International Journal of Rotating Machinery
2001, Vol. 7, No. 2, pp. 79-85
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Studies on Pulse Jet Engine by Wind Tunnel Testing
TOSHIHIRO NAKANO*, MICHAEL ZEUTZIUS, HIDEO MIYANISHI,
TOSHIAKI SETOGUCHI and KENJI KANEKO
Department of Mechanical Engineering, Saga University, 1 Honjo-machi,
Saga-shi, Saga-ken, 840-8502, Japan
(Received in finalform March 1999)
Simple design and efficiency make pulse jet engines attractive for aeronautical short-term
operation applications. An active control system extends the operating range and reduces the
fuel consumption considerably so that this old technology might gain a new interest. The
results on wind tunnel experiments have been reported together with the impact of combus-
tion mode (pulse or steady) on system performance.
Keywords." Active control, Compressible flow, Steady combustion, Propulsion,
Pulse combustion, Wind tunnel testing,
INTRODUCTION
Advantages of pulse jet engines (Foa, 1960) are
their low weight and the generation of thrust even
for start and low flight velocities, where a ramjet
(steady combustion) is not able to generate any
thrust at all. The well-known low specific impulse
and fuel consumption higher than the one of a
turbojet are the disadvantages of the pulse jet
engines due to the missing pre-compression of the
inlet flow. For short-term operation and applica-
tions where the turbojet is not the main propulsion
and used oaly, for the take-off, the obvious advan-
tages of pulse jet engines used as start booster out-
weigh the disadvantages. Moreover, such engines
offer the possibility to operate one combustor
in ram-, pulse- and rocket-mode (Zeutzius et al.,
1998a,b). Last wind tunnel tests with pulse jet
engines were done in Germany by Schmidt (1950)
and Staab (1954), but most of their results were
getting lost during the war time. In addition, con-
temporary work (Barr et al., 1990) concentrates to
pulse combustors as gas generators. Therefore, pre-
sent investigations are focused to the wind tunnel
testing and the development of an active control
system for pulse jet engines to extend their opera-
ting range including ram- and rocket-mode.
EXPERIMENTAL SET-UP
The pulse jet engine used in wind tunnel experi-
ments as shown in Fig. has a length of 80 cm and
a pipe diameter of 34 mm. The engine runs with
Corresponding author. Fax: +81-952-28-8587. E-mail: nakano@me.saga-u.ac.jp.
79
80 T. NAKANO et al.
Driver
Air-Compressor 1-
PC
Pressure Amplifier
Transducer
E: Ejector Thrust
S: Spark Plug Measurement
FIGURE Pulse jet engine with wind tunnel suspension and propellant supply, experimental set-up.
gasoline that is vaporized by an air jet and charged
to the combustor through the inlet equipped with
aerovalve and reed valve, respectively. Because the
state of the flow in front of the inlet influences
considerably charging process and engine opera-
tion, an external flow around the propulsion is
simulated with the Eiffel-type wind tunnel of Saga
University having an open test section. Start and
take-off conditions can be simulated with a free
stream velocity of u= 35 m/s at maximum. The
pressure within the flow field was measured with a
Prandtl tube, the combustion pressure with Piezo
transducers, thrust and drag were obtained from
the motion of the pendulum suspension of the
engine.
The pulse jet engine (two-dimensional version)
used for the feasibility study on the control system
with a movable inlet cone (mass flow control) is
shown in Fig. 2. Actual experiments with fuel and
air rate as controller output were performed in open
loop mode, it means no feed back to the controller.
Kinetic energy of the exhaust flow, pressures, tem-
peratures and flow rates are scheduled to be the feed
back parameters to the controller for the closed
loop run.
RESULTS AND DISCUSSION
Engine Run and Performance
The oscillation of the gas column in the pipe of a
pulse jet engine is driven by the combustion as long
as the charging of fresh mixture through the inlet
is attenuated (Barr et al., 1990; Zeutzius et al.,
1998a,b). Not only the charging but also the strength
of the subsequent compression of the charged
mixture by the gas column in the tailpipe depends
on this pressure drop in the combustion cham-
ber. The rising stagnation pressure at the inlet
due to the increasing flight velocity lowers the
charging attenuation and the obtainable minimum
pressure because a higher air rate is supplied to
the combustion chamber. The pressure ampli-
tude declines and the flow through the engine
approaches the ram-mode (constant pressure com-
bustion) as shown in Fig. 3 for different flight
velocities. Choking the air rate into the engine
improves the performance of the engine. Since the
ch.arging and combustion mode depend strongly
on the ratio of exit areas to combustor volume
Ai/gi and AN/VN, an active engine control bases
STUDIES ON PULSE JET ENGINE 81
Air-Compressor
FuelvIanometer
[Controll
rll Motor L__
E: Ejector
S: Spark Plug
F: Flow Meter
]Controller[
or
4,#4,1
--Pressure
Transducer
t
i ..............
Turbine
+
Generator
FIGURE 2 Two-dimensional combustor with control system; control parameter: fuel rate rhF, inlet cross section Ai; controlled
parameter: turbine speed R; feed back parameters: pressures, flow rates rhA, turbine speed, cross section.
0.15
,-(a) AP =0.029 MPa
0.10 [-..V V V V V VI V, V V V v v V v
0.05
0.15
0.10
0.05
0.15 [.(e) AP=O.O55MPa
0.05
0.15 (d) AP =0.062 MPa
A_A A A A A A.i
0.00 0.02 0.04 0.06
tS
FIGURE 3 Pressure amplitudes AP of a pulse jet engine
with reed valves for (a) uo=33.44m/s, (b) uo=29.02m/s,
(c) uo 24.51 m/s, (d) uo 19.42 m/s.
upon a control of inlet and/or nozzle throat cross
section.
Air and fuel rate as well affect the operation
mode as can be seen from Fig. 4. The kinetic energy
of the fuel gas was measured supplying the gas to a
turbine whose speed is an indicator for the combus-
tion efficiency. The superiority of the pulse com-
bustion to the steady combustion and important
tendencies can be seen from Fig. 4: (1) Low amount
of air enclosed in the tail pipe can be accelerated
to higher speeds (L/d=4.8, pipe length: L, pipe
diameter: d) with the same amount of heat energy
transferred in the combustor. (2) Higher friction in
a longer tailpipe dependent on Lid weakens the
oscillation and the compression as well, so that a
higher amount of air with lower kinetic energy can
be pumped through the combustor. The maximum
pumping capacity is obtained for non-reacting
flows (thermal choking). Because a larger air mass
is enclosed in a tail pipe with L/d= 13.2, a higher
amount of energy would be necessary to accelerate
the gas. (3) If high-pressure gas is supplied to the
propulsion it is obligatory to adjust the fuel rate not
only to keep the mixing ratio inflammable but also
to provide enough energy for the acceleration of the
gas in the tail pipe.
The declination of the thrust shown in Fig. 5
depends on the pressure amplitude in the combus-
tion chamber. Assuming a nearly harmonic nozzle
exit pressure, the nozzle exit velocity UN scales with
82 T. NAKANO et al.
(a) 2500
2000
500
1000
500
0
(u)
(c)
,13- Lr/Dr 4.8
Lr/D 7.6
Lr/Dr=lO.4
.-0--- Lr /Dr =13 .2
RETRANSITION EADYNON-
t/ FLOW .I.. PULSE FLOW ..[.’ .l.. FLOW
0.2 0.4 0.6 0.8 1.2 1.4
thF g/s
"’
-I ----&-- Lr/Dr =7.6
---O--- Lr/Dr 13.2
0 02 0;4 06 08 12 14
g/s
20
15
10
--o--- Lr /D 4.8
Lr/Dr 7.6
_/1....
---V--- Lr/Dr =10.4
Lr/Dr
=131
012 ft.4 g.6 0.8 "1.2-114
&- g/s
the pressure amplitude AP (ratio of specific heats:
n, sound velocity: c, mean combustion pressure: P0)
2 2 cAP
N b/max (1)
7r 7r P0n
Thrust F and turbine speed concerning kinetic
energy R for pulse mode can be calculated with
(air flow rate: rhA, combustion pressure amplitude:
APc)
F- t/TA(b/N b/oe rhAAPc and R rhAAPc.
(2)
The ram thrust is caused by the fuel-air-ejector
system (similar to an ejector pump) and fully
negligible for low speed flight. Aerovalved engines
produce a thrust of about 6 N slightly increasing
with the stagnation pressure what is due to the
increasing air flow rate. Engines with reed valves
generate highest thrust of 12.5 N at a maximum, but
the rising stagnation pressure difference between
inlet and nozzle reduces the combustion pressure
amplitudes due to the high air inflow into the com-
bustor (Fig. 3). Concerning drag, redesigning the
nacelle could reduce the high drag by half.
FIGURE 4 Capacity of pulse combustion for reduction of
fuel consumption, (a) kinetic energy of the exhaust flow,
(b) air flow rate and (c) pressure amplitude in dependence of
fuel rate and tail pipe length.
10
0 1000
-AERO VALoVE(Thrust)o//
=// RAM(Th_.rust)
200 400 600 800
Pu N/m
2
FIGURE 5 Thrust and drag dependent on the dynamic
pressure.
Propulsion Installation Losses
The requirements for an optimization of the pulse
jet engine are contrary. While a choked inlet flow
is necessary to sustain the pulse combustion and
improve the thrust under high subsonic flight con-
ditions, the installation losses might increase due to
a higher spillage rate.
Spilling flow at the inlet can be seen from the flow
field shown in Fig. 6 for a pulse jet engine with
aerovalve. The free stream velocity of 24.8 m/s is
reduced to 14.3 m/s in front of the inlet resulting
in a spilling air rate in the order of magnitude of
about 40% here for the incompressible external
flow estimated with
/-ho,
20
40
6O
80
100
STUDIES ON PULSE JET ENGINE
INLET
2!3 20.5
8O 60 4O 2O 0
X 111111
FIGURE 6 Velocity distribution in the inlet area for an aerovalved pulse jet.
83
A high integration of the propulsion in combina-
tion with a Busemann inlet (Zeutzius et al., 1998a,b;
Staab, 1954) leads to a reduction of spillage drag
and non-uniformity losses as well.
Propulsion Control
The pressure difference between inlet and nozzle
exit is the reason why the flow turns in a steady
mode and therefore, the design targets for the con-
trol system are:
(1) Extension of the engine operating range by
choking the air rate through the engine.
(2) Keeping spillage drag as low as possible.
(3) Raising the nozzle/base pressure.
These requirements can be fulfilled with a control
of the inlet flow rate (adaptation of cross section
with movable inlet cone). The control law for a
combustor run close to the design point is
dR- drhv + drha
( 0ghA
with
(4)
The air rate can be calculated with the mass flow
equation:
rh =pAu
=A v/2PcPc
t \cJ
The fuel derivative OR/OthF is fitted into a higher
order series for constant inlet cross section
OR
OrhF anrh +... + a2rh + a,rhF + a0. (6)
In contrast to the past, the operating range of
flight vehicles propelled with pulse jet engine is
extended using the inlet control system for Eq. (1)
a stabilization of the operation mode and Eq. (2)
an inclusion of rocket and ram mode in the propul-
sion operation (Fig. 7). For fixed geometry (stan-
dard operation) the fuel consumption is reduced by
at least 20% ofthat ofa steady combustion. While a
small area ratio is beneficial for a high efficient
combustion labeled with "E" in Fig. 7(c), it cannot
84 T. NAKANO et al.
(a) 55
40
(b) 15
0
0
--o-- A/Ac =0.138q ,
..,q. -v- A/c=0.211i "" A/Ac=O’284
g/s
(c)
1500
000
500
LF, MP CA
/
... ../,.., ROCKET
.:.;.....
Adc =0.138
v- Adc =0.211
-..-- AAc=0.284
n?F g/s
FIGURE 7 Two-dimensional combustor; (a) air rate,
(b) amplitude, (c) turbine speed dependent on fuel rate, E:
highest efficiency, LF: low fuel consumption, CA: constant area.
be used for a mode change in propulsion appli-
cation. The subsequent drop of the kinetic energy
for steady combustion is too high. For a smoothed
change of the operation mode from pulse to steady
combustion (ramjet), the inlet area should be
enhanced to a higher value until the steady mode
is reached. No drop of the kinetic energy is obeyed
for a transition from pulse to steady mode (ram
combustion) for larger cross sections. The propul-
sion can be switched to rocket mode by closing inlet
and supplying oxygen from tanks to the combustor.
CONCLUSIONS
Wind tunnel tests were performed to show the
impact ofexternal flow on a pulse jet operation. The
thrust of pulse jet engines with reed valve slightly
decreases with increasing flight velocity. High
stagnation pressure in front of the inlet turns the
pulse flow into a steady one so that the benefit
gained by the compression capacity of the gas
column in the tail pipe is getting lost. The inlet air
rate control used for choking the inlet flow
improves thrust, extends operating range and
makes ram (inlet open, steady combustion) and
rocket (closed inlet, steady flow) operation possible.
Using pulse combustion with inlet flow control
reduces the fuel consumption by at least 20%.
NOMENCLATURE
A
d
F
L
rh
P
R
V
Arh
AP
P
area
sound velocity
pipe diameter
thrust
tail pipe length
mass flow rate
order
pressure
rotational speed
time
velocity
volume
coordinates
spilling flow rate
pressure amplitude
ratio of specific heats
density
Subscripts
A
C
F
N
air
combustion chamber
fuel
inlet
nozzle
STUDIES ON PULSE JET ENGINE 85
mean
ambient
stagnation
References
Barr, P.K., Keller, J.O., Bramlette, T.T. and Westbrook, C.K.
(1990), Pulse combustor modeling-demonstration of the
importance of time characteristics, Combustion and Flame,
82(1), 252-269.
Foa, J.V. (1960), Elements of Flight Propulsion, John Wiley &
Sons Inc., New York, London, pp. 368-389.
Schmidt, P. (1950), Die Entwicklung der Zuendung periodisch
arbeitender Strahlgeraete, VDI-Zeitschrift, Germany, 92(6),
393-399.
Staab, F. (1954), Strahltriebwerke auf Grundlage des
Schmidtrohres, Zeitschriftfuer Flugwissenschaften, Germany,
2(6), 129-144.
Zeutzius, M., Setoguchi, T., Terao, K. and Miyanishi, H.
(1998a), A Propulsion for hypersonic space plane, Proc. 8th
Int’l. Space Planes and Hypersonic Systems and Technologies
Con.[., AIAA 98-1531, Norfolk, pp. 185-195.
Zeutzius, M., Setoguchi, T., Terao, K., Matsuo, S., Nakano, T.
and Fujita, Y. (1998b), Active control of twin pulse com-
bustors, AIAA Journal, 36(5), 1-7.
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