Jet%20Propulsion%20and%20Compressor%20Design

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Jet Propulsion and Compressor Design
NASA's X-43A
Keith Larson
IC Engines and Propulsions Systems
Spring 2005
Professor
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.

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.

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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.

• 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)

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Jet 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
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Thrust 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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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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Propeller 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
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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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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.
Exhaust Gas Out
Air Inlet
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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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Turbo 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.
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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 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).
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Step 1. Create the velocity triangle and
calculate the relative speed of the rotor blade
from the rotational velocity.
Blade motion
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Step 2. Calculate the air to blade relative
velocity and the angle between the relative and
actual air speed.
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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)
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Step 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
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The 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.
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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
No initial tangential component to the inlet
velocity.
Power of the shaft is
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Specific work of the stage is then
Now To2 can be calculated from the specific work
To1, and the conversion factor.
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Finally, the Compressor Pressure Ratio can be
calculated!!!
The answer is
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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.
Lockhead SR-71 Blackbird
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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.