Title: Jet%20Propulsion%20and%20Compressor%20Design
1Jet Propulsion and Compressor Design
NASA's X-43A
Keith Larson
IC Engines and Propulsions Systems
Spring 2005
Professor
Dr. Chiang Shih
2Fluid 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.
3Dynamic 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.
4Turbomachines 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
5Turbine Classifications
- Hydraulic Turbines - The working fluid is
WATER. - Flow is incompressible. - Gas and Steam Turbines - Density of the working
fluid may change significantly.
Further Classification
- 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.
6The 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
7Or the load could be a compressor within a
Turbocharger for an automobile, or a compressor
in a jet engine.
8Turbomachines that add energy to the fluid stream
- Pump - when the fluid is a liquid or a slurry.
- Very small to very large pressure rise.
- Rotating element is called an impeller.
- Fans, Blowers, or Compressors when handling a
gas or a vapor.
- Fans - generally have a small pressure rise (lt 1
inch water) - Blowers - moderate pressure rise (1 inch of
mercury) - Compressors - very high pressure rise (up to
150,000 psi)
9Jet Propulsion Principle (Thrust)
Pa
Po
T Thrust Pa Ambient Pressure Po Internal
Pressure ue Exit Velocity
Po
Steady-Flow
.
ua Mass-average Exhaust Velocity
Tmua
10Thrust per Unit Energy Consumption (Rocket vs.
Propeller)
Acceleration of a stream of air through a
Propeller
Propeller Thrust Ratio
Assume a best thermal efficiency of 40, the
maximum possible value of propeller thrust ratio
becomes.
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12Summary 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.
13Propeller Theory
Air 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
14Limitation 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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16The 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.
Exhaust Gas Out
Air Inlet
171. The turbine engine made supersonic flight
possible in aircraft 2. Reduced the cost of air
travel. 3. Lead to great improvements in aircraft
safety.
18Turboprop
Allison T56 Turboshaft
19Turbofan
General Electric CF6 Turbofan
20Turbojet
General Electric J79 Turbojet with Afterburner
21Turbo Engine Comparison
- 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
NOTE Due to the ram compression due to flight
speed, the optimum compressor pressure ratio
(CPR) goes to zero around Mach 4. CPR 301 for
subsonic flight. CPR 101 _at_ Mach 2. Compressor
not needed at Mach 4 Ramjet.
22Comparison 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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24Example Problem
Given 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/s2K, calculate the first stage Compressor
Pressure Ratio (CPR).
25Step 1. Create the velocity triangle and
calculate the relative speed of the rotor blade
from the rotational velocity.
Blade motion
26Step 2. Calculate the air to blade relative
velocity and the angle between the relative and
actual air speed.
27Step 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)
28Step 4. Calculate the portion of the relative
blade speed associated with the actual air
velocity (Uv2), the calculate the actual air
speed (V2).
V2
W2
Vin
29The Compressor Pressure Ratio (CPR) is found from
the isentropic relationship.
To1 is calculated from the following equation.
To2 has to be calculated from the specific work
of the compressor stage.
30Specific 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
No initial tangential component to the inlet
velocity.
Power of the shaft is
31Specific work of the stage is then
Now To2 can be calculated from the specific work
To1, and the conversion factor.
32Finally, the Compressor Pressure Ratio can be
calculated!!!
The answer is
33The 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.
Lockhead SR-71 Blackbird
34NASA 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.