ASTM E458-72(2002)
(Test Method)Standard Test Method for Heat of Ablation
Standard Test Method for Heat of Ablation
SIGNIFICANCE AND USE
The heat of ablation provides a measure of the ability of a material to serve as a heat protection element in a severe thermal environment. The property is a function of both the material and the environment to which it is subjected. It is therefore required that laboratory measurements of heat of ablation simulate the service environment as closely as possible. Some of the parameters affecting the property are pressure, gas composition, heat transfer rate, mode of heat transfer, and gas enthalpy. As laboratory duplication of all parameters is usually difficult, the user of the data should consider the differences between the service and the test environments. Screening tests of various materials under simulated use conditions may be quite valuable even if all the service environmental parameters are not available. These tests are useful in material selection studies, materials development work, and many other areas.
SCOPE
1.1 This test method covers determination of the heat of ablation of materials subjected to thermal environments requiring the use of ablation as an energy dissipation process. Three concepts 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 the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
General Information
Relations
Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation:E458–72(Reapproved2002)
Standard Test Method for
Heat of Ablation
This standard is issued under the fixed designation E 458; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript 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. Scope 3.1.1 heat of ablation—a property that indicates the ability
of a material to provide heat protection when used as a
1.1 This test method covers determination of the heat of
sacrificial thermal protection device.The property is a function
ablation of materials subjected to thermal environments requir-
of both the material and the environment to which it is
ing the use of ablation as an energy dissipation process. Three
subjected. In general, it is defined as the incident heat dissi-
concepts of the property are described and defined: cold wall,
pated by the ablative material per unit of mass removed, or
effective, and thermochemical heat of ablation.
1.2 This standard does not purport to address all of the Q* 5 q/m (1)
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
where:
priate safety and health practices and determine the applica-
Q* = heat of ablation, kJ/kg,
bility of regulatory limitations prior to use. 2
q = incident heat transfer rate, kW/m , and
m = total mass transfer rate, kg/m ·s.
2. Referenced Documents
3.2 The heat of ablation may be represented in three
2.1 ASTM Standards:
different ways depending on the investigator’s requirements:
E 285 Test Method for Oxyacetylene Ablation Testing of
3.3 cold-wall heat of ablation—The most commonly and
Thermal Insulation Materials
easily determined value is the cold-wall heat of ablation, and is
E 341 Practice for Measuring Plasma Arc Gas Enthalpy by
defined as the incident cold-wall heat dissipated per unit mass
Energy Balance
of material ablated, as follows:
E 377 Practice for Internal Temperature Measurements in
Q* 5 q /m (2)
cw
Low Conductivity Materials
E 422 Test Method for Measuring Heat Flux Using a
where:
Water-Cooled Calorimeter
Q* = cold-wall heat of ablation, kJ/kg,
cw
E 457 TestMethodforMeasuringHeat-TransferRateUsing
q = heat transfer rate from the test environment to a
hw
a Thermal Capacitance (Slug) Calorimeter
cold wall, kW/m , and
E 459 TestMethodforMeasuringHeat-TransferRateUsing
m = total mass transfer rate, kg/m ·s.
a Thin-Skin Calorimeter
E 470 Method for Measuring Gas Enthalpy Using Calori-
The temperature of the cold-wall reference for the cold-wall
metric Probes
heat transfer rate is usually considered to be room temperature
E 471 Test Method for Obtaining Char Density Profile of
or close enough such that the hot-wall correction given in Eq
Ablative Materials by Machining and Weighing
7 is less than 5 % of the cold-wall heat transfer rate.
E 511 Test Method for Measuring Heat Flux Using a
3.4 effective heat of ablation—The effective heat of ablation
Copper-Constantan Circular Foil, Heat-Flux Gage
is defined as the incident hot-wall dissipated per unit mass
ablated, as follows:
3. Terminology
Q* 5 q /m (3)
eff hw
3.1 Descriptions of Terms Specific to This Standard:
where:
Q* = effective heat of ablation, kJ/kg,
eff
This test method is under the jurisdiction of ASTM Committee E21 on Space
q = heat transfer rate from the test environment to a
hw
Simulation andApplications of Space Technology and is the direct responsibility of
Subcommittee E21.08 on Thermal Protection. nonablating wall at the surface temperature of the
Current edition approved Aug. 29, 1972. Published November 1972.
material under test, kW/m , and
Annual Book of ASTM Standards, Vol 15.03. 2
m = total mass transfer rate, kg/m ·s.
Discontinued, see 1982 Annual Book of ASTM Standards, Part 41.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E458–72 (2002)
3.5 thermochemical heat of ablation—The thermochemical 5. Determination of Heat Transfer Rate
heat of ablation is defined as the incident hot-wall heat
5.1 Cold-Wall Heat Transfer Rate:
dissipated per unit mass ablated, corrected for reradiation heat
5.1.1 Determine the cold-wall heat-transfer rate to a speci-
rejection processes and material eroded by mechanical re-
men by using a calorimeter. These instruments are available
moval, as follows:
commercially in several different types, some of which can be
Q* 5 ~q 2 q 2 q !m (4)
tc hw rr mech tc readily fabricated by the investigator. Selection of a specific
type is based on the test configuration and the methods used,
m 5 m 2 m (5)
tc mech
and should take into consideration such parameters as instru-
q 5 m ~h ! (6)
mech mech m
ment response time, test duration, and heat transfer rate (6).
where:
5.1.1.1 Thecalorimetersdiscussedin5.1.1measurea“cold-
Q* = thermochemical heat of ablation, kJ/kg,
tc wall” heat-transfer rate because the calorimeter surface tem-
q = reradiative heat transfer rate, kW/m ,
rr
perature is much less than the ablation temperature. The value
m = mass transfer rate due to thermochemical pro-
tc
thus obtained is used directly in computing the cold-wall heat
cesses, kg/m ·s,
of ablation.
m = mass-transfer rate due to mechanical processes,
mech
5.1.2 Install the calorimeter in a calorimeter body that
kg/m ·s,
duplicatesthetestmodelinsizeandconfiguration.Thisisdone
q = heat-transferrateduetoenergycarriedawaywith
mech
in order to eliminate geometric parameters from the heat-
mechanically removed material, kW/m , and
transfer rate measurement and to ensure that the quantity
h = enthalpy of mechanically removed material, kJ/
m
measured is representative of the heat-transfer rate to the test
kg.
model. If the particular test run does not allow an independent
heat-transfer rate measurement, as in some nozzle liner and
Mechanical removal of material takes place in the more severe
pipe flow tests, mount the calorimeter as near as possible to the
test environments where relatively high aerodynamic shear or
location of the mass-loss measurements. Take care to ensure
particle impingement is present. The effects of mechanical
that the nonablating calorimeter does not affect the flow over
removal and theories relating to the mechanism of this process
the area under test. In axisymmetric flow fields, measurements
are presented in Refs (1-5). If the effects of mechanical
of mass loss and heat-transfer rate in the same plane, yet
removal of material cannot be determined or are deemed
diametrically opposed, should be valid.
unimportant, the term q in Eq 4 goes to zero. The
mech
5.2 Computation of Effective and Thermochemical Heats of
investigator should, however, be aware of the existence of this
Ablation:
phenomenon and also note whether this effect was considered
5.2.1 In order to compute the effective and thermochemical
when reporting data.
heatsofablation,correctthecold-wallheat-transferrateforthe
3.6 Thethreeheatofablationvaluesdescribedin3.2require
effect of the temperature difference on the heat transfer. This
two basic determinations: the heat-transfer rate and the mass-
correction factor is a function of the ratio of the enthalpy
transfer rate. These two quantities then assume various forms
potentials across the boundary layer for the hot and cold wall
depending on the particular heat of ablation value being
as follows:
determined.
q 5 q [~h 2 h !/~h 2 h !# (7)
hw cw e hw e cw
4. Significance and Use
where:
4.1 The heat of ablation provides a measure of the ability of
h = gas enthalpy at the boundary layer edge, kJ/kg,
e
a material to serve as a heat protection element in a severe
h = gas enthalpy at the surface temperature of the test
hw
thermal environment. The property is a function of both the
model, kJ/kg, and
material and the environment to which it is subjected. It is
h = gas enthalpy at a cold wall, kJ/kg.
cw
therefore required that laboratory measurements of heat of
5.2.2 This correction is based upon laminar flow in air and
ablation simulate the service environment as closely as pos-
subject to the restrictions imposed in Ref (7). Additional
sible. Some of the parameters affecting the property are
correctionsmayberequiredregardingtheeffectoftemperature
pressure, gas composition, heat transfer rate, mode of heat
on the transport properties of the test gas. The form and use of
transfer, and gas enthalpy. As laboratory duplication of all
these corrections should be determined by the investigator for
parameters is usually difficult, the user of the data should
each individual situation.
consider the differences between the service and the test
5.3 Gas Enthalpy Determination:
environments. Screening tests of various materials under simu-
5.3.1 The enthalpy at the boundary layer edge may be
lated use conditions may be quite valuable even if all the
determined in several ways: energy balance, enthalpy probe,
service environmental parameters are not available.These tests
spectroscopy, etc. Details of the methods may be found
are useful in material selection studies, materials development
elsewhere (8-11). Take care to evaluate the radial variation of
work, and many other areas.
enthalpy in the nozzle.Also, in low-density flows, consider the
effect of nonequilibrium on the evaluation. Determination of
the gas enthalpy at the ablator surface and the calorimeter
surface requires pressure and surface temperature measure-
The boldface numbers in parentheses refer to the references listed at the end of
the standard. ments. The hot-wall temperatures are generally measured by
E458–72 (2002)
optical methods such as pyrometers, radiometers, etc. Other 6.1.1.2 Determine the change in length with the time of a
methods such as infrared spectrometers and monochromators modelundertest,byusingmotionpicturetechniques.Notethat
have been used (12,13). Effects of the optical properties of the observationofthefrontsurfacealonedoesnot,however,verify
boundary layer of an ablating surface make accurate determi- the existence of steady state ablation. Take care, however, to
nations of surface temperature difficult. provide appropriate reference marks for measuring the length
change from the film. Timing marks on the film are also
5.3.2 Determinethewallenthalpyfromtheassumedstateof
required to accurately determine the time parameter. Avoid
the gas flow (equilibrium, frozen, or nonequilibrium), if the
using framing speed as a reference, as it generally does not
pressure and the wall temperature are known. It is further
provide the required accuracy.
assumedthatthewallenthalpyistheenthalpyofthefreestream
gas, without ablation products, at the wall temperature. Make 6.1.1.3 Use the length change measurement of mass-loss
the wall static pressure measurements with an ordinary pitot
rate for non-charring ablators, subliming materials, or with
arrangement designed for the flow regime of interest and by charring ablators under steady state ablation conditions (see
using the appropriate transducers.
Section 7) and only with materials that do not swell or grow in
5.4 Reradiation Correction: length.
5.4.1 Calculate the heat-transfer rate due to reradiation from 6.1.2 Direct Weighing Method:
the surface of the ablating material from the following equa-
6.1.2.1 A second method of determining mass-transfer rate
tion:
is by the use of a pretest and post-test mass measurement. This
procedure yields the mass transfer rate directly.Adisadvantage
q 5esT (8)
rr s
of this method is that the mass-transfer rate obtained is
where:
averaged over the entire test model heated area. The heat-
s = Stefan-Boltzmann constant,
transfer rate is generally varying over the surface and therefore
T = absolute surface temperature of ablating material,
s leadstoerrorsinheatofablation.Themass-transferrateisalso
K, and
averagedovertheinsertionperiodwhichincludestheearlypart
e = thermal emittance of the ablating surface.
of the period when the ablation process is transient and after
5.4.2 Eq 8 assumes radiation through a transparent medium
the specimen has been removed where some mass loss occurs.
to a blackbody at absolute zero. Consider the validity of this
The experimenter should be guided by Section 7 in determin-
assumption for each case and if the optical properties of the
ing the magnitude of these effects.
boundary layer are known and are deemed significant, or the
6.1.2.2 In cases where the mass loss is low, the errors
absolute zero blackbody sink assumption is violated, consider
incurred in mass loss measurements could become large. It is
these effects in the use of Eq 8.
therefore recommended that a significant mass loss be realized
5.5 Mechanical Removal Correction:
to reduce measurement errors. The problem is one of a small
5.5.1 Determine the heat-transfer rate due to the mechanical
difference of two large numbers.
removal of material from the ablating surface from the mass-
6.1.3 Core Sample Method:
loss rate due to mechanical processes and the enthalpy of the
6.1.3.1 Accomplish direct measurement of the mass loss by
material removed as follows:
coringthemodelaftertestingbyusingstandardcoredrills.The
q 5 m h (9)
core size is determined by the individual experiment; however,
mech mech m
core diameters of 5.0 to 10.0 mm should be adequate. Coring
5.5.2 Approximate the enthalpy of the material removed by
the model at the location of the heat-transfer rate measurement
the product of the specific heat of the mechanically removed
makes the mass-transfer rate representative of the measured
material, and the surface temperature (1-5).
environment. Obtain the mass-transfer rate from the core
sample as follows:
6. Determination of Mass-Transfer Rate
m 5 ~r V 2 w !/~tA ! (11)
o o f c
6.1 The determination of the heat of ablation requires the
measurement of the mass-transfer rate of the material under
where:
test. This may be accomplished in several ways depending on
V = original calculated volume of core, m ,
o
the type of material under test. The heat of ablation value can
w = final mass of core, kg, and
f
be affected by the choice of method. A = cross-sectional area of core, m .
c
6.1.1 Ablation Depth Method:
6.1.3.2 Calculate the original core volume using the mea-
6.1.1.1 The simplest method of measurement of mass-loss sured diameter of the core after removal from the test model.
rate is the change in length or ablation depth. Make a pretest
The core drill dimensions should not be used due to drilling
and post-test measurement of the length and calculate t
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.