Warsaw University of Technology

The Faculty of Power and Aeronautical Engineering

Institute of Heat Engineering

Intermediate Engineering Project

Analysis of work cycle of turbofan engine equipped with

intercooler

Supervisor:

dr. inż. Paweł Oleszczak

Prepared by:

Kaushik Gogoi

Warsaw, 2012

i

List of Contents

1. Introduction........................................................................................................................1

2. Description of the thermodynamic cycle............................................................................1

2.1 Brayton cycle ..............................................................................................................1

3. The intercooler....................................................................................................................2

3.1 Description of an intercooler......................................................................................2

4. Analysis of the work cycle...................................................................................................4

4.1 Assumptions for calculation.......................................................................................4

4.2 Calculations (Ideal case).............................................................................................5

4.3 Results.......................................................................................................................6

5. Plots ..................................................................................................................................12

6. Conclusions........................................................................................................................15

References...............................................................................................................................16

ii

Nomenclature

LPC Low pressure compressor

HPC High pressure compressor

CP specific heat at constant pressure

AFR air/fuel ratio

h enthalpy

LHV low heating value of fuel

M mach number

m

mass flow rate

P pressure

P* total pressure

SFC specific fuel consumption

T* total temperature

V absolute velocity

k ratio of specific heats, v

p

c

c

ηth thermal Efficiency

πc compressor pressure ratio (overall pressure ratio)

τ ratio of total temperatures

1

1. Introduction:

Enhancing the performance of an engine without affecting its efficiency is of paramount

importance and the ultimate goal while designing an engine for the aviation industry.

Continuous research has been done and many novel techniques have been applied to

achieve this goal.

One such reliable and proven technique is to employ an intercooler. Although it is still a

relatively new concept for aero engines, intercooled gas turbines have been employed in

the power generation industry and also in marine gas turbines to a great effect - thus

offering a consistent design solution for aero engines as well.

In this project, an analysis of a intercooled turbofan engine will be performed. In other

words, observations will be made on how an intercooler affects the compression cycle

and the efficiency of a turbofan engine.

2. Description of the thermodynamic cycle:

2.1 Brayton cycle The typical (ideal) Joule-Brayton cycle consists of four processes: Two isentropic

processes (Compression & expansion) & two isobaric processes(combustion and

cooling).

Figure 1: A typical Brayton cycle in T,s diagram and its schematic

2

In the particular case of a turbojet engine, the intake air is compressed by the

compressor(step 1→2) and mixed with fuel and converted into high temperature flue

gas by means of combustion(step 3→4) . The nozzle then converts the internal energy of

the hot gas into kinetic energy, or simply thrust.

The main parameter which is an indication of the effectiveness of this cycle and the

turbojet engine in general, is its thermal efficiency, ηth. Thermal efficiency can be

described as the ratio of the amount of energy converted to mechanical energy to the

thermal energy supplied to the system.

or,

3. The intercooler

3.1 Description of the intercooler The intercooler is used to reduce the temperature at the high pressure compressor. It

effectively lowers the work input of the compression process.

Figure 2: Schematic Layout of intercooled engine

After passing through the inlet and the fan, the air is compressed in the low pressure

compressor to some intermediate pressure (P0→P1). This fluid then passes through an

intercooler where it is cooled down to a lower temperature in an isobaric process. This

cooler fluid is then compressed further in an high pressure compressor.

3

Two main types of intercoolers exist : an inline design and the more conventional off

the flow path design. The typical components of an inline intercooler is illustrated in

the following figure:

Figure 3: Typical design of an inline intercooler

In an inline intercooler, fins (4) are located on struts (3) and house between an inner(2) an outer

casing (1). The struts themselves have coolant flow paths (5) located inside them. The fins increase

the heat transfer area and act as heat sinks. Cooling fluid is usually air (for aviation purposes).

The cold flow for the intercooler is extracted from the bypass flow by an additional flow splitter in

the bypass stream.

Due to intercooling, the ideal thermodynamic cycle is changed to one where the inlet temperature

of the HPC is greatly reduced. The main advantage of an inline intercooler over a conventional one

is that it doesn't divert the airflow away from the main flow path, hence reduces pressure loss.

Figure 4: The modified thermodynamic cycle (after intercooling)

After passing through the LPC (step 0→1), the intercooler cools down the air flow to a lower

temperature while maintaining the same pressure (step 1→2). The compressed cooled air has

lesser volume - so it allows HPC size to be made smaller as well. This lowers the work input needed

for the HPC to compress the air and increases the mass flow - resulting in higher specific power.

4

4. Analysis of the work cycle

4.1 Assumptions for calculation For the purpose of the analysis, the following simplified assumptions were made:

No Quantity Notation Value Unit

1 Mach Number 0.82

2 Specific heat capacity at constant pressure

(compressor)

Cpc 1000 J/kg.K

3 Specific heat capacity at constant pressure (turbine)

Cpt 1050 J/kg.K

4 Ratio of specific heats k 1.4

5 Lower heating value LHV 45 MJ/kg

6 Compressor & Turbine efficiency

ηLPC , ηHPC, ηT

0.90

7 Turbine inlet temperature T4 1250 K

Moreover, standard initial calculations were made for the ambient temperatures and

pressures for typical cruising altitudes:

Altitude

To [k] Po [bar] Density (Kg/m3)

8500

232.9

0.33

0.49509

9000

229.65

0.307

0.466348

9500

226.4

0.285

0.438901

10000

223.15

0.264

0.412707

12000

216.65

0.193

0.310828

Temperature T2 should be as low as possible, hence for our calculations, we'll assume T2=T1.

5

4.2 Calculations (Ideal case)

1. Inlet temperature, T0*:

2. inlet pressure, P0*:

3. Intermediate temperature, T1 :

4. Specific work of the Low pressure compressor, WLPC :

5. Specific work of the High pressure compressor, WHPC :

6. Intercooler effectiveness, x :

hence, intercooler effectiveness was assumed in a range of 0.5 to 1 ; with x=0 denoting the lack of an

intercooler. It is to be noted that intercooler effectiveness of x=1 is highly improbable and is

considered for analytical purposes only.

7. Temperature after intercooler, T2 (using equation 6):

6

8. Specific work by the turbine, WT :

9. Net work, Wnet :

10. Heat added, Qadd :

11. Thermal Efficiency, ηth :

12. Air to fuel Ratio, AFR :

13. Specific fuel consumption, SFC :

4.3 Results:

A. For approximation, πc was divided between πLPC and πHPC as follows:

πC 25 27.5 30 32.5 35 40

πLPC + πf 4 4.2 4.4 4.6 4.8 5

πHPC 6.25 6.547619 6.818182 7.065217 7.291667 8

7

B. Results for compressor specific work:

Using formulae (1) & (2):

πc 25 27.5 30 32.5 35 40

Altitude [m]

T0*=T2* [k]

P0* T1*

8500 264.2204 0.33029 406.898 413.022 418.941 424.671 430.225 435.617

9000 260.5333 0.30727 401.220 407.258 413.095 418.745 424.222 429.538

9500 256.8463 0.28525 395.542 401.495 407.249 412.819 418.218 423.459

10000 253.1592 0.26424 389.864 395.731 401.403 406.893 412.215 417.381

12000 245.7851 0.19317 378.507 384.204 389.710 395.040 400.208 405.223

Using formula (4):

πc 25 27.5 30 32.5 35 40

altitude WLPC [J/kg]

8500 142677.3 148801.3 154720.5 160450.4 166005.02 171396.7

9000 140686.3 146724.9 152561.4 158211.4 163688.51 169005

9500 138695.4 144648.5 150402.4 155972.4 161371.99 166613.2

10000 136704.4 142572 148243.3 153733.4 159055.48 164221.5

12000 132722.4 138419.1 143925.2 149255.4 154422.45 159438

From formula (7): Temperature after intercooler

T2*

x 0.5 0.6 0.7 0.8 0.9 1 0

8500 341.5806 326.1086 310.6365 295.1645 279.6924 264.2204 418.9408

9000 336.814 321.5579 306.3018 291.0456 275.7895 260.5333 413.0947

9500 332.0475 317.0072 301.967 286.9267 271.8865 256.8463 407.2486

10000 327.2809 312.4565 297.6322 282.8079 267.9835 253.1592 401.4025

12000 317.7477 303.3552 288.9627 274.5701 260.1776 245.7851 389.7103

8 Temperature after HPC:

T3*

πC 25 27.5 30 32.5 35 40

8500 662.7928 681.0896 698.234 714.3861 729.6736 758.0498

9000 653.5438 671.5853 688.4905 704.4172 719.4914 747.4716

9500 644.2949 662.0811 678.747 694.4483 709.3092 736.8934

10000 635.046 652.5768 669.0035 684.4794 699.1269 726.3152

12000 616.5481 633.5683 649.5165 664.5417 678.7625 705.1589

Using formula (5) , for a particular Pressure ratio, πc = 30:

WHPC [J/kg] (πC =30)

x 0.5 0.6 0.7 0.8 0.9 1 0

8500 277279 264719.5 252160 239600.5 227041.07 214481.6 340076.3

9000 273409.7 261025.5 248641.3 236257 223872.83 211488.6 335330.7

9500 269540.4 257331.4 245122.5 232913.5 220704.59 208495.6 330585.2

10000 265671.1 253637.4 241603.7 229570 217536.35 205502.7 325839.6

12000 257932.5 246249.4 234566.2 222883 211199.87 199516.7 316348.4

Therefore, compressor specific work (WC = WHPC + WLPC) :

WC [J/kg] (πC =30)

x 0.5 0.6 0.7 0.8 0.9 1 0

8500 431999.4 419439.9 406880.5 394321 381761.53 369202.1 494796.8

9000 425971.1 413586.9 401202.7 388818.5 376434.24 364050 487892.2

9500 419942.8 407733.8 395524.9 383315.9 371106.95 358898 480987.5

10000 413914.4 401880.7 389847 377813.4 365779.67 353746 474082.9

12000 401857.8 390174.6 378491.4 366808.3 355125.09 343441.9 460273.6

9 C. Results for turbine specific work:

From formula (8):

WT [J/kg]

πC 25 27.5 30 32.5 35 40

1781897 1863697 1940344 2012556 2080901.4 2207763

For further calculations, the value of WT = 1940344 J/kg (πC =30) was considered.

D. Results for Net specific work:

Using formula (9):

Wnet [J/kg] (πC =30)

x 0.5 0.6 0.7 0.8 0.9 1 0

8500 1508345 1520904 1533464 1546023 1558582.8 1571142 1445548

9000 1514373 1526757 1539142 1551526 1563910.1 1576294 1452452

9500 1520402 1532611 1544819 1557028 1569237.4 1581446 1459357

10000 1526430 1538464 1550497 1562531 1574564.7 1586598 1466261

12000 1538487 1550170 1561853 1573536 1585219.3 1596902 1480071

E. Results for heat added:

Using formula (10):

Qadd [J]

x 0.5 0.6 0.7 0.8 0.9 1 0

8500 2972006.764 2745190 2518373 2291556 2064739 1837922 4106091

9000 2948849.198 2725197 2501546 2277894 2054242 1830590 4067108

9500 2925691.633 2705205 2484718 2264232 2043745 1823258 4028125

10000 2902534.068 2685213 2467891 2250569 2033248 1815926 3989142

12000 2856218.937 2645228 2434236 2223245 2012254 1801262 3911175

10

F. Results for Thermal efficiency:

Using formula (11):

Efficiency ηth

x 0.5 0.6 0.7 0.8 0.9 1 0

8500 0.507517 0.554025 0.608911 0.674661 0.754857 0.854847 0.35205

9000 0.513547 0.560237 0.615276 0.681123 0.7613076 0.861085 0.357122

9500 0.519673 0.566541 0.621728 0.687663 0.7678244 0.867374 0.362292

10000 0.525896 0.572939 0.628268 0.694282 0.7744086 0.873713 0.367563

12000 0.538644 0.586025 0.641619 0.707765 0.787783 0.886546 0.378421

G. Results for different pressure ratios (assuming x = 0.5) :

In order to compare the efficiencies for different pressure ratios, a minimum intercooler effectiveness

x=0.5 was considered:

Wnet (for x = 0.5)

πc 25 27.5 30 32.5 35 40

8500 1349898 1431698 1508345 1580556 1648902 1775764

9000 1355926 1437726 1514373 1586585 1654930 1781792

9500 1361955 1443754 1520402 1592613 1660959 1787820

10000 1367983 1449783 1526430 1598641 1666987 1793849

12000 1380040 1461839 1538487 1610698 1679044 1805905

Efficiency ηth

8500 0.454204 0.481728 0.507517 0.531815 0.554811 0.597497

9000 0.459815 0.487555 0.513547 0.538035 0.561212 0.604233

9500 0.465515 0.493474 0.519673 0.544354 0.567715 0.611076

10000 0.471306 0.499489 0.525896 0.550774 0.574321 0.618028

12000 0.48317 0.511809 0.538644 0.563927 0.587855 0.632271

11

H. Specific fuel consumption:

SFC [kg/kN.s]

8500 0.15763 0.144398 0.131382 0.118578 0.10598 0.093584 0.227241

9000 0.155779 0.142797 0.130023 0.117453 0.105082 0.092906 0.224013

9500 0.153943 0.141208 0.128674 0.116336 0.10419 0.092232 0.220816

10000 0.152121 0.139631 0.127334 0.115227 0.103305 0.091563 0.21765

12000 0.148521 0.136513 0.124685 0.113032 0.101551 0.090238 0.211405

I. Results for different turbine inlet temperatures( T4):

A range of turbine inlet temperatures between 1200k to 1600k was assumed:

Qadd

T4 1200 1250 1300 1400 1500 1600

8500 2919507 2972007 3024507 3129507 3234507 3339507

9000 2896349 2948849 3001349 3106349 3211349 3316349

9500 2873192 2925692 2978192 3083192 3188192 3293192

10000 2850034 2902534 2955034 3060034 3165034 3270034

12000 2803719 2856219 2908719 3013719 3118719 3223719

WT -(in terms of T4)

1862731 1940344 2017958 2173186 2328413 2483641

Wnet -(in terms of T4)

8500 1430731 1520904 1611078 1778865 1946652 2114439

9000 1436759 1526757 1616755 1784367 1951979 2119591

9500 1442788 1532611 1622433 1789870 1957306 2124743

10000 1448816 1538464 1628111 1795372 1962634 2129895

12000 1460873 1550170 1639467 1806377 1973288 2140199

12

Efficiency ηth

T4 1200 1250 1300 1400 1500 1600

8500 0.490059 0.511743 0.532675 0.568417 0.601839 0.633159

9000 0.496059 0.517747 0.538676 0.574426 0.607838 0.639134

9500 0.502155 0.523846 0.544771 0.580525 0.613924 0.645193

10000 0.50835 0.530042 0.550962 0.586716 0.620099 0.651337

12000 0.521048 0.542735 0.563639 0.599385 0.632724 0.663891

5. Plots

Figure 5: Effect of altitude(ambient temperature) and intercooler effectiveness on compressor work

300000

320000

340000

360000

380000

400000

420000

440000

460000

480000

500000

8000 9000 10000 11000 12000

Co

mp

ress

or

wo

rk [

J/kg

]

Altitude [m]

x= 0.5

x=0.6

x= 0.7

x= 0.8

x= 0.9

x= 1.0

x= 0

13

Figure 6: Effect of intercooler on thermal efficiency

Figure 7: Effect of overall pressure ratio on thermal efficiency

0.3

0.35

0.4

0.45

0.5

0.55

0.6

8000 9000 10000 11000 12000

The

rmal

eff

icie

ncy

Altitude [m]

With intercooler

Without intercooler

0.45

0.47

0.49

0.51

0.53

0.55

0.57

0.59

0.61

0.63

0.65

8000 9000 10000 11000 12000

The

rmal

Eff

icie

ncy

Altitude [m]

PR = 25.0

PR = 27.5

PR = 30.0

PR = 32.5

PR = 35.0

PR = 40.0

14

Figure 8: Effect of intercooler effectiveness on thermal efficiency

Figure 9: Effect of turbine inlet temperature on thermal efficiency (for a certain pressure ratio)

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

215 220 225 230 235

The

rmal

Eff

icie

ncy

Ambient Temperature [k]

x= 0.5

x= 0.6

x= 0.7

x= 0.8

x= 0.9

x= 1.0

x=0

0.45

0.5

0.55

0.6

0.65

0.7

8000 9000 10000 11000 12000

The

rmal

Eff

icie

ncy

Altitude [m]

T4 = 1200

T4 = 1250

T4 = 1300

T4 = 1400

T4 = 1500

T4 = 1600

15

Figure 10: Effect of intercooler on specific fuel consumption

6. Conclusions

The following observations were made from the analysis :

Efficiency of the thermodynamic cycle can be substantially increased by the use of an

intercooler. It is particularly evident from Figure 6, which shows that by incorporating an

intercooler even with effectiveness as low as x=0.5 greatly increases the efficiency of the

cycle.

With the increasing altitude (in other words with increasing ambient temperature), the

thermal efficiency tends to decrease.

Compressor pressure ratio too has a significant impact on the efficiency. The efficiency is

notably large for bigger pressure ratios of the compressor.

With increasing ambient temperature (decreasing cruise altitude), total compressor work

increase too, resulting in lower efficiencies.

To summarize, an intercooler provides a relatively easy and reliable way to achieve higher

pressure ratios without the need for a larger HPC, thus saving weight and increasing

efficiency. Intercooling can greatly influence the fuel efficiency of the engine as it is evident

from the results. However, for engines with lower pressure ratios, the effects of intercooling

is less pronounced. Engines with overall pressure ratio above 30 ( such as the General

electric CF-6) can benefit immensely by using intercooling technology.

0.05

0.07

0.09

0.11

0.13

0.15

0.17

0.19

0.21

0.23

0.25

8000 9000 10000 11000 12000

Spe

cifi

c Fu

el C

on

sum

pti

on

[kg

/kN

.s]

Altitude [m]

x=0.5

x=0.6

x=0.7

x=0.8

x=0.9

x=1.0

x=0

16

References:

1. Propulsion systems Lectures - dr. inż. Paweł Oleszczak

2. Patent - US 6,430,931 B1 Gas turbine inline intercooler - Michael W. Horner

3. Intercooled Recuperated Aero Engine - S. Boggia, K. Rüd, Advanced Project Design, MTU Aero Engines

-München, Germany

4. Study on the effective parameter of gas turbine model with intercooled compression process - Thamir

K. Ibrahim1*, M. M. Rahman2 and Ahmed N. Abd Alla

5. Parametric Performance of Gas Turbine Power Plant with Effect Intercooler - Wadhah Hussein Abdul

Razzaq Al- Doori

6. A complete Parametric analysis of ideal turbofan engine - S.L. Yang, Y.K. Siow, K.H. Liew, and E. Urip

Mechanical Engineering – Engineering Mechanics Department, Michigan Technological University

7. Compression cycle of intercooled Gas turbine - Magdalena Milancej, Institut für Thermodynamik und

Energiewandlung Technische Universität Wien & Institute of Turbomachinery International Faculty of

Engineering, Technical University of Lodz