Experimental Investigation on Ablation Characteristic of EPDM Insulator in Different Gas Environment Experimental Investigation on Ablation Characteristic of EPDM Insulator in Different Gas Environment

Experimental Investigation on Ablation Characteristic of EPDM Insulator in Different Gas Environment

  • 期刊名字:兵工学报
  • 文件大小:870kb
  • 论文作者:WANG Shu-xian,HE Guo-qiang,LI
  • 作者单位:National Key Laboratory of Combustion
  • 更新时间:2020-09-13
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论文简介

JOURNAL OF CHINA ORDNANCEExperimental Investigation on ablation Characteristic ofEPDM Insulator in Different Gas EnvironmentsWANG Shu-xian(王书贤), HE GuO- qang(何国强), LI Jiang(李江), LIU Pei-jin(刘佩进)National Key Laboratory of Combustion, Flow and Thermal-structure, Northwestern Polytechnical UniversityXi an 710072 Shaanxi, ChinaAbstract Some ablation experiments of Ethylene-Propylene-Diene Monomer( EPDM) insulator were carried out in quasistatic low temperature gas environment, gas-phase environment, two-phase environment with Al, o, grain and high concen-ration AL,O, grain gas environments. Their charring ablation rate, thickness, surface morphology and main ingredient of theharring layer were analyzed. The experiment results show that the main influent factors for the charring ablation rate are thefor the chsg layerthe gas velocity and environment pressure; and the process of SiO, migrating in the charring layer occur commonly in differ-ent gas environments. They provide a foundation for the ablation mechanism research and modeling of EPDM insulatorKey Words: propulsion system of aviation and aerospace; EPDM insulator; experiment environment; charring ablationrate; charring layer thickness; surface morphology; ingredientCLC Number:V435°.6Document Code: AArticle I:1673002X(2010)02010905Introductioninsulator ablation experiments were carried out. Thecharring ablation rate, the thickness, surface morpholEPDM insulator is a kind of heat-insulating mate- ogy and main ingredient of the charring layer were ana-rial widely used in solid rocket motor chamber as elas. lyzed thoroughltie thermal protection shield. It takes EPDM rubber as 1 Quasi-static Low Temperature Gasthe matrix and contains short fibers, SiO, spheres, vul-canizer and inhibitor. It can shield the engine caseEnvironmentfrom thermal failure by charring ablation which is aThe test motor SSFl, as shown in Fig. 1, and theprehensive result of thermo-chemical ablation, gas composite propellant containing 1% aluminum are usederosion and grain erosion1-3. At present, the thermo- and the gas temperature is 2 700 K. The gas velocity ischemical ablation models in our country usually draw reduced to only 0. 48 m/s by enlarging the section arealessons from the foreign models for throat insert or air- of the test segment. Twelve samples can be put in thecraft reentry atmosphere in 70's4. They consider the test segment. In this experiment environment, the gasinfluence of gas velocity on the heat convective only, is nearly static and the temperature is relatively lowand often neglect the gas and grain erThe operation time is 9.3 s and the pressure is 5.9In order to study the ablation mechanism and MPamodeling for EPDM insulator, it is necessary to under- 1.1 Charring Ablation Rate and Layer Thickstand its ablation characteristic of the material. ThreeAfter the experiment, the ablator was cut off nor-different test motors and four propellants were used to mally to the surface and two layers could be distin-imitate eight kinds of gas environments in which EPDM guished clearly, namely the charring and matrix lay中国煤化工Received 2009-06-12CNMHGSponsored by the National Nature Science Foundation of Chins(50876091)BiographiesWANGShu-xian(1977-),female,doctoralstudent,wangshuxianl977@sina.com:HEGuo-qiang(1962--),male,ProfessorJOURNAL OF CHINA ORDNANCE, 2010, Vol 6, No. 2time. Modified double-base propellant is used, and thegas temperature is 3 289 K. This experiment enviroment is a gas-phase environment almost without thegrain effect, and the experiment results can reflect theffects of both thermo-chemical ablation and gas ero-sIon.Fig 1 Sketch of test motor SSFlers. A puce pyrolysis layer can be seen after the char-ring layers removal, and it can be simplified as a sur-w siedtest segment test segmentface for it is very thin compared with the charring lay-er. Dismantled the charring layer, the thickness inFig 3 Sketch of test motor SSF2three positions can be measured using a micrometer2. 1 Charring Ablation Rate and Layer ThicknessThe average charring ablation rate is 0. 066 mm/s andIn the ablation experiments, the effects of presthe thickness of charring layer is about 0. 8 mm.sure and gas velocity on EPDM insulator are studiedBoth ablation rate and layer thickness are small in After experiment, each sample is measured at threethe quasi-static low temperature gas environment, for points, and Table 1 shows the average results. It canthe gas and grain erosions can be neglected, and at the be seen that the charring ablation rate in 5.9 MPa isame time , the weaker therno-chemical ablation occurs slightly bigger than that in 3. 2 MPa, but the charringin the relatively low temperaturelayer thickness is obviously thicker; also the charring1.2 Surface Morphology of Charring Layerablation rate under the higher gas velocity is slightlyAfter experiment, the charring layers surface hasbigger, but the charring layer thickness is obviouslysome cracks with different lengths, as shown in Fig. 2thinner(b), whose directions are consistent with the gas flowTable 1 Test conditions and results in gas-phaseand still keep the orthogonal texture of the original abenvironmenlator surface, as shown in Fig. 2 (a). It shows that, inpressure/ gas velocity/the quasi-static low temperature gas environment, theconditionMPa(m*s1)mass loss caused by the chemical reaction between theoxidizing component and charring layer surface is very5.90.116127week and can not decrease the thickness of the charringayer33.353.20.117The results show that the influences of gas velocityand pressure on the charming ablation rate are small infa) Before lesttb) Afler lesconstantthe thickness of charring layer deFig 2 Surface of EPDM insulator in test condition 12 Gas-phase EnvironmentThe influence of the velocity increase on the char中国煤化工e1) the convectiveThe test motor SSF2, as shown in Fig 3, is deCNMHG e radiation heatgned to obtain two kinds of gas velocity by changingSTHStne convective heat fluxthe section area of test segments and throat. Eight sarincrease caused by the gas velocity increase can notples can be tested in each test segment at the same fluence the total heat flux greatly, and 2)the surface110WANG Shu-xian, et al./ Experimental Investigation on Ablation Characteristic of EPDM insulator in Different Gas Environmentslayer peeling off takes a great deal of heat, which re- composition distribution has a consistent trend, that isduces the surface temperature. In addition, the char- the contents of element 0 and Si on the charring layerring layer structure dissimilarity in different environ- surface are the highest, the content of the element C isments will also influence the charring ablation ratedecreased and the element 0. Si are increased from the2.2 Surface Morphology of Charring Layertop to the bottom inside the charring layer. As theThe surface of charring layer accquired in test melting and boiling temperatures are 1 996 K and 2 503ondition 2 and 4 keeps the texture of the ablator origi- K respectively for Si02, when the temperature of charnal surface, but it is shallower. It indicates that, be- ring layer exceeds the phase-changing temperature,cause of the weak gas erosion, the surface of charring SiO, migrating toward the surface in the liquid or gaslayer has no recession, and the mass loss is still small form, which makes the contents of 0 and Si change in-in the thermo-chemical reaction time of about 9. 1 sside the charring layer. After quenching at room tem-9.7 s. Compared with the results in the quasi-static low perature, a part of SiO, depositing on the surface leadsemperature gas environment, the gas temperature in. to the highest contents of 0 and Si.fluences the charring ablation rate greatly in the gasTable 2 Main ingredient molar contents of charringphase environmentlayer in test condition 2 and 3The SEM(Scanning Electron Microscopy) x200element molar content/% ingredientphotographs show that the charring layer surfaces pro-condition testduced in the pressure of 5.9 MPa and 3. 2 MPa havesurface68.9820.709.16similar characteristics, as shown in Fig 4 and 5. Un-188.227.682.76der the higher gas velocity, the charring layer surface85.859.722.72has no crack, the pore size is bigger and distributes epoint368.8121.186.28renly, and a great deal of flocculent matter adheres tothe surface. The composition analysis result in Table 2bck50.0529.689.57shows that the main composition of the charring layerurface52.7129.459.51surface deposition is SiO,, but the existence form ispoint183.2812.403.09Edifferent under different gas velocity conditions3 point289.436.642.48poin378.8114.304.26back34,2340.1413.67The main ingredient distribution of the charringlayer in the pressure 3. 2 MPa experiment has the samecharacteristics. It indicates that, in the gas-phase en-Flg. 4 SEM photographs of charring layer surfacevironment with temperature of 3 289 K, under the dif-in test condition 2 and 3ferent conditions of pressure and gas velocity, theprocess of Sio, phase-changing and migrating occurredin the charring layer is basically consistent, but the de-grees are different.3 Two-phase Environment with AL,O3GrainFig 5 SEM photographs of charring layer surface中国煤化工 mposite propellantin test condition 4 and 5CNMHGused in EPDm in2. 3 Main Ingredient of Charring LayerIment.It nas the gas temperatureThe ingredients of charring layer can be analyzed of 3 281 K, action time of 7. 2 s and pressure of 5. 9by EDS. Table 2 lists their molar contents. The main. MPa. The content of Al20,111JOURNAL OF CHINA ORDNANCE, 2010, Vol 6, No. 216. 8% and the gas flow direction parallel to the mate- different, the processes of SiO2 phase-changing and mirial surfacegrating occur in the charring layer of EPDM ablator are3. 1 Charring ablation Rate and Layer Thickness basically consistentThe experiment results in Table 3 show that theharring ablation rate under the gas velocity 42 m/s is15% higher than that under 2. 4 m/s and the layerthickness is 3. 2 times of that under 2. 4 m/sTable 3 Test condition and results in two-phasegas environmenFig 6 SEM photographs of charring layer surfacetest pressure/ gas velocity/ablation rate/ thiekness/in test conditions 6 and(m1)mm·sTable 4 Main ingredient molar contents of charring2.4layer in test conditions 6 and 775.90.236element molar content/%conditionCompared with the experiment in gas-phase envi-nment, these experiment results have following fea-surface66.2327.812.413.03tures.85.8611.242.530.091) The ablation tendency is consistent with that inint269.ll24.694.030.15gas-phase environment, and the influence of gas veloint357.8031.150.10Ity onharring ablation rate is small, but on theback27.7250.207.580.11harring layer thickness significant3813.552)The charring ablation rate increase almost in 2point188.0010.61.150.17times, because of the existence of the Al, O, grain inint282.7613.512.400.13the gas increases the heat radiation, and in addition, ant342.2734.510.48part of the grain contacts with the charring layer andbackresults in the direct heat transfer and the thermal enernversed from the kinetic energy4 Gas Environment with High Con3)The charring layer thicknesses are almostcentration Al o grainequal. It shows that, under the condition of lower density grain near wall, the charring layer thickness de-EPDM insulator ablation experiment is carried outpressin gas environment with high concentration of Al, 0,3.2 Surface Morphology and Main Ingredient grain, and the test motor SSF3, as shown in Fig. 7Distribution of Charring Layerand the composite propellant containing 17% alumiThe SEM x200 photograph, as shown in Fig. 6, of num are used. The gas temperature is 3 313 K and thethe charring layer surface has similar characteristics pressure is 6. 7 MPa. After the gas flows through thewith that in the gas-phase environment experiment, but convergent segment, the Al,O, grain in the gas concen-has more deposition on the surface. Combined with the trates to form a dense bunch and impact the surface ofcomposition analysis results in Table 4, it can be seen sample. The content of AL, O, grain in the gas is aboutthat a lot of oxide of aluminum adheres to the surface28. 6% and the grain concenntration is 71. 6 kg/m, thecoms H中国煤化工mpact velocityCNMHGaun Nat and Layer ThicknessEach sample is measured at thirty points. In thement results show that, although the gas environment is impact area, the charring ablation rate is 0. 38 mm/112WANG Shu-xian, et aL. /Experimental investigation on Ablation Characteristic of EPDM insulator in Different Gas Entironmentsand under thein the two-phase environments istwo times of that in the gas-phase environment.2)If the gas velocity exceeds 40m/s, the me-hanical erosion for charring layer is obvious. If thegas velocity is below 3. 3 m/s, the mechanical erosionFig 7 Sketch of test motor SSF3and the layer thickness is about 1 mm; in the other can be neglected3)The gas temperature, grain concentration andarea,the charring ablation rate is 0. 223 mm/s. Thegrain impact are the main influence factors for the charcharring ablation rate in the other area is close to that ring ablation rate. The gas velocity and pressure areunder the test condition 7, but the charring ablation the main influence factors for the thickness of the charrate in the impact area increases obviously. It indicatnnthat the grain impact has a prominent thermal effec4 )The distributions of main element C, 0 and Sithe charring layer.have the same features, although there is sligIn the impact area, the thickness of the charring ence of gas environment in SSF2. It indicates that thelayer is close to those under the test condition 3 and 7processes of SiO, phase-changing and migrating in theIt implies that the layer thickness still depends on the charring layer of EPDM ablator occur ubiquitouslygas velocity and pressure5)The heat transmission mechanism of Al, O,4.2 Surface Morphology of Charring Layergrain to the charring layer, the relationship betweenFrom SEM 50 photograph of the charring layer the ablation and the charring layer's thickness andsurface, as shown in Fig 8, porous structure and a lot structure, and the cooling effect of Sio, phase-changeof fibers can be seen, but any fiber structure can not should all be taken into account in ablation modelingbe seen on the charring layer surface in previous exper-iments. This phenomenon implies that the grain impact Referenceshas a very strong peeling off effect on the pyrolysis [1] Gerald Wayne Russell. Analytic modeling and experimen-remnant wrapped on the fiber frameworktal validation of intumescent behavior of charring heatshield materials[ D]. Alabama: The University of Alaama in Huntsville, 2002.[2] Milos F S, Chen Y-K. Comprehensive model for multi-component ablation thermochemistry [R]. AIAA 970141,1997[3 Swann Robert T. Approximate analysis of the performarof char-forming ablators[ R]. NASA TR R-195, 1964Fig 8 SEM photograph of carbon layer surface[4 Jumper Jr George y. Thermo-mechanical ablation[R]in test condition 8AD-A021275,1976[5] HE Hong-qing, YAN Hong. Ablation model of EPDM5 Conclusions[J]. Journal of Propulsion Technology, 1999, 20(4): 3639.(in Chinese)1)The charring ablation rate in the quasi-static [6] SUN Bing, LIU Xiao-yong. Computation of ablation oflow temperatureminimum and maximumthermal-protection layer in solid rocket ramjet combustorin high concentration grain impact environment. The[J]. Journal of Propulsion Technology, 2002, 23(5)experiments carried out in the same test motor SSF2中国煤化工CNMHG

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