APPLIED THERMODYNAM ICS GAS TURBINES & JET PROPULSION

6. GAS TURBINES AND JET PROPULSION

Introduction:
Gas turbines are also prime movers producing mechanical power from the heat generated
by the combustion of fuels. They are used in aircraft, some automobile units, industrial
installations and small - sized electrical power generating units. A schematic diagram of a
simple gas turbine power plant is shown below. This is the open cycle gas turbine plant.

[l,qhirurl
r,.+-l
C.'I5 li5

Fig. Open cycle gas turbine engine.

Working:
Air from atmosphere is compressed adiabatically (idealized) in a compressor (usually
rotary) i.e., Process 1-2. This compressed air enters the combustion chamber, where fuel is
injected and undergoes combustion at constant pressure in process 2-3. The hot products of
combustion expand in the turbine to the ambient pressure in process 3-4 and the used up exhaust
gases are let out into the surroundings.
.T
P Cc,ar.F;tl<.t*'"r'. ?

a\
, 1',,
(v.

\..-,
.J

N. SATHEESH KUMAR, DEPT OF MECHANICAL ENGG., Page 1

APPLIED TH ERMODYNAM ICS GAS TURBINES & JET PROPULSION

The compressor is usually coupled to the turbine, so that the work input required by the
compressor comes from the turbine. The turbine produces more work than what is required by
the compressor, so that there is net work output available from the turbine.
Since the products of combustion cannot be re-used, real gas turbines work essentially in
open cycles. The p-v and T-s diagrams of such aplant are shown
t:r..
.r
Brayton Cycle: This is the air-standard cycle for the gas turbine plant. It
consists of two
reversible adiabatic processes and two reversible isobars.(constant pressure processes). The p-v
and T-s diagrams of a Brayton Cycle are as shown below.

T,

.Er

"lout

Process 1 - 2: Reversible adiabatic compresslon.

2 - 3: Reversible constant pressure heat addition.
3 - 4: Reversible adiabatic expansion.
4 - l: Reversible constant pressure heat rejection.
A schematic flow diagram of this somewhat hypothetical gas turbine plant is shown below.
Though this plant works on a closed cycle, each of the four devices in the plant is a
steady-flow device, in the sense that there is a continuous flow of the working fluid (air) through
each device. Hence, the steady-flow energy equation is the basis for analysis, and can be applied
to each of the four processes. Neglecting changes in kinetic and potential energies, the steady
flow energy equation takes the from
Q-W:Ah
: C,.AT (Since air is assumed to be an ideal gas)

Process 1 - 2 is reversible adiabatic, hence Qr-z : 0

Wr-z: - Cp.AT = - Cp (T2 - T1): - ve, work input

Work of compression W": :

lWr - zl Cp (Tz-Tr)

N. SATHEESH KUMAR, DEPT OF MECHANICAL ENGG., Page 2

APPLIED THERMODYNAM ICS GAS TURBINES & JET PROPULSION

Corresponding to rr: (rp)opt i.e., when Wn.1is maximum, cycle efficiency is

1 1-r
,T
t&t' ';11-I-il--l-
' ' )r:x
p

;r"-,",
lt:1-
, I """'
1T
- rri(l}
\

Open Cycle Gas Turbine Plants:

eerr',fuAl,iat)

Yft(

/''
,/ I
foa-td){,

N, SATHEESH KUMAR, DEPT OF MECHANICAL ENGG., Page 20

APPLIED THERMODYNAMICS GAS TURBINES & JET PROPULSION

In practice, a gas turbine plant works on an open cycle. Air from atmosphere is first compressed
to a higher pressure in a rotary compressor, which is usually run by the turbine itself, before it
enters the combustion chamber. Fuel is injected into the bombustion chamber where it undergoes
combustion. The heat released is absorbed by the products of combustion and the resulting high
temperature; high pressure products expand in the turbine producing work output. The used up
combustion products (exhaust gases) are let out into the atmosphere. In the ideal case,
compression and expansion are assumed to be isentropic and combustion is assumed to take
place at constant pressure. The schematic flow diagram and p-v and T-s diagrams of an opetl
cycle gas turbine plant are as shown above. p

Advantages and disadvantages of closed cycle over open cycle

Advantages of closed cycle:

i) Higher thermal efficiencY
ii) Reduced size
iii) No contamination
iv) Improved heat transmission
v) Improved part load q
vi) Lesser fluid friction
vii) No loss of working medium
viii) Greater output and
ix) Inexpensive fuel.

Disadvantages of closed cycle:

i) Complexity
ii) Large amount of cooling water is required. This limits its use of stationary installation
or marine use
iii) Dependent system
iv) The weight of the system per kW developed is high comparatively, .'. not economical
for moving vehicles
v) Requires the use of a very large air heater.

N. SATHEESH KUMAR, DEPT OF MECHANICAL ENGG., Page2l

 I

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3.2 JET PROPULSION

A11 the phenomenon of propulsive devices are based on Newton's second and thirci laws of
motion. In these devices a change in mornenl'um of a mass of fluid (propellanti in:piuts its energy
due to combustion and the reaction of this acueierating fiuid mass results in a propulsive force or
rhrust- The propulsive force resulted in a reaction opposite to thqof the accelerating fluid mass, is
used to propel ttre engine t
The propulsive devices are classified as

l) Air-breathing engines
These engines nray be either reciprocating engines or gas iurbine engines which use
atmospheric air for propulsion. Some of the gar turbine engines are turbo jet, rurbo prop, rarn jet.
pulse jet etc.
2) Rocket engines.
These cngines use a combination of liquid or solid propellants and cairv their oivn suppl;- of
oxidant. They are employed for space applications'
 rl* !{i
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fi6s iurbines ond

3,2.T AIA.BREAT}{ ING ENGINES

A Ttl&IlO.lBT HNGINI:I
3. i 5 Lrelorv. It consists of a difTuser,
A Schenraric di::grarr r:f a rurbojer *ngine is shown in Fig.
cornpressor. coml -::;tici: charnber,
ttiurne and a nozzle. The iiir inclucted in the di.fflusel reduces its
br-rt incrrases thc pressr.tre . 11-,e high pressure
air is compressed again in either an axiatr
".iu.ity The compresseC air p;isses througi. a cornbustion
flsrv comp-.r.essor or & ccn;ifugal campre.rsor.
ol combusiicir enter & gi,*s turbi*e and expand
c-hamber *|^,cr* ct':m6ustion ralis
place. The prociucts
br the turbine is useri to drive the compressor and stlier
,o u io**r pre ssure . The power rleveiop*,1
pump etc. About two rhixl of the energy available at tire
auxiliaries likr: fue I pump, lubricating
gas expa:rds in the nezzle' ihe pressute energy is converted
exhaust- ir rised for this purpose. As thi
ffiil;;;;;;;; ffiil tn,*.y r",igi uerocity. The reaclion due ro rhis which accounts ror one-
prcrduces a proprilsive lurce' An after trun]er is used to
thkti of availahle energy oitr,. *rio,,rr.
bunr rhe leiiner comhustion mix{nre afi*r
e>lpanding ix ttie turbine' P
,Tulbine
iAiler hurncr

*1
#}

High
---+h i velocity
---t, i
I

---ri
**-+"i
-*-+i -{
Ilifluser Combustion
chambct'
,Fig. 3.i5 : Turba P! e.r,gi*e

5 Ttl&i1{}-PlI(}tr, F.l\iGllcu
"fh{.: 'turb{r-prop engine uses a propeller in lronf of the engine as shorvn in Fig' 3'16- This
the mass fl ow rate oi ai r in the engine. The plopeller speed can be controlled
mr:rdi flicatic:n i ncreases
b1' -
uirrloctir:n gear rnrchanism'
"' charnber' a turbine' an extraust
;;. **g,ri consirts of u difiuser, a comllressor. combustior:
cornhustion chambcr ar.rd the comprrDssor
nozrle. a rr:cluctlt.)n gco. au,J propeller. The
difiuser
jet The turtine exttacts much niore porver frorn
furctiorr ii: tirl sa*ir ,r-,innro ,, i,rrrre turbc
€ngine.
of exhar"rst
thr avrrii*hle *nergy at rnu ieaving anly about 107c' of enei'gy far acceleration
t,t'tuo"
the rnaio ngine and ir circuisted
:-n. ,,ir ti'w piocluc,:d b!, ,,re propeller bypasses
e
gas*s in *re n*zzic. anc
is the sum of the rl.uusts pioducei by lire nozzle
around ri:e errgine. r.n* rrtuitl.,.*st pr:oducld axiai flor'v
lhe conpressor {u'iric}i is usually
thl prr:;peiirr. Tire turtlne n,oy ir* t,rupr.d to both
r;,p.1 r::,j iht i.rr:cpe.}Icr Llsrng
lhe reduction year'
 I
194 Applied Thermodynomics "

Free turbine

I
o co o Oq

I
.
oo',t o ao
Propelier ilcduction gear
purbine
Clombustor
w
irg. J,Id; Tarbo-prop eugine

T] RA]!f ,TE? ENGINE

il
is tl;r.: siinplcs{ t-l Pr- rri rrl! I;ie prr,l::lsi"'c dcvicrr us sir,-,$n i:: Fig 3.11.
-i}e
engine eonsists
of a sute rsonic diffuse r { i-11} ;r riilbsixic tljffLrs*r,rrr{ir-ir'l i3.-3), a cotl}b*sti{}n charnlrer (3-4) and a
discliargt iiirzzlc section. {-1-5J. 'l'ir* sirp*l'.io:iic and [uhso:ii.-r elillrsers coi]\,crt kinetic energy of
the entering aii itlio ixe-(srr(l rlieig;v. fhis pressure rise in lirii,:rvn lr r;im pressure and it is sufficient
for comtrurtioir.

Super ;j Subson ic , 1 4J
Ctrt:lbuslor
sonic
diftuser i k- xozzte -+i
rlifli:;er

;.- i-;';: --pl{*- Co[rbustor Nozzi' -;

Ccntral body
Fig . 3.17 : F"anriet engiw
Air 1rc:m the air.nospherrl sfiieis fie engir,;e rvhete its r,e k:cit)'decreases and pressute increa.ses
in rhe super.ronic dilluser. Air i-rcrv eniers the sebscrnic difiiiscr ancl its piessttre is raised furlher and
then il:w-c ilrri.rugh the combilsior. Air gets heated io ii lrigh ttrrperature of about 150C0C tt)
l0rJ0cc in ti"re cornbuslcr hr' bi,rning iur:I. T]re prcxJitcts of combusliotr are then allorved to
t]-,e
expa::ri in th': ::o:,.zlr: u,heie tite pi:clsure enirlly is cotlverted iirrt; kii:ctic energy. The tlirust developed
riuc to ihil, is usr<l ftr prooulsrrrrt.
 gs5 Tyrbines ond Jet Propulsion
135
3.3 ROCKET PROPUL$ION
A rocket propulsion device uses the s:inre principle of, jet pri:puisi;:n device, l:ur r.lr::csn,i
depend on the atmospheric air supplv. The n:aior ditferenur beiioee,r r.ire
rr>cker propi,rlsi.n n,rJl*,
propulsion i.s that rocket propuision carries its entire propellant which ir iin
c^ri<jiser a1d ir.iel r,r,rii
it" It is usually empioyed ibr space traveiling at a tlight ipeeci nruclr si*iti.:ri
rli;rn iet -spt:crl.

The rocket engine in its sirttplest i'onn congisls uf a conrtiusiir.rr chririber

lntJ irn c.tpurcine
nozzle' The tuel and the oxidiint burn logetlrer forrning exhausi rit conbusii6n. The
Fr'oducts
combustion products knos'n as propellants nass tl:rough tl'ie nczzll' g,;-rier:;itinn prgpnlsivr:
a lorcc
or thrxst. Expansion of propellants takes place in the ii,:r,:[: ir;r:ri.;nir,g Lhr) gas vpjjocirv
into
supersonic spced. These high velocit,v- grses passing through r.hc ri::..rle prt,,.j.ik
rhc ,i.rr,,.,,,n,1
prOpel the n:ckef" Generally a conver€ent-clii ercent nur:zle is emploi eii
lix. rhis i,,r1rrr,...
A schanatic diagrarn cf a rocket eligirre is sirou,n in Frg .). iS

Iuel tenk

0xi<lizing tank

>_<*
,\.'...h

+)

Conl:nl r.alve
Fr& 3.18 : Roeke! engin*
The propellants used in the rocket engine may he a solirl propt:iia*t cr ;.i irqLr:d ;,.ri;i:ciialr ui ;i
combination of them.
In a liquid bi-propeilant combination, a cornbinalion +i r.-r,--Ii,, D^*NH-.
oxygen-gasoline, oxygen-methanol, hydrogen peroxide* ethancrl, h1'dir-.p.r:* p,rr,ii:ide - n.,*ii-,unni.
hydrogen peroxide - hydrazine are used.
If m= mass rate afprr;pellant
V" = e.xharrst eas velocity in the nozzle
F = thrust or the propulsive force
P = power required tcl give an exhaust velocit;; V"
a = acceleration of the rocket cf mass trd
then, kinetic energy
rr 2
p=nlY" ,--i t)1 l
I

f"t,r .-$r
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o i; i:':: I -. I 1/f! 1r

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185 Al:,olrett

This is baianced ir,v iiie lhi'tlsl or F

/ fr-rir":e

F=n:\'=l'I.i e
l)l
t:'
Cornbining trl lnd f2l
// nL'
r: \
l
I "'r rr
I] * i l\i -1")j
ij
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