Analysis of work cycle of intercooled turbofan engine
1. 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
2. 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
3. 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
4. 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
5. 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.
6. 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.
7. 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.
8. 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):
9. 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
18. 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
SpecificFuelConsumption[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
19. 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
