
1500 K
Process 3→4 :  
1
1.4 1
3 3 1.4
4
4 4
(1500 ) 6.5
878.679
T P
T K
T P
K



 
   
 

EXAMPLE ON IDEAL BRAYTON CYCLE

Compressor Work :
     2 1 2 1 1.005 (512.132 300) 213.19
.
C P
kJ kJ
W h h C T T K
kg K kg
    
         
   
Steady – Flow – Energy – Equation :
Heat Input :
     3 2 3 2 1.005 (1500 512.132) 992.81
.
in P
kJ kJ
Q h h C T T K
kg K kg
    
         
   
Turbine Work :
     3 4 3 4 1.005 (1500 878.679) 624.43
.
T P
kJ kJ
W h h C T T K
kg K kg
    
         
   
Heat Out :
     4 1 4 1 1.005 (878.679 300) 581.57
.
out P
kJ kJ
Q h h C T T K
kg K kg
    
         
   
EXAMPLE ON IDEAL BRAYTON CYCLE

Net Power In :  624.43 213.19 411.24T C
kJ
W W W
kg
    
      
 
Plant Efficiency :
 
 
411.24 /
41.42 %
992.81 /
th
in
kJ kgW
kJ kgQ



   ….ANS
Alternatively;
 
1.4 11
1.4
1 1
1 1 41.42 %
6.5
th
pr


    
  
  
    
  
….ANS
EXAMPLE ON IDEAL BRAYTON CYCLE…CNTD

18
• Some pressure drop occurs during the heat-
addition and heat rejection processes.
• The actual work input to the compressor is more,
and the actual work output from the turbine is
less, because of irreversibilities or frictional
effects .
Deviation of actual compressor and turbine
behavior from the idealized isentropic behavior can
be accounted for by utilizing isentropic efficiencies
of the turbine and compressor.
Turbine:
Compressor:
ACTUAL GAS-TURBINE CYCLES…CNTD

300 K
1500 K
rp =6.5
In the plant of Example 1, let the compressor and the turbine have the isentropic
efficiencies of 0.8 each. Calculate the performance parameters of the cycle.
From the earlier results,
kg
kJ
W
kg
kJ
Q
kg
kJ
W
KT
KT
T
in
C
43.624
81.992
19.213
679.878
132.512
4
2








EXAMPLE ON ACTUAL BRAYTON CYCLE

 
  12
12
12
12
12
12
,
,
TT
TT
TTC
TTC
hh
hh
W
W
A
S
AP
SP
A
S
ActC
TheorC
C








 


300 K
1500 K
rp =6.5
300
300132.512
8.0
2 


AT
KT A 165.5652 
Similarly;
 
  S
A
SP
AP
S
A
TheorT
ActT
T
TT
TT
TTC
TTC
hh
hh
W
W
43
43
43
43
43
43
,
,








 


679.8781500
1500
8.0
4



AT KT A 94.10024 
EXAMPLE ON ACTUAL BRAYTON CYCLE…CNTD

Compressor Work :
       












kg
kJ
K
Kkg
kJ
TTChhW APAC 49.266300165.565
.
005.11212
Turbine Work :
       












kg
kJ
K
Kkg
kJ
TTChhW APAT 54.49994.10021500
.
005.14343
Heat Input :
       













kg
kJ
K
Kkg
kJ
TTChhQ APAin 51.939165.5651500
.
005.12323
Net Power In :   







kg
kJ
WWW CT 05.23349.26654.499
Plant Efficiency : ….ANS%8.24
51.939
05.233
 

in
th
Q
W


Heat Input :
       













kg
kJ
K
Kkg
kJ
TTChhQ APAin 51.939165.5651500
.
005.12323
Alternatively;









kg
kJW
W
C
SC
AC 49.266
8.0
19.213,
,

Actual Compressor Work :








kg
kJ
WW STTAT 54.49943.6248.0,, Actual Turbine Work :
Net Power In :   







kg
kJ
WWW CT 05.23349.26654.499
Plant Efficiency : ….ANS%8.24
51.939
05.233
 

in
th
Q
W


The fraction of the turbine work
used to drive the compressor is
called the back work ratio.
BWR is defined as the ratio of compressor work to the
turbine work
𝑟𝑏𝑤 =
𝑤𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑜𝑟
𝑤𝑡𝑢𝑟𝑏𝑖𝑛𝑒
The BWR in gas turbine power plant is very high, normally
one-half of turbine work output is used to drive the
compressor
BACK WORK RATIO

The fraction of the turbine work that
becomes the net work is called the
work ratio.
Work Ratio is defined as the ratio of net work to the turbine
work
𝑟𝑤 =
𝑤 𝑛𝑒𝑡
𝑤𝑡𝑢𝑟𝑏𝑖𝑛𝑒
WORK RATIO

The early gas turbines (1940s to 1959s) found only limited use despite their
versatility and their ability to burn a variety of fuels, because its thermal
efficiency was only about 17%. Efforts to improve the cycle efficiency are
concentrated in three areas:
1. Regeneration by preheating the air leaving the compressor with turbine
exhaust gases (regeneration or recuperation).
2. Reheating of gases after each stage of expansion to obtain more work
from the turbine (reheating).
3. Intercooling during compression stages to reduce the work input to the
compressor (intercooling).
IMPROVEMENTS OF GAS TURBINE’S PERFORMANCE

BRAYTON CYCLE WITH REGENERATION
• Temperature of the exhaust gas leaving the turbine is
higher than the temperature of the air leaving the
compressor.
• The air leaving the compressor can be heated by the
hot exhaust gases in a counter-flow heat exchanger
(a regenerator or recuperator) – a process called
regeneration (Fig. 9-38 & Fig. 9-39).
• The thermal efficiency of the Brayton cycle
increases due to regeneration since less fuel is used
for the same work output.
Note:
The use of a regenerator is
recommended only when
the turbine exhaust
temperature is higher than
the compressor exit
temperature.

Effectiveness of the regenerator,
Effectiveness under cold-air standard
assumptions,
Thermal efficiency under
cold-air standard
assumptions,
EFFECTIVENESS OF THE REGENERATOR
Assuming the regenerator is well insulated and changes in kinetic and potential
energies are negligible, the actual and maximum heat transfers from the exhaust
gases to the air can be expressed as
If written in terms of
temperatures only, it is
also called the thermal
ratio

Thermal efficiency of Brayton cycle
with regeneration depends on:
a) ratio of the minimum to
maximum temperatures, and
b) the pressure ratio.
Regeneration is most effective at
lower pressure ratios and small
minimum-to-maximum temperature
ratios.
FACTORS AFFECTING THERMAL EFFICIENCY
Can regeneration be
used at high
pressure ratios?

BRAYTON CYCLE WITH REHEATER
•A high pressure and a low pressure
turbine are used in a Reheat Brayton Cycle
•A reheater is a heat exchanger that increases
the power output without increasing the maximum
operating temperature but it does not increase
the efficiency of the cycle
•The capital cost to build a reheater alone cannot
be justified because the thermal efficiency
• does not increase.

•Multistage compression with
intercooling improves the efficiency of
a compression process.
•A Brayton Cycle with Intercooling uses
two or more compression stages with one
or more intercoolers, as shown below.
The power requirement
for compression is reduced,
but QH also increases.
•Again, the capital cost to build
an intercooled compressor alone cannot
be justified because the thermal
efficiency does not increase.
BRAYTON CYCLE WITH INTERCOOLING

A gas-turbine engine
with two-stage
compression with
intercooling, two-stage
expansion with
reheating, and
regeneration and its T-s
diagram.
For minimizing work input to compressor and
maximizing work output from turbine:
Tmax limited by materials,
Tmin limited by environment
BRAYTON CYCLE WITH INTERCOOLING, REHEATING & REGENERATION

32
The work input to a two-stage compressor is
minimized when
a) equal pressure ratios are maintained across
each stage.
b) Complete intercooling is performed
This procedure also maximizes the turbine work
output.
Thus, for best performance we have,
CONDITIONS FOR BEST PERFORMANCE
𝑇3 = 𝑇1

The net work output of a gas-turbine cycle
can be increased by either:
a) decreasing the compressor work, or
b) increasing the turbine work, or
c) both.
The compressor work input can be decreased
by carrying out the compression process in
stages and cooling the gas in between (Fig.
9-42), using multistage compression with
intercooling.
The work output of a turbine can be
increased by expanding the gas in stages
and reheating it in between, utilizing a
multistage expansion with reheating.

As the number of compression and expansion
stages increases, the gas-turbine cycle with
intercooling, reheating, and regeneration
approaches the Ericsson cycle.
Intercooling and reheating always
decreases thermal efficiency unless
accompanied by regeneration. Why?
Therefore, in gas turbine power
plants, intercooling and reheating are
always used in conjunction with
regeneration.

• The thermal efficiency of an ideal Brayton cycle
depends on the pressure ratio, rp of the gas
turbine and the specific heat ratio, k of the
working fluid.
• The thermal efficiency increases with both of
these parameters, which is also the case for
actual gas turbines.
PARAMETERS AFFECTING THERMAL EFFICIENCY

• The highest temperature in the cycle is
limited by the maximum temperature
that the turbine blades can withstand.
This also limits the pressure ratios that
can be used in the cycle.
• The air in gas turbines supplies the
necessary oxidant for the combustion of
the fuel, and it serves as a coolant to
keep the temperature of various
components within safe limits. An air–
fuel ratio of 50 or above is not
uncommon.

Thank You !

Chapter 4 Gas Turbine

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