ASTM E341-96
(Practice)Standard Practice for Measuring Plasma Arc Gas Enthalpy by Energy Balance
Standard Practice for Measuring Plasma Arc Gas Enthalpy by Energy Balance
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: E 341 – 96
Standard Practice for
Measuring Plasma Arc Gas Enthalpy by Energy Balance
This standard is issued under the fixed designation E 341; 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.
n p
1. Scope
¯ ¯
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
i 5 1 j 5 1
1.1 This practice covers the measurement of total gas
5 Energy to gas
enthalpy of an electric-arc-heated gas stream by means of an
overall system energy balance. This is sometimes referred to as
where:
a bulk enthalpy and represents an average energy content of the
C 5 water, specific heat,
p
test stream which may differ from local values in the test
E 5 plasma arc voltage,
stream.
H 5 exhaust gas enthalpy,
g
1.2 This standard does not purport to address all of the
H 5 inlet gas enthalpy,
in
safety concerns, if any, associated with its use. It is the
H 5 heat of vaporization corresponding to the ma-
j
responsibility of the user of this standard to establish appro-
terial M ,
j
priate safety and health practices and determine the applica-
I 5 plasma arc current,
bility of regulatory limitations prior to use.
M 5 mass loss rate of electrode insulator, interior
j
metal surface, etc.
2. Summary of Test Method
Q 5 energy convected and radiated from external
CR
2.1 A measure of the total or stagnation gas enthalpy of
surface of plasma generator,
plasma-arc heated gases (nonreacting) is based upon the
DT 5 T − T 5 water temperature rise during
0 0 0
H2O 2 1
following measurements:
plasma arc operation,
2.1.1 Energy input to the plasma arc,
DT 5 T −T 5 water temperature rise before
1 2 1
H2O
2.1.2 energy losses to the plasma arc hardware and cooling plasma arc operation,
water, and T 5 water exhaust temperature during plasma arc
operation,
2.1.3 gas mass flow.
T 5 inlet water temperature during plasma arc op-
2.2 The gas enthalpy is determined numerically by dividing
eration,
the gas mass flow into the net power input to the plasma arc
T 5 water exhaust temperature before plasma arc
(power to plasma arc minus the energy losses). 2
operation,
2.3 The technique for performing the overall energy balance
T 5 inlet water temperature before plasma arc op-
is illustrated schematically in Fig. 1. The control volume for 1
eration,
the energy balance can be represented by the entire envelope of
W 5 gas flow rate,
g
this drawing. Gas enters at an initial temperature, or enthalpy,
W 5 mass flow rate of coolant water, and
H O
and emerges at a higher enthalpy. Water or other coolant enters
¯ ¯
E I 5 average of the product of voltage, E, and
the control volume at an initial temperature and emerges at a
current, I.
higher temperature. Across the arc, electrical energy is dissi-
2.4 An examination of Eq 1 shows that, in order to obtain an
pated by virtue of the resistance and current in the arc itself. A
evaluation of the energy content of the plasma for a specified
heat balance of the system requires that the energy gained by
set of operating conditions, measurements must be made of the
the gas must be defined by the difference between the incoming
voltage and current, the mass-flow rate and temperature rise of
energy (electrical input) and total coolant and external losses.
the coolant, the mass-flow rate and inlet ambient temperature
This is a direct application of the First Law of Thermodynam-
of the test gas, and the external surface temperature and
ics and, for the particular control volume cited here, can be
housing of the arc chamber. For all practical purposes, the
written as follows:
external surface temperature of the water-cooled plasma arc is
Energy In 2 Energy Out 5 Energy to Gas (1)
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
This method is under the jurisdiction of ASTM Committee E-21 on Space
convective or radiative mechanisms and that the internal loss of
Simulation and Applications of Space Technology and is the direct responsibility of
Subcommittee E21.08 on Thermal Protection. electrode or plasma generator material is small compared with
Current edition approved Oct. 10, 1996. Published December 1996. Originally
the energy input. In addition, as some plasma generators utilize
published as E 341 – 68 T. Last previous edition E 341 – 81 (1992).
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
E 341
FIG. 1 Schematic Energy Balance Method for Determining Gas
Enthalpy
magnetic fields in their design, the magnetic field coil electrical steady-state conditions must exist both in plasma performance
power and ohmic-heating dissipation should be included in the and fluid flow.
over-all heat balance. Precautions should be taken to assure
3.4 In particular it is noted that total enthalpy as determined
that only a negligible portion of magnetic energy is being
by the energy balance technique is most useful if the plasma
dissipated in hardware not within the heat balance circuit. For
generator design minimizes coring affects. If nonuniformity
the purposes of this discussion, the magnetic field power input
exists the enthalpy determined by energy balance gives only
and loss aspects have been omitted because of their unique
the average for the entire plasma stream, whereas the local
applicability to specific plasma generator designs.
enthalpy experienced by a model in the core of the stream may
2.5 The energy balance is given by Eq 2 when these factors
be much higher. More precise methods are needed to measure
are taken into account:
local variations in total enthalpy.
n
¯
EI 2 W C ~DT 2DT ! (2)
( H O p 0 1 H O
2 i 2 i
4. Apparatus
i 5 1
The exhaust enthalpy, H , of the effluent as defined by Eq 1
g 4.1 General—The apparatus shall consist of the plasma-arc
and 2 is a measure of the average total (stagnation) enthalpy at
facility and the necessary instrumentation to measure the
the nozzle exit plane of the plasma-arc heater. This enthalpy
power input to the arc, gas stream and coolant flow rates, inlet
does not necessarily apply to the plasma downstream of the
gas temperature and net coolant temperature rise of the plasma
nozzle exit plane.
generator hardware. Although the recommended instrumenta-
tion accuracies are state-of-the-art values, higher accuracy
3. Significance and Use
instruments (than those recommended) may be required for
3.1 The purpose of this practice is to measure the total or
low efficiency plasma generators.
stagnation gas enthalpy of a plasma-arc gas stream in which
4.2 Input Energy Measurements—The energy input term,
nonreactive gases are heated by passage through an electrical
EI, to a large degree may be time dependent. Fluctuations in
discharge device during calibration tests of the system.
the power input can produce errors as large as 50 % under
3.2 The plasma arc represents one heat source for determin-
certain conditions. The magnitude of the error will depend on
ing the performance of high temperature materials under
the amplitude of the unsteady compared with the steady portion
simulated hyperthermal conditions. As such the total or stag-
of the current and voltage and also on the instantaneous phase
nation enthalpy is one of the important parameters for corre-
relationship between current and voltage. The power input
lating the behavior of ablation materials.
portion term should be written:
3.3 The most direct method for obtaining a measure of total
t
enthalpy, and one which can be performed simultaneously with
¯
EI 5 1/t EI dt (3)
*
each material test, if desired, is to perform an energy balance
on the arc chamber. In addition, in making the energy balance,
As a consequence each plasma generator should make use of
accurate measurements are needed since the efficiencies of
some plasma generators are low (as low as 15 to 20 % or less oscilloscopic voltage-current traces during operation in order
to ascertain the time variation of the voltage-current input. If
in which case the enthalpy depends upon the difference of two
quantities of nearly equal magnitude). Therefore, the accuracy these traces show significant unsteadiness it is recommended
of the measurements of the primary variables must be high, all that additional methods of input power measurements be
energy losses must be correctly taken into account, and pursued, such as an integrating device if available. In order to
E 341
measure power directly, a wattmeter as cited by Dawes (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 than6 1 percent. 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 of the plasma arc, care should be taken to minimize deposits on
voltage across the circuit. Suitable instruments for such voltage the sensors and to eliminate any possibility of sensor heating
measurements are presented by the Instrument Socie
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