ASTM E458-72(1996)e1
(Test Method)Standard Test Method for Heat of Ablation
Standard Test Method for Heat of Ablation
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.
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e1
Designation: E 458 – 72 (Reapproved 1996)
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.
e NOTE—Section 13 was added editorially in May 1996.
1. Scope sacrificial thermal protection device. The property is a function
of both the material and the environment to which it is
1.1 This test method covers determination of the heat of
subjected. In general, it is defined as the incident heat dissi-
ablation of materials subjected to thermal environments requir-
pated by the ablative material per unit of mass removed, or
ing the use of ablation as an energy dissipation process. Three
concepts of the property are described and defined: cold wall, Q* 5 q/m (1)
effective, and thermochemical heat of ablation.
where:
1.2 This standard does not purport to address all of the
Q* 5 heat of ablation, kJ/kg,
safety concerns, if any, associated with its use. It is the
q 5 incident heat transfer rate, kW/m , and
responsibility of the user of this standard to establish appro-
m 5 total mass transfer rate, kg/m ·s.
priate safety and health practices and determine the applica-
3.2 The heat of ablation may be represented in three
bility of regulatory limitations prior to use.
different ways depending on the investigator’s requirements:
3.3 cold-wall heat of ablation—The most commonly and
2. Referenced Documents
easily determined value is the cold-wall heat of ablation, and is
2.1 ASTM Standards:
defined as the incident cold-wall heat dissipated per unit mass
E 285 Test Method for Oxyacetylene Ablation Testing of
of material ablated, as follows:
Thermal Insulation Materials
Q* 5 q /m (2)
E 341 Practice for Measuring Plasma Arc Gas Enthalpy by cw
Energy Balance
where:
E 377 Practice for Internal Temperature Measurements in
Q* 5 cold-wall heat of ablation, kJ/kg,
cw
Ablative Materials
q 5 heat transfer rate from the test environment to a
hw
E 422 Test Method for Measuring Heat Flux Using a
cold wall, kW/m , and
Water-Cooled Calorimeter
m 5 total mass transfer rate, kg/m ·s.
E 457 Test Method for Measuring Heat-Transfer Rate Using
The temperature of the cold-wall reference for the cold-wall
a Thermal Capacitance (Slug) Calorimeter
heat transfer rate is usually considered to be room temperature
E 459 Test Method for Measuring Heat-Transfer Rate Using
or close enough such that the hot-wall correction given in Eq
a Thin-Skin Calorimeter
7 is less than 5 % of the cold-wall heat transfer rate.
E 470 Method for Measuring Gas Enthalpy Using Calori-
3.4 effective heat of ablation—The effective heat of ablation
metric Probes
is defined as the incident hot-wall dissipated per unit mass
E 471 Test Method for Obtaining Char Density Profile of
ablated, as follows:
Ablative Materials by Machining and Weighing
Q* 5 q /m (3)
eff hw
E 511 Test Method for Measuring Heat Flux Using a
Copper-Constantan Circular Foil, Heat-Flux Gage
where:
Q* 5 effective heat of ablation, kJ/kg,
eff
3. Terminology
q 5 heat transfer rate from the test environment to a
hw
3.1 Descriptions of Terms Specific to This Standard:
nonablating wall at the surface temperature of the
3.1.1 heat of ablation—a property that indicates the ability
material under test, kW/m , and
of a material to provide heat protection when used as a
m 5 total mass transfer rate, kg/m ·s.
3.5 thermochemical heat of ablation—The thermochemical
heat of ablation is defined as the incident hot-wall heat
This test method is under the jurisdiction of ASTM Committee E-21 on Space
Simulation and Applications of Space Technology and is the direct responsibility of
dissipated per unit mass ablated, corrected for reradiation heat
Subcommittee E21.08 on Thermal Protection.
rejection processes and material eroded by mechanical re-
Current edition approved Aug. 29, 1972. Published November 1972.
2 moval, as follows:
Annual Book of ASTM Standards, Vol 15.03.
Discontinued, see 1982 Annual Book of ASTM Standards, Part 41.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 458
Q* 5 ~q 2 q 2 q !m (4) type is based on the test configuration and the methods used,
tc hw rr mech tc
and should take into consideration such parameters as instru-
m 5 m 2 m (5)
tc mech
ment response time, test duration, and heat transfer rate (6).
q 5 m ~h ! (6)
mech mech m
5.1.1.1 The calorimeters discussed in 5.1.1 measure a “cold-
wall” heat-transfer rate because the calorimeter surface tem-
where:
Q* 5 thermochemical heat of ablation, kJ/kg, perature is much less than the ablation temperature. The value
tc
q 5 reradiative heat transfer rate, kW/m , thus obtained is used directly in computing the cold-wall heat
rr
m 5 mass transfer rate due to thermochemical pro-
tc of ablation.
cesses, kg/m ·s,
5.1.2 Install the calorimeter in a calorimeter body that
m 5 mass-transfer rate due to mechanical processes,
mech duplicates the test model in size and configuration. This is done
kg/m ·s,
in order to eliminate geometric parameters from the heat-
q 5 heat-transfer rate due to energy carried away with
mech
transfer rate measurement and to ensure that the quantity
mechanically removed material, kW/m , and
measured is representative of the heat-transfer rate to the test
h 5 enthalpy of mechanically removed material, kJ/
m
model. If the particular test run does not allow an independent
kg.
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.
heats of ablation, correct the cold-wall heat-transfer rate for the
3.6 The three heat of ablation values described in 3.2 require
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 5 gas enthalpy at the boundary layer edge, kJ/kg,
e
a material to serve as a heat protection element in a severe h 5 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 5 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 corrections may be required regarding the effect of temperature
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
5. Determination of Heat Transfer Rate
the gas enthalpy at the ablator surface and the calorimeter
surface requires pressure and surface temperature measure-
5.1 Cold-Wall Heat Transfer Rate:
ments. The hot-wall temperatures are generally measured by
5.1.1 Determine the cold-wall heat-transfer rate to a speci-
optical methods such as pyrometers, radiometers, etc. Other
men by using a calorimeter. These instruments are available
commercially in several different types, some of which can be methods such as infrared spectrometers and monochromators
have been used (12,13). Effects of the optical properties of the
readily fabricated by the investigator. Selection of a specific
boundary layer of an ablating surface make accurate determi-
nations of surface temperature difficult.
5.3.2 Determine the wall enthalpy from the assumed state of
The boldface numbers in parentheses refer to the references listed at the end of
the standard. the gas flow (equilibrium, frozen, or nonequilibrium), if the
E 458
pressure and the wall temperature are known. It is further 6.1.1.3 Use the length change measurement of mass-loss
assumed that the wall enthalpy is the enthalpy of the freestream rate for non-charring ablators, subliming materials, or with
gas, without ablation products, at the wall temperature. Make
charring ablators under steady state ablation conditions (see
the wall static pressure measurements with an ordinary pitot
Section 7) and only with materials that do not swell or grow in
arrangement designed for the flow regime of interest and by
length.
using the appropriate transducers.
6.1.2 Direct Weighing Method:
5.4 Reradiation Correction:
6.1.2.1 A second method of determining mass-transfer rate
5.4.1 Calculate the heat-transfer rate due to reradiation from
is by the use of a pretest and post-test mass measurement. This
the surface of the ablating material from the following equa-
procedure yields the mass transfer rate directly. A disadvantage
tion:
of this method is that the mass-transfer rate obtained is
q 5esT (8)
rr s
averaged over the entire test model heated area. The heat-
transfer rate is generally varying over the surface and therefore
where:
leads to errors in heat of ablation. The mass-transfer rate is also
s5 Stefan-Boltzmann constant,
averaged over the insertion period which includes the early part
T 5 absolute surface temperature of ablating material,
s
of the period when the ablation process is transient and after
K, and
e5 thermal emittance of the ablating surface. the specimen has been removed where some mass loss occurs.
5.4.2 Eq 8 assumes radiation through a transparent medium The experimenter should be guided by Section 7 in determin-
to a blackbody at absolute zero. Consider the validity of this ing the magnitude of these effects.
assumption for each case and if the optical properties of the
6.1.2.2 In cases where the mass loss is low, the errors
boundary layer are known and are deemed significant, or the
incurred in mass loss measurements could become large. It is
absolute zero blackbody sink assumption is violated, consider
therefore recommended that a significant mass loss be realized
these effects in the use of Eq 8.
to reduce measurement errors. The problem is one of a small
5.5 Mechanical Removal Correction:
difference of two large numbers.
5.5.1 Determine the heat-transfer rate due to the mechanical
6.1.3 Core Sample Method:
removal of material from the ablating surface from the mass-
6.1.3.1 Accomplish direct measurement of the mass loss by
loss rate due to mechanical processes and the enthalpy of the
coring the model after testing by using standard core drills. The
material removed as follows:
core size is determined by the individual experiment; however,
q 5 m h (9)
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
6. Determination of Mass-Transfer Rate
sample as follows:
6.1 The determination of the heat of ablation requires the
m 5 ~r V 2 w !/~tA ! (11)
o o f c
measurement of the mass-transfer rate of the material under
where:
test. This may be accomplished in several ways depending on
V 5 original calculated volume of core, m ,
the type of material under test. The heat of ablation value can o
w 5 final mass of core, kg, and
f
be affected by the choice of method.
A 5 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
and post-test measurement of the length and calculate the The core drill dimensions should not be used due to drilling
inaccuracies.
mass-loss rate from the following relationship:
6.1.4 Shrouded Core Method—A second core sample
m5r ~dL/t! (10)
o
method used in measuring ablation properties of materials
where:
involves the use of a model that includes a core and model
r 5 virgin material density, kg/m ,
o
shroud of the same material where the core has been prepared
dL 5 change in length or ablation depth, m, and
prior to testing. This method is described in
...
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