1、Designation: E 458 72 (Reapproved 2002)Standard 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 paren
2、theses indicates the year of last reapproval. Asuperscript epsilon (e) 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 mate
3、rials subjected to thermal environments requir-ing the use of ablation as an energy dissipation process. Threeconcepts of the property 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,
4、associated with its 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:E 285 Test Method for Oxyacetylene Ablation Testing of
5、Thermal Insulation Materials2E 341 Practice for Measuring Plasma Arc Gas Enthalpy byEnergy Balance2E 377 Practice for Internal Temperature Measurements inLow Conductivity Materials2E 422 Test Method for Measuring Heat Flux Using aWater-Cooled Calorimeter2E 457 Test Method for Measuring Heat-Transfer
6、 Rate Usinga Thermal Capacitance (Slug) Calorimeter2E 459 Test Method for Measuring Heat-Transfer Rate Usinga Thin-Skin Calorimeter2E 470 Method for Measuring Gas Enthalpy Using Calori-metric Probes3E 471 Test Method for Obtaining Char Density Profile ofAblative Materials by Machining and Weighing2E
7、 511 Test Method for Measuring Heat Flux Using aCopper-Constantan Circular Foil, Heat-Flux Gage23. Terminology3.1 Descriptions of Terms Specific to This Standard:3.1.1 heat of ablationa property that indicates the abilityof a material to provide heat protection when used as asacrificial thermal prot
8、ection device. The property is a functionof 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/kg,q = incident heat transfer rate, k
9、W/m2, andm = total mass transfer rate, kg/m2s.3.2 The heat of ablation may be represented in threedifferent ways depending on the investigators requirements:3.3 cold-wall heat of ablationThe most commonly andeasily determined value is the cold-wall heat of ablation, and isdefined as the incident col
10、d-wall heat dissipated per unit massof material ablated, as follows:Q* 5 qcw/m (2)where:Q*cw= cold-wall heat of ablation, kJ/kg,qhw= heat transfer rate from the test environment to acold wall, kW/m2, andm = total mass transfer rate, kg/m2s.The temperature of the cold-wall reference for the cold-wall
11、heat transfer rate is usually considered to be room temperatureor close enough such that the hot-wall correction given in Eq7 is less than 5 % of the cold-wall heat transfer rate.3.4 effective heat of ablationThe effective heat of ablationis defined as the incident hot-wall dissipated per unit massa
12、blated, 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, kg/m2s.1This test method is under the jurisdiction of
13、 ASTM Committee E21 on SpaceSimulation and Applications of Space Technology and is the direct responsibility ofSubcommittee E21.08 on Thermal Protection.Current edition approved Aug. 29, 1972. Published November 1972.2Annual Book of ASTM Standards, Vol 15.03.3Discontinued, see 1982 Annual Book of AS
14、TM Standards, Part 41.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.3.5 thermochemical heat of ablationThe thermochemicalheat of ablation is defined as the incident hot-wall heatdissipated per unit mass ablated, corrected for rerad
15、iation heatrejection processes and material eroded by mechanical re-moval, as follows:Q*tc5 qhw2 qrr2 qmech!mtc(4)mtc5 m 2 mmech(5)qmech5 mmechhm! (6)where:Q*tc= thermochemical heat of ablation, kJ/kg,qrr= reradiative heat transfer rate, kW/m2,mtc= mass transfer rate due to thermochemical pro-cesses
16、, kg/m2s,mmech= mass-transfer rate due to mechanical processes,kg/m2s,qmech= heat-transfer rate due to energy carried away withmechanically removed material, kW/m2, andhm= enthalpy of mechanically removed material, kJ/kg.Mechanical removal of material takes place in the more severetest environments
17、where relatively high aerodynamic shear orparticle impingement is present. The effects of mechanicalremoval and theories relating to the mechanism of this processare presented in Refs (1-5).4If the effects of mechanicalremoval of material cannot be determined or are deemedunimportant, the term qmech
18、in Eq 4 goes to zero. Theinvestigator should, however, be aware of the existence of thisphenomenon and also note whether this effect was consideredwhen reporting data.3.6 The three heat of ablation values described in 3.2 requiretwo basic determinations: the heat-transfer rate and the mass-transfer
19、rate. These two quantities then assume various formsdepending on the particular heat of ablation value beingdetermined.4. Significance and Use4.1 The heat of ablation provides a measure of the ability ofa material to serve as a heat protection element in a severethermal environment. The property is
20、a function of both thematerial and the environment to which it is subjected. It istherefore required that laboratory measurements of heat ofablation simulate the service environment as closely as pos-sible. Some of the parameters affecting the property arepressure, gas composition, heat transfer rat
21、e, mode of heattransfer, and gas enthalpy. As laboratory duplication of allparameters is usually difficult, the user of the data shouldconsider the differences between the service and the testenvironments. Screening tests of various materials under simu-lated use conditions may be quite valuable eve
22、n if all theservice environmental parameters are not available. These testsare useful in material selection studies, materials developmentwork, and many other areas.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
23、by using a calorimeter. These instruments are availablecommercially in several different types, some of which can bereadily fabricated by the investigator. Selection of a specifictype is based on the test configuration and the methods used,and should take into consideration such parameters as instru
24、-ment response time, test duration, and heat transfer rate (6).5.1.1.1 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-w
25、all heatof ablation.5.1.2 Install the calorimeter in a calorimeter body thatduplicates the test model in size and configuration. This is donein order to eliminate geometric parameters from the heat-transfer rate measurement and to ensure that the quantitymeasured is representative of the heat-transf
26、er rate to the testmodel. If the particular test run 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 do
27、es not affect the flow overthe area under test. In axisymmetric 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 therm
28、ochemicalheats of ablation, correct the cold-wall heat-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:qhw5 qcwhe2 hhw!/he2 hcw!# (
29、7)where:he= gas enthalpy at the boundary layer 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 (7). Additionalcorrections
30、 may be required regarding the effect of temperatureon 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
31、 several ways: energy balance, enthalpy probe,spectroscopy, etc. Details of the methods may be foundelsewhere (8-11). 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 entha
32、lpy at the ablator surface and the calorimetersurface requires pressure and surface temperature measure-ments. The hot-wall temperatures are generally measured by4The boldface numbers in parentheses refer to the references listed at the end ofthe standard.E 458 72 (2002)2optical methods such as pyro
33、meters, radiometers, etc. Othermethods such as infrared spectrometers and monochromatorshave been used (12,13). Effects of the optical properties of theboundary layer of an ablating surface make accurate determi-nations of surface temperature difficult.5.3.2 Determine the wall enthalpy from the assu
34、med state ofthe gas flow (equilibrium, frozen, or nonequilibrium), if thepressure and the wall temperature are known. It is furtherassumed that the wall enthalpy is the enthalpy of the freestreamgas, without ablation products, at the wall temperature. Makethe wall static pressure measurements with a
35、n ordinary pitotarrangement designed for the flow regime of interest and byusing the appropriate transducers.5.4 Reradiation Correction:5.4.1 Calculate the heat-transfer rate due to reradiation fromthe surface of the ablating material from the following equa-tion:qrr5esTs4(8)where:s = Stefan-Boltzma
36、nn constant,Ts= absolute surface temperature of ablating material,K, ande = thermal emittance of the ablating surface.5.4.2 Eq 8 assumes radiation through a transparent mediumto a blackbody at absolute zero. Consider the validity of thisassumption for each case and if the optical properties of thebo
37、undary layer are known and are deemed significant, or theabsolute zero blackbody sink assumption is violated, considerthese effects in the use of Eq 8.5.5 Mechanical Removal Correction:5.5.1 Determine the heat-transfer rate due to the mechanicalremoval of material from the ablating surface from the
38、mass-loss rate due to mechanical processes and the enthalpy of thematerial removed as follows:qmech5 mmechhm(9)5.5.2 Approximate the enthalpy of the material removed bythe product of the specific heat of the mechanically removedmaterial, and the surface temperature (1-5).6. Determination of Mass-Tra
39、nsfer 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 test. The heat of ablation value canbe affected by the choice of method.6.1.1 Ablation
40、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 following relationship:m 5rodL/t! (10)where:ro= virgin material density, kg/m3,dL = chang
41、e in length or ablation depth, m, andt = test time, s.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, to
42、provide 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 length change measure
43、ment of mass-lossrate for non-charring ablators, subliming materials, or withcharring ablators under steady state ablation conditions (seeSection 7) and only with materials that do not swell or grow inlength.6.1.2 Direct Weighing Method:6.1.2.1 A second method of determining mass-transfer rateis by
44、the use of a pretest and post-test mass measurement. Thisprocedure yields the mass transfer rate directly. A disadvantageof this method is that the mass-transfer rate obtained isaveraged over the entire test model heated area. The heat-transfer rate is generally varying over the surface and therefor
45、eleads to errors in heat of ablation. The mass-transfer rate is alsoaveraged over the insertion period which includes the early partof the period when the ablation process is transient and afterthe specimen has been removed where some mass loss occurs.The experimenter should be guided by Section 7 i
46、n determin-ing the magnitude of these effects.6.1.2.2 In cases where the mass loss is low, the errorsincurred in mass loss measurements could become large. It istherefore recommended that a significant mass loss be realizedto reduce measurement errors. The problem is one of a smalldifference of two
47、large numbers.6.1.3 Core Sample Method:6.1.3.1 Accomplish direct measurement of the mass loss bycoring the model after testing by using standard core drills. Thecore size is determined by the individual experiment; however,core diameters of 5.0 to 10.0 mm should be adequate. Coringthe model at the l
48、ocation of the heat-transfer rate measurementmakes the mass-transfer rate representative of the measuredenvironment. Obtain the mass-transfer rate from the coresample as follows:m 5 roVo2 wf!/tAc! (11)where:Vo= original calculated volume of core, m3,wf= final mass of core, kg, andAc= cross-sectional
49、 area of core, m2.6.1.3.2 Calculate the original core volume using the mea-sured diameter of the core after removal from the test model.The core drill dimensions should not be used due to drillinginaccuracies.6.1.4 Shrouded Core MethodA second core samplemethod used in measuring ablation properties of materialsinvolves the use of a model that includes a core and modelshroud of the same material where the core has been preparedprior to testing. This method is described in detail in Ref. (5).This type of test model offers the advantages of ease ofinstallation of thermal
