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Axial-Flow Turbines• Axial-flow turbines power most gas turbine units, except the

smaller horsepower turbines, and they are more efficient than radial-inflow turbines in most operational ranges.

• There are two types of axial turbines: (1) impulse type turbine has its entire enthalpy drop in the nozzle; therefore it has a very high velocity entering the rotor. (2) reaction type turbine divides the enthalpy drop in the nozzle and the rotor.

• Most axial flow turbines consist of more than one stage, the front stages are usually impulse (zero reaction) and the later stages have about 50% reaction.

• The impulse stages produce about twice the output of a comparable 50% reaction stage.

• The efficiency of an impulse stage is less than that of a 50% reaction stage.

2Schematic of an axial flow turbine flow characteristics

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Impulse Turbine• The impulse turbine is the simplest type of

turbine. It consists of a group of nozzles followed by a row of blades.

• The gas is expanded in the nozzle, converting all the high thermal energy into kinetic energy.

• The high-velocity gas impinges on the blade where a large portion of the kinetic energy of the moving gas stream is converted into turbine shaft work.

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• The static pressure decreases in the nozzle with a corresponding increase in the absolute velocity. The absolute velocity is then reduced in the rotor, but the static pressure and the relative velocity remain constant.

• To get the maximum energy transfer, the blades must rotate at about one-half the velocity of the gas jet velocity.

Schematic of an impulse turbine

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• Two or more rows of moving blades are sometimes used in conjunction with one nozzle to obtain wheels with low blade tip speeds and stresses.

• In-between the moving rows of blades are guide vanes that redirect the gas from one row of moving blades to another. This type of turbine is sometimes called a Curtis turbine.

Pressure and velocity distributions in a Curtis-type impulse turbine

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• Another impulse turbine is the pressure compound or Ratteau turbine. In this turbine the work is broken down into various stages. Each stage consists of a nozzle and blade row where the kinetic energy of the jet is absorbed into the turbine rotor as useful work.

• The air that leaves the moving blades enters the next set of nozzles where the enthalpy decreases further, and the velocity is increased and then absorbed in an associated row of moving blades.

• The total pressure and temperature remain unchanged in the nozzles, except for minor frictional losses.

7Pressure and velocity distributions in a Ratteau-type

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Reaction Degree• The axial-flow reaction turbine is the most widely

used turbine. • By definition, the impulse turbine has a degree of

reaction equal to zero.• This degree of reaction means that the entire

enthalpy drop is taken in the nozzle, and the exit velocity from the nozzle is very high.

• Since there is no change in enthalpy in the rotor, the relative velocity entering the rotor equals the relative velocity exiting from the rotor blade. For the maximum utilization factor (ratio of ideal work to the energy supplied), the absolute exit velocity must be axial.

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Reaction Turbine• In a reaction turbine both the nozzles and blades act as

expanding nozzles. Therefore, the static pressure decreases in both the fixed and moving blades.

• The fixed blades act as nozzles and direct the flow to the moving blades at a velocity slightly higher than the moving blade velocity.

• In the reaction turbine, the velocities are usually much lower, and the entering blade relative velocities are nearly axial.

• The 50% reaction turbine has been used widely and has special significance. The velocity diagram for a 50% reaction is symmetrical and, for the maximum utilization factor, the exit velocity must be axial.

• The 50% reaction turbine has the highest efficiency of all the various types of turbines.

10Schematic of a reaction-type turbine

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The effect of inlet velocity and air angle on the utilization factor

Comparison Impulse & Reaction Turbine• The work produced in an impulse turbine with a

single stage running at the same blade speed is twice that of a reaction turbine. Hence, the cost of a reaction turbine for the same amount of work is much higher, since it requires more stages.

• It is a common practice to design multistage turbines with impulse stages in the first few stages to maximize the pressure decrease and to follow it with 50% reaction turbines. The reaction turbine has a higher efficiency due to blade suction effects. This type of combination leads to an excellent compromise, since otherwise an all-impulse turbine would have a very low efficiency, and an all-reaction turbine would have an excessive number of stages.

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Turbine Blade Cooling Concepts• The turbine inlet temperatures of gas turbines have

increased considerably over the past years. This trend has been made possible by advancement in materials and technology, and the use of advanced turbine blade cooling techniques.

• The first stage blade must withstand the most severe combination of temperature, stress, and environment; it is generally the limiting component in the machine.

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Firing temperature increase with blade material improvement

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• Importance of this increase can be appreciated by noting that an increase of 100 F (56 C) in turbine firing temperature can provide a corresponding increase of 8-13% in output and 2-4% improvement in simple-cycle efficiency.

• The cooling air is bled from the compressor and is directed to the stator, the rotor, and other parts of the turbine rotor and casing to provide adequate cooling.

• The effect of the coolant on the aerodynamics depends on the type of cooling involved, the temp. of the coolant compared to the mainstream temperature, the location and direction of coolant injection, and the amount of coolant.

• Pressure of the cooling air must be at higher pressure than that part of flow path to be cooled

• Blades are cooled by combination of internal and external cooling– Internal cooling : Convection driven– External cooling : Film Cooling

COOLING SYSTEMS

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Turbine Blade Cooling Methods1. Convection cooling, flowing air inside the turbine blade. The

most common technique.2. Film cooling, allowing the cooling air to established an

insulating layer between the hot gas and the blades.3. Water cooling, passing water through tubes embedded in the

blade. The most promising technique as keeping temperature below 540C

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4. Impingement cooling, blasting air on the inner surface of the blades by high velocity air jets.

5. Transpiration cooling, passing the cooling through the porous wall of the blade

INTERNAL COOLING

• Cooling air is pumped through inside of blades– Air is pumped in at root and

makes multiple passes before exiting at root

• Material is cooled by forced convection on inside surface and by conduction through blade

• Different regions of blades can have different cooling profiles

• Large surface area on inside• Many designs employ roughened

internal microfin structure

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The strut insert (convection and impingement cooling) design has a midchord section that is convection-cooled through horizontal fins, and a leading edge that is impingement cooled. The coolant is discharged through a split trailing edge. The air flows up the central cavity formed by the strut insert and through holes at the leading edge of the insert to impingement cool the blade leading edge. The air then circulates through horizontal fins between the shell and strut, and discharges through slots in the trailing edge.

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The midchord region is convection-cooled, and the leading edges are both convection and film-cooled. The cooling air is injected through the blade base into two central and one leading edge cavity. The air then circulates up and down a series of vertical passages. At the leading edge, the air passes through a series of small holes in the wall of the adjacent vertical passages, and then impinges on the inside surface of the leading edge and passes through film cooling holes. The trailing edge is convection-cooled by air discharging through slots.

FILM COOLING

FILM COOLING BEHAVIOR

FILM COOLING OPTIONS

FILM COOLING OPTIONS

EXAMPLE OF FILM-COOLING DATA

cr

rfr

TTTT

• Cooling Effectiveness, (often called adiabatic film effectiveness)• Tr = Adiabatic recovery temperature = the temperature wall would reach if

adiabatic (no heat transfer) in the absence of film cooling• Trf = recovery temperature in the presence of film cooling

x/D

TREMENDOUS AMOUNT OF DATA AVAILABLEB

≡ (r

U) je

t / (r

U) ∞

~ 0

.5B

≡ (r

U) je

t / (r

U) ∞

~ 1

.0

COOLING JET BEHAVIOR by CFD

-100

2780

2492

2204

1916

1628

1340

1052

764

476

188

°F K

200

1800

1640

1480

1320

1160

1000

840

680

520

360

A-A: x/D = 10

A-A

x/D = 10

A-A

Attached Jet: B ≡ (rU)jet / (rU)∞ ~ 0.5

Lifted Jet: B ≡ (rU)jet / (rU)∞ ~ 2.0

TRANSPIRATION COOLING• Wire cloth or mesh is used

for exterior of blade and air is leaked uniformly through it– Consists of a plurality of

wires made of metal, ceramic or other materials, and arranged with their longitudinal axes generally and not necessarily precisely, parallel to the blade axis, either with or without a stiffener insert

• Ample porosity is provided for transportation cooling

• Cools surface and provides a protective layer

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The shell attached to the strut is of wire from porous material. Cooling air flows up the central plenum of the strut, which is hollow with various-size metered holes on the strut surface. The metered air then passes through the porous shell. The shell material is cooled by a combination of convection and film cooling. This process is effective due to the infinite number of pores on the blade surface.

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Temperature distribution for a multiple small-hole design (F, cooled)

With this particular design, primary cooling is achieved by film cooling with cold air injected through small holes over the airfoil surface. These holes are considerably larger than holes formed with porous mesh for transpiration cooling. Also, because of their larger size, they are less susceptible to clogging by oxidation. In this design, the shell is supported by cross ribs and is capable of supporting itself without a strut under engine operating conditions.

Water-Cooled

33Internal of the frame FA blades, showing cooling passages

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Turbine Losses• The primary cause of efficiency losses in an axial-

flow turbine is the build-up of boundary layer on the blade and end walls.

• The losses associated with a boundary layer (profile losses) are viscous losses, mixing losses, and trailing edge losses. The blade shape and the pressure gradient to which the flow is subjected are major factors in this type of loss.

• The profile loss from this type of boundary-layer build-up results in losses of stagnation pressure.

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• The endwall losses are also due to a loss of momentum. Endwall losses are often combined with secondary losses, since adjacent blade profiles cause a pressure gradient from the pressure surface to the suction surface.

• The blade loading is produced by the different pressures on the opposite side of the same blade. The pressure gradient across the blade passage induces flow from the higher to the lower-pressure regions. This secondary flow causes losses and results in vorticity in the exit flow.

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• Tip clearance loss occurs when the blade tip is mechanically free of the shroud casing, and the pressure gradient across the blade thickness induces flow leakage through the clearance space. This flow across the tip causes turbulence, a pressure drop, and interferes with the main stream flow.

• Another loss is caused by flow incidence when the gas angle and the blade angle of the flow do not coincide, resulting in a disruption of the flow at the blade leading edge.

• Wheel friction loss occurs in an axial-flow compressor because of the close clearances between the casing and the rotor disc.

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