A 2D-laser doppler velocimeter was used to  measure velocities of reacting boron (B) particles during the  combustion of a metallised solid fuel slab inside a 20-combustion  chamber. The solid fuel hydroxyl-terminated polybutadiene (HTPB) was  enriched with B particles to increase its specific heat. To obtain  information on the combustion process and on the movement of B  particles, their velocities were measured. The experiments were  performed at ambient pressure. The behaviour of the B particles  concerning the exit velocities from the fuel slab has been discussed on  the basis of the experimental results

Ciezki, H.K.

Sender, J.

English

Defence Science Journal

Defence Science Journal. Vo148, No 4, October 1998, pp. 343-349@ 1998, DESIDQCVelocities of Reacting Boron Particles within a Solid FuelRamjet Combustion ChamberI. Sender and H. K. CiezkiDLR, German Aerospace Centre, D- 74 239 Hardthausen, Germany.ABSTRACTA 2D-Iaser doppler velocimeter was used to measure velocities of reacting boron (B) particlesduring the combustion of a metallised solid fuel slab inside a 2D-combustion chamber. The solid fuelhydroxyl-terminated polybutadiene (HTPB) was enriched with B particles to increase its specific heat.To obtain information on the combustion process and on the movement of B particles, their velocitieswere measured. The experiments were performed at ambient pressure. The behaviour of the Bparticles concerning the exit velocities from the fuel slab has been discussed on the basis of theexperimental results.I. INTRODUCTIONAddition of boron (B) to the commonly usedhydrocarbon binders with the application of airbreathing propulsion systems like solid fuel ramjetsis of special interest due to its high votumetric andgravimetric heat capacity (Table I). However, fullusage of that heat capacity is limited due to theignition and combustion of B, which may lead toincomplete combustion I. Several reasons can beattributed for this behaviour:(a) The combustion is inhibited by a molten layer ofB203 immediately after ignition, whose boilingtemperature is about 2580 K.(b) The boiling temperature of solid B is very high(about 2900 K), which results in relatively slowcombustion rates if the oxide layer is removed.(c) Agglomeration increases the size of B particles,slowing down their heating and combustion.(d) Possible kinetic trapping of parts of thecombustion products in gaseous form,particularly in the presence of hydrogen, resultingin appreciable loss of heat of condensation.N atan and Gany2 showed that for B particles,depending on their size, a minimum residencyperiod of some milliseconds within the flame zoneare required before they ignite. Experimentalinvestigations by ClaussJ using single B particleswithin a hot gas flow showed the particles to bestrongly affected by the combustion products afterignition. Their flight path tends to form some kindof swirl curves. A recent study by Meinkohn4 on thebehaviour of thin films on reacting surfaces showedthat the fluidised B2OJ layer may break up locally.Scanning electron microscope (SEM) picturestaken by Ciezki and Schweins seem to confirmthese results. At these free surface regions, thecombustion process may restart, leading to localproduction of gas and acceleration of the particlemovement.The objectives of this investigation were toascertain as to what extent this effect influences theReceived 04 September) 997, revised 2) October) 997343DEF SCI J, VOL 48, NO 4, OCfOBER 1998combustion, to measure the velocities of exit of Bparticles from the solid fuel slab and to examine theexisted without any fuel to prevent thewindows from getting covered with soot.Hydroxyl-terminated polybutadiene (HTPB) wa$used as (uel with the addition of 30 per centparticles by weight. The particle size of B was1.75 J.lm (sauter size D2.3 = 0.96 J.lm). A secondhydrogen/oxygen bummer whose flame passed for ashort time through a slit at the lower edge of the stepwas used to ignite the fuel slab. Optical access forthe LDV at arbitrary- positions downstream the stepwas provided by movable window elements on eachside wall. To prevent deposition of particles andsoot on the windows, a special design was used.Each element was built from a converging tube oflOO mm length. The window on the side wall was of17 mm width and 45 mm height. The outer side ofthe tube of size 45 mm x 45 mm was closed by aquartz glass window. A slight nitrogen flow wasused to purge the windows. The influence of purgeflow on combustion flow is negligible, as can beseen from the velocity data. Table 2 shows theoverall dimensions of the combustion chamber .Table I. Heating values of fuels, assuming H20 (gas), C02 (gas)and metal oxides (0) as productsFuel Gravimetric heating Volumetric heating(kJ/g) (kJ/m3)JP5 425 344SHELLDYN-H 413 418Coal 328 741At 310 837(CHVn 435 402B 578 1349Mg .247 431influence, if any, on the velocity of the particleswhen ignited. For making the measurements, a laserDoppler vefocimeter (LDV) was used.2. EXPERIMENTAL SETUP2.1 Combustion ChamberA 2D-combustion chamber with a backwardfacing step and a rectangular cross-section of150 mm width and 45 mm height, was used toperform the experiments (Fig. I ). The flameholding step height H was 20 mm. Air, vitiated by aTable 2. Characteristic dimensions of the 2D-dump combustorof test section Mll.2Dimensions(mm)1502204520171Q5100CharacterWidthLengthHeightStep heightInner window widthLength of the fuel slabWidth of the fuel slabNo nozzle was attached to the exit of thecombustion chamber. Thus, nearly ambientpressure was maintained inside. The velocity of theincoming vitiated air above the step was about90 nils with a mass flux of ISQ gls at 500 °C. Twoinlets attached to the side of the air heater suppliedTiO2 seeding particles with a low air mass flux tofacilitate LDV measurements. The average particlesize of the seeding was 1 ~m. To avoidFigure I. Cross-section or 2D-dump combustorhydrogen/oxygen burner, was heated up to 500 °Cand fed via rectifiers and meshes into thecombustion chamber. Behind the step, the solidwas inserted into the lower wall of the test section.The fuel slab was 100 mm in width and 198 mm inlength, i.e., a border of 25 mm on each side wall344SENDER & CIEZKI: VELOCI11ES OF REAcnNG BORON PAR11CLESFigure 2. Entrance velocity profile atxlH = .0.25; Uo = 94 m/sJD-traversing unit controlled by a PC. Datareduction was done using standard software.agglomeration, the particles were dried befo~e useThe seeding was performed using a swirl generator .Figure 2 shows the normalised velocitydistribution directly above the flame holding step atx/H=-0.25 corresponding to a Reynolds number(ReM) of 22800.2.2 InstrumentationA 2D-Iaser Doppler velocimeter from Dantec,equipped with a 5W argon ion laser (Innova 90,Coherent) and two burst spectrum analysers (BSA,Dantec) was used to measure the velocities usingforward scatter sampling. The focal length of thetransmitting lens was 300 mm. The conversionfactor for this configuration was 8.67 ms-I/MHz for514 nm and 8.23 ms-I/MHz for 488 nm. The beampairs were rotated at 45° along the z-axis to improveresolution for both x and y velocity components. Atotal of 6100 samples were taken at eachmeasurement point, scanning the incoming LDVsignal at 5k Hz. The LDV was mounted on a2.3 Experimental ProcedureFour configurations (a,b,c,d) with differentmass fractions of both TiO2 and B particles wereselected to clarify the visibility of burning Bparticles with LDV measurements (Table 3). Foreach. configuration, velocities for one cross-section(at xlH=4.75) were measured.The coordinate system used was located at thelower edge of the step in the ceriireline of thecombustion chamber at ignition time. During runtime, the surface of the fuel slab was lowered due tocombustion. The regression rate was about0.06 mm/s, i.e. the overall difference before andafter the experiment which took 2 min was about7 mm. Two runs were necessary to complete onecross-section velocity profile. The reproducibilityof flow conditions and thus velocities fromrun-to-run was excellent. The velocity profiles weretaken vertically starting at the ground in each case,345DEF SCI J, VOL 48, NO 4, OCTOBER 1998COW'ARlOON OF V-RMSFigure 3. Comparison of velocity and RMS values with and without seeding of the plug now at xlH ~ 4.75; Uo ~91 mls; fuel: 30%B/HTPB.to reduce the influence of regression on the 3. RESULTScoordinate origin during one vertical profilemeasurement. Thus, the change in geometry was 3.1 Boron Particles used as Seedingnot considered for the data reduction.Figure 3 shows two-velocity and RMS profilesTable 3. Configuration of experiments (giving mass fractions of .particle additives by weight) at x/H=4.75 measured for configura(lons I (d) and2 ( d), .i.e. solid fuel with B particles without seedingof the plug flow i.e I (d) and the plug flow seeded in2 ( d).. Both cases show an increase in meanvelocities with increase in distance from the fuelslab. The agreement for the profiles within all fourdiagrams is good, i.e. there is nO'. significantdifference whether the outer flow is seeded or not.The RMS values show the typical shape of a stepflow. Comparing the horizontal and verticalvelocity components, it can be seen that the abilityof the B particles to follow the flow is comparableto that of TiO2 particles. On comparison of the RMSprofiles, it appears that. there is no additionalvelocity component due to ignition and combustionof the particles, as was observed earlier by ClaussJwith a laboratory setup. This means that it is correct2Without WithTiOl seeding TiOl seedingMain flow(Solid fuel with)a No additive xb B (30 %) xc TiO2 (30 %) xd TiO2( 5 %) and B (25 %) x xFor Case I. cross-section velocity profileswere measured atx/H= -0.25. 0.75. 2.75. 4.75. 6.75 and 8.55.The measurement grid for each cross-sectionylH = 0.25, 0.6, 0.95,zlH = -3.0, -1.5, 0.0,.3, and 1.651.5, and 3.0SENDER & CIEZKI: VELOCITIES OF REACfING BOR()~ PARTICLESFigure 4. Comparison of velocities and RMS for 30% B/HTPB and 30% T;Oz /HTPB; xlH = 4.75; Uo = 93 m/sper cent TiO2 (Case 2 (c) and Table 3) at theposition xlH = 4.75. No significant differences forthe U component between the two cases can beto measure velocities by means of LDV using Bparticles while additionally seeding the air flowunder the conditions adopted in the present study...~ ~~~~~ <~~~~~~~~===,Figure 5. Vedor plot with streamlines at z IH = 0; Uo = 91 m/s; fuel: 30% B/HTPBseen. Close to the fuel surface, the U is negative,indicating the re-circulation zone behind the step.With increasing distance from the lower wall, Uvalues increase reaching finally the value of themain flow. The RMS values for both cases show3.2 Comparison of Reacting and Non-ReactingBoron Particle PhasesFigure 4 shows the mean velocities and RMSvalues of the U and y components for combustionof solid fuel with 30 per cent B (Case 2 (b» and 30347DEF SCI J, VOL 48, NO 4, OCTOBER 1998comparison to measurement without seeding of theincoming air. Under the flow conditions adopted,the B particles followed the flow without slip. Nosignificant proper motion due to ignition and localcombustion on the particle surface could be seen.Orientation of the velocity vectors behind there-circulation zone was horizontaf or slightlydownward indicating that B particles in the outerflow originate from the re-circulation zone directlybehind the step. No vertical outlet velocity from thefuel slab in the region 0.2 < y/H <1.65 was noted.similarity close to the wall. Abovey/H=I, the RMSfor Case 2(b) shows significantly higher values.This seems to be caused by the combustion processand the increased energy production and not by anadditional velocity component due to motion of theB particles, as mentioned above. The V-mean isconstantly decreasing and is in good agreementclose to the wall. For y/H >1, the velocity forCase 2 (b) (B additive) is still decreasing, whereasfor Case 2 (c) (inert additive), the trend changes andthe values become positive for y/H > 1.6.Figure 5 depicts the velocity vectors in thez/H = 0 plane (centre of the channel from x/H = 0 tox/H=10 measured for Case. 2 (b) (B-additive).Additionally, calculated streamlines were plotted.The re-circulation zone behind the step can berecognised clearly. The region after that zone showsthe velocity vectors being orieRted horizontal orslightly slanted towards the lower wall. Thisindicates that B particles that leave the fuel slab donot intrude into the far-region of the flow, butremain close to the wall from where they aretransported downstream. This is in agreement withthe results of Gany and Netzer6, who identifiedparticle trajectories by means of a high speedcamera. It was shown for similar flow conditionsthat particles of size 100 Jlm reached a maximumdistance of 3 mm from the wall within the regionscanned. However, the possibility to measurevelocities in the outer flow for Case I (b)(B-additives without seeding outer flow) and theresults of Ciezki and Schweins indicate that thereare B particles in that region and that these particlesmust have originated from the re-circulation zone.REFERENCES1. King, M. K. A review of studies of boron ignitionand combustion phenomena at Atlantic ResearchCorporation over the past decade. In Combustionof boron-based solid propellants and solidfuels,edited by K.K. Kuo & R. Pein. CRC Press,Boca Raton, USA,1993.2.3.4.s4. SUMMARYThe velocity field in a 2D-dump combustorusing solid fuel with B-additives was measured withLDV. The added B particles were used successfullyas tracer particles. Additional seeding of theincoming air flow did not reveal differences in6.Natan, B. & Gany, A. Ignition and combustion ofindividual boron particles in the flow field of asolid fuel ramjet. J. Propul. Power, 1991, 7(1),37-43.Clauss, Wo Verbrennung von borstaubwolkenin wasserdampfhaltigem heisgas. GermanAerospace Research Establishment (DLR), June1988, DLR-Intemal Report IB-643- 88/6.Meinkohn, D. The dynamics of thin films onreaction surfaces. Combus. Sci. Tech., 1995, 105.Ciezki, Ho & Schwein, B. Investigation ofgaseous and solid reaction products in a stepcombustor using a water-cooled sampling probe.American Institute of Aeronautics andAstronautics Joint Propulsion Conference,Orlando, USA. AIAA-96-2768oGany, A. & Netzer, D. W. Combustion studies ofmetallised fuels for solid fuel ramjets. AmericanInstitute of Aeronautics and Astronautics JoiqtPropulsion Conference, 21, Monterey, USA.AIAA-85-1177.348SENDER & CIEZKI: VELOCmES OF REACflNG BORON PARTICLESContributorMr J Sender studied Chemical Engineering at the University of Erlangen/N"urnberg, Germany. Hehas worked on several engineering and scientific projects at the Institute of Fluid Mechanics of theUniversity Erlangen/N"urnberg. Presently, he is Head, Spray Combustion Group at DLR, GermanAerospace Centre. His areas of research include cryogenic combustion of hydrogen/oxygen. He is amember of the Optical Diagnostics Group of German Aerospace Centre, in Lampoldshausen.349

Velocities of Reacting Boron Particles within a Solid Fuel Ramjet Combustion Chamber

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Velocities of Reacting Boron Particles within a Solid Fuel Ramjet Combustion Chamber

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