Turbine and Compressor Design

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Turbine and Compressor Design

• Team
• Kevin Garvey
• Alex von Oetinger

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Major Topics

• Compressor and Turbine Design
• Cooling
• Dynamic Surge
• Stall Propagation

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Background

• History
• First gas turbine was developed in 1872 by Dr. F.
  Stolze.
• Gas Turbine EngineWhat does it do?
• Generates thrust by mixing compressed ambient air
  with fuel and combusting the mixture through a
  nozzle to propel an object forward or to produce
  shaft work.

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How Does it Work?

• Newtons third law
• For every action, there is an equal and
  opposite reaction.
• As the working fluid is exhausted out the nozzle
  of the gas turbine engine, the object that the
  engine is attached to is pushed forward. In the
  case of generating shaft work, the shaft turns a
  generator which produces electrical power.

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How Does it Work? Cont.
Exhaust Gas
Ambient Air In
Shaft
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Operation

• Compressor is connected to the turbine via a
  shaft. The turbine provides the turning moment to
  turn the compressor.
• The turning turbine rotates the compressor fan
  blades which compresses the incoming air.
• Compression occurs through rotors and stators
  within the compression region.
• Rotors (Rotate with shaft)
• Stators (Stationary to shaft)

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Types of Gas Turbines

• Centrifugal
• Compressed air output is around the outer
  perimeter of engine
• Axial
• Compressed air output is directed along the
  centerline of the engine
• Combination of Both
• Compressed air output is initially directed along
  center shaft of engine and then is compressed
  against the perimeter of engine by a later stage.

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Example of Centrifugal Flow
Airflow being forced around body of engine
Centrifugal Compressor
Intake airflow is being forced around the outside
perimeter of the engine.
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Example of Axial Flow
Multistage Axial Compressor
Center Shaft
Intake airflow is forced down the center shaft of
the engine.
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Example of Combination Flow
Centrifugal Compressor
Intake Air Flow
Axial Compressor
Intake air flow is forced down the center shaft
initially by axially compressor stages, and then
forced against engine perimeter by the
centrifugal compressor.
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Major Components of Interest

• Compressor
• Axial
• Centrifugal
• Turbine
• Axial
• Radial

Axial Compressor
Centrifugal Compressor
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Axial Compressor Operation
Axial compressors are designed in a divergent
shape which allows the air velocity to remain
almost constant, while pressure gradually
increases.
Average Velocity
AP Technician Powerplant Textbook published by
Jeppesen Sanderson Inc., 1997
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Axial Compressor Operation cont.

• The airflow comes in through the inlet and first
  comes to the compressor rotor.
• Rotor is rotating and is what draws the airflow
  into the engine.
• After the rotor is the stator which does not move
  and it redirects the flow into the next stage of
  the compressor.
• Air flows into second stage.
• Process continues and each stage gradually
  increases the pressure throughout the compressor.

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Axial Compressor Staging

• An axial compressor stage consists of a rotor and
  a stator.
• The rotor is installed in front of the stator and
  air flows through accordingly. (See Fig.)

www.stanford.edu/ group/cits/simulation/
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Centrifugal Compressor Operation

• Centrifugal compressors rotate ambient air about
  an impeller. The impeller blades guide the
  airflow toward the outer perimeter of the
  compressor assembly. The air velocity is then
  increased as the rotational speed of the impeller
  increases.

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Axial Turbine Operation
Hot combustion gases expand, airflow pressure and
temperature drops. This drop over the turbine
blades creates shaft work which rotates the
compressor assembly.
Airflow through stator
Airflow around rotor
Axial Turbine with airflow
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Radial Turbine Operation

• Same operation characteristics as axial flow
  turbine.
• Radial turbines are simpler in design and less
  expensive to manufacture.
• They are designed much like centrifugal
  compressors.
• Airflow is essentially expanded outward from the
  center of the turbine.

Radial Flow Turbine
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Gas Turbine Issues

• Gas Turbine Engines Suffer from a number of
  problematic issues
• Thermal Issues
• Blade (airfoil) Stalls
• Dynamic Surge

http//www.turbosolve.com/index.html
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Thermal Issues

• Gas Turbines are limited to lower operating
  temperatures due to the materials available for
  the engine itself.
• Operating at the lower temperature will decrease
  the efficiency of the gas turbine so a means of
  cooling the components is necessary to increase
  temperatures at which engine is run.

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Cooling Methods

• Spray (Liquid)
• Passage
• Transpiration

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Spray Cooling

• The method of spraying a liquid coolant onto the
  turbine rotor blades and nozzle.
• Prevents extreme turbine inlet temperatures from
  melting turbine blades by direct convection
  between the coolant and the blades.

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Passage Cooling

• Hollow turbine blades such that a passage is
  formed for the movement of a cooling fluid.
• DOE has relatively new process in which excess
  high-pressure compressor airflow is directed into
  turbine passages.

http//www.eere.energy.gov/inventions/pdfs/fluidth
erm.pdf
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Transpiration Cooling

• Method of forcing air through a porous turbine
  blade.
• Ability to remove heat at a more uniform rate.
• Result is an effusing layer of air is produced
  around the turbine blade.
• Thus there is a reduction in the rate of heat
  transfer to the turbine blade.

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Blade (airflow) Stalls

• When airflow begins separating from the
  compressor blades over which it is passing as the
  angle of attack w.r.t. the blades exceeds the
  design parameters.
• The result of a blade stall is that the blade(s)
  no longer produce lift and thus no longer
  produces a pressure rise through the compressor.

Separation Regions
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Dynamic Surge

• Occurs when the static (inlet) air pressure rises
  past the design characteristics of the
  compressor.
• When there is a reversal of airflow from the
  compressor causing a surge to propagate in the
  engine.
• Essentially, the flow is exhausted out of the
  compressor, or front, of the engine.
• Result, is the compressor no longer able to
  exhaust as quickly as air is being drawn in and a
  bang occurs.

Turbine Exit
Compressor Inlet
http//www.turbosolve.com/index.html
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Dynamic Surge Effects

• Cause Inlet flow is reversed
• Effect Mass flow rate is reduced into engine.
• Effect Compressor stages lose pressure.
• Result Pressure drop allows flow to reverse back
  into engine.
• Result Mass flow rate increases
• Cause Increased mass flow causes high pressure
  again.
• Effect Surge occurs again and process continues.
• Result Engine surges until corrective actions
  are taken.

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Dynamic Surge Process
Surge Point, Flow Reverses
Compressor Pressure Loss Occurs
P
No Surge Condition
Flow reverses back into engine
Corrective Action Taken
V
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Axial Compressor Design

• Assumption of Needs
• Determination of Rotational Speed
• Estimation of number of stages
• General Stage Design
• Variation of air angles

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Assumption of Needs

• The first step in compressor design in the
  determination of the needs of the system
• Assumptions
• Standard Atmospheric Conditions
• Engine Thrust Required
• Pressure Ratio Required
• Air Mass Flow
• Turbine inlet temperature

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Rotational Speed Determination

• First Step in Axial Compressor Design
• Process for this determination is based on
  assumptions of the system as a whole
• Assumed Blade tip speed, axial velocity, and
  hub-tip ratio at inlet to first stage.

Rotational Speed Equation
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Derivation of Rotational Speed

• First Make Assumptions
• Standard atmospheric conditions
• Axial Velocity
• Tip Speed
• No Intake Losses
• Hub-tip ratio 0.4 to 0.6

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Compressor Rotational Speed

• Somewhat of an iterative process in conjunction
  with the turbine design.
• Derivation Process
• First Define the mass flow into the system
• is the axial velocity range from the root
  of the compressor blades to the tips of the
  blades.

where U
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Axial Velocity Relationship
Radius to root of blade
Radius to tip of blade
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Tip Radius Determination

• By rearranging the mass flow rate equation we
  can obtain an iterative equation to determine the
  blade tip radius required for the design.

• Now Looking at the energy equation, we can
  determine the entry temperature of the flow.

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Isentropic Relationships

• Now employing the isentropic relation between the
  temperatures and pressures, then the pressure at
  the inlet may be obtained.
• Now employ the ideal gas law to obtain the
  density of the inlet air.

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Finally Obtaining Rotational Speed

• Using the equation for tip speed.
• Rearranging to obtain rotational speed.
• Finally an iterative process is utilized to
  obtain the table seen here.

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Determining Number of Stages

• Make keen assumptions
• Polytropic efficiency of approximately 90.
• Mean Radius of annulus is constant through all
  stages.
• Use polytropic relation to determine the exit
  temperature of compressor.

n 1.4, Ratio of Specific Heats, Cp/Cv
is the pressure that the compressor outputs To1
is ambient temperature
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Determine Temperature Change

• Assuming that Ca1Ca
• ? is the work done factor
• Work done factor is estimate of stage efficiency
• Determine the mean blade speed.
• Geometry allows for determining the rotor blade
  angle at the inlet of the compressor.

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Temperature Rise in a Stage

• Determine the speed of the flow over the blade
  profile.

Velocity flow over blade V1.

• This will give an estimate of the maximum
  possible rotor deflection.
• Finally obtain the temperature rise through the
  stage.

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Number of Stages Required

• The number of stages required is dependent upon
  the ratio of temperature changes throughout the
  compressor.

is the temperature change within a stage is the
average temperature change over all the stages
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Designing a Stage

• Make assumptions
• Assume initial temperature change through first
  stage.
• Assume the work-done factors through each stage.
• Ideal Gas at standard conditions
• Determine the air angles in each stage.

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Stages 1 to 2

• Determine the change in the whirl velocity.
• Whirl Velocity is the tangential component of the
  flow velocity around the rotor.

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Stage 1 to 2

• Change in whirl velocity through stage.

Alpha 1 is zero at the first stage.
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Compressor Velocity Triangles
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Pressure ratio of the Stage

• The pressure ratio in the stage can be
  determined through the isentropic temperature
  relationship and the polytropic efficiency
  assumed at 90.

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Stage Attributes

• The analysis shows that the stage can be
  outlined by the following attributes

1.) Pressure at the onset of the stage. 2.)
Temperature at the onset of the stage. 3.) The
pressure ratio of the stage. 4.) Pressure at the
end of the stage. 5.) Temperature at the end of
the stage. 6.) Change in pressure through the
stage.
Example of a single stage
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Variation in Air Angles of Blade

• Assume the free vortex condition.
• Determine stator exit angle.
• Then determine the flow velocity.

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Air Angle Triangle
Alpha 1 is 0 at the inlet stage because there are
no IGVs.
Thus, Ca1C1, and Cw1 is 0
Note This is the whirl velocity component and
not a blade spacing!
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Velocity Triangle
Red is Green is Blue is
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Variation in Air Angles of Blade

• Determine the exit temp., pressure, and density
  of stage 1
• Determine the blade height at exit.
• Finally determine the radii of the blade at
  stator exit.

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Variation in Air Angles of Blade

• Determine the radii at the rotor exit.
• Determine the whirl velocities at the blade root
  and tip.

Note That is the radius of the blade at
the tip at rotor inlet.
Note That is the radius of the blade at
the root at rotor inlet.
Note because
there is no other whirl velocity component in the
first stage.
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Finally determine the Air Angles

• Stator air angle at root of blade
• Stator air angle at middle of blade
• Stator air angle at tip of blade
• Deflection air angle at root of blade
• Deflection air angle at middle of blade
• Deflection air angle at tip of blade

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Compressor Design Example
Design of a 5 stage axial compressor
Givens
Use this and chart to get Rotational speed of
engine.
Once rotational speed is found, determine mean
blade tip speed.
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Example
Determine the total temperature rise through the
first stage.
We are designing for more than just one stage, so
we need to define an average temperature rise per
stage
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Example (Air Angle Determination)
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Example (Air Angle Determination)
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Questions???