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TurbinesSteam and Gas Units

Ch 5 in El-Wakil

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Turbines

Consider the following schematic of a vapour power plant.

Until now, we have focused most of our attention onsubsystems A and B

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Turbines

Within subsystem B is the turbine, which merits special

attention. We will now consider steam and gas turbines inmore detail.

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Steam Turbines

Before 1900, steam power plants werereciprocating engines to convert enthalpy to work

1909: Steam turbines started to get implemented

with 12 MW units being installed

siemens.com

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Steam Turbines

1929: A 208 MW unit is installed in NYC

1950s: 450 MW steam turbines were being

installed

Capacities have now risen to over 1000 MW

siemens.com

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Gas Turbines

1903: The first successful gas turbine was built in

France (with = 3% it was not practical)

1939: First flight of a jet plane powered by a gas

turbine

flightglobal.com

http://www.google.ca/imgres?q=747&um=1&hl=en&rlz=1T4SKPB_enUS300US300&biw=1020&bih=535&tbm=isch&tbnid=-qtpQDftk6k2rM:&imgrefurl=http://www.flightglobal.com/news/articles/boeing-predicts-passenger-747-8-order-this-year-205071/&docid=see27bP4wmR3lM&imgurl=http://www.flightglobal.com/assets/getasset.aspx%253Fitemid%253D11691&w=445&h=297&ei=_-RTT836KqHd0QGfk6HxDQ&zoom=1&iact=rc&dur=0&sig=102898375750689644062&page=3&tbnh=157&tbnw=198&start=20&ndsp=12&ved=1t:429,r:7,s:20&tx=64&ty=62

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Gas Turbines

WWII: Swiss engineers develop gas turbines for

power generation that are efficient enough for use

After WWII, jet fighters and passenger jets became

widespread in many countries

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Turbine Uses for Power

Steam turbines are used for municipal power

generation, normally providing base load power from

large installations

Gas turbines: Used as peaking units

Power isolated locations such as oil rigs

Power oil pipeline routes

Used with steam turbines in combined cyclepower plants

To understand the physics of blade motion, we

revisit the Impulse Principle

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The Impulse Principle

Consider a horizontal jet impinging in the +x

direction on a flat plate.

The fluid imparts a horizontal force, called an

impulseto the plate

F= the force of the impulse (N)

= mass flow rate of the jet (kg/s)

= velocity of the jet in the x-direction (m/s)

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The Impulse Principle

If the plate is moving as the jet hits it, than the

velocity of the jet relativeto the plate is Vs- V

b

Now:

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The Impulse Principle

Work is being done on the plate, because the force

is being applied over a distance (recall W=Fd.

Power is work done per unit time =Fd/t

Alternatively:

Or:

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The Impulse Principle

The efficiency of the transfer or power from the jet

to the plate, can be found by dividing the power

by the power of the water jet

to get the efficiency

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The Impulse Principle

In a gas turbine with a steam jet impinging on a

blade, maximizing power transfer is an objective

This can be done by differentiating w/r/t and

setting it equal to zero (see El-Wakil 5.2).

This gives:

Note that this is50% of the power

in the incoming

water

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The Impulse Principle

Turbine blades are rounded. Consider a surface

that is rounded instead of flat, which causes the jet to

change direction.

The fluid velocity, relative to the surface is still

Vs - Vb. The fluid exit velocity, relative to the surface isalso V

s- V

b.

This will increase the impulse that is imparted to

the rounded surfaces

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The Impulse Principle

Using the same analysis as with the flat plate,

impulse, power, and blade efficiency are:

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The Impulse Principle

Again, the blade velocity which maximizes power

transfer can be found by differentiating power and

setting the expression equal to zero.

Note that this is100%of the

power in the

incoming water

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Example 14

A jet of water with a mass flow rate of 0.84 kg/s and

a velocity of 0.66 m/s impinges on a curved surfaces

and propels it forward. The efficiency with which

power is transferred from the jet to the surface is

78%.A) Calculate the velocity of the surface

B) Calculate the power transferred from the jet to

the surface

C) Calculate the velocityof the surface and power

transferred as functions of

varying efficiency

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The Impulse Principle

In a turbine, the blades are rotating away from a jet,

so there must be a continuous series of blades.

Turbines typically have 30 - 60 blades per rotor.

In the confined space of a gas turbine, it is

impossible to have the jet impinging directly on theblades

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The Impulse Principle

The jet impinges at an angle

The jet also exits at an angle close to so that it

can be redirected to another row of blades

General FlowDirection