experimental investigation of a multi-cycle pulsed

Copyright ©1996, American Institute of Aeronautics and Astronautics, Inc.

AIAA Meeting Papers on Disc, January 1996A9618308, AIAA Paper 96-0346

Experimental investigation of a multi-cycle pulsed detonation wave engine

W. S. StuessyTexas Univ., Arlington

D. R. WilsonTexas Univ., Arlington

AIAA 34th Aerospace Sciences Meeting and Exhibit, Reno, NV Jan 15-18, 1996

The Detonation Wave Facility at the University of Texas at Arlington has been modified to run in a multi-cycle modeas a Pulse Detonation Engine simulator. A fuel injection system which injects repetitively was incorporated, as well asan ignition system which could be used to repetitively ignite the combustible mixture. A description of the facility andresults from pressure measurements from the initial multi-cycle operation are presented for hydrogen/oxygen mixturesat low frequencies. Comparison with results from the facility when in the single-shot mode of operation are also made.(Author)

Experimental Investigation of a Multi-cyclePulsed Detonation Wave Engine

W. S. Stuessy* and D. R. Wilson§

Aerodynamics Research CenterThe University of Texas at Arlington

Arlington, Texas 76019

ABSTRACT

The Detonation Wave Facility at the Universityof Texas at Arlington has been modified to run in amulti-cycle mode as a Pulse Detonation Enginesimulator. A fuel injection system to injectrepetitively was incorporated as well as an ignitionsystem which could be used repetitively to ignitethe combustible mixture. A description of thefacility and results from pressure measurementsfrom initial multi-cycle operation are presented forhydrogen/oxygen mixtures at low frequencies.Comparison with results from the facility operatedin the single shot mode of operation are alsomade.

INTRODUCTION

Pulsating engines have been around for quitesome time and found us in World War II in theGerman V-1. These engines took in fuel andoxidizer, burnt then and expelled the product athigher pressure out the back for thrust. Thecombustion process in these engines took place atsubsonic speeds as a deflagration. The pulsedetonation wave engine utilizes combustion atsupersonic velocities by detonation waves. Theengine operates in a cyclic fashion like a pulse jetbut operates at a higher pressure level due to thedetonation waves and potentially at higherfrequency. These concepts of a pulsating

This material is based in part upon worksupported by the Texas Advanced TechnologyProgram under Grant No. 003656-056

* Faculty Research Associate, Member§ Professor of Aerospace Engineering,

Associate Fellow

Copyright © 1996 by the American Institute ofAeronautics and Astronautics, Inc. All rightsreserved.

propulsion device have not found wide spreaduse, but have received attention ofresearchers. "I'2

The use of detonation waves for propulsiveapplications show great promise. Detonationwaves can be used in a propulsion system foraerospace applications as a Pulse DetonationEngine (PDE) and in other industrial applications.The airbreathing propulsion system use is thoughtto have applications from static conditions toaround Mach 10 conditions, perhaps more, andhave substantially higher efficiency than conven-tional propulsion systems.^ The complexity of theengine could be reduced. The detonation wavewould generate the high pressure so thecompressor would no longer be required. Withoutthe compressor, the turbine is also not required.With out the rotating machinery the enginebecomes much less complicated.

Detonation waves can also be used in rocketengines'* to increase performance and eliminatemany of the heavy and complex components suchas the high pressure turbo pumps, since the fueland oxidizer is injected at lower pressure insteadof the current high operating pressure.

THEORY OF OPERATION

A Pulse Detonation Engine operates by filling achamber with a fuel and oxidizer combination thendetonating the mixture for combustion. Thedetonation wave and combustion products areallowed to exit the chamber to provide thrust. Thechamber can then be refilled with fuel and oxidizerand the cycle repeated. The fuel and oxidizer areinjected at low pressure and then detonated toobtain the high pressure to produce the thrust, in aconstant volume mode of operation. The injectionpressure is much lower than required forconventional propulsion systems which operate atconstant pressure. The fuel and oxidizer can beinjected during the low pressure portion of thecycle.

The detonation wave is a coupled shock waveand combustion process as Figure 1 illustrates.The shock front increases the pressure and

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temperature similar to a normal shock wave.Immediately following the shock the combustionprocess adds heat at constant volume. TheZeldovich-Von Neumann-Doring (2ND) spike seenin Figure 1 is the initial shock wave and thecombustion process takes a finite amount of timeso it follows behind the initial wave. Thissignificantly increases the temperature andreduces the pressure significantly. Thecombustion region is followed by an expansionregion which brings the supersonic flow behind thedetonation wave to rest at the end wall.

Combustion waves traveling at supersonicvelocity tend to approach the Chapman-Jouguetdetonation velocity. Even a subsonic deflagrationwill transition to a supersonic C-J detonation in atube given sufficient length. The confined spacecontains the heat until it builds to a level sufficientfor a detonation.

A detonation wave can also be created byadding sufficient energy in a sufficiently shortperiod of time, such as with an arc discharge.This results in an almost immediate detonationwave but requires a very large amount of energy.

The addition of turbulence generator can alsospeed the process of generating a detonationwave by increasing the mixing of the fuel andoxidizer. The detonation must be generated in arelatively short distance in order to make a pulsedetonation engine feasible.

FACILITY

A facility specifically designed to studydetonation waves at the University of Texas atArlington is being utilized to study repetitivedetonation waves as a propulsion device afterpreviously being used for single shot detonationwave research.5-6-7 The injection portion of thefacility has been replaced with one designed forrepetitive use. The ignition system has also beenupgraded for repetitive use. A control circuit inFigure 2 was also designed and built to sensethe injection of fuel and oxidizer, provide a shorttime delay, fire the ignition source, recharge theignition capacitor bank, and provide asynchronized signal to the data acquisition ifrequired or desired.

TEST CHAMBER

The cylindrical facility has a fixed internaldiameter of 7.62 cm (3 inch) and sections of 7.62cm (3 inch), 15.24 cm (6 inch), and 30.48 cm (12

inch) in length for a total length of up to 91.44 cm(36 inch). The sections can be utilized in differentcombinations to provide various length to diameter(L/D) ratios of between 3 and 12. The diameter isnot changed. Each section of the facility hasprovisions for mounting pressure transducers,thermocouples, thin film gauges, and heat fluxgauges every 7.62 cm (3 inch). Figure 3 showsthe various sections.

The ignition plug is mounted in a 7.62 cm (3inch) section of its own and can be inserted anywhere along the length of the tube between othersections.

One end of the chamber is sealed with a blindflange. The fuel and oxidizer is injected throughthis plate. The various sections of the chamberare flanged and bolted together at each joint. Theopen end of the chamber is bolted to a thruststand to hold the chamber in place. Figure 4shows the overall schematic.

INJECTION SYSTEM

The pneumatic gas control system used inthe single shot experiments was also used in themulti-cycle test with minimal modifications. Thefuel, oxidizer, and air lines were used as beforewhile the others lines were not required. Theremote control valves were retained as a measureof positive control over fuel and oxidizer flowthrough the rotary valves for safety reasons. Theinjection system has been designed for hydrogen,propane, or methane as the fuel and oxygen or airas the oxidizer. Other fuels or oxidizers could beused if they were compatible with the valvematerials and seals.

The fuel and oxidizer are injected through anendplate which closes one end of the tube. Thefuel and oxidizer is measured by setting the valvesupply pressure according to regulator flow ratecharts and injected using rotary valves. Bufferair is also synchronized between cycles to providea buffer between the hot products of one cycleand the unburnt reactants of the next cycle. Thebuffer air is injected through an unused port in theignition section, perpendicular to the axis of thechamber and against the opposite wall by meansof a solenoid valve. An air line is also installed forpurging of the explosive gases for safetypurposes. The opposite end is open for theexhaust of the detonation wave and combustionproducts.

The rotary injection valves are connectedtogether by pulleys and a timing belt turned by a

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variable speed electric motor controlledremotely from the control room for frequencycontrol. A magnetic pickup is located nearby tosense the closure of the valves and initiate theignition process.

The fuel and oxidizer are injectedperpendicular to the axis of the detonationchamber and in such a way to impinge uponeach other during the injection process but notinto the supply line of the other. This is in aneffort to mix the fuel with the oxidizer.

IGNITION SYSTEM

The ignition source is one high voltage, highcurrent arc plug driven by a discharge capacitorbank and initiated by a high frequency arc weldingsource. The arc plug is mounted in a 7.62 cm (3inch) section of the facility which allowsplacement at nearly any location along thelength of the facility. The arc welder ionizes apath between the two electrodes of the arc plug.When the path is ionized sufficiently the dischargecapacitors discharge through this path in the formof a high current arc. An electrical schematic ofthe ignition system is show in Figure 5.

The discharge capacitor bank consists of two11000 micro farad 75 VDC capacitors connectedin series and charged to about 100 VDC. Asecond charge capacitor bank, identical to thedischarge capacitor bank, is used to rechargethe discharge capacitor bank between cycles andis kept at 100 VDC by a 1.2 kVA variabletransformer and a rectifying diode bridge. Thetwo capacitor banks are isolated by means ofa thyristor. The thyristor turns on just longenough to recharge the discharge capacitor bankand then turns off. If the two capacitor banks arenot isolated during the arc discharge bothcapacitor banks will discharge and then thevariable transformer will begin driving the arc in awelding mode. This draws large amounts ofcurrent and leads to rapid heating and destructionof certain components. Minimizing thedischarge time results in more energy transferredto the gas and less to the structure of the arc plugfor the same energy discharge from thecapacitor. The charge capacitor bank is used toeven out the current flow through the variabletransformer and allow the discharge capacitorbank to be recharged more quickly. The outputsof the discharge capacitor bank were connectedtogether with a diode to eliminate ringing of thedischarge current. This eliminates reverse

voltage on the capacitor bank and reduces themaximum voltage differential seen by the thyristor.

The thyristor is controlled by a timer circuit thatalso initiates the high frequency welding unit,provides a delay for recharging the dischargecapacitor bank, and the signal to the thyristor.The timer circuit also provides a signal to asolenoid valve for purge air.

The energy from the discharge capacitorbank is discharged through a arc plug which isconstructed from two tungsten electrodesmounted in ceramic and the assembly mounted ina threaded steel housing. The end of theelectrodes are flush with the surface of theceramic. The threaded housing assembly is theninstalled into the ignition section of the facility sothe ceramic and ends of the electrodes are nearlyflush with the inner wall of the chamber. Theenergy discharges in an arc between the twoelectrodes.

INSTRUMENTATION

The instrumentation used to obtain theexperimental data are seven pressure transducerswhich are water cooled for continuous multi-cycleoperation. The instrumentation sensors can bemounted in the sidewall at 7.62 cm (3 inch)increments with the capability for all types of sen-sors to be mounted at the same axial locations.The pressure transduces are PCB model 111A24dynamic pressure transducers with a full scalerange of 6.89 MPa (1000 psi), rise time of 1microsecond, and a time constant of 100 seconds.

The initial reference pressure is atmosphericpressure and is measured by a Baratron PressureTransducer from MKS, model number 127A. Thistransducer has a maximum pressure range of1333.22 kPa (10000 Torr). This transducer isused as it was part of the facility when it was usedfor enclosed single detonation test and it providesa very accurate measure of atmospheric pressure.

The pressure transducers are connected to aDSP Technology data acquisition system whichhas the capability of 100 kHz sampling rate, 12bits of accuracy, and 48 channels each with itsown amplifier and analog to digital converter toallow for simultaneous sampling of all channels.The system has 512 Kilobytes of memory avail-able for distribution to the channels being utilized.The data acquisition system is controlled by a PCwhich retrieves the data, stores it on a harddrive,and analyzes the data.


CONFIGURATION

The facility was configured using a 30.48 cm(12 inch) section, a 15.24 cm (6 inch) section, andthe 7.62 cm (3 inch) ignition section. The ignitionsection as located nearest the closed end ofthe chamber. This length was chosen as itcontained 6 instrumentation locations which wasall the pressure sensors available and it providedthe approximate length required to contain theexpected fuel and oxidizer charge.

DATA ANALYSIS

Voltage readings from the pressuretransducers are converted into pressure readingsand plotted against time. The pressure plots wereused to obtain an experimental wave diagram.The time interval between the observed abruptrise in pressure from adjacent transducers wasused to calculate wave propagation speed.

The effect of the expansion wave generatedbehind the detonation wave was analyzed todetermine how long it takes to die out and how itaffects the next detonation wave in particular.

Pressure data will be analyzed to determinebasic system operating parameters such aspressure level, detonation velocity, expansionwave velocity, and cycle time in general.

RESULTS & DISCUSSION

Initially only fuel and oxidizer was injectedduring each cycle. The fuel and oxidizer in thechamber burned almost continuously andproduced a very low pressure after the firstsupersonic combustion wave. There was nonoticeable increase during or following theinjection, ignition, or combustion periods. Purge airwas then introduced between cycles andsupersonic combustion was sucessful. Asolenoid valve was used to introducecompressed air to provide a this buffer airbetween the combustion products of one cycleand the fresh charge of hydrogen and oxygen ofthe following cycle. A solenoid valve was used toexpedite the test program and will be replaced bya rotary valve. The solenoid valve was controlledby the control system and was set to open forapproximately 100 ms each cycle. At thisopening time it was found the supply air pressurerequired to provide a sufficient air buffer wasapproximately 2.76 MPa (400 psi). Pressures

below 2.41 MPa (350 psi) resulted in a continuousburning process and no detonation wave as if noair at ail were added. Pressure above up to 3.45MPa (500 psi) produced no significantimprovement in the detonation process. The airwas injected through the ignition section using anunused ignition plug port. The air was injectedagainst the opposite wall.

Plotting the time of arrival of each detonationwave, the reflected compression wave, expansionwave, and compression wave from ambientconditions produce the experimental wavediagram in Figure 6 and is obtained from Figures 7and 8.

The path of the detonation wave is clearlyevident but not straight forward to plot. Before atrue detonation wave is created the pressurebegins to rise well ahead of the combustion wave.The combustion process is still supersonic in thecore flow but the boundary layer behind thecombustion wave causes enhanced mixing andcombustion and accelerates the process ahead ofthe boundary layer near the wall. The less wellmixed core flow lags behind in the combustionprocess but creates the larger pressure rise. Thetime passage of the detonation wave isapproximated by ignoring the precompression andlocating the location of the steep pressure jump.The detonation wave proceeds from the ignitionsource towards the open end of the chamber. Awave is also started which travels in the oppositedirection towards the closed end of the chamber.This wave travels only 3.81 cm (1.5 inch) before itreflects off the solid wall and proceeds towards theopen end of the chamber behind the initialdetonation wave. After reflecting off of the endwall this wave is traveling in burnt products ofcombustion. The wave would produce noadditional combustion if the detonation wave itfollows were a C-J detonation wave in a perfectlymixed stoichiometric mixture of hydrogen andoxygen. The pressure traces illustrate a moregradual rise in pressure than one would expectfrom a shock wave so it is suggested that it is aseries of compression waves. The 3.81 cmdistance from the ignition source to the end wall isprobably not of sufficient length to obtainsupersonic combustion and after reflection there isnot enough fuel available to continue combustionso the wave continues as a compression wave.This wave travels 7.62 cm (3 inches) further thanthe initial detonation wave when it is picked up byeach sensor. An expansion wave follows thedetonation wave down the tube and is created in


order to bring the high velocity flow behind thedetonation wave to rest at the end plate. Thecompression wave follows the expansion wave togenerate the secondary pressure increase butthen the pressure again begins to drop due toadditional expansion waves. The first combustionwave is continuously producing expansion wavesbehind it and these interact with the trailingcompression wave to produce several smallerrises and decreases in pressure as they traveldown the length of the chamber.

After the detonation wave exits the chamberthe high pressure generated by the detonationwave expands to match the atmosphere pressure.This generates a series of expansion waves whichenter the chamber and progress towards theclosed end. The pressure transducer nearest theopen end sense the greatest effect of this series ofexpansion waves.

The second transducer from the open endsenses the expansion wave as well but thereflected compression waves from the closed endreaches this location at about the save time. Thetwo waves interact and the result is a plateau inthe pressure trace and very little rise in pressure.

The transducer closest to the open end doesn'tpick up the reflected compression wave as theexpansion wave more than overwhelms thecompression waves. This results in a significantlylower pressure at this location relative to theothers for approximately 0.5 milliseconds.

The series of expansion waves continue topropagate up the chamber, reflect off the solid wallopposite end, and return to the open end. Duringthe downstream motion towards the open end thepressure drops to a sub-atmospheric level. Afterthe returning expansion waves reach the exit ofthe tube the pressure is below the ambientatmospheric pressure so a compression waveenters the chamber to correct this. Thiscompression wave increases the pressure to aslightly positive gage pressure at the first sensorfrom the exit of the chamber. Sensors further fromthe exit record lower pressures before thecompression wave pass as they experience theexpansion waves for longer periods of time beforethe compression wave arrives at the closed end ofthe chamber. The compression wave then returnsto the exit of the chamber. This process ofexpansion and compression waves is repeateduntil the pressure in the tube matches the ambientpressure. The low frequency covered in thispaper allows plenty of time for this series of

expansion and compression waves to play out andthe pressure to return to ambient. At higherfrequencies these waves will have an effect on theinjection of fuel and oxidizer for the next cycle.

The velocity of the initial supersoniccombustion wave initially starts out at over 670m/s (2200 ft/s) then drops but remains supersonicbefore beginning to accelerate again throughoutthe length of the chamber. The velocity obtainedfrom time of flight measurements from thepressure sensors is shown in Figure 9. The flatlines between symbols represent average velocitybetween pressure transducer locations. All thevelocities observed were well below the C-Jdetonation velocity calculated with the TEP^computer code, a Windows™ version of the NASACEC769 code.

A comparison of pressure traces from singleshot and multi-cycle operation is made in Figures10 and 11. Both figures are taken from the samelocation in the tube, which is midway between theopen and closed ends. Figure 10 is from the multi-cycle operation and shows the initial combustionwave and extends for fifty milliseconds andincludes several cycles of expansion andcompression waves. Figure 11 shows the sameportion of the waves for operation in the singleshot mode. The two are very similar in wavepattern and peak pressure. Figure 12 illustrates acomplete cycle with two pressure pulses that areat the same chamber location as Figures 10 and11.

UNCERTAINTIES

The pressure plots show no definitive shock ordetonation wave for the last two instrumentationlocations near the open end of the chamber. Thefuel and oxidizer here was the first injected duringthe cycle and has mixed with the buffer air anddiluted. The detonation wave has basically diedout. The way the air and the combustiblereactants are injected has a significant influenceon their mixing. Different techniques of injectingthe buffer air could reduce the amount requiredand reduce the turbulence created and reduce themixing with the combustible reactants. Themixture ratio of hydrogen to oxygen is notprecisely known so it may not be very nearstoichiometric.


CONCLUSIONS & RECOMMENDATIONS

The operation at the low frequencies describedhere appear to be nearly identical to the operationin single shot mode. The chamber pressure hasequalized out to ambient conditions inapproximately 50 milliseconds. Frequencies of 20Hz appear possible but injection of an air bufferwill require some time. Frequencies of at least 10Hz should be obtained before the interaction ofone cycle's wave with the injection of the nextcycle's fuel becomes a problem. At this pointwhen the interactions begin the frequency may begoverned by the wave location during purge air,fuel, or oxidizer injection and ignition.

The length of the chamber will affect theinteraction of the detonation waves and expansionwaves, so the length will have to be considered tounderstand how it affects the performance atvarious frequencies. This length requires a finiteamount of time for either the detonation orexpansion wave to traverse it. The effect ofthese waves on the previous or following cyclemust be known to optimize performance.

The pressure level measured and time of flightvelocity indicate a C-J detonation wave was notgenerated. Methods to create a near C-Jdetonation wave must be developed. Improvingthe mixing and measurement of fuel and oxidizer,creating turbulent flow including the use of atubulator spiral, and increasing wall area must beaddressed.

Increasing the operating frequency will be donein the future in incremental steps to understandthe wave interaction phenomenon. A 100+ Hzignition system has already been developed andwill be incorporated. The present fuel injectionsystem will be enlarged and improved for high flowrates as the frequency increases.

ACKNOWLEDGMENT

Lockheed, Fort Worth Division, andRocketdyne provided a great deal of insight andinspiration for this study while aiding in its support.

REFERENCES

1. Smith, D.E., "The Synchronous InjectionIgnition Valveless Pulsejet," University of Texas atArlington, TX, May 1987.

2. Bussing, T. R. and Pappas, G., "An Intro-duction to Pulse Detonation Engines," AIAAPaper 94-0263, Jan., 1994.

3. Nicholls, J. A., Wilkinson, H. R., Morrison, R.B., "Intermittent Detonation as a Thrust-Producing Mechanism," Jet Propulsion, May1957, pp. 534-541.

4. Kim, K., Varsi, G., and Back, L. H., "Blast waveAnalysis for Detonation Propulsion," AIAA Journal,Vol. 15, #10, 1977, P. 1500-1502.

5. Stanley, S. B., "Experimental Investigation ofFactors Influencing the Evolution of a DetonationWave," M. S. Thesis, University of Texas atArlington, 1995.

6. Stanley, S. B., Stuessy, W. S. and Wilson, D.R., "Experimental Investigation of PulseDetonation Wave Phenomenon," AIAA Paper 95-2197,1995.

7. Stanley, S., Burge, K., and Wilson, D.,"Experimental Wave Phenomenon as Related toPropulsion Application," AIAA Paper 95-2580,1995.

8. "Thermal Equilibrium Program for WindowsUser's Manual," Software and EngineeringAssociates, Inc., 1992.

9. Gordon, S., and McBride, B. J., "ComputerProgram for Calculation of Complex ChemicalEquilibrium Compositions, Rocket Performance,Incident and Reflected Shocks, and Chapman-Jouguet Detonations," NASA SP 273,1971.

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