Aircraft Propulsion
AENG 31102
Aircraft Gas Turbine Performance
& Design
Revision Lecture
Lecture 7
Fundamentals ~Turbojet
• The compressor raises the
pressure of the air before
combustion
• The turbine extracts work from the
hot high pressure combustion
products to drive the compressor
Each stage consists of a row of rotor
blades followed by a row of stator blades
The first stage is often preceded by an inlet guide vane
Each stage consists of a row of
Nozzle Guide Vanes which direct
the gas onto the rotor blade
Total or Stagnation Temperature
The stagnation temperature is the temperature at the stagnation point
in a fluid flow. At a stagnation point the speed of the fluid is zero and
all of the kinetic energy has been converted to internal energy and is
added to the local static enthalpy.
Hence ho = h + V2/2
For Constant Cp; Enthalpy = Cp T
Hence To = T + V2/2 Cp
or
To/T = 1 + (γ – 1)/2 M2
= 1 + 0.2 M 2
The Practical Turbojet Cycle
Joule or Brayton Cycle:
• Calculate Total Temperatures & pressures
at inlet
• Step by step calculation of Total Temperatures
& Pressures through engine
• Check whether nozzle is choked
• Calculate Jet Velocity & static pressure at
nozzle plane
• Thrust rate of change of momentum
4
Variations
• Remove turbo machinery ~
RAMJET
• Add power turbine ~ TURBOSHAFT/TURBO-PROP
• Split the flow after initial
compression & add more stages ~
TURBOFAN
• Add heat exchanger
~ AIR BREATHING ROCKET
Different Turbofan Types
Civil Turbofan~ Trent
High By-pass Ratio
5 ~ 12
“Low” Specific Thrust
Specific Thrust
Thrust per unit mass flow
Units: N/kg/sec or m/s
Military Turbofan ~ EJ200
Low By-pass Ratio
0.3 ~ 1
“High” Specific Thrust
Axial Compressors
Rotor Speed = U
Combining the vectors of C1 and U to give
V1 and 1
Axial velocity = 𝑪𝒂𝟏
Whirl velocity = 𝑪𝒘𝟏
Assume Constant Axial Velocity i.e.
𝐶𝑎1 = 𝐶𝑎2 = 𝐶𝑎3 = 𝐶𝑎
Hence at Exit 3 and C3 are the same as 1
and C1
Compressors ~ diverging passages
Turbines ~ converging passages
Factors effecting Stage Pressure Ratio
•
Blade Speed
•
Axial velocity
•
High Deflection in
Rotor Blades (β1 - β2 )
•
Efficiency
UCa
p03
1
p01
p
1
tan 1 tan 2
C pT
01
de Haller Number = V2 / V1
For minimum losses V2 / V1 > 0.72
Effect of increasing deflection
Typical Transient Compressor Working Lines
9
Centrifugal Compressors
pO 3 isenU 2 1
1
pO1
Cp TO1
𝑪
Slip Factor:
𝝈 = 𝒘𝟐
𝑼
Power Input Factor:
𝝍 = 𝑅𝑎𝑡𝑖𝑜 𝑜𝑓 𝑎𝑐𝑡𝑢𝑎𝑙 𝑤𝑜𝑟𝑘 𝑖𝑛𝑝𝑢𝑡 𝑡𝑜 𝑡ℎ𝑒𝑜𝑟𝑒𝑡𝑖𝑐𝑎𝑙 𝑤𝑜𝑟𝑘 𝑖𝑛𝑝𝑢𝑡
Tip Clearance Issues
Core
CF6 Thrust 50,000 Lb
Fan Diameter 2.2m
OPR 29
TFE 731 4000 Lb
Fan Diameter 0.72 m
OPR 15
11
Tip Clearances
Stator Inlet
Stator Outlet SOT
Turbine Entry (Rotor) TET
Rotor Inlet RIT
Deflection of flow in rotor = β2 + β3
Axial Flow Turbines
Turbine Layout & Cooling
A typical uncooled
turbine blade showing
twisted contour
Cooling Effectiveness
𝑇𝑏– 𝑇𝑐𝑟
=
𝑇𝑔 𝑟𝑒𝑙 – 𝑇𝑐𝑟
Aircraft Flight Envelope
16
Cycle Choice
SPECIFIC THRUST (Thrust per unit Mass Flow)
= (𝑪𝒋 − 𝑪𝒂 ) (Unit of velocity)
Specific Thrust - Variation with design flight speed
AIRCRAFT PROPULSION
AENG 31102
• Length of Exam 2 hours (max time 2 hrs 30
minutes to cover connectivity issues.
• The paper contains questions of the type:
–
–
–
–
Fill in the blanks
Multiple Choice
True or False
Essay Questions*
• * Questions on specific topics covered during the
lectures – short answers in bullet point form.
• The maximum for this paper is 60 marks.
Hints
• For the calculation questions be careful to
fill into the stated accuracy – there is some
latitude for rounding issues.
• For the essay questions – bullet points
covering the main points are fine
• The exam is “open book” therefore you do
not need to memorise equations.
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