IN - DTIC
INTHE IGNITION AND COMBUSTIONOF A LIQUID MONOPROPELLANT"*\By B. L. KarhanTechnical MemorandumFile No. TM 705.9161-01May 18, 1967Contract NOw 65-0123-dCopy No.ATHIS DOCUMENT HAS BEEN APPROVEDFOR PUBLIC RELEASE AND SALE;ITS DISTRIBUTION ISUNLIMITEDThe Pennsylvania State UniversityInstitute for Science and EngineeringORDNANCE RESEARCH LABORATORYUniversity Park, PennsylvaniaDGCJUNNAVY DEPARTMENT NAVAL ORDNANCEReproduced by theCLEARINGHOUSEfor Federal Scientific & TechnicaInformation Springfield Va 221517 1968SYSTEMS COMMAND
VUNCLASSIFIEDIiReferences:Abstract:See page 64An investigation was made of the heat-up, ignition andcombustion of a nitrate ester monopropellant droplet whensuddenly placed in a high temperature gas. At moderatepressures a combustion zone became noticeably developedas the droplet approached its wet bulb temperature and aAt high pressures reactivesteady burning period followed.effects became noticeable early in the heat-up period andIn this regime the droplet spentburning was not steady.most of its lifetime in the preignition period. A heat-uptheory of ignition is developed and found to be in agreement with the moderate pressure data. Burning rates alsoreported over the test range.UNCLASSIFIED
iiACKNOWLEDGMENTSThe author wishes to thank Professor Gerard M. Faeth, whosupervised the research, for his encouragement and invaluableguidance throughout the course of the investigation.as well,Thanks go,to George Yanyecic for his assistance in the experimenta-tion.The author also wishes to express his sincere appreciationto the Ordnance Research Laboratory for its total financial supportof the investigation.
2i1TABLE OF CONTENTSPageAcknowledgments .List of Figures .Nomenclature .1.2.3.4.5.ii.iv.vINTRODUCTION1.1 General Statement of the Problem .1.2 Previous Related Studies .1.3 Specific Statement of the Problem .i.113EXPERIMENTAL APPARATUS AND PROCEDURE2.1 Test Facility .2.2 Operation of the Appazatus . .51.IGNITION3.1 Preliminary Results .3.2 Theoretical Corsiderations . .3.3 Experimental Results and Discussion . 14. 21. 28CO B.USTION4.1 Theoretical Considerations.4.2 Experimental Results and Discussion .38. 41SUMMARY .APPENDIX A - Derivation of the Finite Conductivity.Heat-Up Model .APPENDIX B - Fluid and Gas PropertiesB.1 Fluid Properties . .B.2 Gas Properties .BIBLIOGRAPHY .47. 50.62. 63. 64
ivLIST Or itURESfi&urePage2.1Experimental Apparatus.2.2Internal Assembly .2.3Droplet Mount Assembly .2.4Sequence3.1Ignition at Various Temperatures .3.2Ignition at Various Pressurevs .3.3Sample Photograph - Visible Flame .3.4Sample Photograph - Flame not: Visible .193.5Finite Cor.ductivity Model .273.6Vapor Pressure Assumptiun3.1Vapor3.8Ignit.ion Delay at Various3.9Ignition Delay at Various Ambient. Temperatures.323.10Ignition Delay at Various Total iPressures343.11Liquid Temperature at Various Pressures .3.12Liquid Temperature Assumption4.1Burning Rates at Various Ambient Temperatures424.2Burning Rates at Various Total Pressures.434.3Activation Energy Comparison .of Operation .ressure Data.6.7.9.12.15.16.1829.30Initial Diameters. .31.35.36.45
FNOMENC LATUREA-Surface Area of Droplet (FT 23-Radiation Absorption CoefficientBi-Dimensionless Bior NumberC-Burning Rate ConstantC3,4-(Dirrensionless)(F I2SEC)Integration Constants0C-Specific Heat of DropletD-Droplet DiameterDo-;.7itiaiE-Average Droplet Diameter During Burning Feriod (FT,E-Activation Energy (BTIU/MOI.;Gr-Dimensionless Grashof NumberH-Heat of Combustionh-Heat Transfer CoefficientK-Rare Constantk-L'uidk-Gas Thermal Cor, uctivity (BTU/FT SEC 0 RL(BTU/LB R)JF.,Droplet Diameter(FT)(:BU/LBM1)tBTU/FT2 SEC 0 R;in Arrhenius Eq.Thermal ConductivityHeat of Vaporization(BTU;FT(BTIU/LBM,1-Minor Axis of an Eilpsoid,Fl)12-Major Axis of an Ellipsoid(Fr)m-Mass Burning RateNu-Dimensionless Nusselt Numbert:BM/SEC)5&:CR.I
viN-Order of Reaction (Dimensionless)P-Pressure (LBF/FT2 )PR-Dimensionless Prandtl NumberQ-Radiation Absorbed Per Unit Volume (BTU/FT )q-H/C T Gr-Radial Distance (FT)rE-Outer Radius of Dropl'ec (FT)ro-Initial Droplet Radius (FT)R-Universal Gas Constant (BTUhL.-Transform Variable from Laplace TransformationT-Droplet Temperature ( R)T-Flame Temperature ( R)TE-Temperature of Droplet Surface ( R)T0-Ynitial Droplet Temperature ( R)Tt-Furnace Temperature ( R)rý'al Time Constant (SEC)t-Time (SEC)V-Volume of Droplet (F1 )v-Velocity of Flame Surface (FT/SEC)X-Frequency Factor in Arrhenius ExpressionY-Root of Trancendental Eq. TAN YnndimensionlessT0Cnz--Yn( Bi/2-1)
viia-Constant Bi/2, Dimensionless-Constant-ccL/C pT wX-I Bi/2 -1, DimensionlessDimensionlessa-Stefan-Boltzman Coefficienta Dimensionless Ratio o' Flame Radius to Drop RadiusO Density of Droplet (LBMIFTI 3.)- Density cf Gas at Flame Ssrface (LBM/FI0Ce O Dimensionless Temperature DifferenceDimensionless Temperature Difference in Tranbformed S-Domain Dimensionless Radius Dimensionless Time Rad4,tion Factor, DimensionlessU
CHAl.ER 1INTRM UCT ION1.1General Statement of the ProblemIn recent years monopropellant droplet combustion hasreceived an increasing amount of attention.I17While liquid monopro-pellantp h-ave not been found to be promising for use in the primarypropulsion units of aircraft or missiles, they may be adapted tosecondary applications.A thorough understanding of the combustionprocesses of a single monopropellant droplet is an Important firststep in comprehending the combustion process of these fuel32 inI-rocket motors.This thesis is concerned with an experimental andanalytical investigation of the heat-up and burning rates cf aparticular liquid monoprcpellant when suddenly subjected to a hightempera ture environment.1.2Previous Related qrudiesRegarding droplet combusticn 3tudies in general, &obayasi 12and Nishiwak-i 17introduced the use of an e ectrically h atedmovable furnace in their studies of bipropellant ignition.Theheated furnace was moved ever a drop!:ýt supported on a qu.artzfiber to provide rapid introduction of the droplet into the high
2Motion pictures were taken of the heat-temperature environment.up and ignition of the droplet.A technique similar to that of Kobayasi12.and Nishivakiwas applied to monopropellant combustion by Barrere3 for a varietyof monopropellants.Barrere3 tested in a nitrogen environment atatmosphere pressure with ambient temperatures between 10000F and1600 F.Droplets with diameters between .030 and .080 inches weretested.However,primary emphasis was on the quasi-steady stateburning period of the droplet and no ignition mea3urement5 weremade.Barrere's3 test isthe orly study to date in wbich the mona-propellant was apparently allowed to burn in an iner: atmosphereas isthe case in an engine where the monopropellant burns in its26products of combustion.Tarifa,et al,20and Rcsserwere ableto support combustion only with the 3dditicn of oxygen to theenvironment thereby creating a system not truly represented asmonopropellant combustion.An early attempt at theotetically modeling the czmbastionof a motionless monopropellant droplet was :ndertaken by lorelland Wise 14.Their model considered a chemicalreaction diffusedthroughout the gas phase with the fuel vapor decomposing exothermically into product gases at a rate dependent on the temperatureand weight fraction of zhe fuel vapor.By taking ejuai vaiues ofmolecular weights and by assuming a Lewis number of unity, the
problem was solved by integrating numerically the resultingdifferential equations of the process.Later works of Tarifa, ec al26, and Williams28considereddiffusion of species of different molecular weights and Lewis28numbers.Williamspresents exact results for his model byapplying the assumption of a thin flame zone for reactions withsufficiently large activation energies.attributed to Spaldingthe system equationas.24This model,first, allows direct simultaneous solution ofHowever,the general lack of experimentalresults has prevented any verification or revision of existingtheories.1.3Specific Statement of the ProblimThe preceding discussion has indicated that knowledge ofthe combustion processes of liquid mcnopropellants is, at best,very limited.Concerning the existing theories of monopropellant dropletburning,if any,it wculd be of interest tc determine wha: limitations,they present.Without this k-cwledge,the accuracy ofburning lifetimes predicted by these theories issubject toquestion.Also, itisof importatce to understand the heat-up mechanismof a monopropallant.ultimately,Its relationship to the heat-up time and,to the total droplet lifetime must be a primary consid-eration in rocket engine design.
4IThe technique employed by Barrere3 offers a means ofstudying the droplet combustion process.However,by enclosingthe apraratus in a pressure vessel equipped with a means of evacuation, a pure inert atmosphere may be maintained at various totalpressures.To summarize,the specific objectives of this thesis areas follows:1.To formulate a theory for the heat-up and igniticn ofa motionless monopropellant droplet, and check ,experimentallyover a range of ambient temperatures and pre3sures.2.To apply a suitable existing monopropellant burningtheory to the experimental results, and thereby determine itslimitations.
CHAPTER 2EXPERIhENTAL APPARATUS AND PROCEDURE2.1Test FacilityThe primary requirement of the experimental apparatus wasto provide a means of rapidly placing a droplet into a hightemperature inert atmosphere at various total gas pressures.Ameans of photographically recording the droplet's behavior anda means of measuring the droplet's temperature were also required.A photograph of the experimental apparatus used for thisstudy of the ignition and combustion of monopropellant dropletsis shown in Figure 2.1.Figure 2.2 is a schematic diagram of theinternal chamber assembly.The upper part of the apparatus was enclosed with a pressurevessel which allowed operation up to 3,000 PSI.A hand-operatedwinch was used to raise and lower the upper cover and,when thecover was bolted to the base, the chamber was gas tight.a vacuwuBy usingpump and a removable hose attached to a pressure sealedconnector,inert gas.the chamber was evacuated and then pressurized with theFor operating pressures below one atmosphere,thepressure within the chamber was measured with a differentialmercu-y manometer.Measurement of chamber pressur-.s between 1 and
IFIGURE 2.1EXPERIMENTAL APPARATUS-!
LETTIOINGSWSWITCHREBOUNDCATCHABSORBERFIGURE 2.2lNTERNAL ASSEMBLY
5 atmospheres was made with an Ashcroft laboratory gauge with1/2 PSI subdivisions.For tests above 5 atmospheres,gauge with 50 PSI subdivisions was used.an AshcroftConmercially purenitrogen was used to pressurize the chamber.The monopropellantused in the testing program was propylene glycol dinitrate.The droplet was supported on either a quartz filament orthe bead of a thermocouple junction.isshown in Figure 2.3.A sketch of the mount assemblyThe thermocouple was constructed fromchromel-alumel wire having an outside diameter of .003 inches.The somewhat enlarged thermocouple junction had an average diameterof between .008 and .016 inches.When only one thermocouple was used,it'soutput was feddirectly into the vertical terminals of a Model 130B HewlettPackard oscilloscope.A Dumont,Type 2614. oscillo3cops cameraemploying Type 47 Polaroid film was used to record the trace.the dual thermocouple arrangement,inthe thermocouple outputs werefed into the terminals of two galvanometers in a CcnsclidatedElectrodynamics Corporation Model 5-124 recording o cillograph.The quartz filament-single thermocouple arrangement was usedprimarily in the larger droplet sizes (.060 - .080 in.: becausethe small thermocouple junction could not support the large droplets.The furnace consisted of a heating coil wound between anouter stainless steel cover and an inner ceramic core.The furnacewas rylindrical in shape with an inside diameter of 2 in.and an
9GAS PHASETHERMOCOUPLEQUARTFIDROPLETMOUNT/QUARTZ FIBER OR THERMOCOUPLEWIRE"SHOCKFIGUFE 2.3DROPLET MOUNT ASSEMBLYABSORBER
outside diameter of 3-1/2 in.The over-all height of the furnacewas 6-1/2 in. with an actively heated inside length of 5 in.After the furnace was heated to the desired temperature,a solenoid was energized releasing it from its initial position.The furnace slid down guide rods to the dropleL location where itwas locked itt place by two larches, thus providing rapid immersionof the droplet into the high temperature media.The time requiredfor the furnace bottom to travel from "he droplet location to thelatches was about 50 ms.The shock of the fall was absorbed bytwo pieces of Resilite mounted on a false base.Resilite is anenergy absorber sometimes used in athletic equipment to reduceshock.The Resilite shock absorbers combined with the latchingmechanism eliminated extraneous motion of the gas around the droplet during the test period.The bottom of the furnace was op,ýn, while the top wasclosed with a quartz window.This window allowed the passage ofbackground illumination for photographic putposes, while preventingthe escape of high temperature gas f em the fuTnace irtc thechamber.The gas temperature within the furn3ce was m,---asured with a1/8 in. O.D. sheathed chremel alumel thermocouplethermo-couple junction was located in a position that would corresponJto the droplet position when the furnace was lock.:!d in place.This was about 2-1/2 in. from the base of the furnace."he
11temperature indicated by this thermocouple just before each testThe thermocouple was allowedwas taKen as the test temperature.to stabilize at a constant temperature before each test to insurea uniform furnace tzemperature.The droplet was photographed through a quartz window inthe base of the chamber using a 16mm Fastair missile camera.Kodak Tri-X negative film was used in the camera at a speed ofapproximately 80 frames per second.a small incandescent lamp.The background light wasThis illumination passed through thequartz window in the top of the furnace giving a shadoivgraph ofthe droplet.A neon timing light in the camera allowed synchruni-zation of the motion picture film and the oscilloscope trace.the furnace reached the droplet location,Asit closed two switcheswhich simultaneously completed the circuits of the timing markerand the oscilloscope trace, thereby indicating the beginning ofthe heat-up process.2.2Operation of the ApparatusThe ove,.-all operation of the apparatus is illustrated inFigure 2.4.Conditions within the chamber at the beginning ofa test are represented in the first diagram of Figure 2.4.this point the furnace temperature has stabilized,Atthe droplethas beer pliced on its support, and the chamber hl-s been closedand pressurized.camera is off.The backgroundlight has been turned on and thed
12RELEASESWITCH-- RELEASESOLENOID ,,Z. SWITCHA,k(I LOCKINGMECHAN IS MA(2)FIGURE 2.4SEQUENCE OF OPERAT(3
13To begin operation the oscilloscope sweep was triggered,the camera was started, and the furnace release solenoid was actuated allowing rhe furnace to fail.in the second diagram ofFigure 2.4, the bottom edge of the furnace is just passing thedroplet location.At this point the furnace closed two switcheswhich simultaneously completed the oscilloscope trace circuit andthe timing light circuit on the camera,The third diagram of Figure 2.4 shows the furnace in itsfinal locked position at the droplet location.
ICHAPTER 3ICGNIIO3.1Preliminary ResultsThe exact nature of the ignition process of a particularfuel varies with the chemical composition, environment,the fuel being investigated.Consequently,etcofpreiiminary experi-mental tests seemed advisable to assist in the selection of theignition model.Typical results of these and later tests areshown in Figures 3.1 and 3.2.Figures 3.ia and 3.lb which present da-a at ! atm.totalpressure over the experimental temperature range demonstrate acorrelation between the beginning of diameter change s:d sn inflection point in the liquid phase thermocouple record.change isTh-. diame.rerthe point in the diameter versus time plct vhi.re thediameter begins to decrease rapidly."t cccurs very slightlybefore, or about the same time as the inflection point.A gasphase thermocouple was then introduced to indicate when a flameappeared in the gas phase,low pressures.Itissince the flame was not visible atseen that the inflection in thr gas phasethermocouple, which indicates thr: presence cf a flame,exhibitsthe saoe correlation with the diameter change that the !iq',i
15800'A000 0Soo0-RRT460P 1 02-",,I-TIME-SECONDSwI1wS800To005w 80 o o o 00(3PTon 2060 -R 1 ATM00i-0 .04-0- oC0n0.0360000050('-0.40.811.211.6TI ME-SECONDSFIGURE 3.1IGNITION AT VARIOUS TEMPERATURES1 0.020
*16AzTcD 1460 'RP 40 ATM10051600 w 800-a:TL0I1200- Ua700-T-1004wz 80oot600z,w,5- END0,.Iw.OFmDROPLETc. 400L -7 50004--08212TIME-SECONDS4-G-1003FLAME :crVISIBLE10(zww 0 o oniw- m0c-P 1 ATM0a00041200-0--,0800-0 03o00102.0TIME-SECONDS30FIGURE 3.2IGNITION AT VARIOUS PRESSURES
K17thermocouple had, as evidenced in Figure 3.2b.That is,theliquid and gas traces inflect at the same time, or very slightlyafter, the diameter change occurred,This correlation extendsover the entire test range.Figure 3.2a, which presents dataat 40 atmospheres pressure,shows excellent agreement betweenthe inflections of the two curves and the appearance of theluminous flame.The diameter of the droplet during the pre-ignition period could not be reasured accurately at high pressures,due to the scattering of the ;zckgroind light rays by naturalconvection patterns on the furnace wa.cl.o.On the basis of the precedinginto -ton, a criterionfor the ignition time was established.Eachi acceptAble datepoint had to have at least two separate indications oC iwr.tionthat occurred at very nearly the same time.couple inflection and diamet-,C-as or liquid thermo-change or, at high pressures, gasor liquid thermocouple inflection and luminous flame were thecombinations used.Figures 3.3 and 3.4 show sample photographs of the ignitionand combustion processes.They illustrate the extremes of theproblem of ignition identification.in Figure 3.3, where thevisible flame was used as the ignition indicator. thL film recordbetween .31 and .85 seconds was eliminated since no significantchanges in the droplet were observed during that period.Notehow the natural convection patterns on the furnace window in
*18IIt-.ZoV [-.,*U,CDna:0C-.44zJ* *'I*
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20Figure 3.3, at 40 atmospheres total pressure,the droplet during the pre-ignition period.tend to obscureNo such problemwas encountered in the I atmosphere test of Figure 3.4.droplet reained essentially unchanged between .17so this file was excluded.at 1.40 seconds.and .88 seconds,Shortly after .88 seconds the dropletbegins to steadily decrease in size until itconsumed at 1.03 seconds.Theis almost completelyThe empty probe is all that remainsAs noted by other experimenters,3,7,12 thedroplet is not exactly spherical during the test period.retain a degree.Toof uniformity with previous workers in the field,the droplet was assumed to have the shape of an ellipsoid.Thediameter of the droplet was then defined ro be the diameter of asp
propulsion units of aircraft or missiles, they may be adapted to . Barrere's3 test is the orly study to date in wbich the mona-propellant was apparently allowed to burn in an iner: atmosphere as is the case in an engine where the monopropellant burns in its . eration in rocket engine design. 4I
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