ASTM E458-2008 516 Standard Test Method for Heat of Ablation《烧蚀的热标准试验方法》.pdf

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1、Designation: E 458 08Standard Test Method forHeat of Ablation1This standard is issued under the fixed designation E 458; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of last revision. A number in parentheses indicates t

2、he year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.This standard has been approved for use by agencies of the Department of Defense.1. Scope1.1 This test method covers determination of the heat ofablation of materials subjected to

3、thermal environments requir-ing the use of ablation as an energy dissipation process. Threeconcepts of the parameter are described and defined: cold wall,effective, and thermochemical heat of ablation.1.2 This standard does not purport to address all of thesafety concerns, if any, associated with it

4、s use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2E 285 Test Method for Oxyacetylene Ablation Testing ofThermal Insulatio

5、n MaterialsE 422 Test Method for Measuring Heat Flux Using aWater-Cooled CalorimeterE 457 Test Method for Measuring Heat-Transfer Rate Usinga Thermal Capacitance (Slug) CalorimeterE 459 Test Method for Measuring Heat Transfer Rate Usinga Thin-Skin CalorimeterE511 Test Method for Measuring Heat Flux

6、Using aCopper-Constantan Circular Foil, Heat-Flux TransducerE 617 Specification for Laboratory Weights and PrecisionMass Standards3. Terminology3.1 Descriptions of Terms Specific to This Standard:3.1.1 heat of ablationa parameter that indicates the abilityof a material to provide heat protection whe

7、n used as asacrificial thermal protection device. The parameter is a func-tion of both the material and the environment to which it issubjected. In general, it is defined as the incident heat dissi-pated by the ablative material per unit of mass removed, orQ* 5 q/m (1)where:Q* = heat of ablation, kJ

8、/kg,q = incident heat transfer rate, kW/m2, andm = total mass transfer rate, kg/m2s.3.1.2 The heat of ablation may be represented in threedifferent ways depending on the investigators requirements:3.1.3 cold-wall heat of ablationThe most commonly andeasily determined value is the cold-wall heat of a

9、blation, and isdefined as the incident cold-wall heat dissipated per unit massof material ablated, as follows:Q*cw5 qcw/m (2)where:Q*cw= cold-wall heat of ablation, kJ/kg,qcw= heat transfer rate from the test environment to acold wall, kW/m2, andm = total mass transfer rate, kg/m2s.The temperature o

10、f the cold-wall reference for the cold-wallheat transfer rate is usually considered to be room temperatureor close enough such that the hot-wall correction given in Eq8 is less than 5 % of the cold-wall heat transfer rate.3.1.4 effective heat of ablationThe effective heat of abla-tion is defined as

11、the incident hot-wall heat dissipated per unitmass ablated, as follows:Q*eff5 qhw/m (3)where:Q*eff= effective heat of ablation, kJ/kg,qhw= heat transfer rate from the test environment to anonablating wall at the surface temperature of thematerial under test, kW/m2, andm = total mass transfer rate, k

12、g/m2s.3.1.5 thermochemical heat of ablationThe derivation ofthe thermochemical heat of ablation originated with thesimplistic surface energy equation employed in the early 60s todescribe the effects of surface ablation, that is:qhw2 qrr5 qcond1 qabl1 qblock(4)1This test method is under the jurisdict

13、ion of ASTM Committee E21 on SpaceSimulation and Applications of Space Technology and is the direct responsibility ofSubcommittee E21.08 on Thermal Protection.Current edition approved May 1, 2008. Published July 2008. Originally approvedin 1972. Last previous edition approved in 2002 as E 45872(2002

14、)2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box

15、 C700, West Conshohocken, PA 19428-2959, United States.where:qrr= energy re-radiated from the heated surface, kW/m2,qcond= net energy conducted into the solid during steady-state ablation = mcp(TwTo), kW/m2,qabl= energy absorbed by surface ablation which, insimple terms, can be represented by mDHv,

16、kW/m2,qblock= energy dissipated (blockage) by transpiration ofablation products into the boundary layer, which,in simple terms, can be represented bymh(hr hw), kW/m2,Tw= absolute surface temperature of ablating material,K,cp= specific heat at constant pressure of ablatingmaterial, kJ/kgK,To= initial

17、 surface temperature of ablating material, K,DHv= an effective heat of vaporization, kJ/kg,h = a transpiration coefficient,hr= gas recovery enthalpy, kJ/kg, andhw= the wall enthalpy, kJ/kg.Using the definitions above, Eq 4 can be rewritten as:qhw2 qrr5 mcpTw2 To! 1 mDHv1 mhhr2 hw! (5)where it should

18、 be apparent that the definition of the thermo-chemical heat of ablation is obtained by dividing Eq 4 by m,where it is understood that m is a steady-state ablation rate. Theresult is:Q*tc5 qhw2 qrr!/m 5 cpTw2 To! 1DHv1hhr2 hw! (6)As seen from Eq 6, definition of the thermochemical heat ofablation re

19、quires an ability to measure the cold-wall heat flux,an ability to define the recovery enthalpy, an ability to measurethe surface temperature, knowledge of the total hemisphericalemittance (at the temperature and state of the ablating surface),and the ability to determine the steady-state mass loss

20、rate.Assuming these parameters can be measured (or estimated),the right hand side of Eq 6 implies that the thermochemicalheat of ablation is a linear function of the enthalpy differenceacross the boundary layer, that is, (hr hw). Consequently, aplot of Q*tc(determined from several tests at different

21、 condi-tions) versus (hr hw) should allow a linear fit of the datawhere the slope of the fit is interpreted as h, the transpirationcoefficient, and the y-intercept is interpreted as cpDT + DHv.Ifthe specific heat of the material is known, the curve fit allowsthe effective heat of vaporization to be

22、empirically derived.3.2 The three heat of ablation values described in 3.1.2require two basic determinations: the heat transfer rate and themass transfer rate. These two quantities then assume variousforms depending on the particular heat of ablation value beingdetermined.4. Significance and Use4.1

23、GeneralThe heat of ablation provides a measure ofthe ability of a material to serve as a heat protection element ina severe thermal environment. The parameter is a function ofboth the material and the environment to which it is subjected.It is therefore required that laboratory measurements of heat

24、ofablation simulate the service environment as closely as pos-sible. Some of the parameters affecting the heat of ablation arepressure, gas composition, heat transfer rate, mode of heattransfer, and gas enthalpy. As laboratory duplication of allparameters is usually difficult, the user of the data s

25、houldconsider the differences between the service and the testenvironments. Screening tests of various materials under simu-lated use conditions may be quite valuable even if all theservice environmental parameters are not available. These testsare useful in material selection studies, materials dev

26、elopmentwork, and many other areas.4.2 Steady-State ConditionsThe nature of the definition ofheat of ablation requires steady-state conditions. Variancesfrom steady-state may be required in certain circumstances;however, it must be realized that transient phenomena make thevalues obtained functions

27、of the test duration and thereforemake material comparisons difficult.4.2.1 Temperature RequirementsIn a steady-state condi-tion, the temperature propagation into the material will move atthe same velocity as the gas-ablation surface interface. Aconstant distance is maintained between the ablation s

28、urfaceand the isotherm representing the temperature front. Understeady-state ablation the mass loss and length change arelinearly related.mt5rodL1 ro2rc!dc(7)where:t = test time, s,ro= virgin material density, kg/m3,dL= change in length or ablation depth, m,rc= char density, kg/m3, anddc= char depth

29、, m.This relationship may be used to verify the existence ofsteady-state ablation in the tests of charring ablators.4.2.2 Exposure Time RequirementsThe exposure timerequired to achieve steady-state may be determined experimen-tally by the use of multiple models by plotting the total massloss as a fu

30、nction of the exposure time. The point at which thecurve departs significantly from linearity is the minimumexposure time required for steady-state ablation to be estab-lished. Cases exist, however, in the area of very high heatingrates and high shear where this type of test for steady-state maynot

31、be possible.5. Determination of Heat Transfer Rate5.1 Cold-Wall Heat Transfer Rate:5.1.1 Determine the cold-wall heat transfer rate to a speci-men by using a calorimeter. These instruments are availablecommercially in several different types, some of which can bereadily fabricated by the investigato

32、r. Selection of a specifictype is based on the test configuration and the methods used,and should take into consideration such parameters as instru-ment response time, test duration, and heat transfer rate (13).3The boldface numbers in parentheses refer to the references listed at the end ofthe stan

33、dard.E4580825.1.1.1 The calorimeters discussed in 5.1.1 measure a “cold-wall” heat transfer rate because the calorimeter surface tem-perature is much less than the ablation temperature. The valuethus obtained is used directly in computing the cold-wall heatof ablation.5.1.2 Install the calorimeter i

34、n a calorimeter body thatduplicates the test model in size and configuration. This is donein order to eliminate geometric parameters from the heattransfer rate measurement and to ensure that the quantitymeasured is representative of the heat transfer rate to the testmodel. If the particular test run

35、 does not allow an independentheat transfer rate measurement, as in some nozzle liner andpipe flow tests, mount the calorimeter as near as possible to thelocation of the mass-loss measurements. Take care to ensurethat the nonablating calorimeter does not affect the flow overthe area under test. In a

36、xisymmetric flow fields, measurementsof mass loss and heat transfer rate in the same plane, yetdiametrically opposed, should be valid.5.2 Computation of Effective and Thermochemical Heats ofAblation:5.2.1 In order to compute the effective and thermochemicalheats of ablation, correct the cold-wall he

37、at transfer rate for theeffect of the temperature difference on the heat transfer. Thiscorrection factor is a function of the ratio of the enthalpypotentials across the boundary layer for the hot and cold wallas follows:qhw/qcw5 he2 hhw!/he2 hcw!# (8)where:he= gas recovery enthalpy at the boundary l

38、ayer edge,kJ/kg,hhw= gas enthalpy at the surface temperature of the testmodel, kJ/kg, andhcw= gas enthalpy at a cold wall, kJ/kg.5.2.2 This correction is based upon laminar flow in air andsubject to the restrictions imposed in Ref (2). Additionalcorrections may be required regarding the effect of te

39、mperatureon the transport properties of the test gas. The form and use ofthese corrections should be determined by the investigator foreach individual situation.5.3 Gas Enthalpy Determination:5.3.1 The enthalpy at the boundary layer edge may bedetermined in several ways: energy balance, enthalpy pro

40、be,spectroscopy, etc. Details of the methods may be foundelsewhere (3-6). Take care to evaluate the radial variation ofenthalpy in the nozzle.Also, in low-density flows, consider theeffect of nonequilibrium on the evaluation. Determination ofthe gas enthalpy at the ablator surface and the calorimete

41、rsurface requires pressure and surface temperature measure-ments. The hot-wall temperatures are generally measured byoptical methods such as pyrometers, radiometers, etc. Othermethods such as infrared spectrometers and monochromatorshave been used (7,8). Effects of the optical properties of thebound

42、ary layer of an ablating surface make accurate determi-nations of surface temperature difficult.5.3.2 Determine the wall enthalpy from the assumed state ofthe gas flow (equilibrium, frozen, or nonequilibrium), if thepressure and the wall temperature are known. It is furtherassumed that the wall enth

43、alpy is the enthalpy of the freestreamgas, without ablation products, at the wall temperature. Makethe wall static pressure measurements with an ordinary pitotarrangement designed for the flow regime of interest and byusing the appropriate transducers.5.4 Reradiation Correction:5.4.1 Calculate the h

44、eat transfer rate due to reradiation fromthe surface of the ablating material from the following equa-tion:qrr5sTw4(9)where:s = Stefan-Boltzmann constant, and, = thermal emittance of the ablating surface.5.4.2 Eq 9 assumes radiation through a transparent mediumto a blackbody at absolute zero. Consid

45、er the validity of thisassumption for each case and if the optical properties of theboundary layer are known and are deemed significant, or theabsolute zero blackbody sink assumption is violated, considerthese effects in the use of Eq 9.5.5 Mechanical Removal Correction:5.5.1 Determine the heat tran

46、sfer rate due to the mechanicalremoval of material from the ablating surface from the mass-loss rate due to mechanical processes and the enthalpy of thematerial removed as follows:qmech5 mmechhm(10)5.5.2 Approximate the enthalpy of the material removed bythe product of the specific heat of the mecha

47、nically removedmaterial, and the surface temperature (9-13).6. Determination of Mass Transfer Rate6.1 The determination of the heat of ablation requires themeasurement of the mass transfer rate of the material undertest. This may be accomplished in several ways depending onthe type of material under

48、 test. The heat of ablation value canbe affected by the choice of method.6.1.1 Ablation Depth Method:6.1.1.1 The simplest method of measurement of mass-lossrate is the change in length or ablation depth. Make a pretestand post-test measurement of the length and calculate themass-loss rate from the f

49、ollowing relationship:m 5rodL/t! (11)6.1.1.2 Determine the change in length with the time of amodel under test, by using motion picture techniques. Note thatobservation of the front surface alone does not, however, verifythe existence of steady state ablation. Take care, however, toprovide appropriate reference marks for measuring the lengthchange from the film. Timing marks on the film are alsorequired to accurately determine the time parameter. Avoidusing framing speed as a reference, as it generally does notprovide the required accuracy.6.1.1.3 Use the