ASTM E341-08(2015)

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it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: E341 − 08 E341 − 08 (Reapproved 2015)
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. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. 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.
2. Summary of Test Method
2.1 A measure of the total or stagnation gas enthalpy of plasma-arc heated gases (nonreacting) is based upon the following
measurements:
2.1.1 Energy input to the plasma arc,
2.1.2 Energy losses to the plasma arc hardware and cooling water, and
2.1.3 Gas mass flow.
2.2 The gas enthalpy is determined numerically by dividing the gas mass flow into the net power input to the plasma arc (power
to plasma arc minus the energy losses).
2.3 The technique for performing the overall energy balance is illustrated schematically in Fig. 1. The control volume for the
energy balance can be represented by the entire envelope of this drawing. Gas enters at an initial temperature, or enthalpy, and
emerges at a higher enthalpy. Water or other coolant enters the control volume at an initial temperature and emerges at a higher
temperature. Across the arc, electrical energy is dissipated by virtue of the resistance and current in the arc itself. A heat balance
of the system requires that the energy gained by the gas must be defined by the difference between the incoming energy (electrical
input) and total coolant and external losses. This is a direct application of the First Law of Thermodynamics and, for the particular
control volume cited here, can be written as follows:
Energy In 2 Energy Out 5 Energy to Gas (1)
n p
¯
EI 2 Q 2 W C ΔT 2 ΔT 2 M H
~ !
CR ( p 0 1 ( j j
H O H O
2 i 2 i
i51 j51
5W H 2 H
~ !
g g in
where:
C = water, specific heat,
p
E = plasma arc voltage,
H = exhaust gas enthalpy,
g
H = inlet gas enthalpy,
in
H = heat of vaporization corresponding to the material M ,
j j
I = plasma arc current,
M = mass loss rate of electrode insulator, interior metal surface, etc.
j
Q = energy convected and radiated from external surface of plasma generator,
CR
This practice is under the jurisdiction of ASTM Committee E21 on Space Simulation and Applications of Space Technology and is the direct responsibility of
Subcommittee E21.08 on Thermal Protection.
Current edition approved May 1, 2008May 1, 2015. Published May 2008June 2015. Originally approved in 1968. Last previous edition approved in 20022008 as E341 – 96
(2002).E341 – 08. DOI: 10.1520/E0341-08.10.1520/E0341-08R15.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E341 − 08 (2015)
FIG. 1 Schematic Energy Balance Method for Determining Gas
Enthalpy
ΔT = T − T = water temperature rise during plasma arc operation,
0 0 0
H2O 2 1
ΔT = T − T = water temperature rise before plasma arc operation,
1 2 1
H2O
T = water exhaust temperature during plasma arc operation,
T = inlet water temperature during plasma arc operation,
T = water exhaust temperature before plasma arc operation,
T = inlet water temperature before plasma arc operation,
W = gas flow rate,
g
W = mass flow rate of coolant water, and
H O
¯
= average of the product of voltage, E, and current, I.
EI
2.4 An examination of Eq 1 shows that, in order to obtain an evaluation of the energy content of the plasma for a specified set
of operating conditions, measurements must be made of the voltage and current, the mass-flow rate and temperature rise of the
coolant, the mass-flow rate and inlet ambient temperature of the test gas, and the external surface temperature and housing of the
arc chamber. For all practical purposes, the external surface temperature of the water-cooled plasma arc is minimum. Consequently,
it will be assumed throughout this discussion that negligible energy (compared to the input energy) is lost from the external plasma
generator surface by convective or radiative mechanisms and that the internal loss of electrode or plasma generator material is
small compared with the energy input. In addition, as some plasma generators utilize magnetic fields in their design, the magnetic
field coil electrical power and ohmic-heating dissipation should be included in the over-all heat balance. Precautions should be
taken to assure that only a negligible portion of magnetic energy is being dissipated in hardware not within the heat balance circuit.
For the purposes of this discussion, the magnetic field power input and loss aspects have been omitted because of their unique
applicability to specific plasma generator designs.
2.5 The energy balance is given by Eq 23 when these factors are taken into account:
n
¯
EI 2 W C ΔT 2 ΔT 5 W H 2 H (2)
~ ! ~ !
( H O p 0 1 H O g g in
2 i 2 i
i51
The exhaust enthalpy, H , of the effluent as defined by Eq 1 and 23 is a measure of the average total (stagnation) enthalpy at
g
the nozzle exit plane of the plasma-arc heater. This enthalpy does not necessarily apply to the plasma downstream of the nozzle
exit plane.
3. Significance and Use
3.1 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.
3.2 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.
3.3 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
E341 − 08 (2015)
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.
3.4 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 effects. 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.
4. Apparatus
4.1 General—The apparatus shall consist of the plasma-arc facility and the necessary instrumentation to measure the power
input to the arc, gas stream and coolant flow rates, inlet gas temperature and net coolant temperature rise of the plasma generator
hardware. Although the recommended instrumentation accuracies are state-of-the-art values, higher accuracy instruments (than
those recommended) may be required for low efficiency plasma generators.
4.2 Input Energy Measurements—The energy input term, EI, to a large degree may be time dependent. Fluctuations in the power
input can produce errors as large as 50 % under certain conditions. The magnitude of the error will depend on the amplitude of
the unsteady compared with the steady portion of the current and voltage and also on the instantaneous phase relationship between
current and voltage. The power input portion term should be written:
t
¯
EI 5 1/t EI dt (3)
*
As a consequence each plasma generator should make use of oscilloscopic voltage-current traces during operation in order to
ascertain the time variation of the voltage-current input. If these traces show significant unsteadiness it is recommended that
additional methods of input power measurements be pursued, such as an integrating device if available. In order to measure power
directly, a wattmeter as cited by Dawes (1) can be employed. As a precaution in the use of the wattmeter, reversed readings of
current and voltage should be taken and the average of the two readings used. For those plasma generator facilities which operate
under known and steady input power the use of a voltmeter and ammeter is recommended owing to their high degree of accuracy.
4.2.1 Voltage Measurement—The determination of power input to the plasma generator requires the measurement of the voltage
across the circuit. Suitable instruments for such voltage measurements are presented by the Instrument Society of America (ISA)
(2). The measurement techniques to be used can be either a voltage divider network or a direct reading instrument. It is highly
desirable to be able to record the voltage such that time variations are a part of the test data. Accuracy of the voltage measurements
shall be within 61 %. The voltage measurement shall be taken at the electrode terminals of the plasma generator circuit.
4.2.2 Current Measurement—The measurement of plasma arc current shall be accomplished with an ammeter equipped with a
precision shunt and the reading shall be within 61 %. Ref (2) lists other instruments suitable for measuring arc current. If a
precision shunt is utilized, the temperature across the shunt shall be constant and within the stated limits as given by the
manufacturer. Arc current shall be measur
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