ASTM E2684-17 Standard Test Method for Measuring Heat Flux Using Surface-Mounted One-Dimensional Flat Gages.pdf

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1、Designation: E2684 17Standard Test Method forMeasuring Heat Flux Using Surface-Mounted One-Dimensional Flat Gages1This standard is issued under the fixed designation E2684; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the year o

2、f last 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 test method describes the measurement of the netheat flux normal to a surface using flat gages mounted onto the

3、surface. Conduction heat flux is not the focus of this standard.Conduction applications related to insulation materials arecovered by Test Method C518 and Practices C1041 and C1046.The sensors covered by this test method all use a measurementof the temperature difference between two parallel planesn

4、ormal to the surface to determine the heat that is exchanged toor from the surface in keeping with Fouriers Law. The gagesoperate by the same principles for heat transfer in eitherdirection.1.2 This test method is quite broad in its field of application,size and construction. Different sensor types

5、are described indetail in later sections as examples of the general method formeasuring heat flux from the temperature gradient normal to asurface (1).2Applications include both radiation and convec-tion heat transfer. The gages have broad application fromaerospace to biomedical engineering with mea

6、surements rang-ing form 0.01 to 50 kW/m2. The gages are usually square orrectangular and vary in size from 1 mm to 10 cm or more ona side. The thicknesses range from 0.05 to 3 mm.1.3 The values stated in SI units are to be regarded as thestandard. The values stated in parentheses are provided forinf

7、ormation only.1.4 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility 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.1

8、.5 This international standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to

9、Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:C518 Test Method for Steady-State Thermal TransmissionProperties by Means of the Heat Flow Meter ApparatusC1041 Practice for In-Situ Measurements of Heat Flux inIndustrial Thermal Insulation Using Heat Flux Transduc-ersC1046 Practice fo

10、r In-Situ Measurement of Heat Flux andTemperature on Building Envelope ComponentsC1130 Practice for Calibrating Thin Heat Flux Transducers3. Terminology3.1 Definitions of Terms Specific to This Standard:3.1.1 heat fluxthe heat transfer per unit area, q, with unitsof W/m2(Btu/ft2-s). Heat transfer (o

11、r alternatively heat-transfer rate) is the rate of thermal-energy movement across asystem boundary with units of watts (Btu/s). This usage isconsistent with most heat-transfer books.3.1.2 heat-transfer coeffcient, (h)an important parameterin convective flows with units of W/m2-K (Btu/ft2-s-F). This

12、isdefined in terms of the heat flux q as:h 5qT(1)where T is a prescribed temperature difference between thesurface and the fluid. The resulting value of h is intended tobe only a function of the fluid flow and geometry, not thetemperature difference. If the surface temperature is non-uniform or if t

13、here is more than a single fluid free streamtemperature, the proper definition of T may be difficult tospecify (2). It is always important to clearly define T whencalculating the heat-transfer coefficient.3.1.3 surface emissivity, ()the ratio of the emitted thermalradiation from a surface to that of

14、 a blackbody at the sametemperature. Surfaces are assumed to be gray bodies where theemissivity is equal to the absorptivity.4. Summary of Test Method4.1 A schematic of the sensing technique is illustrated inFig. 1. Temperature is measured on either side of a thermal1This test method is under the ju

15、risdiction of ASTM Committee E21 on SpaceSimulation and Applications of Space Technology and is the direct responsibility ofSubcommittee E21.08 on Thermal Protection.Current edition approved Sept. 1, 2017. Published October 2017. Originallyapproved in 2009. Last previous edition approved in 2009 as

16、E268409. DOI:10.1520/E2684-17.2The boldface numbers in parentheses refer to the list of references at the end ofthis test method.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accordance wi

17、th internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.1resistance layer of thickness, . This is

18、 the heat-flux sensingmechanism of this test method. The measured heat flux is in thesame direction as the temperature difference and is propor-tional to the temperature gradient through the thermal-resistance layer (TRL). The resistance layer is characterized byits thickness, , thermal conductivity

19、, k, and thermal diffusivity,. The properties are generally a weak function of temperature.q 5kT12 T2! (2)From this point the different gages may vary substantially inhow the temperature difference T1 T2is measured and thethickness of the thermal resistance layer used. These aspects ofeach different

20、 type of sensor are discussed along with theimplications for measurements.4.2 Heat-flux gages using this test method generally useeither thermocouple elements or resistance-temperature ele-ments to measure the required temperatures.4.2.1 Resistance temperature detectors (RTDs) generallyhave greater

21、sensitivity to temperature than thermocouples, butrequire separate temperature measurements on each side of thethermal-resistance layer. The temperature difference must thenbe calculated as the small difference between two relativelylarge values of temperature.4.2.2 Thermocouples can be arranged in

22、series across thethermal-resistance layer as differential thermocouple pairs thatmeasure the temperature difference directly. The pairs can alsobe put in series to form a differential thermopile to increase thesensitivity to heat flux.S 5Eq5NTk(3)Here N represents the number of thermocouple pairs fo

23、rmingthe differential thermopile and Tis the effective temperaturesensitivity (Seebeck coefficient) of the two thermocouplematerials. Although the voltage output is directly proportionalto the heat flux, the sensitivity may be a function of the gagetemperature.5. Significance and Use5.1 This test me

24、thod will provide guidance for the measure-ment of the net heat flux to or from a surface location. Todetermine the radiant energy component the emissivity orabsorptivity of the gage surface coating is required and shouldbe matched with the surrounding surface. The potential physi-cal and thermal di

25、sruptions of the surface due to the presenceof the gage should be minimized and characterized. For thecase of convection and low source temperature radiation to orfrom the surface it is important to consider how the presence ofthe gage alters the surface heat flux. The desired quantity isusually the

26、 heat flux at the surface location without thepresence of the gage.5.1.1 Temperature limitations are determined by the gagematerial properties and the method of application to thesurface. The range of heat flux that can be measured and thetime response are limited by the gage design and construction

27、details. Measurements from 10 W/m2to above 100 kW/m2areeasily obtained with current sensors. Time constants as low as10 ms are possible, while thicker sensors may have responsetimes greater than 1 s. It is important to choose the sensor styleand characteristics to match the range and time response o

28、f therequired application.5.2 The measured heat flux is based on one-dimensionalanalysis with a uniform heat flux over the surface of the gagesurface. Because of the thermal disruption caused by theplacement of the gage on the surface, this may not be true.Wesley (3) and Baba et al. (4) have analyze

29、d the effect of thegage on the thermal field and heat transfer within the surfacesubstrate and determined that the one-dimensional assumptionis valid when:kRks.1 (4)where:ks= the thermal conductivity of the substrate material,R = the effective radius of the gage, = the combined thickness, andk = the

30、 effective thermal conductivity of the gage andadhesive layers.5.3 Measurements of convective heat flux are particularlysensitive to disturbances of the temperature of the surface.Because the heat transfer coefficient is also affected by anynon-uniformities of the surface temperature, the effect of

31、asmall temperature change with location is further amplified, asexplained by Moffat et al. (2) and Diller (5). Moreover, thesmaller the gage surface area, the larger is the effect on theheat-transfer coefficient of any surface temperature non-uniformity. Therefore, surface temperature disruptions ca

32、usedby the gage should be kept much smaller than the surface toFIG. 1 Layered Heat-Flux GageE2684 172environment temperature difference causing the heat flux. Thisnecessitates a good thermal path between the gage and thesurface onto which it is mounted.5.3.1 Fig. 2 shows a heat-flux gage mounted ont

33、o a platewith the surface temperature of the gage of Tsand the surfacetemperature of the surrounding plate of Tp. The goal is to keepthe gage surface temperature as close as possible to the platetemperature to minimize the thermal disruption of the gage.This requires the thermal resistance of the ga

34、ge and adhesive tobe minimized along the thermal pathway from Tsand Tp.5.3.2 Another method to avoid the surface temperaturedisruption problem is to cover the entire surface with theheat-flux gage material. This effectively ensures that thethermal resistance through the gage is matched with that of

35、thesurrounding plate. It is important to have independent mea-sures of the substrate surface temperature and the surfacetemperature of the gage. The gage surface temperature must beused for defining the value of the heat-transfer coefficient.When the gage material does not cover the entire surface,

36、thetemperature measurements are needed to ensure that the gagedoes indeed provide a small thermal disruption.5.4 The time response of the heat-flux gage can be estimatedanalytically if the thermal properties of the thermal-resistancelayer are well known. The time required for 98 % response toa step

37、input (6) based on a one-dimensional analysis is:t 51.5 2(5)where is the thermal diffusivity of the TRL. Covering orencapsulation layers must also be included in the analysis.Uncertainties in the gage dimensions and properties require adirect experimental verification of the time response. If thegag

38、e is designed to absorb radiation, a pulsed laser or opticallyswitched Bragg cell can be used to give rise times of less than1s(7,8). However, a mechanical wheel with slits can be usedwith a light to give rise times on the order of 1 ms (9), whichis generally sufficient.5.4.1 Because the response of

39、 these sensors is close to anexponential rise, a measure of the time constant for the sensorcan be obtained by matching the experimental response to stepchanges in heat flux with exponential curves.q 5 qss1 2 e2t/! (6)The value of the step change in imposed heat flux is repre-sented by qss. The resu

40、lting time constant characterizes thefirst-order sensor response.6. Apparatus-Sensor Construction6.1 Temperature sensors are mounted or deposited on eitherside of the thermal-resistance layer (TRL), which is usually athin material which can be mounted on the test object. Themethod of construction an

41、d details of operation varies for eachdifferent type of gage. Although most of the gages place thetemperature sensors directly over top of each other across theTRL, it is not a requirement for proper measurement. Thebottom temperature sensors simply need to be at a uniformtemperature and the top tem

42、perature sensors need to be at atemperature dictated by the heat flux perpendicular to thesurface. This can be accomplished on a high-conductivitysubstrate by separate thermal-resistance pads for the toptemperature measurements. Several examples are given of thethermopile and RTD based types of gage

43、s.6.2 Thermopile GagesThermopile gages are based onthermocouples forming multiple junctions on either side of theTRL. If properly mounted and designed for the application, theoperation of these heat-flux gages is simple. There is noactivation current or energy required for the thermocouplesensor uni

44、ts. The output voltage is continuously generated bythe gage in proportion to the number of thermocouple pairswired in series. The output can be directly connected to anappropriate differential amplifier and voltage readout device.6.2.1 An early report of the layered sensor (6) used a singlethermocou

45、ple pair across the resistance layer. Ortolano andHines (10) used a number of thermocouple pairs as describedby Eq 3 to give a larger voltage output. The thermocouples areplaced as foils around a polyimide thermal-resistance layer andbutt welded on either side, as illustrated in Fig. 3. Polyimideshe

46、ets are used around the gage for encasement and protection.The resulting Micro-Foil gage3is 75 to 400 m thick andflexible for easy attachment to surfaces, but the low conduc-tivity (high thermal resistance) of the materials must beconsidered when used for convection measurements. Thesensors are limi

47、ted to temperatures below (250 C) and heatfluxes less than 100 kW/m2. The time response can be as fastas 20 ms, but transient signals may be attenuated unless thefrequency of the disturbance is less than a few hertz.6.2.2 The gSKIN heat flux sensor by greenTEG4is athermopile made by depositing bismu

48、th telluride semiconduc-tor materials. These thermocouples give a particularly high3The sole source of supply of the apparatus known to the committee at this timeis RdF Corporation. If you are aware of alternative suppliers, please provide thisinformation to ASTM International Headquarters. Your com

49、ments will receivecareful consideration at a meeting of the responsible technical committee,1whichyou may attend.4The sole source of supply of the apparatus known to the committee at this timeis greenTEG. If you are aware of alternative suppliers, please provide thisinformation to ASTM International Headquarters. Your comments will receivecareful consideration at a meeting of the responsible technical committee,1whichyou may attend.FIG. 2 Diagram of an Installed Surface-Mounted Heat-Flux GageE2684 173thermoelectric output. The sensors