ASTM E458-08(2020)
(Test Method)Standard Test Method for Heat of Ablation
Standard Test Method for Heat of Ablation
SIGNIFICANCE AND USE
4.1 General—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 parameter 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 heat of ablation 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.
4.2 Steady-State Conditions—The nature of the definition of heat of ablation requires steady-state conditions. Variances from steady-state may be required in certain circumstances; however, it must be realized that transient phenomena make the values obtained functions of the test duration and therefore make material comparisons difficult.
4.2.1 Temperature Requirements—In a steady-state condition, the temperature propagation into the material will move at the same velocity as the gas-ablation surface interface. A constant distance is maintained between the ablation surface and the isotherm representing the temperature front. Under steady-state ablation the mass loss and length change are linearly related.
where:
t = test time, s, ρo = virgin material density, kg/m3, δL = change in length or ablation depth, m, ρc = char density, kg/m3, and δc = char depth, m. This relationship may be used to verify the existence of steady-state ablation in the tests of charring ablators.
4.2.2 Exposure T...
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 parameter 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.3 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
General Information
Relations
Standards Content (Sample)
This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: E458 − 08 (Reapproved 2020)
Standard Test Method for
Heat of Ablation
This standard is issued under the fixed designation E458; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
This standard has been approved for use by agencies of the U.S. Department of Defense.
1. Scope 3. Terminology
1.1 This test method covers determination of the heat of
3.1 Descriptions of Terms Specific to This Standard:
ablationofmaterialssubjectedtothermalenvironmentsrequir-
3.1.1 heatofablation—aparameterthatindicatestheability
ing the use of ablation as an energy dissipation process. Three
of a material to provide heat protection when used as a
conceptsoftheparameteraredescribedanddefined:coldwall,
sacrificial thermal protection device. The parameter is a func-
effective, and thermochemical heat of ablation.
tion of both the material and the environment to which it is
subjected. In general, it is defined as the incident heat dissi-
1.2 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the pated by the ablative material per unit of mass removed, or
responsibility of the user of this standard to establish appro-
Q* 5 q/m (1)
priate safety, health, and environmental practices and deter-
where:
mine the applicability of regulatory limitations prior to use.
Q* = heat of ablation, kJ/kg,
1.3 This international standard was developed in accor-
q = incident heat transfer rate, kW/m , and
dance with internationally recognized principles on standard-
m = total mass transfer rate, kg/m ·s.
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
3.1.2 The heat of ablation may be represented in three
mendations issued by the World Trade Organization Technical
different ways depending on the investigator’s requirements:
Barriers to Trade (TBT) Committee.
3.1.3 cold-wall heat of ablation—The most commonly and
easilydeterminedvalueisthecold-wallheatofablation,andis
2. Referenced Documents
defined as the incident cold-wall heat dissipated per unit mass
2.1 ASTM Standards:
of material ablated, as follows:
E285Test Method for Oxyacetylene Ablation Testing of
Q* 5 q /m (2)
cw cw
Thermal Insulation Materials
E422Test Method for Measuring Heat Flux Using a Water- where:
Cooled Calorimeter
Q* = cold-wall heat of ablation, kJ/kg,
cw
E457Test Method for Measuring Heat-Transfer Rate Using
q = heattransferratefromthetestenvironmenttoacold
cw
a Thermal Capacitance (Slug) Calorimeter
wall, kW/m , and
E459Test Method for Measuring Heat Transfer Rate Using m = total mass transfer rate, kg/m ·s.
a Thin-Skin Calorimeter
The temperature of the cold-wall reference for the cold-wall
E511TestMethodforMeasuringHeatFluxUsingaCopper-
heat transfer rate is usually considered to be room temperature
Constantan Circular Foil, Heat-Flux Transducer
or close enough such that the hot-wall correction given in Eq
E617Specification for Laboratory Weights and Precision
8 is less than 5% of the cold-wall heat transfer rate.
Mass Standards
3.1.4 effective heat of ablation—The effective heat of abla-
tion is defined as the incident hot-wall heat dissipated per unit
mass ablated, as follows:
This test method is under the jurisdiction of ASTM Committee E21 on Space
Simulation andApplications of SpaceTechnology and is the direct responsibility of
Q* 5 q /m (3)
eff hw
Subcommittee E21.08 on Thermal Protection.
Current edition approved Nov. 1, 2020. Published December 2020. Originally
where:
approvedin1972.Lastpreviouseditionapprovedin2015asE458–08(2015).DOI:
Q* = effective heat of ablation, kJ/kg,
10.1520/E0458-08R20. eff
q = heat transfer rate from the test environment to a
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
hw
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
nonablating wall at the surface temperature of the
Standards volume information, refer to the standard’s Document Summary page on 2
material under test, kW/m , and
the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E458 − 08 (2020)
4. Significance and Use
m = total mass transfer rate, kg/m ·s.
4.1 General—Theheatofablationprovidesameasureofthe
3.1.5 thermochemical heat of ablation—The derivation of
ability of a material to serve as a heat protection element in a
the thermochemical heat of ablation originated with the
severe thermal environment. The parameter is a function of
simplisticsurfaceenergyequationemployedintheearly60sto
both the material and the environment to which it is subjected.
describe the effects of surface ablation, that is:
Itisthereforerequiredthatlaboratorymeasurementsofheatof
q 2 q 5 q 1q 1q (4)
hw rr cond abl block ablation simulate the service environment as closely as pos-
sible. Some of the parameters affecting the heat of ablation are
where:
pressure, gas composition, heat transfer rate, mode of heat
q = energy re-radiated from the heated surface, kW/m ,
rr
transfer, and gas enthalpy. As laboratory duplication of all
q = net energy conducted into the solid during steady-
cond
parameters is usually difficult, the user of the data should
state ablation = mc (T −T ), kW/m ,
p w o
consider the differences between the service and the test
q = energy absorbed by surface ablation which, in
abl
2 environments.Screeningtestsofvariousmaterialsundersimu-
simple terms, can be represented by m∆ H,kW/m ,
v
lated use conditions may be quite valuable even if all the
q = energy dissipated (blockage) by transpiration of
block
serviceenvironmentalparametersarenotavailable.Thesetests
ablation products into the boundary layer, which, in
are useful in material selection studies, materials development
simple terms, can be represented by mη(h −h ),
r w
work, and many other areas.
kW/m ,
T = absolutesurfacetemperatureofablatingmaterial,K,
w
4.2 Steady-StateConditions—Thenatureofthedefinitionof
c = specific heat at constant pressure of ablating
p
heat of ablation requires steady-state conditions. Variances
material, kJ/kg·K,
from steady-state may be required in certain circumstances;
T = initial surface temperature of ablating material, K,
o
however,itmustberealizedthattransientphenomenamakethe
∆H = an effective heat of vaporization, kJ/kg,
v
values obtained functions of the test duration and therefore
η = a transpiration coefficient,
make material comparisons difficult.
h = gas recovery enthalpy, kJ/kg, and
r
4.2.1 Temperature Requirements—In a steady-state
h = the wall enthalpy, kJ/kg.
w
condition, the temperature propagation into the material will
moveatthesamevelocityasthegas-ablationsurfaceinterface.
Using the definitions above, Eq 4 can be rewritten as:
Aconstant distance is maintained between the ablation surface
q 2 q 5 mc ~T 2 T !1m∆H 1mη~h 2 h ! (5) and the isotherm representing the temperature front. Under
hw rr p w o v r w
steady-state ablation the mass loss and length change are
where it should be apparent that the definition of the ther-
linearly related.
mochemical heat of ablation is obtained by dividing Eq 4 by
mt 5 ρ δ 1~ρ 2 ρ !δ (7)
o L o c c
m, where it is understood that m is a steady-state ablation
rate. The result is:
where:
Q* 5 q 2 q /m 5 c T 2 T 1∆H 1η h 2 h (6)
~ ! ~ ! ~ ! t = test time, s,
tc hw rr p w o v r w
ρ = virgin material density, kg/m ,
o
As seen from Eq 6, definition of the thermochemical heat
δ = change in length or ablation depth, m,
L
of ablation requires an ability to measure the cold-wall heat
ρ = char density, kg/m , and
c
flux, an ability to define the recovery enthalpy, an ability to
δ = char depth, m.
c
measure the surface temperature, knowledge of the total
This relationship may be used to verify the existence of
hemispherical emittance (at the temperature and state of the
steady-state ablation in the tests of charring ablators.
ablating surface), and the ability to determine the steady-
4.2.2 Exposure Time Requirements—The exposure time re-
state mass loss rate. Assuming these parameters can be mea-
quired to achieve steady-state may be determined experimen-
sured (or estimated), the right hand side of Eq 6 implies that
tally by the use of multiple models by plotting the total mass
the thermochemical heat of ablation is a linear function of
loss as a function of the exposure time. The point at which the
the enthalpy difference across the boundary layer, that is,
curve departs significantly from linearity is the minimum
(h −h ). Consequently, a plot of Q* (determined from sev-
r w tc
exposure time required for steady-state ablation to be estab-
eral tests at different conditions) versus (h − h ) should
r w
lished. Cases exist, however, in the area of very high heating
allow a linear fit of the data where the slope of the fit is in-
ratesandhighshearwherethistypeoftestforsteady-statemay
terpreted as η, the transpiration coefficient, and the
not be possible.
y-intercept is interpreted as c ∆ T + ∆H . If the specific heat
p v
of the material is known, the curve fit allows the effective
5. Determination of Heat Transfer Rate
heat of vaporization to be empirically derived.
5.1 Cold-Wall Heat Transfer Rate:
3.2 The three heat of ablation values described in 3.1.2
5.1.1 Determine the cold-wall heat transfer rate to a speci-
require two basic determinations: the heat transfer rate and the
men by using a calorimeter. These instruments are available
mass transfer rate. These two quantities then assume various
commercially in several different types, some of which can be
forms depending on the particular heat of ablation value being
readily fabricated by the investigator. Selection of a specific
determined. type is based on the test configuration and the methods used,
E458 − 08 (2020)
and should take into consideration such parameters as instru- have been used (7,8). Effects of the optical properties of the
ment response time, test duration, and heat transfer rate (1 ). boundary layer of an ablating surface make accurate determi-
5.1.1.1 Thecalorimetersdiscussedin5.1.1measurea“cold- nations of surface temperature difficult.
wall” heat transfer rate because the calorimeter surface tem-
5.3.2 Determinethewallenthalpyfromtheassumedstateof
perature is much less than the ablation temperature. The value
the gas flow (equilibrium, frozen, or nonequilibrium), if the
thus obtained is used directly in computing the cold-wall heat
pressure and the wall temperature are known. It is further
of ablation.
assumedthatthewallenthalpyistheenthalpyofthefreestream
5.1.2 Install the calorimeter in a calorimeter body that
gas, without ablation products, at the wall temperature. Make
duplicatesthetestmodelinsizeandconfiguration.Thisisdone
the wall static pressure measurements with an ordinary pitot
in order to eliminate geometric parameters from the heat
arrangement designed for the flow regime of interest and by
transfer rate measurement and to ensure that the quantity
using the appropriate transducers.
measured is representative of the heat transfer rate to the test
5.4 Reradiation Correction:
model. If the particular test run does not allow an independent
5.4.1 Calculatetheheattransferrateduetoreradiationfrom
heat transfer rate measurement, as in some nozzle liner and
the surface of the ablating material from the following equa-
pipeflowtests,mountthecalorimeterasnearaspossibletothe
tion:
location of the mass-loss measurements. Take care to ensure
q 5σεT (9)
that the nonablating calorimeter does not affect the flow over
rr w
the area under test. In axisymmetric flow fields, measurements
where:
of mass loss and heat transfer rate in the same plane, yet
σ = Stefan-Boltzmann constant, and,
diametrically opposed, should be valid.
ε = thermal emittance of the ablating surface.
5.2 Computation of Effective and Thermochemical Heats of
5.4.2 Eq 9 assumes radiation through a transparent medium
Ablation:
to a blackbody at absolute zero. Consider the validity of this
5.2.1 In order to compute the effective and thermochemical
assumption for each case and if the optical properties of the
heatsofablation,correctthecold-wallheattransferrateforthe
boundary layer are known and are deemed significant, or the
effect of the temperature difference on the heat transfer. This
absolute zero blackbody sink assumption is violated, consider
correction factor is a function of the ratio of the enthalpy
these effects in the use of Eq 9.
potentials across the boundary layer for the hot and cold wall
as follows: 5.5 Mechanical Removal Correction:
5.5.1 Determine the heat transfer rate due to the mechanical
q /q 5 h 2 h / h 2 h (8)
@~ ! ~ !#
hw cw e hw e cw
removal of material from the ablating surface from the mass-
where:
loss rate due to mechanical processes and the enthalpy of the
h = gas recovery enthalpy at the boundary layer edge,
material removed as follows:
e
kJ/kg,
q 5 m h (10)
mech mech m
h = gas enthalpy at the surface temperature of the test
hw
model, kJ/kg, and 5.5.2 Approximate the enthalpy of the material removed by
h = gas enthalpy at a cold wall, kJ/kg.
cw the product of the specific heat of the mechanically removed
material, and the surface temperature (9-13).
5.2.2 This correction is based upon laminar flow in air and
subject to the restrictions imposed in Ref (2). Additional
6. Determination of Mass Transfer Rate
correctionsmayberequiredregardingtheeffectoftemperature
on the transport properties of the test gas.The form and use of
6.1 The determination of the heat of ablation requires the
these corrections should be determined by the investigator for
measurement of the mass transfer rate of the material under
each individual situation.
test. This may be accomplished in several ways depending on
the type of material under test. The heat of ablation value can
5.3 Gas Enthalpy Determination:
be affected by the choice of method.
5.3.1 The enthalpy at the boundary layer edge may be
6.1.1 Ablation Depth Method:
determined in several ways: energy balance, enthalpy probe,
spectroscopy, etc. Details of the methods may be found
6.1.1.1 The simplest method of measurement of mass-loss
elsewhere (3-6). Take care to evaluate the radial variation of rate is the change in length or ablation depth. Make a pretest
enthalpyinthenozzle.Also,inlow-densityflows,considerthe
and post-test measurement of th
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