Lect-38
Lect-38
In this lecture...
• Thermodynamic cycle and
performance parameters of ramjets
and pulsejets
Lect-38
• Ramjet is the simplest of all the airbreathing engines.
• 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
Lect-38
Ramjet engines
Diffuser Combustion chamber Nozzle
Supersonic
compression Subsonic compression
Flame holders
Lect-38
Ramjet engines
s
T 4
2
a
7
Ideal ramjet cycle on a T-s diagram
a-2: isentropic
compression in the intake 2-4: combustion at
constant pressure
4-7: Isentropic expansion
through the nozzle
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, P a =P 7 =P e
• We shall use the isentropic relations to determine the variation of pressure and temperature in the intake and nozzle.
Ramjet engines
Lect-38
Ramjet engines
T u u T
T u T
a u a
or , M M
, therefore and
P P P
, P equations above
the From
2 M 1 1
P , P and 2 M
1 1 P
, P Similarly
T M T
2 1 1
T T T
, T T Also
M T 2
1 1 T
T
: Intake
02 04 a
e e e
e
a a 0 e
07
) 1 2 /(
e e
07 )
1 /(
2 a
a 0
a 2 04
e 6
06 a
e 0 a
2 02 a
a 0
=
=
=
=
=
+ −
=
−
+
=
− = +
=
=
− = +
=
−
− γ γ
γ
γ
γ
γ
γ
γ
Lect-38
Ramjet engines
) T c / T
c ( )
T c / Q (
1 )
T c / T
c f (
, therefore
T c ) f 1
( fQ
T c
h ) f 1
( fQ
h
h ) m m
( Q
m h
m
, gives combustor
the across
balance Energy
: Combustor
a 0 p 04
p a
0 p
a 0 p 04
p 04 p
02 p
04 02
04 f
f 02
−
= − +
= +
+
= +
+
=
+
Lect-38
Ramjet engines
{ }
f m
1 2 M
1 1 T
/ T
) f 1
( RT m M
F
as written
re be
can equation
thrust The
2 M 1 1
1 T
u T T
u T u
, that know
We
u u
) f 1
( m
/ F or u
u ) f 1
( m F
: is developed Thrust
2 / 1 a 2
04 a
a 2 04 a
0 e 04
e e
−
−
+ +
=
−
+ −
=
=
− +
=
− +
=
γ
−γ
γ
Lect-38
• A real or actual ramjet cycle will have irreversibilities like pressure drop and efficiencies of intake, combustor and nozzle.
Ramjet engines
04 n 07
02 b 04
0a d 02
P , P
nozzle the
in ratio pressure
Stagnation
P , P
combustor the
in loss essure
Pr
P , intake, P
the of
recovery Pressure
π π
π
=
=
=
Lect-38
Ramjet engines
s
T 4
2
a
7
a-2: Compression in the intake
2-4: Combustion at constant pressure
4-7: Expansion through the nozzle
P
02P
04P
aP
7Lect-38
Ramjet engines
( )
(
02)
pa 04
pg
e a
e
n b
d
/ ) 1 ( e n a b 2 d
2e
1 T
c / T
f c
, balance energy
by , f , e min er
det can
We
. cycle ramjet
Ideal M
M then ,
P P
and 1
if , ression exp
above the
In
P 1 M P
2 1 1
1 M 2
as number Mach
inlet the
to related be
can number
Mach exhaust
The
= −
→
=
=
=
=
=
−
−
+
= −
−
π π
π π π γ π
γ
γ γ
Lect-38
Ramjet engines
[ ]
f
) P P
m ( u A
u ) f 1
m ( F
ramjet.
ideal the
for adopted
that to
similar manner
a in
calculated be
can SFC
and thrust
specific The
a e e
e
= + − + −
( )
(
2104 2e)
e 04
04 e e
e e
e
2e 21 e
04 7
07
M 1
M RT T
RT T M
RT M
u
, M T 1
T T
T
−
−
= +
=
=
+
=
=
γ γ
γ γ
γ
Lect-38
Ramjet engines
Mach number
m F
m / T
f
Specific thrust T
04TSFC
T
04Variation of specific thrust and TSFC
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
Lect-38
Pulsejet engines
Valves Fuel supply Combustion chamber
Tail pipe
a 2 3 7
Lect-38
Pulsejet engines
=
=
=
−
+
=
=
−
+
=
=
−
03
01 02
01 02
a 2 a
0 01
) 1 /(
a 2 a
0 01
P T P
Therefore,
cycle).
(ideal volume
constant at
place takes
Combustion : Combustor
T T
and P
P , pulsejet ideal
an For
2 M 1 1
T T
T , Similarly
2 M 1 1
P P
P
: Intake
γ
γ
γ γLect-38
Pulsejet engines
( )
1 P T
c 2 u
: as calculated is
velocity exhaust
The
P P T
, T P P
, g min Assu
: Tailpipe
T c Q
T c T
f c , g Simplifyin
Q m T
c m T
c m m
: chamber combustion
the across
balance Energy
/ ) 1 ( 03 a
p e
/ ) 1 (
7 03 7
a 03 7
03 p
02 p 03
p
f 02
p 03
p f
−
=
=
=
−
= −
+
= +
−
−
γ γ
γ γ
Lect-38
Pulsejet engines
s T
3 2
a
7
a-2: Stagnation pressure loss
2-3: Non 2-3
3-7: Non-isentropic
process
Lect-38
Pulsejet engines
• A real or actual pulsejet cycle will have irreversibilities like pressure drop and efficiencies of intake, combustor and nozzle.
04 n 07
02 b 04
0a d 02
P , P
tailpipe the
in ratio pressure
Stagnation
P , P
combustor the
in loss essure
Pr
P , intake, P
the of
recovery Pressure
π π
π
=
=
=
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.
03 p b
02 p 03
p
T c Q
T c T
f c
−
= −
η
Lect-38
In this lecture...
• Thermodynamic cycle and
performance parameters of ramjets
and pulsejets
Lect-38
In the next lecture...
• Components of ramjets and
pulsejets
