ASTM E341-96(2002)
(Practice)Standard Practice for Measuring Plasma Arc Gas Enthalpy by Energy Balance
Standard Practice for Measuring Plasma Arc Gas Enthalpy by Energy Balance
SIGNIFICANCE AND USE
The purpose of this practice is to measure the total or stagnation gas enthalpy of a plasma-arc gas stream in which nonreactive gases are heated by passage through an electrical discharge device during calibration tests of the system.
The plasma arc represents one heat source for determining the performance of high temperature materials under simulated hyperthermal conditions. As such the total or stagnation enthalpy is one of the important parameters for correlating the behavior of ablation materials.
The most direct method for obtaining a measure of total enthalpy, and one which can be performed simultaneously with each material test, if desired, is to perform an energy balance on the arc chamber. In addition, in making the energy balance, accurate measurements are needed since the efficiencies of some plasma generators are low (as low as 15 to 20 % or less in which case the enthalpy depends upon the difference of two quantities of nearly equal magnitude). Therefore, the accuracy of the measurements of the primary variables must be high, all energy losses must be correctly taken into account, and steady-state conditions must exist both in plasma performance and fluid flow.
In particular it is noted that total enthalpy as determined by the energy balance technique is most useful if the plasma generator design minimizes coring affects. If nonuniformity exists the enthalpy determined by energy balance gives only the average for the entire plasma stream, whereas the local enthalpy experienced by a model in the core of the stream may be much higher. More precise methods are needed to measure local variations in total enthalpy.
SCOPE
1.1 This practice covers the measurement of total gas enthalpy of an electric-arc-heated gas stream by means of an overall system energy balance. This is sometimes referred to as a bulk enthalpy and represents an average energy content of the test stream which may differ from local values in the test stream.
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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Designation:E341–96 (Reapproved 2002)
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 (e) indicates an editorial change since the last revision or reapproval.
EnergyIn 2EnergyOut 5EnergytoGas (1)
1. Scope
n p
1.1 This practice covers the measurement of total gas
¯ ¯
E I 2 Q 2 W C ~DT 2DT ! 2 M H
( (
CR p 0 1 H O j j
H O 2 i
2 i
enthalpy of an electric-arc-heated gas stream by means of an i 51 j 51
overallsystemenergybalance.Thisissometimesreferredtoas
5Energytogas
abulkenthalpyandrepresentsanaverageenergycontentofthe
test stream which may differ from local values in the test
where:
stream.
C = water, specific heat,
p
1.2 This standard does not purport to address all of the
E = plasma arc voltage,
safety concerns, if any, associated with its use. It is the
H = exhaust gas enthalpy,
g
responsibility of the user of this standard to establish appro-
H = inlet gas enthalpy,
in
priate safety and health practices and determine the applica-
H = heat of vaporization corresponding to the ma-
j
bility of regulatory limitations prior to use.
terial M,
j
I = plasma arc current,
2. Summary of Test Method
M = mass loss rate of electrode insulator, interior
j
2.1 A measure of the total or stagnation gas enthalpy of
metal surface, etc.
plasma-arc heated gases (nonreacting) is based upon the
Q = energy convected and radiated from external
CR
following measurements:
surface of plasma generator,
2.1.1 Energy input to the plasma arc,
DT = T − T = water temperature rise during
0 0 0
H2O 2 1
2.1.2 Energy losses to the plasma arc hardware and cooling
plasma arc operation,
water, and
DT = T −T =water temperature rise before plasma
1 2 1
H2O
2.1.3 Gas mass flow.
arc operation,
2.2 The gas enthalpy is determined numerically by dividing
T = water exhaust temperature during plasma arc
the gas mass flow into the net power input to the plasma arc operation,
(power to plasma arc minus the energy losses). T = inlet water temperature during plasma arc op-
2.3 Thetechniqueforperformingtheoverallenergybalance eration,
T = water exhaust temperature before plasma arc
is illustrated schematically in Fig. 1. The control volume for
operation,
theenergybalancecanberepresentedbytheentireenvelopeof
T = inlet water temperature before plasma arc op-
this drawing. Gas enters at an initial temperature, or enthalpy,
eration,
andemergesatahigherenthalpy.Waterorothercoolantenters
W = gas flow rate,
the control volume at an initial temperature and emerges at a g
W = mass flow rate of coolant water, and
H O
higher temperature. Across the arc, electrical energy is dissi- 2
¯ ¯
E I = average of the product of voltage, E, and
pated by virtue of the resistance and current in the arc itself.A
current, I.
heat balance of the system requires that the energy gained by
2.4 AnexaminationofEq1showsthat,inordertoobtainan
thegasmustbedefinedbythedifferencebetweentheincoming
evaluation of the energy content of the plasma for a specified
energy (electrical input) and total coolant and external losses.
setofoperatingconditions,measurementsmustbemadeofthe
This is a direct application of the First Law of Thermodynam-
voltage and current, the mass-flow rate and temperature rise of
ics and, for the particular control volume cited here, can be
the coolant, the mass-flow rate and inlet ambient temperature
written as follows:
of the test gas, and the external surface temperature and
housing of the arc chamber. For all practical purposes, the
This practice is under the jurisdiction of ASTM Committee E21 on Space
external surface temperature of the water-cooled plasma arc is
Simulation andApplications of SpaceTechnology and is the direct responsibility of
minimum. Consequently, it will be assumed throughout this
Subcommittee E21.08 on Thermal Protection.
discussion that negligible energy (compared to the input
Current edition approved Oct. 10, 1996. Published December 1996. Originally
energy) is lost from the external plasma generator surface by
published as E341–68 T. Last previous edition E341–81(1992).
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
E341
FIG. 1 Schematic Energy Balance Method for Determining Gas
Enthalpy
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 affects. 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
local variations in total enthalpy.
n
¯
EI 2 W C ~DT 2DT ! (2)
(
H O p 0 1 H O
2 i 2 i
i 51
4. Apparatus
4.1 General—The apparatus shall consist of the plasma-arc
The exhaust enthalpy, H , of the effluent as defined by Eq 1
g
facility and the necessary instrumentation to measure the
and 2 is a measure of the average total (stagnation) enthalpy at
power input to the arc, gas stream and coolant flow rates, inlet
the nozzle exit plane of the plasma-arc heater. This enthalpy
gas temperature and net coolant temperature rise of the plasma
does not necessarily apply to the plasma downstream of the
generator hardware. Although the recommended instrumenta-
nozzle exit plane.
tion accuracies are state-of-the-art values, higher accuracy
instruments (than those recommended) may be required for
3. Significance and Use
low efficiency plasma generators.
3.1 The purpose of this practice is to measure the total or
4.2 Input Energy Measurements—The energy input term,
stagnation gas enthalpy of a plasma-arc gas stream in which
EI, to a large degree may be time dependent. Fluctuations in
nonreactive gases are heated by passage through an electrical
the power input can produce errors as large as 50% under
discharge device during calibration tests of the system.
certain conditions. The magnitude of the error will depend on
3.2 Theplasmaarcrepresentsoneheatsourcefordetermin-
theamplitudeoftheunsteadycomparedwiththesteadyportion
ing the performance of high temperature materials under
of the current and voltage and also on the instantaneous phase
simulated hyperthermal conditions. As such the total or stag-
relationship between current and voltage. The power input
nation enthalpy is one of the important parameters for corre-
portion term should be written:
lating the behavior of ablation materials.
t
3.3 The most direct method for obtaining a measure of total
¯
enthalpy,andonewhichcanbeperformedsimultaneouslywith EI 51/t EIdt (3)
*
each material test, if desired, is to perform an energy balance
on the arc chamber. In addition, in making the energy balance, Asaconsequenceeachplasmageneratorshouldmakeuseof
accurate measurements are needed since the efficiencies of oscilloscopic voltage-current traces during operation in order
some plasma generators are low (as low as 15 to 20% or less to ascertain the time variation of the voltage-current input. If
E341
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. The temperature measurements shall be
mended owing to their high degree of accuracy. recorded continuously. Ref (2) lists a variety of commercially
4.2.1 Voltage Measurement—The determination of power available temperature sensors. During the course of operation
input to the plasma generator requires the measurement of the oftheplasmaarc,cares
...
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