1、Designation: E 341 96 (Reapproved 2002)Standard Practice forMeasuring Plasma Arc Gas Enthalpy by Energy Balance1This standard is issued under the fixed designation E 341; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year of
2、last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon (e) 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
3、system 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 there
4、sponsibility 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
5、 thefollowing 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
6、 energy 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. Wa
7、ter 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 define
8、d by 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(i 5
9、1nWH2 OiCpDT02DT1!H2Oi2(j 5 1pMjHj5 Energy to gaswhere:Cp= water, specific heat,E = plasma arc voltage,Hg= exhaust gas enthalpy,Hin= inlet gas enthalpy,Hj= heat of vaporization corresponding to the ma-terial Mj,I = plasma arc current,Mj= mass loss rate of electrode insulator, interiormetal surface,
10、etc.QCR= energy convected and radiated from externalsurface of plasma generator,DT0H2O= T02 T01= water temperature rise duringplasma arc operation,DT1H2O= T2T1= water temperature rise before plasmaarc operation,T02= water exhaust temperature during plasma arcoperation,T01= inlet water temperature du
11、ring plasma arc op-eration,T2= water exhaust temperature before plasma arcoperation,T1= inlet water temperature before plasma arc op-eration,Wg= gas flow rate,WH2O= mass flow rate of coolant water, andEI= average of the product of voltage, E, andcurrent, I.2.4 An examination of Eq 1 shows that, in o
12、rder to obtain anevaluation 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 su
13、rface temperature andhousing of the arc chamber. For all practical purposes, theexternal surface temperature of the water-cooled plasma arc isminimum. Consequently, it will be assumed throughout thisdiscussion that negligible energy (compared to the inputenergy) is lost from the external plasma gene
14、rator surface byconvective or radiative mechanisms and that the internal loss ofelectrode or plasma generator material is small compared with1This practice is under the jurisdiction of ASTM Committee E21 on SpaceSimulation and Applications of Space Technology and is the direct responsibility ofSubco
15、mmittee E21.08 on Thermal Protection.Current edition approved Oct. 10, 1996. Published December 1996. Originallypublished as E 341 68 T. Last previous edition E 341 81 (1992).1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.the energy
16、 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 being
17、dissipated 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 2 when these factorsare tak
18、en into account:EI2(i 5 1nWH2OiCpDT02DT1!H2Oi(2)The exhaust enthalpy, Hg, of the effluent as defined by Eq 1and 2 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 of thenozzle ex
19、it 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 arc represents one
20、 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 obtaining a measure of t
21、otalenthalpy, 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 low as 15 to 20 %
22、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 both in plasma per
23、formanceand 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 affects. If nonuniformityexists the enthalpy determined by energy balance gives onlythe average for the entire plasma st
24、ream, 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 instrumentation to measure t
25、hepower 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) may be required fo
26、rlow 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 unsteady compare
27、d 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:EI5 1/t*0tEI dt (3)As a consequence each plasma generator should make use ofoscilloscopic voltage-current traces during opera
28、tion 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 beFIG. 1 Schematic Energy Balance Method for Determining GasEnthalpyE 3412pursued, such as an integrating devic
29、e 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 facilities which operate under
30、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 instruments for such voltagemeasu
31、rements 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 test data.Accuracy of th
32、e 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 the reading shall be with
33、in 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 in the lead lines from t
34、he 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 is the coolant used in mos
35、t 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 flowmeters, etc. Care mus
36、tbe 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 shall be properly adjusted
37、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 Coolant Temperature Meas
38、urementThe 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 calibration and preparati
39、on 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 devices shall be placed
40、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 sensors. During the course
41、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.4.3.3 Coolant Flow Pressure
42、 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 gas inletsmay be used in
43、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 measured and the errorshall
44、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 are furnished withthe flo
45、wmeters. 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 differ substantially from roo
46、mtemperature, 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 the temperature of the i
47、nlet 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-state device, all calcul
48、a-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 abovementioned parameters shall be continuously recorded such thatinstantaneous measurements are available to establish a mea-s
49、ure 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 cooling2The boldface numbers in parentheses refer to the list of references appended tothis practice.E 3413water temperature rises shall be automatically recordedthroughout the calibration period. Recording speed will dependon the variations of the parameters being recorded. Theresponse time of the recorder shall be1sorless for f
