compressor design
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Jet Propulsion and Compressor Design
NASA's X-43A
Keith LarsonIC Engines and Propulsions Systems
Spring 2005Professor
Dr. Chiang Shih
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Fluid Machinery
Positive Displacement
• Working fluid is confined within a boundary.
•Energy transfer is by volume changes due to the movement of the boundary.
Dynamic• Working fluid is not confine
within a boundary.
• Energy transfer is by dynamic effects of the rotor on the fluid stream.
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Dynamic Machine
A.K.A. Turbomachines
* Radial-Flow - Also called Centrifugal.- Radial flow path.- Large change in radius from inlet to outlet.
* Axial-Flow - Flow path nearly parallel to the axis of rotation.- Radius of the flow path does not very significantly.
* Mixed-Flow - Flow path radius changes only moderately.
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Turbomachines that extract energy from the fluid stream
Turbines
Turbines use Vanes, Blades, or Buckets attached to the turbine shaft.
This assembly is called the Rotor, Wheel, or Runner.
Bourn, Cambridgeshire, England
Colvin Run Mill near Dranesville, Virginia
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Turbine Classifications* Hydraulic Turbines - The working fluid is WATER.
- Flow is incompressible.
* Gas and Steam Turbines - Density of the working fluid may change significantly.
• Impulse Turbines - Driven by one or more high-speed free jets. - Each jet is accelerated in an external nozzle. - Fluid acceleration and pressure drop is
external to the blades.
• Reaction Turbines - Part of the pressure change takes place externally and part takes place within the moving blades.
Further Classification
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The turbine extracts energy from the fluid stream and converts it into mechanical energy, which is then transmitted through a shaft to some load.
The Steam Turbine Generator Satsop Development Park
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Or the load could be a compressor within a Turbocharger for an automobile, or a compressor in a jet engine.
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Turbomachines that add energy to the fluid stream
Pump - when the fluid is a liquid or a slurry.
• Fans - generally have a small pressure rise (< 1 inch water)
• Blowers - moderate pressure rise (1 inch of mercury)
• Compressors - very high pressure rise (up to 150,000 psi)
Very small to very large pressure rise.
Rotating element is called an impeller.
Fans, Blowers, or Compressors when handling a gas or a vapor.
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TPa
TPa
ue
Po
Po
TPa
ua
Po
Pa Po Ai
Jet Propulsion Principle (Thrust)
T=Ai(po-pa)
T: Thrust
Pa: Ambient Pressure
Po: Internal Pressure
ue: Exit Velocity
ua: Mass-average Exhaust Velocity
Steady-Flow
T=mua
.
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Acceleration of a stream of air through a Propeller
Engineu ue
Thrust per Unit Energy Consumption (Rocket vs. Propeller)
Propeller Thrust Ratio
.T=ma(ue-u). .E
ma
e
uc2
2ua
2
2
.TE
2 e
ue u
Assume a best thermal efficiency of 40%, the maximum possible value of propeller thrust ratio becomes.
.TE
25u
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TTpropeller
rocket
1000u
Rocket
Tmpuer• • •Emp
uer2
2
•TE
2
uer2
Rocket Thrust Ratio
Estimate ratio of propeller and rocket thrusts
TTpropeller
rocket
uer
5u
Assume that the rocket exhaust velocity is 5000m/s.
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Summary of Propeller and Rocket Thrust
For Aircraft propulsion the big advantage of using a propeller is that less fuel must be carried on board.• The rate of airflow through the propeller can be as much as
three orders of magnitude larger than the rate of fuel consumption of the driving engine.
Propulsion using a propeller has much better efficiency when compared to propulsion with a rocket.• The aircraft using a propeller can travel much greater
distances before having to refuel.
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w1t
Ut
u
ueD
u
c2w1t
w2t
u
Blade Motion
Air MotionAxis of Rotation
Propeller TheoryAir Velocity (u)
Blade Speed (Ut)
Relative Approach Velocity (w1t)
Relative Leaving Velocity (w2t)
Swirling Velocity (u)
Axial Component of Leaving Velocity (ue)
Leaving Velocity (c2)
Turning Angle ()Ut
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Limitation of the Propeller in Propulsion
In order to maintain good flow over the blade certain conditions must be meet.1. The relative approach angle and the blade leading
edge angle must be close to prevent flow separation from the blade.
2. The turning angle must be keep quite small, or the flow will also separate from the blade.
3. The relative approach velocity must not be too close to the speed of sound. This is to prevent shock waves from forming on the blade.
Thus conventional propellers are used for flight speeds well below the speed of sound; usually at or below 135 m/s (300 mph).
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Blade Motion
Air Motion
Axis w1t
Ut
u
Blade Motion
Air Motion
Axis w1t
Ut
u
Blade speed too high
Flight speed too slow
Operating outside of design parameters
Poor design: Turning angle is too large
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The Importance of the Compressor/Turbine in Modern Flight
It was not until 1939 that a compressor, combuster, and turbine were coupled together to create the first turbo engine for aircraft propulsion.
Air Inlet Exhaust Gas Out
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1. The turbine engine made supersonic flight possible in aircraft
2. Reduced the cost of air travel.
3. Lead to great improvements in aircraft safety.
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Turboprop
Allison T56 Turboshaft
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Turbofan
General Electric CF6 Turbofan
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Turbojet
General Electric J79 Turbojet with Afterburner
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Turboprop
• Medium-speed
•Moderate-size craft
•High efficiency
•Limited flight speed
•Geared transmission
Turbofan
• Internal Propeller
• Supersonic speeds
• High bypass airflow
• Med/High efficiency
• No gearbox
Turbojet
• High speed
• Mach 4
• Low airflow rate
• Low efficiency
• High op temps
Turbo Engine Comparison
NOTE: Due to the ram compression due to flight speed, the optimum compressor pressure ratio (CPR) goes to zero around Mach 4.
CPR 30:1 for subsonic flight.
CPR 10:1 @ Mach 2.
Compressor not needed at Mach 4; Ramjet.
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Comparison of the Axial-Flow and Radial-Flow Compressors
Axial-Flow compressors do not significantly change the direction of the flow stream, thus Axial-Flow Compressor allows for multiple stages. Radial-Flow Compressors can not be staged.
While the Radial-Flow Compressor has a larger Compressor Pressure Ratio (CPR) per stage, the multi-stages of the Axial-Flow compressor allows for a larger overall CPR.
The frontal area for a given air flow rate is smaller for an Axial-Flow Compressor than for a Radial-Flow Compressor.
The Axial-Flow Compressor has a higher efficiency.
Disadvantages are the higher cost to manufacture the Axial-Flow Compressor, and the Radial-flow Compressor is more durable than the Axial-Flow Compressor.
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Example ProblemGiven a first single stage of an Axial Compressor with the following
conditions: ambient pressure (Pin) 1 atmosphere, ambient temperature (Tin) 300K, aircraft cruising speed (Vin) 170m/s, median blade diameter (D) 0.5m, rotor rpm (Urotor) 8000rpm, turning angle () 15 degrees, specific heat ratio () 1.4, air mass flow rate (mdot) 35kg/s, and (Cp) conversion factor 1004 m2/s2*K, calculate the first
stage Compressor Pressure Ratio (CPR).
Pin 1atm Tin 300K Vin 170ms
D .5m
Urotor 8000rpm 15deg 1.4 Cp 1004m2
s2 K
kg 1000gm mdot 35kgs
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U
Vin
W1 1
Blade motion
U r UD2
2 60 s
8000
U 209.44ms
Wx U Wx 209.44ms
Step 1.
Create the velocity triangle and calculate the relative speed of the rotor blade from the rotational velocity.
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1 atanW x
V in
1 50.934 deg
U
Vin
W1 1
W1 Wx2 Vin
2 W1 269.75ms
Step 2.
Calculate the air to blade relative velocity and the angle between the relative and actual air speed.
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2
U w2
Vin
W2
Step 3.
Axial velocity (Vin) does not change. Calculate relative exit angle(2), then portion of the relative blade speed (Uw2). Calculate relative air speed (W2)
2 1
2 35.934 deg
U w2 V in tan 2
U w2 123.214ms
W 2V in
cos 2
W 2 209.956ms
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V2 2
U w2U v2
Vin
W2
Step 4.
Calculate the portion of the relative blade speed associated with the actual air velocity (Uv2), the calculate the actual air speed (V2).
Uv2 Wx Uw2 Uv2 86.226ms
V2 Vin2 Uv2
2 V2 190.617ms
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P o2
P o1
T o2
T o1
1
The Compressor Pressure Ratio (CPR) is found from the isentropic relationship.
To1 TinVin
2
2 Cp To1 314.392K
To1 is calculated from the following equation. To2 has to be calculated from the specific work of the compressor stage.
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wstageTshaft
mdot
Tshaft mdotD2
Uv1 Uv2
Specific work of the stage is calculated from the torque of the shaft, angular velocity of the blade, and mass flow rate of the air.
Torque of the shaft is:
Tshaft 754.476J
Power of the shaft is:
Power Tshaft2 60 s
8000
Power 632.068kW
Uv1 0ms
No initial tangential component to the inlet velocity.
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wstagePowermdot
wstage 1.806 104J
kg
Specific work of the stage is then:
To2 To1wstage
Cp
CPRTo2
To1
1
Now To2 can be calculated from the specific work To1, and the conversion factor.
To2 332.38K
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Finally, the Compressor Pressure Ratio can be calculated!!!To2 To1
wstage
Cp
CPRTo2
To1
1
CPR 1.215
The answer is:
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Lockhead SR-71 Blackbird
The engines on the blackbird are turbojets and are used as such up to about Mach 4; when the air flow is bypassed around the compressor and the engines become ramjets.
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NASA X-43A
This is where we are today. The X-43A is an experimental aircraft that uses a scramjet (supersonic ramjet) for its propulsion. The X-43A has reach speeds of about Mach 10.