ASTM E341-08(2020)

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: E341 − 08 (Reapproved 2020)
Standard Practice for
Measuring Plasma Arc Gas Enthalpy by Energy Balance
This standard is issued under the fixed designation E341; 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.
1. Scope higher temperature. Across the arc, electrical energy is dissi-
pated by virtue of the resistance and current in the arc itself.A
1.1 This practice covers the measurement of total gas
heat balance of the system requires that the energy gained by
enthalpy of an electric-arc-heated gas stream by means of an
thegasmustbedefinedbythedifferencebetweentheincoming
overallsystemenergybalance.Thisissometimesreferredtoas
energy (electrical input) and total coolant and external losses.
abulkenthalpyandrepresentsanaverageenergycontentofthe
This is a direct application of the First Law of Thermodynam-
test stream which may differ from local values in the test
ics and, for the particular control volume cited here, can be
stream.
written as follows:
1.2 This standard does not purport to address all of the
EnergyIn 2 EnergyOut 5 EnergytoGas (1)
safety concerns, if any, associated with its use. It is the
responsibility of the user of this standard to establish appro-
n p
priate safety, health, and environmental practices and deter- ¯
EI 2 Q 2 W C ∆T 2∆T 2 M H
~ !
CR ( p 0 1 H O ( j j
H O 2 i
2 i
i51 j51
mine the applicability of regulatory limitations prior to use.
1.3 This international standard was developed in accor-
5W H 2 H
~ !
g g in
dance with internationally recognized principles on standard-
where:
ization established in the Decision on Principles for the
Development of International Standards, Guides and Recom-
C = water, specific heat,
p
mendations issued by the World Trade Organization Technical E = plasma arc voltage,
Barriers to Trade (TBT) Committee. H = exhaust gas enthalpy,
g
H = inlet gas enthalpy,
in
H = heat of vaporization corresponding to the material
2. Summary of Test Method
j
M ,
j
2.1 A measure of the total or stagnation gas enthalpy of
I = plasma arc current,
plasma-arc heated gases (nonreacting) is based upon the
M = mass loss rate of electrode insulator, interior metal
j
following measurements:
surface, etc.
2.1.1 Energy input to the plasma arc,
Q = energy convected and radiated from external sur-
CR
2.1.2 Energy losses to the plasma arc hardware and cooling
face of plasma generator,
water, and
∆T = T − T =water temperature rise during plasma
0 0 0
H2O 2 1
2.1.3 Gas mass flow.
arc operation,
2.2 The gas enthalpy is determined numerically by dividing ∆T = T −T =water temperature rise before plasma arc
1 2 1
H2O
operation,
the gas mass flow into the net power input to the plasma arc
T = water exhaust temperature during plasma arc
(power to plasma arc minus the energy losses).
operation,
2.3 Thetechniqueforperformingtheoverallenergybalance
T = inlet water temperature during plasma arc
is illustrated schematically in Fig. 1. The control volume for
operation,
theenergybalancecanberepresentedbytheentireenvelopeof
T = water exhaust temperature before plasma arc
this drawing. Gas enters at an initial temperature, or enthalpy,
operation,
andemergesatahigherenthalpy.Waterorothercoolantenters
T = inlet water temperature before plasma arc
the control volume at an initial temperature and emerges at a
operation,
W = gas flow rate,
g
W O = mass flow rate of coolant water, and
H
This practice is under the jurisdiction of ASTM Committee E21 on Space
¯
= averageoftheproductofvoltage, E,andcurrent, I.
EI
Simulation andApplications of SpaceTechnology and is the direct responsibility of
Subcommittee E21.08 on Thermal Protection.
2.4 AnexaminationofEq1showsthat,inordertoobtainan
Current edition approved Nov. 1, 2020. Published December 2020. Originally
evaluation of the energy content of the plasma for a specified
approvedin1968.Lastpreviouseditionapprovedin2015asE341–08(2015).DOI:
10.1520/E0341-08R20. setofoperatingconditions,measurementsmustbemadeofthe
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E341 − 08 (2020)
FIG. 1 Schematic Energy Balance Method for Determining Gas
Enthalpy
voltage and current, the mass-flow rate and temperature rise of nation enthalpy is one of the important parameters for corre-
the coolant, the mass-flow rate and inlet ambient temperature lating the behavior of ablation materials.
of the test gas, and the external surface temperature and
3.3 The most direct method for obtaining a measure of total
housing of the arc chamber. For all practical purposes, the
enthalpy,andonewhichcanbeperformedsimultaneouslywith
external surface temperature of the water-cooled plasma arc is
each material test, if desired, is to perform an energy balance
minimum. Consequently, it will be assumed throughout this
on the arc chamber. In addition, in making the energy balance,
discussion that negligible energy (compared to the input
accurate measurements are needed since the efficiencies of
energy) is lost from the external plasma generator surface by
some plasma generators are low (as low as 15 to 20% or less
convectiveorradiativemechanismsandthattheinternallossof
in which case the enthalpy depends upon the difference of two
electrode or plasma generator material is small compared with
quantities of nearly equal magnitude). Therefore, the accuracy
theenergyinput.Inaddition,assomeplasmageneratorsutilize
of the measurements of the primary variables must be high, all
magneticfieldsintheirdesign,themagneticfieldcoilelectrical
energy losses must be correctly taken into account, and
power and ohmic-heating dissipation should be included in the
steady-state conditions must exist both in plasma performance
over-all heat balance. Precautions should be taken to assure
and fluid flow.
that only a negligible portion of magnetic energy is being
3.4 In particular it is noted that total enthalpy as determined
dissipated in hardware not within the heat balance circuit. For
by the energy balance technique is most useful if the plasma
the purposes of this discussion, the magnetic field power input
generator design minimizes coring effects. If nonuniformity
and loss aspects have been omitted because of their unique
exists the enthalpy determined by energy balance gives only
applicability to specific plasma generator designs.
the average for the entire plasma stream, whereas the local
2.5 The energy balance is given by Eq 2 when these factors
enthalpyexperiencedbyamodelinthecoreofthestreammay
are taken into account:
be much higher. More precise methods are needed to measure
n
local variations in total enthalpy.
¯
EI 2 W C ∆T 2∆T 5 W H 2 H (2)
~ ! ~ !
H O p 0 1 g g in
( H O
2 i 2 i
i51
4. Apparatus
The exhaust enthalpy, H , of the effluent as defined by Eq 1
g
and 2 is a measure of the average total (stagnation) enthalpy at 4.1 General—The apparatus shall consist of the plasma-arc
the nozzle exit plane of the plasma-arc heater. This enthalpy facility and the necessary instrumentation to measure the
does not necessarily apply to the plasma downstream of the power input to the arc, gas stream and coolant flow rates, inlet
nozzle exit plane. gas temperature and net coolant temperature rise of the plasma
generator hardware. Although the recommended instrumenta-
3. Significance and Use
tion accuracies are state-of-the-art values, higher accuracy
3.1 The purpose of this practice is to measure the total or instruments (than those recommended) may be required for
low efficiency plasma generators.
stagnation gas enthalpy of a plasma-arc gas stream in which
nonreactive gases are heated by passage through an electrical
4.2 Input Energy Measurements—The energy input term,
discharge device during calibration tests of the system.
EI, to a large degree may be time dependent. Fluctuations in
3.2 Theplasmaarcrepresentsoneheatsourcefordetermin- the power input can produce errors as large as 50% under
ing the performance of high temperature materials under certain conditions. The magnitude of the error will depend on
simulated hyperthermal conditions. As such the total or stag- theamplitudeoftheunsteadycomparedwiththesteadyportion
E341 − 08 (2020)
of the current and voltage and also on the instantaneous phase not present. If practical, the water flowmeters shall be placed
relationship between current and voltage. The power input upstream of the plasma generator in straight portions of the
portion term should be written: piping. The flowmeter device shall be checked and calibrated
t periodically.
¯
EI 51/t EIdt (3)
*
4.3.2 Coolant Temperature Measurement—The method of
temperature measurement must be sufficiently sensitive and
Asaconsequenceeachplasmageneratorshouldmakeuseof
reliable to ensure accurate measurement of the coolant water
oscilloscopic voltage-current traces during operation in order
temperature rise. Procedures similar to those given in the
to ascertain the time variation of the voltage-current input. If
Annual Book of ASTM Standards, Part 44, and Ref (3) should
these traces show significant unsteadiness it is recommended
be adhered to in the calibration and preparation of temperature
that additional methods of input power measurements be
sensors. The bulk or average temperature of the coolant shall
pursued, such as an integrating device if available. In order to
be measured at the inlet and output lines of each cooled unit.
measurepowerdirectly,awattmeterascitedbyDawes(1) can
The error in measurement of temperature difference between
be employed. As a precaution in the use of the wattmeter,
inlet and outlet shall be not more than 61 %. The water
reversed readings of current and voltage should be taken and
temperature-indicating devices shall be placed as close as
the average of the two readings used. For those plasma
practical to the plasma arc in the inlet and outlet lines. No
generator facilities which operate under known and steady
additional apparatus shall be between the temperature sensor
input power the use of a voltmeter and ammeter is recom-
and the plasma arc. Th
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