11-pt12-turb_comp matching 30 [compatibility mode]

8/17/2019 11-PT12-Turb_Comp Matching 30 [Compatibility Mode]

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RMD510

Session delivered by:Session delivered by:

. . .. . .

1© M.S.Ramaiah School of Advanced Studies, Bengaluru15


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RMD510

Session Objectives

• To discuss the operating

and turbines• To understand the basic

conditions for compressor and

turbine matching

• in a single shaft gas turbine

• To discuss the matching of gas

generator with free powerturbine and nozzle



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RMD510Introduction

The main components of a gas turbine engine are: inlet diffuser,

compressor, combustion chamber, turbine, and exhaust nozzle.

The individual components are designed based on established procedures and their performances are obtained from actual tests.

en ese componen s are n egra e n an eng ne, e range o

possible operating conditions is considerably reduced.

The roblem is to find corres ondin o eratin oints on the

characteristics of each component when the engine is running at a

steady speed or in equilibrium.

The equilibrium running points for a series of speeds may be plottedto on the compressor characteristics and joined up to form an

equilibrium running line or equilibrium running diagram.



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RMD510Introduction

The equilibrium running

diagram also shows the proximity

of the operating line to the

compressor surge line. If it intersects the surge line, the

gas ur ne w no e capa e o

being brought up to full speed

without some remedial action.

It also shows whether the engine

is operating in a region of

Ideally the operating line should

lie near the locus of the points of


maximum compressor efficiency.


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RMD510Test Cases

Three cases are discussed here:

A single shaft gas turbine delivering shaft power

A free turbine engine where the gas generator turbine drivesthe compressor and the power turbine drives the load

A simple jet engine with a propelling nozzle

The gas generator performs exactly the same function for both theree tur ne eng ne an t e et eng ne.

The flow characteristics of a free turbine and a nozzle are similar.

Hence, the free turbine engine and the jet engine arethermodynamically similar and differ only in the manner in which

the output is utilised.



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RMD510Test Cases

6© M.S.Ramaiah School of Advanced Studies, Bengaluru15Brayton cycle for case (b) and (c)


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RMD510Component Characteristics

Compressor characteristics Turbine characteristics



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RMD510Assumptions

Turbine characteristic is represented by a single line,

ecause s oun n prac ce a ur nes o no ex

any significant variation in non-dimensional flow with non-dimensional speed.

Inlet and exhaust losses are considered negligible.

of the compressor delivery pressure.



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RMD510Single Shaft Gas Turbine

Pressure ratio across the turbine is determined by the compressor pressure

ratio and combustor pressure loss.

–

Fuel flow.

Procedure for obtainin an e uilibrium runnin oint is as follows:



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Compressor and turbine are directly coupled, hence

Speed compatibility (1)

Flow compatibility

From combustor pressure loss:

Assuming m1 = m3 = m(2)



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RMD510Single Shaft Gas TurbineIf T 01 is specified, then obtain T 03 from eqn (2) and from eqn (1).

Obtain turbine efficiency from turbine characteristics using the known

03 04.

Turbine temperature drop (3)

(4)

(5)



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If the engine is coupled to a dynamometer on the test bed, then the load could beset independently of the speed and it would be possible to operate at any point on

.

If a propeller is the load, then Power N 3.

The roblem is to find the sin le oint on each constant s eed line of com ressor

characteristic which will give the required net power output at that speed.

This can only be done by trial and error, taking several operating points on the


.


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Generator runs at constant speed

with load varied electrically.

Each point on this line represents a

erent va ue o tur ne n et

temperature and power output.


oa c arac er s c

of a propeller

qu r um runn ng nes


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RMD510Equilibrium Running of a Gas Generator

A gas generator performs the same function for the free turbine engine and the jetengine. It generates high pressure, high temperature gas for expansion through the

turbine or the nozzle.

Eqns. 1 and 2 are applicable for speed and mass flow compatibility.

The turbine pressure ratio is not known and can be determined by

(6)

. ,03 01

to determine (by trial and error) the turbine inlet temperature required for operation

at any arbitrary point on the compressor performance map.


ssuming t at t e tur ine non- imensiona ow is in epen ent o t e non-

dimensional speed , the procedure is as follows:


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(4)

(2)

(1)

(6)(3)

(2)



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(2) (6)

Calculations can be carried out for a large number of points and the

results can be presented on the compressor characteristics by the locus ofconstant T 03/T 01 (see figure in slide 18).

However, the flow compatibility with the component downstream

power ur ne or nozz e w res r c e opera ng zone on ecompressor characteristic.



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Note:

(1) (2)

(3) (6)



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RMD510Matching of Gas Generator with Free Turbine

The mass flow leaving the gas generator is equal to that entering the powerturbine.

Pressure ratio across the ower turbine is fixed b the ressure ratios across the

compressor and gas generator turbine.

The characteristic of the power turbine will have the same form as of the gas

,

(7)

where (8)



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The corresponding pressure ratio across the power turbine can be given as

For stationary gas turbines (ignoring the inlet and exit duct losses), po1 = pa andthe power turbine outlet pressure is also pa.

(7)

in slide 21.



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RMD510

Generator with Free

(a) Iteration for gas generator


for free power turbine


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in Slide 18.

in Slide 13.



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RMD510Matching of Gas Generator with Nozzle

The propelling nozzle area for a jet engine is fixed from design point calculations.

Propelling Nozzle Characteristics

Once the nozzle size is fixed, it has major influence on off-design operation. The mass flow parameter is given by

(12)

nozzle area

(13)

(14)



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(14)(15)

in Slide 25

(14)


(16)


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Likewise, with the nozzle unchoked,

is given by eqn.13; whereas when it is choked,

C 5 is the sonic velocity and M 5 is unity.

Recallin that

we have the general relation

(17)

and when the nozzle is choked, we have



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The flow characteristics of nozzle and free turbine are similar.

For operation of a jet engine under static conditions, the behaviour of nozzle is

same as that of a free turbine.

Hence, the equilibrium running line can be determined according to the flow chart

of slide 21, with the nozzle characteristic replacing the power turbine characteristic.

, .

Forward speed produces a ram pressure ratio, which is a function of both flight

Mach number and intake efficiency.

e compressor e very pressure an nozz e n e pressure ncrease, ea ng o

increase in nozzle pressure ratio.

If the nozzle chokes, then the mass flow rate becomes maximum and is independent

o nozz e pressure ra o an orwar spee .

Hence, the turbine operating point will also remain unchanged.

Therefore, under choked nozzle condition, the equilibrium running line will be


uniquely determined by the fixed turbine operating point and will be independent of

the flight speed.


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Usually the nozzle is choked during take-off, climb and cruise, and may remain

unchoked while preparing to land or taxiing, when the thrust is significantly reduced.

Hence, the running line is affected at low forward speeds when the engine

rotational speed is also low and the running line is close to the surge line.

The nozzle pressure ratio p04/ pa is linked to the ram pressure ratio

The ram pressure ratio is given by

(19)

(20)

Now the procedure of flow chart (slide 21) can be followed with eqn. 19 substitutedfor eqn. 8, but for each compressor speed line the calculation is repeated for several

values of M coverin the desired ran e of fli ht s eed.



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The result is a fan of

equilibrium running lines of

constant M a, merging into a

single running line at higher

compressor speeds when thenozzle is choked.

Increasing the Mach number

pushes the equilibrium running

line away from the surge line atlow compressor speeds,

because the ram pressure rise

allows the compressor to utilise

a ower pressure ra o or

pushing the required flow

through the nozzle.



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RMD510Session Summary

Gas turbine component characteristics have been explained.

The procedure for matching of turbine and compressor in a

simple single shaft gas turbine is discussed.

.

The procedures of matching the gas generator to a power turbine

and a propelling nozzle are explained.



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RMD510

T an you


11-pt12-turb_comp matching 30 [compatibility mode]

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