1

Lect-38

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay2

Lect-38

In this lecture...

• Thermodynamic cycle and performance parameters of ramjets and pulsejets

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay3

Lect-38

• Ramjet is the simplest of all the airbreathingengines.

• It consists of a diffuser, combustion chamber and a nozzle.

• Ramjets are most efficient when operated at supersonic speeds.

• When air is decelerated from a high Mach number to a low subsonic Mach number, it results in substantial increase in pressure and temperature.

• Hence ramjets do not need compressors and consequently no turbines as well.

Ramjet engines

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay4

Lect-38

Ramjet enginesDiffuser Combustion chamber Nozzle

Supersonic compression

Subsonic compression

Flame holders

Schematic of typical ramjet engine

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay5

Lect-38

Ramjet engines

s

T4

2

a

7

Ideal ramjet cycle on a T-s diagram

a-2: isentropic compression in the intake2-4: combustion at constant pressure4-7: Isentropic expansion through the nozzle

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay6

Lect-38

• In an ideal ramjet cycle, there are no irreversibilities considered.

• Therefore there are no pressure drops or efficiencies of the components comprising a ramjet.

• If we assume complete expansion in the nozzle, Pa=P7=Pe

• We shall use the isentropic relations to determine the variation of pressure and temperature in the intake and nozzle.

Ramjet engines

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay7

Lect-38

Ramjet engines

uTTu

TTu

aauor,MM,thereforeand

PP

PP,equationsabovetheFrom

M2

11PP,andM

211

PP,Similarly

TTM

211

TT

TT,Also

TTM

211

TT

:Intake

02

04

a

eeee

a

a0

e

07

)1/(2

ee

07)1/(

2

a

a0

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042e

6

06

a

e0

a

022

a

a0

====

=

−

+=

−

+=

=−

+===−

+=

−− γγγγ γγ

γγ

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay8

Lect-38

Ramjet engines

)Tc/Tc()Tc/Q(1)Tc/Tc(

f,therefore

Tc)f1(fQTch)f1(fQh

h)mm(Qmhm,givescombustortheacrossbalanceEnergy

:Combustor

a0p04pa0p

a0p04p

04p02p

0402

04ff02

−−

=

+=+

+=+

+=+

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay9

Lect-38

Ramjet engines

{ }

m/Ff

FmTSFCand

1M2

11T/T)f1(RTMmF

aswrittenrebecanequationthrustThe

M2

11

1TTu

TTuu,thatknowWe

uu)f1(m/Foruu)f1(mF:isdevelopedThrust

f

2/12

a04a

2a

04

a0

04e

ee

==

−

−

++=

−

−+

==

−+=−+=

−γγ

γ

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay10

Lect-38

• A real or actual ramjet cycle will have irreversibilities like pressure drop and efficiencies of intake, combustor and nozzle.

Ramjet engines

nbd0a

07

04

07n

02

04b

0a

02d

PP,ratiopressureOverall

PP,nozzletheinratiopressureStagnation

PP,combustortheinlossessurePr

,PP intake, the of recovery Pressure

πππ

π

π

π

=

=

=

=

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay11

Lect-38

Ramjet engines

s

T 4

2

a

7

Real ramjet cycle on a T-s diagram

a-2: Compression in the intake2-4: Combustion at constant pressure4-7: Expansion through the nozzle

P02

P04

Pa

P7

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay12

Lect-38

Ramjet engines

( )( )02pa04pg02pab

02pa04pg

eae

nbd

/)1(

e

anbd

22e

Tc/Tc)Tc/Q(1Tc/Tc

f

,balanceenergyby,f,eminerdetcanWe.cycleramjetIdealMMthen,PP

and1if,ressionexpabovetheIn

1PPM

211

12M

asnumber Mach inlet the to related be cannumber Mach exhaust The

−−

=

→==

===

−

−

+

−

=−

η

πππ

πππγγ

γγ

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay13

Lect-38

Ramjet engines

[ ]

m/FfTSFCand

)PP(mAuu)f1(

mFramjet.

ideal thefor adopted that tosimilar manner a in calculated be can SFC and thrust specific The

aee

e

=

−+−+=

( )

( )2e2

104

e04

e04eeee

2e2

1

e

04

7

07

M1RTM

TTRTMRTMu

,M1TT

TT

−

−

+===

+==

γ

γ

γγγ

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay14

Lect-38

Ramjet engines

Mach number

mF

m/Tf

Specific thrustTSFC

T04

T04

Variation of specific thrust and TSFC with Mach number

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay15

Lect-38

• Pulsejet is a very simple engine like a ramjet.

• Comprises of an intake, combustion chamber and an acoustically resonating exhaust pipe.

• Combustion occurs in pulses resulting in a pulsating thrust.

• Two types of pulsejet engines: valved and valveless engines.

• Pulse Detonation Engines (PDE) is being evolved conceptually.

Pulsejet engines

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay16

Lect-38

Pulsejet engines

Valves

Schematic of typical pulsejet engine

Fuel supply Combustion chamber

Tail pipe

a 2 3 7

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay17

Lect-38

Pulsejet engines

=

==

−

+==

−

+==−

02

030203

01020102

2aa001

)1/(2

aa001

TTPP Therefore,

cycle). (ideal volume constant at place takes Combustion:Combustor

TTandPP,pulsejetidealanFor

M2

11TTT,Similarly

M2

11PPP

:Intake

γ

γ γγ

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay18

Lect-38

Pulsejet engines

( )

.calculated be therefore can TSFCand thrust The

PP1Tc2u

:as calculated is velocity exhaust ThePP

TT,PP,gminAssu

:TailpipeTcQ

TcTcf,gSimplifyin

QmTcmTcmm:chambercombustiontheacrossbalanceEnergy

/)1(

03

a03pe

/)1(

7

03

7

03a7

03p

02p03p

f02p03pf

−=

==

−−

=

+=+

−

−

γγ

γγ

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay19

Lect-38

Pulsejet engines

s

T3

2

a

7

Real pulsejet cycle on a T-s diagram

a-2: Stagnation pressure loss2-3: Non 2-33-7: Non-isentropic process

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay20

Lect-38

Pulsejet engines• A real or actual pulsejet cycle will have

irreversibilities like pressure drop and efficiencies of intake, combustor and nozzle.

nbd0a

07

04

07n

02

04b

0a

02d

PP,ratiopressureOverall

PP,tailpipetheinratiopressureStagnation

PP,combustortheinlossessurePr

,PP intake, the of recovery Pressure

πππ

π

π

π

=

=

=

=

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay21

Lect-38

Pulsejet engines

• Besides this combustion process may have an efficiency associated with it.

• The thrust and fuel consumption will be affected as a result of the irreversibilities.

03pb

02p03p

TcQTcTc

f−−

=η

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay22

Lect-38

In this lecture...

• Thermodynamic cycle and performance parameters of ramjets and pulsejets

Prof. Bhaskar Roy, Prof. A M Pradeep, Department of Aerospace, IIT Bombay23

Lect-38

In the next lecture...

• Components of ramjets and pulsejets