1、Designation: E2684 09E2684 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, th
2、e year of 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 net heat flux normal to a surface using flat gages mounted
3、 onto thesurface. Conduction heat flux is not the focus of this standard. Conduction applications related to insulation materials are coveredby Test Method C518 and Practices C1041 and C1046. The sensors covered by this test method all use a measurement of thetemperature difference between two paral
4、lel planes normal to the surface to determine the heat that is exchanged to or from thesurface in keeping with Fouriers Law. The gages operate by the same principles for heat transfer in either direction.1.2 This test method is quite broad in its field of application, size and construction. Differen
5、t sensor types are described in detailin later sections as examples of the general method for measuring heat flux from the temperature gradient normal to a surface (1).2Applications include both radiation and convection heat transfer. The gages have broad application from aerospace to biomedicalengi
6、neering with measurements ranging form 0.01 to 50 kW/m2. The gages are usually square or rectangular and vary in size from1 mm to 10 cm or more on a side. The thicknesses range from 0.05 to 3 mm.1.3 The values stated in SI units are to be regarded as the standard. The values stated in parentheses ar
7、e provided for informationonly.1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatorylimitations
8、 prior to use.1.5 This international standard was developed in accordance with internationally recognized principles on standardizationestablished in the Decision on Principles for the Development of International Standards, Guides and Recommendations issuedby the World Trade Organization Technical
9、Barriers to Trade (TBT) Committee.2. Referenced Documents2.1 ASTM Standards:C518 Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter ApparatusC1041 Practice for In-Situ Measurements of Heat Flux in Industrial Thermal Insulation Using Heat Flux TransducersC104
10、6 Practice for In-Situ Measurement of Heat Flux and Temperature 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 units of W/m2 (Btu/ft2-s).
11、Heat transfer (or alternatively heat-transferrate) is the rate of thermal-energy movement across a system boundary with units of watts (Btu/s). This usage is consistent withmost heat-transfer books.3.1.2 heat-transfer coeffcient, (h)an important parameter in convective flows with units of W/m2-K (Bt
12、u/ft2-s-F). This isdefined in terms of the heat flux q as:h 5 qT (1)where T is a prescribed temperature difference between the surface and the fluid. The resulting value of h is intended to be1 This test method is under the jurisdiction of ASTM Committee E21 on Space Simulation and Applications of S
13、pace Technology and is the direct responsibility ofSubcommittee E21.08 on Thermal Protection.Current edition approved June 15, 2009Sept. 1, 2017. Published August 2009October 2017. Originally approved in 2009. Last previous edition approved in 2009 asE268409. DOI: 10.1520/E2684-09.10.1520/E2684-17.2
14、 The boldface numbers in parentheses refer to the list of references at the end of this test method.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Becauseit may not be technicall
15、y possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the official document.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C7
16、00, West Conshohocken, PA 19428-2959. United States1only a function of the fluid flow and geometry, not the temperature difference. If the surface temperature is non-uniform or ifthere is more than a single fluid free stream temperature, the proper definition of T may be difficult to specify (2). It
17、 is al-ways important to clearly define T when calculating the heat-transfer coefficient.3.1.3 surface emissivity, ()the ratio of the emitted thermal radiation from a surface to that of a blackbody at the sametemperature. Surfaces are assumed to be gray bodies where the emissivity is equal to the ab
18、sorptivity.4. Summary of Test Method4.1 A schematic of the sensing technique is illustrated in Fig. 1. Temperature is measured on either side of a thermal resistancelayer of thickness, . This is the heat-flux sensing mechanism of this test method. The measured heat flux is in the same directionas th
19、e temperature difference and is proportional to the temperature gradient through the thermal-resistance layer (TRL). Theresistance layer is characterized by its thickness, , thermal conductivity, k, and thermal diffusivity, . The properties are generallya weak function of temperature.q 5k T12T2! (2)
20、From this point the different gages may vary substantially in how the temperature difference T1 T2 is measured and thethickness of the thermal resistance layer used. These aspects of each different type of sensor are discussed along with the im-plications for measurements.From this point the differe
21、nt gages may vary substantially in how the temperature difference T1 T2 is measured and thethickness of the thermal resistance layer used. These aspects of each different type of sensor are discussed along with theimplications for measurements.4.2 Heat-flux gages using this test method generally use
22、 either thermocouple elements or resistance-temperature elements tomeasure the required temperatures.4.2.1 Resistance temperature detectors (RTDs) generally have greater sensitivity to temperature than thermocouples, but requireseparate temperature measurements on each side of the thermal-resistance
23、 layer. The temperature difference must then becalculated as the small difference between two relatively large values of temperature.4.2.2 Thermocouples can be arranged in series across the thermal-resistance layer as differential thermocouple pairs thatmeasure the temperature difference directly. T
24、he pairs can also be put in series to form a differential thermopile to increase thesensitivity to heat flux.S 5Eq 5NTk (3)Here N represents the number of thermocouple pairs forming the differential thermopile and T is the effective temperaturesensitivity (Seebeck coefficient) of the two thermocoupl
25、e materials. Although the voltage output is directly proportional to theheat flux, the sensitivity may be a function of the gage temperature.Here N represents the number of thermocouple pairs forming the differential thermopile and T is the effective temperaturesensitivity (Seebeck coefficient) of t
26、he two thermocouple materials.Although the voltage output is directly proportional to the heatflux, the sensitivity may be a function of the gage temperature.5. Significance and Use5.1 This test method will provide guidance for the measurement of the net heat flux to or from a surface location. To d
27、eterminethe radiant energy component the emissivity or absorptivity of the gage surface coating is required and should be matched withthe surrounding surface. The potential physical and thermal disruptions of the surface due to the presence of the gage should beFIG. 1 Layered Heat-Flux GageE2684 172
28、minimized and characterized. For the case of convection and low source temperature radiation to or from the surface it is importantto consider how the presence of the gage alters the surface heat flux. The desired quantity is usually the heat flux at the surfacelocation without the presence of the g
29、age.5.1.1 Temperature limitations are determined by the gage material properties and the method of application to the surface. Therange of heat flux that can be measured and the time response are limited by the gage design and construction details.Measurements from 10 W/m2 to above 100 kW/m2 are eas
30、ily obtained with current sensors. Time constants as low as 10 ms arepossible, while thicker sensors may have response times greater than 1 s. It is important to choose the sensor style andcharacteristics to match the range and time response of the required application.5.2 The measured heat flux is
31、based on one-dimensional analysis with a uniform heat flux over the surface of the gage surface.Because of the thermal disruption caused by the placement of the gage on the surface, this may not be true. Wesley (3) and Babaet al. (4) have analyzed the effect of the gage on the thermal field and heat
32、 transfer within the surface substrate and determinedthat the one-dimensional assumption is valid when:kRks1 (4)where:ks = the thermal conductivity of the substrate material,R = the effective radius of the gage, = the combined thickness, andk = the effective thermal conductivity of the gage and adhe
33、sive layers.5.3 Measurements of convective heat flux are particularly sensitive to disturbances of the temperature of the surface. Becausethe heat transfer coefficient is also affected by any non-uniformities of the surface temperature, the effect of a small temperaturechange with location is furthe
34、r amplified, as explained by Moffat et al. (2) and Diller (5). Moreover, the smaller the gage surfacearea, the larger is the effect on the heat-transfer coefficient of any surface temperature non-uniformity. Therefore, surfacetemperature disruptions caused by the gage should be kept much smaller tha
35、n the surface to environment temperature differencecausing the heat flux. This necessitates a good thermal path between the gage and the surface onto which it is mounted.5.3.1 Fig. 2 shows a heat-flux gage mounted onto a plate with the surface temperature of the gage of Ts and the surfacetemperature
36、 of the surrounding plate of Tp. The goal is to keep the gage surface temperature as close as possbiblepossible to theplate temperature to minimize the thermal disruption of the gage. This requires the thermal resistance of the gage and adhesiveto be minimized along the thermal pathway from Ts and T
37、p.5.3.2 Another method to avoid the surface temperature disruption problem is to cover the entire surface with the heat-flux gagematerial. This effectively ensures that the thermal resistance through the gage is matched with that of the surrounding plate. It isimportant to have independent measures
38、of the substrate surface temperature and the surface temperature of the gage. The gagesurface temperature must be used for defining the value of the heat-transfer coefficient. When the gage material does not cover theentire surface, the temperature measurements are needed to ensure that the gage doe
39、s indeed provide a small thermal disruption.5.4 The time response of the heat-flux gage can be estimated analytically if the thermal properties of the thermal-resistance layerare well known. The time required for 98 % response to a step input (6) based on a one-dimensional analysis is:t 51.5 2 (5)Th
40、e time response of the heat-flux gage can be estimated analytically if the thermal properties of the thermal-resistance layerare well known. The time required for 98 % response to a step input (6) based on a one-dimensional analysis is:t 51.5 2 (5)where is the thermal diffusivity of the TRL. Coverin
41、g or encapsulation layers must also be included in the analysis. Uncer-tainties in the gage dimensions and properties require a direct experimental verification of the time response. If the gage isFIG. 2 Diagram of an Installed Surface-Mounted Heat-Flux GageE2684 173designed to absorb radiation, a p
42、ulsed laser or optically switched Bragg cell can be used to give rise times of less than 1 s(7,8). However, a mechanical wheel with slits can be used with a light to give rise times on the order of 1 ms (9), which isgenerally sufficient.5.4.1 Because the response of these sensors is close to an expo
43、nential rise, a measure of the time constant for the sensor canbe obtained by matching the experimental response to step changes in heat flux with exponential curves.q 5qss 12e2t/! (6)The value of the step change in imposed heat flux is represented by qss. The resulting time constant characterizes t
44、he first-order sensor response.6. Apparatus-Sensor Construction6.1 Temperature sensors are mounted or deposited on either side of the thermal-resistance layer (TRL), which is usually a thinmaterial which can be mounted on the test object. The method of construction and details of operation varies fo
45、r each differenttype of gage. Although most of the gages place the temperature sensors directly over top of each other across the TRL, it is nota requirement for proper measurement. The bottom temperature sensors simply need to be at a uniform temperature and the toptemperature sensors need to be at
46、 a temperature dictated by the heat flux perpendicular to the surface. This can be accomplishedon a high-conductivity substrate by separate thermal-resistance pads for the top temperature measurements. Several examples aregiven of the thermopile and RTD based types of gages.6.2 Thermopile GagesTherm
47、opile gages are based on thermocouples forming multiple junctions on either side of the TRL.If properly mounted and designed for the application, the operation of these heat-flux gages is simple.There is no activation currentor energy required for the thermocouple sensor units. The output voltage is
48、 continuously generated by the gage in proportion tothe number of thermocouple pairs wired in series. The output can be directly connected to an appropriate differential amplifier andvoltage readout device.6.2.1 An early report of the layered sensor (6) used a single thermocouple pair across the res
49、istance layer. Ortolano and Hines(10) used a number of thermocouple pairs as described by Eq 3 to give a larger voltage output. The thermocouples are placed asfoils around a Kaptonpolyimide thermal-resistance layer and butt welded on either side, as illustrated in Fig. 3. KaptonPolyimidesheets are used around the gage for encasement and protection. The resulting Micro-Foil gage3 is 75 to 400 m thick and flexiblefor easy attachment to surfaces, but the low conductivity (high thermal resistance) of the materials must be considered when usedfor c
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