Using Burner Exit Temperature in the Control of a Geared
Turbofan Engine with NPSS
by
David P. Shore
An Engineering Project Submitted to the Graduate
Faculty of Rensselaer Polytechnic Institute
in Partial Fulfillment of the
Requirements for the degree of
MASTER OF ENGINEERING IN MECHANICAL ENGINEERING
Approved:
_________________________________________
Ernesto Gutierrez-Miravete, Project Adviser
Rensselaer Polytechnic Institute
Hartford, CT
December, 2010
1
Abstract
This project presents a large matrix of burner temperature (T4) targets across the
entire operating envelope of a Pratt and Whitney customer’s aircraft for use in thrust
output control. The design of this matrix is geared around hitting a number of thrust
requirements agreed upon with the customer for maximum takeoff, cruise, and continuous power requirements. Additionally, the ratings will be defined for 23k, 21k, and
18.9k pounds of thrust, and for a number of different customer bleed off take combinations. The ratings will be created using the Numerical Propulsion System Simulation
(NPSS), which utilizes turbomachinery theory to determine various engine parameters
based on a specific set of inputs. The entire work through of the rating creation process
will be shown and will result in a final set of ratings that will meet customer demands.
These final ratings will be the input for a customer simulation to be used in the aircraft
design process.
2
Table of Contents
Abstract
ii
List of Tables
iv
List of Figures
v
Nomenclature
vi
1. Introduction
8
1.1
Geared Turbofan Basics
1.2
Numerical Propulsion System Simulation
2. Methodology
xx
3. Results and Discussion
xx
4. Conclusions
xx
5. References
xx
6. Appendices
xx
3
LIST OF TABLES
Table 1 – Burner Exit Temperature Schedule Summary
4
LIST OF FIGURES
Figure 1 - Two-Spool Turbofan Cross Section with Station Designations
Figure 2 - Simple Conceptual NPSS Model
Figure 3 - Typical T4 / Thrust Lapse
Figure 4 - Modified Bombardier Operating Envelopes
5
LIST OF SYMBOLS
BPR = fan bypass ratio
Cp = constant pressure specific heat
Cv = constant volume specific heat
Fn = Raw Thrust (lbs)
FNTST = Stream tube corrected thrust (lbs)
h = enthalpy (BTU/lbm)
ISA = International Standard Atmosphere
LHV = Lower Heating Value of fuel (BTU/lb)
MN = Mach number
N1 = Low rotor speed (RPM)
N1F = Fan speed (RPM)
N2 = High rotor speed (RPM)
P0, Pamb = Outside free-stream air temperature (psia)
P2 = Pressure at the face of the fan, core stream (psia)
P12 = Pressure at the face of the fan, fan stream (psia)
P2.5 = Pressure in between the LPC and HPC (psia)
P3 = Pressure at the exit of the HPC (psia)
P4 = Pressure at the exit of the burner (psia)
P4.5 = Pressure in between the HPT and LPT (psia)
P4.9 = Pressure at exit of the LPT (psia)
R = gas constant (BTU/°F/lbm)
T0, Tamb = Outside free-stream air temperature (°F)
T2 = Temperature at the face of the fan, core stream (°F)
T12 = Temperature at the face of the fan, fan stream (°F)
T2.5 = Temperature in between the LPC and HPC (°F)
T3 = Temperature at the exit of the HPC (°F)
T4 = Temperature at the exit of the burner (°F)
T4.5 = Temperature in between the HPT and LPT (°F)
T4.9 = Temperature at exit of the LPT (°F)
TSFC = Thrust Specific Fuel Consumption
6
Wf = Fuel flow (lbm/sec)
Wa = Airflow (lbm/sec)
ρ = Density (lbm/ft3)
η = efficiency
γ = Ratio of constant pressure specific heat to constant volume specific heat (Cp/Cv)
7
1. Introduction
The Pratt and Whitney PW1500G is a high-bypass geared turbofan engine currently
selected as the exclusive engine for the Bombardier C-Series aircraft. At the time of this
project creation, the first engine is currently being tested in Pratt and Whitney’s outdoor
testing facilities in Florida. With the C-Series estimated entry in to service date of 2013
rapidly approaching, there is a substantial amount of work that needs to be completed.
One such task is defining a rating that will be used to determine performance (thrust
targets) of the engine. This rating will eventually be defined by low turbine rotor speed
(N1), but as an intermediate step, will be defined by burner exit temperature (T4).
The final goal of this project will be a number of T4 matrices organized into a
comprehensive ratings file. This file will be packaged in a customer simulation and
shipped to Bombardier for use in their aircraft design process. The customer simulation
is essential for accurate expectations on thrust targets, station temperatures and pressures, bleed pressures and temperatures, fuel burn, etc. Additional to the design process,
it will be used by the Bombardier engine performance group to quote aircraft thrust
capabilities to potential aircraft buyers.
8
1.1 Geared Turbofan Basics
The PW1500G Geared Turbofan engine is a typical two spooled engine design with
a twist. Normal two spool engine designs are driven by two rotors, called the high spool
and low spool. The low spool is composed of the fan, low pressure compressor (LPC),
and low pressure turbine (LPT). The high spool, often called the core when combined
with the burner, is composed of the high pressure compressor (HPC) and high pressure
turbine (HPT). In the typical two spool design, all components on the low spool spin at
the same rate, referred to as the low rotor speed (N1). All components on the high spool
also spin at the same rate, referred to as the high rotor speed (N2). The figure below
shows a simplified cartoon of a two-spool engine. Additionally, the figure has numeric
station designations that will be used in the remainder of this project.
Figure 1 – Two-Spool Turbofan Cross Section with Station Designations
In the geared turbofan design, the low spool components are separated by a reduction
gearbox. In this design, the LPC and LPT will spin at the same rate, but the reduction
gearbox will create a slower spinning fan. The reason behind this design is fan bypass
9
ratio (BPR). For subsonic commercial aircraft, the goal in engine design is multipronged,
but all aircraft suppliers would agree the engine needs to be as lightweight as possible
with low thrust specific fuel consumption (TSFC), while still being able to meet thrust
requirements. A high bypass ratio gives a lower exhaust speed, which helps reduce
TSFC. Unfortunately for the two spool engine design, a high bypass ratio requires a slow
spinning fan. Since the fan is driven by the LPT, to slow the fan down to an optimal
speed would require more stages on the LPT, which increases size and, more importantly, weight of the engine. By adding the reduction gearbox, the fan can spin at an optimal
speed (N1F) without needing to increase the size of the LPT.
1.2 Numerical Propulsion System Simulation
The Numerical Propulsion System Simulation, hereby referred to as NPSS, is a
detailed aerothermomechanical computer simulation developed by the NASA Glenn
Research Center with support from the US aeropropulsion industry. It is used in the
aircraft engine design process to realistically model the physical interactions that take
place inside an engine, which can not only reduce the need for expensive and complicated testing, but to also verify results of such testing as it is required [1].
At its core, NPSS is a component-based object-oriented engine cycle simulator
designed to perform cycle design, steady state and transient off-design performance
prediction, test data matching, and many other traditional tasks of engine cycle simulation codes [1]. For the purpose of this project, only the steady state performance
predictions will be used. Transient simulations will not be necessary for rating table
definitions. NPSS is composed of a series of individual, interconnected components,
linked together through mechanical, fluid & fuel ports. The overall assembly of components is controlled by a solution algorithm by way of a solver object that will impose
fundamental conservation laws (continuity, energy, momentum) as well as any other
user defined constraints. Additionally, NPSS incorporates a full-featured user interface
via a built-in programming language modeled after C++.
All NPSS simulations are created from a collection of six (6) basic object types
(classes) or building blocks. These classes (building blocks) represent the engine com-
10
ponents (hardware) and how those components are linked together, as well as the more
abstract features that a simulation comprises. The six basic object types are elements,
subelements, sockets, flow stations, ports and tables. A conceptual figure illustrating
how these concepts are used to create an engine simulation is included below in Figure
2. This particular figure is simplified for a single shaft turbofan. The highlighted area
represents the portion of the simulation that would be doubled for the typical dual shaft
engine design. Objects represent both engine components (compressor, turbine, shaft,
nozzle, etc.) as well as the more abstract features such as “FlightConditions”, “Solver”,
“DataViewers”, etc.
Simple Conceptual
Engine Model
Model
FlightConditions
Element
s
FuelStart
Element
Fluid Port
Fuel Port
Assembly
F
P
Inlet
Element
F
P
F
F
P Compressor P
Element
F
P
Fuel Port
F
F
P Burner P
Element
F
P
Turbine
Element
F
P
F
P
F
Duct P
Element
F
P CDNozzle
Element
F
P
Shaft Port
Fluid Port
Shaft Port
Bleed
Fluid Port
S
P
Shaft
Element
S
P
FlowEnd
Element
CompressorMap
Subelement
F
P
Socket
Compressor
Element
Shaft Port
F
P
F
P
Compressor, Shaft, and Turbine will have both LP and HP
objects for a 2-spool engine
Solver
Data Viewer(s)
Figure 2 – Simple Conceptual NPSS Model
For the purposes of this project, a baseline NPSS simulation for the Bombardier
engine has already been developed. Various “building blocks” from a number of departments within Pratt and Whitney have been supplied to create this baseline
simulation. These building blocks include input files for component maps, component
setup, subsystems (bleeds, drag), engine control, ratings, and installation effects (customer bleeds, port losses, flight envelope, etc.). The ratings input file is what will be
modified to meet the customer requirements for the new simulation.
11
2. Methodology
2.1 Review of Gas Generator Turbomachinery
Methods of engine control in past engine designs at Pratt and Whitney have varied
quite a bit over the years. Legacy engine models have used engine pressure ratio (EPR),
or PT7/PT2 as a method of controlling the engine powersetting. Newer engine models,
such as the Engine Alliance GP7000 and PW6000 have used low rotor speed (N1) as the
means for setting engine power. The Bombardier engine will also eventually use an N1
rating to define powersetting, but in advance of this, combustor exit temperature will be
used for preliminary customer simulations. T4 can be directly linked to engine thrust
with basic thermodynamics of turbofan engines. For a turbofan engine, we can approximate specific thrust with the following equation [3]:
FN
 (1  f )  u e  BPR  u ef  (1  BPR )  u
m a
where
BPR = fan bypass ratio
FN
= specific thrust
m a
f = fuel air ratio
ue = core engine exit velocity
uef =
fan nozzle exit velocity
u = free stream velocity
With the above equation, we assume that the core engine flow and the bypass flow
expand separately to the ambient conditions behind the engine. Since we are concerned
with burner exit temperatures for control, we want to focus on the core engine flow.
Allowing for the work done by the engine core on the fan stream, and assuming the rate
of mass flow in both the turbine and compressor to be equal, we can write the turbine
power balance as [3]:
T4.9  T4  (T3  T2 )  BPR (T18  T0 )
12
where
T4.9 = LPT exit temperature
T3 = LPC exit temperature
T2 = Fan inlet temperature
T18 = Fan bypass air temperature
T0 = Ambient temperature
We can also model the core engine exit velocity as such [3]:
1
 

 pa   



ue  2n
R  T4.9 1  

 1
p4.9  



where
 n = Fan nozzle efficiency
γ = Ratio of constant pressure specific heat to constant volume specific
heat (Cp/Cv)
R = Gas Constant
pa = Ambient pressure
p4.9 = LPT exit pressure
Since LPT exit temperature is dependent on upstream burner exit temperature, we
can make the conclusion that core engine exit velocity is dependent on the square of
burner exit temperature, which corresponds to specific thrust. Thus, engine thrust for a
given flight condition (MN, ALT, ambient pressures and temperatures) is directly
proportional to T4 as well as ambient conditions. A simplified cartoon of a typical thrust
/ T4 lapse with respect to delta ambient temperature is presented below.
Figure 3 – Typical T4 / Thrust Lapse
13
Figure 3 shows that for a given DTAMB from ISA standard day, the T4 remains
constant for all higher DTAMB values. This is referred to as the break-point. For
DTAMB values hotter than the breakpoint temperature, T4 will remain constant and
thrust will fall off. This is called the “hot-day line”, and the rating files we will create
will need to model this type of behavior. Furthermore, subtracting flow out of the
compressor for engine bleeds will have a noticeable effect on thrust. By reducing core
flow, if all other conditions are held constant, the overall thrust value will decrease, and
in most cases, the breakpoint value will move to a lower DTAMB [2]. This will be
evident in the results section when we apply constant T4 schedules to different bleed
configurations.
2.2 Aircraft Flight Envelope, Power Settings, and Derates
The initial requirement for the PW1500G engine was a maximum take-off (MTO)
thrust capability of 23,000 pounds of thrust. This requirement is outlined in the Technical Requirements Document (TRD) agreed upon between Pratt and Whitney and
Bombardier. Further discussions with Bombardier have led to the requirements for thrust
derates on the engine for the purpose of prolonging engine life on wing. Derates have
been defined at 21k and 18.9k pounds of thrust respectively. These derates have been
added to the TRD. Specific requirements for these derates have been supplied by Bombardier and have to be met for this particular engine simulation.
Power settings required for the simulation are maximum take-off (MTO), maximum
continuous (MCT), and maximum climb (MCL), ordered in descending levels of thrust.
The baseline simulation to be used in this project has 23k and 21k levels of MTO, MCT,
and MCL thrust defined for only a small number of engine bleed configurations. The
new simulation will have MTO, MCT, and MCL power settings defined for 23k, 21k,
and 18.9k levels of thrust at a larger number of bleed conditions.
There are three different engine bleeds that will be taken into account for the rating
definition. These bleeds are the environmental control (ECS), wing anti-ice (WAI), and
cowl anti-ice (CAI). ECS bleeds are responsible for feeding bleed off-take air into a
number of different systems that are required for a commercial flight with passengers,
including breathable air, cabin pressure regulation, heat and air conditioning systems,
14
etc. WAI and CAI bleeds are fairly self-explanatory, in that their specific purpose is to
combat the effects of wing and cowl icing in conditions in which icing can become a
potential problem. Engine bleeds can be utilized in both single and dual (nominal)
engine operation. Single operation occurs if there is only one engine on the aircraft
producing bleed flow for these systems. This will incur a higher bleed flow for a particular engine, and hence the performance of that engine will be more restricted than an
engine flowing less bleed for dual operation. There are a large number of different bleed
configuration permutations, but only eleven configurations were outlined in the requirements between P&W and BA. The eleven different bleed configurations that will be
taken into account for rating design are as follows:

Bleeds OFF

ECS OFF / CAI ON

ECS OFF / CAI ON WAI ON DUAL

ECS OFF / CAI ON WAI ON SINGLE

ECS ON DUAL

ECS ON DUAL / CAI ON

ECS ON DUAL / CAI ON WAI ON DUAL

ECS ON SINGLE, ECS ON SINGLE / CAI ON

ECS ON SINGLE / CAI ON WAI ON DUAL

ECS ON SINGLE / CAI ON WAI ON SINGLE.
All schedules must be defined for the entire operating envelope of the Bombardier
C-series aircraft. Since the operating envelope is an internally controlled and proprietary
document, for the purposes of this project, it has been modified to eliminate any exposure. Figure 3 describes the modified operator envelopes.
15
CSeries Operating Envelopes
40000.00
39,000 ft
35000.00
0.77M
30000.00
Pressure Altitude (ft)
25000.00
20000.00
15000.00
14,500 ft
12,000 ft
10000.00
Take-Off
Envelope
5000.00
0.00
-2,000 ft
-5000.00
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Mach Number
Figure 4 – Modified Bombardier Operating Envelopes
Notice there are two operating envelopes above. It is only required to hit MTO target thrusts within the take-off envelope, which accounts for realistic airports around the
globe. For MCT and MCL target thrusts, all points within the operating envelope are
required. To hit specific thrusts for every point in the envelope would create ratings
tables too large for the simulation, and fidelity on a level which is not required. As a
result, grids of nodes were agreed upon with Bombardier to meet requirements. Typically, this resulted in a node at every 0.05 mach number and every 2000 feet.
Creating individual schedules for all possible permutations of derate, power setting,
and bleed configuration would require a total of 99 different schedules. If we include all
other bleed configurations outside the 11 defined previously, the number of schedules
required gets out of control in a hurry. Combine this with the fact that there are nodes
every 0.05 mach and every 2000 feet across the entire envelope, and the size of the
ratings file balloons very quickly. As a result, schedules were targeted for specific bleed
conditions which were most important to Bombardier. For bleed conditions in which a
specific schedule was not targeted, the bleed configuration with the next highest bleed
16
flow was chosen to represent this case. For example, for the 21k MTO rating, bleed
configurations that were specifically targeted were Bleeds OFF, ECS ON SINGLE, and
ECS ON SINGLE / CAI ON WAI ON SINGLE. Obviously, bleed flow for the Bleeds
OFF configuration is zero. Total bleed flow for ECS ON SINGLE will be less than ECS
ON SINGLE / CAI ON WAI ON SINGLE, as the later is flowing bleed for the anti-ice
system. Logically, it would also follow that ECS ON SINGLE / CAI ON would be
flowing greater bleed than ECS ON SINGLE. As such, the next targeted bleed configuration with the highest flow would be ECS ON SINGLE / CAI ON WAI ON SINGLE,
so that schedule would be used. This will result in the same T4 schedule being used for
both the targeted and non-targeted condition. Since the non-targeted condition is operating with less bleed flowing, but the T4 remains the same, the corresponding output thrust
will be greater.
Table 1 below illustrates the T4 schedule summary. The numbers below the bleed
configurations correspond to the bleed flow ranking, with (1) being the least amount of
bleed flow, and (11) being the max case.
Table 1 – Burner Exit Temperature Schedule Summary
2.3 Customer Requirements and Thrust Logic
This section will discuss the agreed upon thrust logic and requirements proposed by
Bombardier.
17
2.4 Method Used to Develop Ratings Tables
This section will discuss the actual methods used to generate the tables, creating
runin files, excel spreadsheets, etc.
18
3. Results and Discussion
3.1 Technical Requirements Document Thrust Points
This section will discuss hitting TRD required points
3.2 Derates and Meeting Customer Requirements
This section will discuss hitting agreed upon customer requirements for derates,
percentage deltas from 23 to 21k, 21 to 18.9k.
3.3 Behavior Checks
This section will check to ensure engine performance is behaving as expected with
respect to thrust lapses, fir tree plots, and ensuring MTO>MCT>MCL for 23, 21,
and 18.9k ratings.
19
4. Conclusion
20
5. References
1. NPSS User Guide, NASA John H. Glenn Research Center at Lewis Field, April
4, 2005
2. Lewis, John H., Fundamental Engineering Principles as Applied to Gas Turbine
Performance Analysis, 2nd Edition, September 2003
3. Hill, Philip, Mechanics and Thermodynamics of Propulsion, 2nd Edition, Addison-Wesley Publishing Company, 1992
21