Chap 9 Lecture

7/23/2019 Chap 9 Lecture

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Chapter 9GAS POWER CYCLES

Mehmet Kanoglu

Universit o! Ga"iantep

Copright # $he M%Gra&'(ill Companies) *n%+ Permission re,uire- !or repro-u%tion or -ispla+

$hermo-nami%s. An Engineering Approa%hSeventh E-ition in S* Units

Yunus A+ Cengel) Mi%hael A+ /oles

M%Gra&'(ill) 0122


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O34e%tives Evaluate the performance of gas power cycles for which the

working fluid remains a gas throughout the entire cycle.

Develop simplifying assumptions applicable to gas power

cycles.

Review the operation of reciprocating engines.

Analyze both closed and open gas power cycles.

olve problems based on the !tto" Diesel" tirling" and

Ericsson cycles.

olve problems based on the #rayton cycle$ the #rayton cycle

with regeneration$ and the #rayton cycle with intercooling"

reheating" and regeneration. Analyze %et&propulsion cycles.

'dentify simplifying assumptions for second&law analysis of

gas power cycles.

(erform second&law analysis of gas power cycles.


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/AS*C CO5S*6ERA$*O5S *5 $(E A5ALYS*S

O7 POWER CYCLES

*odeling is a

powerful

engineering toolthat provides great

insight and

simplicity at the

e+pense of some

loss in accuracy.

*ost power&producing devices operate on cycles.

*-eal %%le.A cycle that resembles the actual cycle

closely but is made up totally of internally reversible

processes+

Reversi3le %%les such asCarnot %%lehave the

highest thermal efficiency of all heat engines

operating between the same temperature levels.,nlike ideal cycles" they are totally reversible" and

unsuitable as a realistic model.

-hermal efficiency of

heat engines

-he analysis of many comple+

processes can be reduced to

a manageable level by

utilizing some idealizations.


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-he ideal cycles are internally reversible, but" unlike the 0arnot cycle" they are not

necessarily e+ternally reversible. -herefore" the thermal efficiency of an ideal

cycle" in general" is less than that of a totally reversible cycle operating between

the same temperature limits. 1owever" it is still considerably higher than the

thermal efficiency of an actual cycle because of the idealizations utilized.


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$he i-eali"ations an- simpli!i%ations in the

analsis o! po&er %%les.

3. -he cycle does not involve any friction.

-herefore" the working fluid does not

e+perience any pressure drop as it flows in

pipes or devices such as heat e+changers.

2. All e+pansion and compression processes

take place in a quasi-equilibrium manner.

). -he pipes connecting the various

components of a system are well

insulated" and heat transfer through them

is negligible.

0are should be e+ercised

in the interpretation of the

results from ideal cycles.

!n both P-v and T&s diagrams" the area enclosed

by the process curve represents the net work of the

cycle.

!n a T&s diagram" the ratio of the

area enclosed by the cyclic curve to

the area under the heat&addition

process curve represents the thermal

efficiency of the cycle.Any

modification that increases the ratio

of these two areas will also increase

the thermal efficiency of the cycle.


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$(E CAR5O$ CYCLE A56 *$S

8ALUE *5 E5G*5EER*5G

P-v and T&s diagrams of

a 0arnot cycle.

-he 0arnot cycle is composed of four totally reversible

processes isothermal heat addition" isentropice+pansion" isothermal heat re%ection" andisentropic

compression.

For both ideal and actual cycles:Thermal efficiency

increases with an increase in the average temperature

at which heat is supplied to the system or with a

decrease in the average temperature at which heat isrejected from the system.

A steady&flow 0arnot engine.


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A*R'S$A56AR6 ASSUMP$*O5S

-he combustion process is replaced by

a heat&addition process in ideal cycles.

Air'stan-ar- assumptions

3. -he working fluid is air" whichcontinuously circulates in a closed loop

and always behaves as an ideal gas.

2. All the processes that make up the

cycle are internally reversible.

). -he combustion process is replaced by

a heat&addition process from an

e+ternal source.

/. -he e+haust process is replaced by a

heat&re%ection process that restores the

working fluid to its initial state.

Col-'air'stan-ar- assumptions6hen the working fluid is considered

to be air with constant specific heats at room temperature72809.

Air'stan-ar- %%le.A cycle for which the air&standard assumptions are

applicable.


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A5 O8ER8*EW O7 REC*PROCA$*5G E5G*5ES

;omenclature for reciprocating engines.

Spar'ignition :S*; engines

Compression'ignition :C*; engines

0ompression ratio

*ean effectivepressure


9/33 / stroke > 2 revolution

$&o'stroe %%le

3 cycle > 2 stroke > 3 revolution


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-he thermal efficiency of the

!tto cycle increases with the

specific heat ratio k of the

working fluid.

-hermal efficiency of the ideal

!tto cycle as a function of

compression ratio 7k =3./9.

'n ' engines"

the

compression

ratio is limited

byautoignition

or engine

no%.


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6*ESEL CYCLE. $(E *6EAL CYCLE

7OR COMPRESS*O5'*G5*$*O5 E5G*5ES

'n diesel engines" the spark plug is replaced

by a fuel in%ector" and only air is compressed

during the compression process.

'n diesel engines" only air is compressed during the

compression stroke" eliminating the possibility ofautoignition 7engine knock9. -herefore" diesel engines

can be designed to operate at much higher compression

ratios than ' engines" typically between 32 and 2/.

2'0isentropic

compression0'


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-hermal

efficiency of the

ideal Diesel cycle

as a function of

compression and

cutoff ratios

7k=3./9.

0utoff

ratio

for the same compression ratio


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>UES$*O5S ???

Diesel engines operate at

higher air&fuel ratios than

gasoline engines. 6hy?

Despite higher power to

weight ratios" two&stroke

engines are not used in

automobiles. 6hy?

-he stationary diesel

engines are among the

most efficient power

producing devices 7about

=@9. 6hy?

6hat is a turbocharger?6hy are they mostly used

in diesel engines

compared to gasoline

engines.

P-v diagram of an ideal dual cycle.

6ual %%le.A more realisticideal cycle model for modern"

high&speed compression ignition

engine.


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S$*RL*5G A56 ER*CSSO5 CYCLES

A regenerator is a device that

borrows energy from the working

fluid during one part of the cycle

and pays it back 7without interest9

during another part.

Stirling %%le2'0T =constant e+pansion 7heat addition from the e+ternal source9

0'


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-he e+ecution of the tirling cycle. A steady&flow Ericsson engine.

-he Ericsson cycle is very much like the

tirling cycle" e+cept that the two constant&

volume processes are replaced by two

constant&pressure processes.

#oth the tirling and Ericsson cycles are

totally reversible" as is the 0arnot cycle"

and thus

-he tirling and Ericsson cycles

give a message egeneration

can increase efficiency.


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/RAY$O5 CYCLE. $(E *6EAL CYCLE

7OR GAS'$UR/*5E E5G*5ES

An open&cycle gas&turbine engine. A closed&cycle gas&turbine engine.

-he combustion process is replaced by a constant&pressure heat&additionprocess from an e+ternal source" and the e+haust process is replaced by a

constant&pressure heat&re%ection process to the ambient air.

3&2 'sentropic compression 7in a compressor9

2&) 0onstant&pressure heat addition

)&/ 'sentropic e+pansion 7in a turbine9

/&3 0onstant&pressure heat re%ection


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Chap 9 Lecture

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