ASTM E2683-09
(Test Method)Standard Test Method for Measuring Heat Flux Using Flush-Mounted Insert Temperature-Gradient Gages
Standard Test Method for Measuring Heat Flux Using Flush-Mounted Insert Temperature-Gradient Gages
SIGNIFICANCE AND USE
The purpose of this test method is to measure the net heat flux to or from a surface location. For measurement of the radiant energy component the emissivity or absorptivity of the surface coating of the gage is required. When measuring the convective energy component the potential physical and thermal disruptions of the surface must be minimized and characterized. Requisite is to consider how the presence of the gage alters the surface heat flux. The desired quantity is usually the heat flux at the surface location without the presence of the gage.
Temperature limitations are determined by the gage material properties, the method of mounting the sensing element, and how the lead wires are attached. The range of heat flux that can be measured and the time response are limited by the gage design and construction details. Measurements of a fraction of 1 kW/m2 to above 10 MW/m2 are easily obtained with current gages. With thin film sensors a time response of less than 10 μs is possible, while thicker sensors may have response times on the order of 1 s. It is important to choose the gage style and characteristics to match the range and time response of the required application.
When differential thermocouple sensors are operated as specified for one-dimensional heat flux and within the corresponding time response limitations, the voltage output is directly proportional to the heat flux. The sensitivity, however, may be a function of the gage temperature.
The measured heat flux is based on one-dimensional analysis with a uniform heat flux over the surface of the gage. Measurements of convective heat flux are particularly sensitive to disturbances of the temperature of the surface. Because the heat-transfer coefficient is also affected by any non-uniformities in the surface temperature, the effect of a small temperature change with location is further amplified as explained by Moffat et al. (2) and Diller (3). Moreover, the smaller the gage surface area, the larger...
SCOPE
1.1 This test method describes the measurement of the net heat flux normal to a surface using gages inserted flush with the surface. The geometry is the same as heat-flux gages covered by Test Method E 511, but the measurement principle is different. The gages covered by this standard all use a measurement of the temperature gradient normal to the surface to determine the heat that is exchanged to or from the surface. Although in a majority of cases the net heat flux is to the surface, the gages operate by the same principles for heat transfer in either direction.
1.2 This general test method is quite broad in its field of application, size and construction. Two different gage types that are commercially available are described in detail in later sections as examples. A summary of common heat-flux gages is given by Diller (1). Applications include both radiation and convection heat transfer. The gages used for aerospace applications are generally small (0.155 to 1.27 cm diameter), have a fast time response (10 μs to 1 s), and are used to measure heat flux levels in the range 0.1 to 10 000 kW/m2. Industrial applications are sometimes satisfied with physically larger gages.
1.3 The values stated in SI units are to be regarded as the standard. The values stated in parentheses are provided for information only.
1.4 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: E2683 − 09
Standard Test Method for
Measuring Heat Flux Using Flush-Mounted Insert
Temperature-Gradient Gages
This standard is issued under the fixed designation E2683; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope 2. Referenced Documents
1.1 This test method describes the measurement of the net 2.1 ASTM Standard:
E511TestMethodforMeasuringHeatFluxUsingaCopper-
heatfluxnormaltoasurfaceusinggagesinsertedflushwiththe
surface. The geometry is the same as heat-flux gages covered Constantan Circular Foil, Heat-Flux Transducer
by Test Method E511, but the measurement principle is
3. Terminology
different. The gages covered by this standard all use a
3.1 Definitions of Terms Specific to This Standard:
measurementofthetemperaturegradientnormaltothesurface
3.1.1 heat flux—the heat transfer per unit area, q, with units
to determine the heat that is exchanged to or from the surface.
2 2
ofW/m (Btu/ft -s). Heat transfer (or alternatively heat transfer
Although in a majority of cases the net heat flux is to the
rate) is the rate of thermal energy movement across a system
surface, the gages operate by the same principles for heat
boundary with units of watts (Btu/s). This usage is consistent
transfer in either direction.
with most heat transfer books.
1.2 This general test method is quite broad in its field of
3.1.2 heat transfer coeffıcient, (h)—an important parameter
application, size and construction. Two different gage types
2 2
inconvectiveflowswithunitsofW/m -K(Btu/ft -s-F).Thisis
that are commercially available are described in detail in later
defined in terms of the heat flux q as
sections as examples. A summary of common heat-flux gages
is given by Diller (1). Applications include both radiation and
q
h 5 (1)
convection heat transfer. The gages used for aerospace appli-
∆T
cations are generally small (0.155 to 1.27 cm diameter), have where ∆T is a prescribed temperature difference between the
surface and the fluid. The resulting value of h is intended to
afasttimeresponse(10µsto1s),andareusedtomeasureheat
be only a function of the fluid flow and geometry, not the
flux levels in the range 0.1 to 10 000 kW/m . Industrial
temperature difference. If the surface temperature is non-
applications are sometimes satisfied with physically larger
uniform or if there is more than a single fluid free stream
gages.
temperature, the proper definition of ∆ T may be difficult to
1.3 The values stated in SI units are to be regarded as the specify (2). It is always important to clearly define ∆T when
standard. The values stated in parentheses are provided for calculating the heat transfer coefficient.
information only.
3.1.3 surface emissivity, (ε)— the ratio of the emitted
1.4 This standard does not purport to address all of the thermal radiation from a surface to that of a blackbody at the
safety concerns, if any, associated with its use. It is the same temperature. Surfaces are assumed to be gray bodies
responsibility of the user of this standard to establish appro- where the emissivity is equal to the absorptivity.
priate safety and health practices and determine the applica-
4. Summary of Test Method
bility of regulatory limitations prior to use.
4.1 A schematic of the sensing technique is illustrated in
Fig. 1. Temperature difference is measured across a thermal-
This test method is under the jurisdiction of ASTM Committee E21 on Space
resistance layer of thickness, δ. This is the heat flux sensing
Simulation andApplications of SpaceTechnology and is the direct responsibility of
Subcommittee E21.08 on Thermal Protection.
Current edition approved June 15, 2009. Published August 2009. DOI: 10.1520/ For referenced ASTM standards, visit the ASTM website, www.astm.org, or
E2683-09. contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
Theboldfacenumbersinparenthesesrefertothelistofreferencesattheendof Standards volume information, refer to the standard’s Document Summary page on
this test method. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
E2683 − 09
FIG. 1 Layered Heat-Flux Gage
mechanism of this method following Fourier’s law. The mea- with current gages. With thin film sensors a time response of
sured heat flux is in the same direction as the temperature less than 10 µs is possible, while thicker sensors may have
difference and is proportional to the temperature gradient responsetimesontheorderof1s.Itisimportanttochoosethe
through the thermal-resistance layer (TRL). The resistance gage style and characteristics to match the range and time
layer is characterized by its thickness, δ, thermal conductivity, response of the required application.
k, and thermal diffusivity, α. The properties are generally a 5.1.2 When differential thermocouple sensors are operated
weak function of temperature. as specified for one-dimensional heat flux and within the
corresponding time response limitations, the voltage output is
k
q 5 T 2 T (2)
~ ! directly proportional to the heat flux. The sensitivity, however,
1 2
δ
may be a function of the gage temperature.
From this point the different gages may vary in how the
temperature difference T − T is measured, the thickness of
1 2
5.2 The measured heat flux is based on one-dimensional
the thermal-resistance layer used, and how the sensing ele-
analysis with a uniform heat flux over the surface of the gage.
ment is mounted in the gage. These three aspects of each
Measurementsofconvectiveheatfluxareparticularlysensitive
different type of gage are discussed along with the implica-
to disturbances of the temperature of the surface. Because the
tions for measurements. In all of the cases considered in this
heat-transfer coefficient is also affected by any non-
standard the gage housing is a circular cylinder that is in-
serted into a hole in the material of the test object flush with uniformities in the surface temperature, the effect of a small
the surface.
temperature change with location is further amplified as
explained by Moffat et al. (2) and Diller (3). Moreover, the
4.2 Gages using this test method generally use differential
smallerthegagesurfacearea,thelargeristheeffectontheheat
thermocouple pairs that give an output that is directly propor-
transfer coefficient of any surface temperature non-uniformity.
tional to the required temperature difference. The differential
Therefore, surface temperature disruptions caused by the gage
thermocouple pairs are put in series to form a differential
should be kept much smaller than the surface to environment
thermopile to increase the sensitivity to heat flux.
temperaturedifferencedrivingtheheatflux.Thisnecessitatesa
E Nσ δ
T
good thermal path between the sensor and the surface into
S 5 5 (3)
q k
which it is mounted. If the gage is not water cooled, a good
Here N represents the number of thermocouple pairs forming
thermal pathway to the system’s heat sink is important. The
the differential thermopile and σ is the effective temperature
T
gage should have an effective thermal conductivity as great or
sensitivity (Seebeck coefficient) of the two thermocouple
materials. greater than the surrounding material. It should also have good
physical contact insured by a tight fit in the hole and a method
5. Significance and Use
to tighten the gage into the surface. An example method used
5.1 The purpose of this test method is to measure the net to tighten the gage to the surface material is illustrated in Fig.
heatfluxtoorfromasurfacelocation.Formeasurementofthe
2. The gage housing has a flange and a separate tightening nut
radiant energy component the emissivity or absorptivity of the tapped into the surface material.
surface coating of the gage is required. When measuring the 5.2.1 Ifthegageiswatercooled,thethermalpathwaytothe
convective energy component the potential physical and ther- plate is less important. The heat transfer to the gage enters the
mal disruptions of the surface must be minimized and charac- water as the heat sink instead of the surrounding plate.
terized. Requisite is to consider how the presence of the gage Consequently, the thermal resistance between the gage and
alters the surface heat flux. The desired quantity is usually the plate may even be increased to discourage heat transfer from
heat flux at the surface location without the presence of the the plate to the cooling water. Unfortunately, this may also
gage. increasethethermalmismatchbetweenthegageandsurround-
5.1.1 Temperature limitations are determined by the gage ing surface.
material properties, the method of mounting the sensing 5.2.2 Fig. 2 shows a heat flux gage mounted into a plate
element,andhowtheleadwiresareattached.Therangeofheat with the surface temperature of the gage of T and the surface
s
flux that can be measured and the time response are limited by temperature of the surrounding plate of Tp. As previously
the gage design and construction details. Measurements of a discussed, a difference in temperature between the gage and
2 2
fraction of 1 kW/m to above 10 MW/m are easily obtained plate may also increase the local heat transfer coefficient over
E2683 − 09
FIG. 2 Diagram of an Installed Insert Heat-Flux Gage
the gage. This amplifies the measurement error. Consequently, 6. Apparatus-Sensor Constructions
a well designed heat flux gage will keep the temperature
6.1 While the principle of operation is similar, the method
difference between the gage surface and the plate to a
ofconstructionanddetailsofoperationvariesforeachdifferent
minimum, particularly if any convection is being measured.
type of gage. The two popular commercially available types
5.2.3 Under transient or unsteady heat transfer conditions a
are described in detail below.
different thermal capacitance of the gage than the surrounding
6.2 Thin-film Sensors—The thermal resistance and thermo-
material may also cause a temperature difference that affects
couple layers can all be deposited directly onto a substrate to
the measured heat flux. Independent measures of the substrate
give more design and manufacturing flexibility. Such a thin-
and the gage surface temperatures are advantageous for defin-
film device has been described in detail by Diller and Onishi
ing the heat transfer coefficient and ensuring that the gage
(8) and was first produced by Hager et al. (5) using sputtering
thermal disruption is acceptably small.
techniques. The thermal resistance layer of 1 µm silicon
5.3 The heat flux gages described here may also be water
monoxide is deposited directly onto the surface. Microfabrica-
cooledtoincreasetheirsurvivabilitywhenintroducedintohigh
tion methods are used to deposit hundreds of thermocouple
temperature environments. By limiting the rise in gage
pairs around the silicon monoxide layer to create the desired
temperature, however, a large disruption of the measured heat
differential thermopile as specified for Eq 3. Because of the
flux may result, particularly if convection is present. For
thin-films used, it has been named the Heat Flux Microsensor
convection measurements to match the heat flux experienced
(HFM). Either photolithography or stencil masks can be used
by the surrounding surface, the gage temperature must match
to define the patterns. Precise registration of the elements in
the temperature of that surface. This will usually require the
each of the five layers allows a fine pattern to be created in a
surrounding surface to also be water cooled.
small surface area.Across-section of the gage, which does not
5.4 The time response of the heat flux sensor can be
need an adhesive layer, is illustrated in Fig. 3. The resulting
estimated analytically if the thermal properties of the thermal
physical and thermal disruption of the surface due to the
resistance layer are well known. The time required for 98 %
presence of the sensor is extremely small because of the low
response to a step input (4) based on a one-dimensional
sensor mass.
analysis is:
6.2.1 While the original version of these sensors placed the
temperature sensors almost directly over top of each other
1.5δ
t 5 (4)
across a single TRL, it is not a requirement. The bottom
α
where α is the thermal diffusivity of the TRL. Covering or
temperaturesensorssimplyneedtobeatauniformtemperature
encapsulation layers must also be included in the analysis.
and the top temperature sensors need to be at a temperature
The calibrated gage sensitivity in Eq 3 applies only under
dictated by the heat flux perpendicular to the surface. This can
steady-state conditions.
be accomplished on a high conductivity substrate by separate
thermal resistance pads for the top temperature measurements.
5.4.1 For thin-film sensors theTRLmaterial properties may
The pattern is illustrated in Fig. 4 (7).The bottom temperature
be much different from those of bulk materials. Therefore, a
sensors can be placed directly on the substrate with or without
direct experimental verification of the time response is desir-
thermal resistance pads on top. If the thermal resistance of the
able. If the gage is designed to absorb radiation, a pulsed laser
pads is large relative to the lateral thermal resistance in the
or optically switched Bragg cell can be used to give rise times
substrate between individual temperature sensors, the pads on
of less than 1 µs (5,6).Arise time on the order of 5 µs can be
the lower thermocouple junctions are redundant and not
provided in a convective flow with a shock tunnel (7).
necessary. For the Heat Flux Microsensor this is accomplished
5.4.2 Because the response of these gages is close to an
using aluminum nitride as the substrate material. With a
exponential rise, a measure of the first-order time constant, τ,
for the gage can be obtained by matching the experimental thermal conductivity of approximately 170 W/m-K, which is
severalordersofmagnitudehigherthantheconductivityofthe
response to step changes in heat flux with exponential curves.
siliconmonoxide,andexcellentelectricalinsulationproperties,
2t/τ
q 5 q 1 2 e (5)
~ !
ss
it forms an ideal substrate material. Leads are taken down the
The value of the step change in imposed heat flux is repre-
side and attached to wires on the side or behind the sensor
sented by q . The resulting time constant characterizes the
ss
first-order sensor response. substrate, which is then press fit into a high conductivity metal
E2683 − 09
FIG. 3 Isometric View
...
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