1、Designation: E341 08 (Reapproved 2015)Standard Practice forMeasuring Plasma Arc Gas Enthalpy by Energy Balance1This standard is issued under the fixed designation E341; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of la
2、st revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This practice covers the measurement of total gasenthalpy of an electric-arc-heated gas stream by means of anoverall sys
3、tem energy balance. This is sometimes referred to asa bulk enthalpy and represents an average energy content of thetest stream which may differ from local values in the teststream.1.2 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is therespo
4、nsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Summary of Test Method2.1 A measure of the total or stagnation gas enthalpy ofplasma-arc heated gases (nonreacting) is based upon th
5、efollowing measurements:2.1.1 Energy input to the plasma arc,2.1.2 Energy losses to the plasma arc hardware and coolingwater, and2.1.3 Gas mass flow.2.2 The gas enthalpy is determined numerically by dividingthe gas mass flow into the net power input to the plasma arc(power to plasma arc minus the en
6、ergy losses).2.3 The technique for performing the overall energy balanceis illustrated schematically in Fig. 1. The control volume forthe energy balance can be represented by the entire envelope ofthis drawing. Gas enters at an initial temperature, or enthalpy,and emerges at a higher enthalpy. Water
7、 or other coolant entersthe control volume at an initial temperature and emerges at ahigher temperature. Across the arc, electrical energy is dissi-pated by virtue of the resistance and current in the arc itself. Aheat balance of the system requires that the energy gained bythe gas must be defined b
8、y the difference between the incomingenergy (electrical input) and total coolant and external losses.This is a direct application of the First Law of Thermodynam-ics and, for the particular control volume cited here, can bewritten as follows:Energy In 2 Energy Out 5 Energy to Gas (1)EI2 QCR2(i51nWH2
9、 OiCpT02 T1!H2Oi2(j51pMjHj5WgHg2 Hin!where:Cp= water, specific heat,E = plasma arc voltage,Hg= exhaust gas enthalpy,Hin= inlet gas enthalpy,Hj= heat of vaporization corresponding to the materialMj,I = plasma arc current,Mj= mass loss rate of electrode insulator, interior metalsurface, etc.QCR= energ
10、y convected and radiated from external sur-face of plasma generator,T0H2O= T02 T01= water temperature rise during plasmaarc operation,T1H2O= T2T1= water temperature rise before plasma arcoperation,T02= water exhaust temperature during plasma arcoperation,T01= inlet water temperature during plasma ar
11、coperation,T2= water exhaust temperature before plasma arcoperation,T1= inlet water temperature before plasma arcoperation,Wg= gas flow rate,WH2O= mass flow rate of coolant water, andEI= average of the product of voltage, E, and current, I.2.4 An examination of Eq 1 shows that, in order to obtain an
12、evaluation of the energy content of the plasma for a specifiedset of operating conditions, measurements must be made of thevoltage and current, the mass-flow rate and temperature rise ofthe coolant, the mass-flow rate and inlet ambient temperatureof the test gas, and the external surface temperature
13、 andhousing of the arc chamber. For all practical purposes, theexternal surface temperature of the water-cooled plasma arc is1This practice is under the jurisdiction of ASTM Committee E21 on SpaceSimulation and Applications of Space Technology and is the direct responsibility ofSubcommittee E21.08 o
14、n Thermal Protection.Current edition approved May 1, 2015. Published June 2015. Originallyapproved in 1968. Last previous edition approved in 2008 as E341 08. DOI:10.1520/E0341-08R15.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States1min
15、imum. Consequently, it will be assumed throughout thisdiscussion that negligible energy (compared to the inputenergy) is lost from the external plasma generator surface byconvective or radiative mechanisms and that the internal loss ofelectrode or plasma generator material is small compared withthe
16、energy input. In addition, as some plasma generators utilizemagnetic fields in their design, the magnetic field coil electricalpower and ohmic-heating dissipation should be included in theover-all heat balance. Precautions should be taken to assurethat only a negligible portion of magnetic energy is
17、 beingdissipated in hardware not within the heat balance circuit. Forthe purposes of this discussion, the magnetic field power inputand loss aspects have been omitted because of their uniqueapplicability to specific plasma generator designs.2.5 The energy balance is given by Eq 3 when these factorsa
18、re taken into account:EI2(i51nWH2OiCpT02 T1!H2Oi5 WgHg2 Hin! (2)The exhaust enthalpy, Hg, of the effluent as defined by Eq 1and 3 is a measure of the average total (stagnation) enthalpy atthe nozzle exit plane of the plasma-arc heater. This enthalpydoes not necessarily apply to the plasma downstream
19、 of thenozzle exit plane.3. Significance and Use3.1 The purpose of this practice is to measure the total orstagnation gas enthalpy of a plasma-arc gas stream in whichnonreactive gases are heated by passage through an electricaldischarge device during calibration tests of the system.3.2 The plasma ar
20、c represents one heat source for determin-ing the performance of high temperature materials undersimulated hyperthermal conditions. As such the total or stag-nation enthalpy is one of the important parameters for corre-lating the behavior of ablation materials.3.3 The most direct method for obtainin
21、g a measure of totalenthalpy, and one which can be performed simultaneously witheach material test, if desired, is to perform an energy balanceon the arc chamber. In addition, in making the energy balance,accurate measurements are needed since the efficiencies ofsome plasma generators are low (as lo
22、w as 15 to 20 % or lessin which case the enthalpy depends upon the difference of twoquantities of nearly equal magnitude). Therefore, the accuracyof the measurements of the primary variables must be high, allenergy losses must be correctly taken into account, andsteady-state conditions must exist bo
23、th in plasma performanceand fluid flow.3.4 In particular it is noted that total enthalpy as determinedby the energy balance technique is most useful if the plasmagenerator design minimizes coring effects. If nonuniformityexists the enthalpy determined by energy balance gives onlythe average for the
24、entire plasma stream, whereas the localenthalpy experienced by a model in the core of the stream maybe much higher. More precise methods are needed to measurelocal variations in total enthalpy.4. Apparatus4.1 GeneralThe apparatus shall consist of the plasma-arcfacility and the necessary instrumentat
25、ion to measure thepower input to the arc, gas stream and coolant flow rates, inletgas temperature and net coolant temperature rise of the plasmagenerator hardware. Although the recommended instrumenta-tion accuracies are state-of-the-art values, higher accuracyinstruments (than those recommended) ma
26、y be required forlow efficiency plasma generators.4.2 Input Energy MeasurementsThe energy input term,EI, to a large degree may be time dependent. Fluctuations inthe power input can produce errors as large as 50 % undercertain conditions. The magnitude of the error will depend onthe amplitude of the
27、unsteady compared with the steady portionof the current and voltage and also on the instantaneous phaserelationship between current and voltage. The power inputportion term should be written:FIG. 1 Schematic Energy Balance Method for Determining GasEnthalpyE341 08 (2015)2EI5 1/t *0tEI dt (3)As a con
28、sequence each plasma generator should make use ofoscilloscopic voltage-current traces during operation in orderto ascertain the time variation of the voltage-current input. Ifthese traces show significant unsteadiness it is recommendedthat additional methods of input power measurements bepursued, su
29、ch as an integrating device if available. In order tomeasure power directly, a wattmeter as cited by Dawes (1)2canbe employed. As a precaution in the use of the wattmeter,reversed readings of current and voltage should be taken andthe average of the two readings used. For those plasmagenerator facil
30、ities which operate under known and steadyinput power the use of a voltmeter and ammeter is recom-mended owing to their high degree of accuracy.4.2.1 Voltage MeasurementThe determination of powerinput to the plasma generator requires the measurement of thevoltage across the circuit. Suitable instrum
31、ents for such voltagemeasurements are presented by the Instrument Society ofAmerica (ISA) (2).The measurement techniques to be used canbe either a voltage divider network or a direct readinginstrument. It is highly desirable to be able to record thevoltage such that time variations are a part of the
32、 test data.Accuracy of the voltage measurements shall be within 61%.The voltage measurement shall be taken at the electrodeterminals of the plasma generator circuit.4.2.2 Current MeasurementThe measurement of plasmaarc current shall be accomplished with an ammeter equippedwith a precision shunt and
33、the reading shall be within 61%.Ref (2) lists other instruments suitable for measuring arccurrent. If a precision shunt is utilized, the temperature acrossthe shunt shall be constant and within the stated limits as givenby the manufacturer. Arc current shall be measured taking intoaccount any losses
34、 in the lead lines from the metering shunt tothe ammeter recorder. It is highly desirable to be able to recordthe plasma-arc current so that time variations are a part of thetest data.4.3 Coolant Energy Loss Measurements:4.3.1 Coolant Flow MeasurementThe discussion that fol-lows assumes that water i
35、s the coolant used in most plasmaarcs. The water flow rate to each water-cooled component ofthe plasma arc shall be measured. The error in measurementtechniques shall be not more than 62 %. Suitable equipmentthat can be used is listed in Ref (2) and includes turbineflowmeters, heat flowmeters, area
36、flowmeters, etc. Care mustbe exercised in the use of all of these devices. In particular, itis recommended that appropriate filters be placed in all waterinlet lines to prevent particles or unnecessary deposits frombeing carried to the water cooling passages, pipe and meterwalls. Water flow rates sh
37、all be properly adjusted in such a waythat bubbles are eliminated and that water vapor formation isnot present. If practical, the water flowmeters shall be placedupstream of the plasma generator in straight portions of thepiping. The flowmeter device shall be checked and calibratedperiodically.4.3.2
38、 Coolant Temperature MeasurementThe method oftemperature measurement must be sufficiently sensitive andreliable to ensure accurate measurement of the coolant watertemperature rise. Procedures similar to those given in theAnnual Book of ASTM Standards, Part 44, and Ref (3) shouldbe adhered to in the
39、calibration and preparation of temperaturesensors. The bulk or average temperature of the coolant shallbe measured at the inlet and output lines of each cooled unit.The error in measurement of temperature difference betweeninlet and outlet shall be not more than 61 %. The watertemperature-indicating
40、 devices shall be placed as close aspractical to the plasma arc in the inlet and outlet lines. Noadditional apparatus shall be between the temperature sensorand the plasma arc. The temperature measurements shall berecorded continuously. Ref (2) lists a variety of commerciallyavailable temperature se
41、nsors. During the course of operationof the plasma arc, care should be taken to minimize deposits onthe sensors and to eliminate any possibility of sensor heatingbecause of specimen radiation to the sensor. In addition, allwater lines should be shielded from direct radiation from thetest specimen.4.
42、3.3 Coolant Flow Pressure GagesIf apparatus such asflowmeters require the use of pressure gages, they shall be usedin accordance with the manufacturers instructions and calibra-tion charts that are furnished with the flowmeters.4.4 Gas Stream Measurements:4.4.1 GeneralInasmuch as single or multiple
43、gas inletsmay be used in the plasma arc the following pertains to eachgas supply system in use. In particular, for each gas supplysystem the measurements shall include inlet gas temperatureand gas flow rate.4.4.2 Gas Stream Flow RateThe flow rate of each gasentering the plasma generator shall be mea
44、sured and the errorshall be no greater than 64.0 %. Ref (2) gives suitablecommercially available gas flowmeters. For most applications,the orifice or rotameter will suffice. Pressure and temperaturemeasurements shall be made in accordance with manufactur-ers instructions and calibration charts that
45、are furnished withthe flowmeters. The flowmeter should be placed in that portionof the gas line where disturbances are at a minimum. In allcases the flowmeter device shall be checked and calibratedperiodically.4.4.3 Gas Stream Temperature MeasurementInasmuch asthe inlet gas temperature will not diff
46、er substantially from roomtemperature, it will usually have negligible effects on thecalculation of total enthalpy. However, if it is practical, athermal sensor such as those commercially available in Ref (2)shall be installed in an inlet section where the gas flow is freeof disturbances. Preferably
47、 the temperature of the inlet gasstream shall be measured in a settling or plenum chamberwhere the gas velocity has been minimized. The error inmeasurement shall not be greater than 61%.4.5 Recording Means:4.5.1 Since the energy balance technique requires that theplasma generator operate as a steady
48、-state device, all calcula-tions will use only measurements taken after it has beenestablished that the device has achieved steady operatinglevels. To assure steady flow or operating conditions the above2The boldface numbers in parentheses refer to the list of references appended tothis practice.E34
49、1 08 (2015)3mentioned parameters shall be continuously recorded such thatinstantaneous measurements are available to establish a mea-sure of steady-state operation. Wherever possible it is highlydesirable that separate measurements be made of the desiredparameters.4.5.2 In all cases, parameters of interests, such as arcvoltage, arc current, gas and water flow rates, and coolingwater temperature rises shall be automatically recordedthroughout the calibration period. Recording speed will dependon the variations of the parameters being recorded. Theresponse
