studies on pulse engine by tunnel testingpulse combustion, windtunnel testing, introduction...

International Journal of Rotating Machinery2001, Vol. 7, No. 2, pp. 79-85

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Studies on Pulse Jet Engine by Wind Tunnel TestingTOSHIHIRO 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-termoperation applications. An active control system extends the operating range and reduces thefuel consumption considerably so that this old technology might gain a new interest. Theresults 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) aretheir low weight and the generation of thrust evenfor start and low flight velocities, where a ramjet(steady combustion) is not able to generate anythrust at all. The well-known low specific impulseand fuel consumption higher than the one of a

turbojet are the disadvantages of the pulse jetengines due to the missing pre-compression of theinlet flow. For short-term operation and applica-tions where the turbojet is not the main propulsionand used oaly, for the take-off, the obvious advan-tages of pulse jet engines used as start booster out-weigh the disadvantages. Moreover, such enginesoffer the possibility to operate one combustorin ram-, pulse- and rocket-mode (Zeutzius et al.,

1998a,b). Last wind tunnel tests with pulse jetengines 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 topulse combustors as gas generators. Therefore, pre-sent investigations are focused to the wind tunneltesting and the development of an active controlsystem 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 anda pipe diameter of 34 mm. The engine runs with

Corresponding author. Fax: +81-952-28-8587. E-mail: [email protected].

79

80 T. NAKANO et al.

DriverAir-Compressor 1-

PC

Pressure AmplifierTransducer

E: Ejector ThrustS: 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 chargedto the combustor through the inlet equipped withaerovalve and reed valve, respectively. Because thestate of the flow in front of the inlet influencesconsiderably charging process and engine opera-tion, an external flow around the propulsion issimulated with the Eiffel-type wind tunnel of SagaUniversity having an open test section. Start andtake-off conditions can be simulated with a freestream velocity of u= 35 m/s at maximum. Thepressure within the flow field was measured with aPrandtl tube, the combustion pressure with Piezotransducers, thrust and drag were obtained fromthe motion of the pendulum suspension of theengine.The pulse jet engine (two-dimensional version)

used for the feasibility study on the control systemwith a movable inlet cone (mass flow control) isshown in Fig. 2. Actual experiments with fuel andair rate as controller output were performed in openloop 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 feedback parameters to the controller for the closedloop 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 longas the charging of fresh mixture through the inletis attenuated (Barr et al., 1990; Zeutzius et al.,1998a,b). Not only the charging but also the strengthof the subsequent compression of the chargedmixture by the gas column in the tailpipe dependson this pressure drop in the combustion cham-ber. The rising stagnation pressure at the inletdue to the increasing flight velocity lowers thecharging attenuation and the obtainable minimumpressure because a higher air rate is supplied tothe combustion chamber. The pressure ampli-tude declines and the flow through the engineapproaches the ram-mode (constant pressure com-bustion) as shown in Fig. 3 for different flightvelocities. Choking the air rate into the engineimproves the performance of the engine. Since thech.arging and combustion mode depend stronglyon the ratio of exit areas to combustor volumeAi/gi and AN/VN, an active engine control bases

STUDIES ON PULSE JET ENGINE 81

Air-Compressor

FuelvIanometer

[Controllrll Motor L__

E: EjectorS: Spark PlugF: Flow Meter

]Controller[or

4,#4,1 --PressureTransducer

ti ..............

Turbine+

Generator

FIGURE 2 Two-dimensional combustor with control system; control parameter: fuel rate rhF, inlet cross section Ai; controlledparameter: 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.i0.00 0.02 0.04 0.06

tS

FIGURE 3 Pressure amplitudes AP of a pulse jet enginewith 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 crosssection.

Air and fuel rate as well affect the operationmode as can be seen from Fig. 4. The kinetic energy

of the fuel gas was measured supplying the gas to aturbine whose speed is an indicator for the combus-tion efficiency. The superiority of the pulse com-bustion to the steady combustion and importanttendencies can be seen from Fig. 4: (1) Low amountof air enclosed in the tail pipe can be acceleratedto higher speeds (L/d=4.8, pipe length: L, pipediameter: d) with the same amount of heat energytransferred in the combustor. (2) Higher friction ina longer tailpipe dependent on Lid weakens theoscillation and the compression as well, so that a

higher amount of air with lower kinetic energy canbe pumped through the combustor. The maximumpumping capacity is obtained for non-reactingflows (thermal choking). Because a larger air massis enclosed in a tail pipe with L/d= 13.2, a higheramount of energy would be necessary to acceleratethe gas. (3) If high-pressure gas is supplied to thepropulsion it is obligatory to adjust the fuel rate notonly to keep the mixing ratio inflammable but alsoto provide enough energy for the acceleration of thegas 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 nozzleexit 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 4.8Lr/D 7.6Lr/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 1.4thF g/s

"’ -I ----&-- Lr/Dr =7.6

---O--- Lr/Dr 13.2

0 02 0;4 06 08 12 14g/s

20

15

10

--o--- Lr /D 4.8Lr/Dr 7.6 _/1....

---V--- Lr/Dr =10.4Lr/Dr

=131012 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 cAPN 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-ejectorsystem (similar to an ejector pump) and fullynegligible for low speed flight. Aerovalved enginesproduce a thrust of about 6 N slightly increasingwith the stagnation pressure what is due to theincreasing air flow rate. Engines with reed valvesgenerate highest thrust of 12.5 N at a maximum, butthe rising stagnation pressure difference betweeninlet and nozzle reduces the combustion pressureamplitudes due to the high air inflow into the com-bustor (Fig. 3). Concerning drag, redesigning thenacelle could reduce the high drag by half.

FIGURE 4 Capacity of pulse combustion for reduction offuel consumption, (a) kinetic energy of the exhaust flow,(b) air flow rate and (c) pressure amplitude in dependence offuel rate and tail pipe length.

10

0 1000

-AERO VALoVE(Thrust)o//

=// RAM(Th_.rust)200 400 600 800

Pu N/m2

FIGURE 5 Thrust and drag dependent on the dynamicpressure.

Propulsion Installation Losses

The requirements for an optimization of the pulsejet engine are contrary. While a choked inlet flowis necessary to sustain the pulse combustion andimprove the thrust under high subsonic flight con-

ditions, the installation losses might increase due toa higher spillage rate.

Spilling flow at the inlet can be seen from the flowfield shown in Fig. 6 for a pulse jet engine withaerovalve. The free stream velocity of 24.8 m/s isreduced to 14.3 m/s in front of the inlet resultingin a spilling air rate in the order of magnitude ofabout 40% here for the incompressible externalflow estimated with

/-ho,

20

40

6O

80100

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 dragand non-uniformity losses as well.

Propulsion Control

The pressure difference between inlet and nozzleexit is the reason why the flow turns in a steadymode and therefore, the design targets for the con-trol system are:

(1) Extension of the engine operating range bychoking 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 controlof the inlet flow rate (adaptation of cross sectionwith movable inlet cone). The control law for acombustor run close to the design point is

dR- drhv + drha

( 0ghA

with

(4)

The air rate can be calculated with the mass flowequation:

rh =pAu

=A v/2PcPct \cJ

The fuel derivative OR/OthF is fitted into a higherorder series for constant inlet cross section

OROrhF anrh +... + a2rh + a,rhF + a0. (6)

In contrast to the past, the operating range offlight vehicles propelled with pulse jet engine isextended 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 byat least 20% ofthat ofa steady combustion. While a

small area ratio is beneficial for a high efficientcombustion labeled with "E" in Fig. 7(c), it cannot

84 T. NAKANO et al.

(a) 55

40

(b) 15

00

--o-- A/Ac =0.138q ,..,q. -v- A/c=0.211

i "" A/Ac=O’284

g/s

(c)

1500

000

500

LF, MP CA

/... ../,.., ROCKET.:.;.....

Adc =0.138v- 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 energyfor steady combustion is too high. For a smoothedchange of the operation mode from pulse to steadycombustion (ramjet), the inlet area should beenhanced to a higher value until the steady modeis reached. No drop of the kinetic energy is obeyedfor a transition from pulse to steady mode (ramcombustion) for larger cross sections. The propul-sion can be switched to rocket mode by closing inletand supplying oxygen from tanks to the combustor.

CONCLUSIONS

Wind tunnel tests were performed to show theimpact ofexternal flow on a pulse jet operation. Thethrust of pulse jet engines with reed valve slightlydecreases with increasing flight velocity. Highstagnation pressure in front of the inlet turns thepulse flow into a steady one so that the benefitgained by the compression capacity of the gascolumn in the tail pipe is getting lost. The inlet airrate control used for choking the inlet flowimproves thrust, extends operating range andmakes ram (inlet open, steady combustion) androcket (closed inlet, steady flow) operation possible.Using pulse combustion with inlet flow controlreduces the fuel consumption by at least 20%.

NOMENCLATURE

A

dFLrh

PR

V

ArhAP

P

area

sound velocitypipe diameterthrusttail pipe lengthmass flow rateorderpressurerotational speedtimevelocityvolumecoordinatesspilling flow ratepressure amplituderatio of specific heatsdensity

Subscripts

ACF

N

aircombustion chamberfuelinletnozzle

STUDIES ON PULSE JET ENGINE 85

mean

ambientstagnation

References

Barr, P.K., Keller, J.O., Bramlette, T.T. and Westbrook, C.K.(1990), Pulse combustor modeling-demonstration of theimportance 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 periodischarbeitender Strahlgeraete, VDI-Zeitschrift, Germany, 92(6),393-399.

Staab, F. (1954), Strahltriebwerke auf Grundlage desSchmidtrohres, Zeitschriftfuer Flugwissenschaften, Germany,2(6), 129-144.

Zeutzius, M., Setoguchi, T., Terao, K. and Miyanishi, H.(1998a), A Propulsion for hypersonic space plane, Proc. 8thInt’l. Space Planes and Hypersonic Systems and TechnologiesCon.[., 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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