9.5 Measurement of the Wall Heat Transfer
221
9.5.3 Infrared Thermography
All bodies emit radiation whose intensity is a function of the temperature T, distinction has to be made between the total intensity radiated over the entire spectrum and
that radiated at a particular wavelength λ. For the black body, the radiated energy is
given by Planck’s law which states that the energy per unit of time (power) radiated
in a certain direction OX by a unit surface of the black body—or radiative flux—is
given by:
L
0
λ =
2h c
2
λ
−5
exp
hc
kλ T
− 1
where h is the Planck constant, k is the Boltzmann constant, and c is the speed of
light. The radiative property of a real body is characterised by its emissivity defined
as the ratio between the intensity of the light emitted by this body and the intensity
of the light emitted by the black body at the same temperature.
In infrared thermography (IR), the model is observed by an infrared camera containing a detector element sensible to infrared radiation at a certain wavelength (the
band 3–5 μm is the most used). Since the signal delivered by the camera is proportional to the radiative flux, this information must be converted into temperature in
order to build the temperature map of the model. This can be done by calibrating
the system by observing a black body whose temperature and emissivity are known.
The calibration can also be done by placing thermocouples on the model that provide
the temperature at selected points. The test section must be equipped with windows
that allow infrared radiation, such as germanium for the 8–12 μm band, silicon for
the 3–5 μm band or zinc sulphide. This becomes a serious constraint for infrared
thermography.
Figure 9.27 shows an infrared camera mounted outside the test section of a supersonic wind tunnel. A thermal image of the model is obtained where the regions at
different temperatures are represented by grey or false colour scales. This qualitative
aspect of thermography is very useful for detecting the laminar to turbulent transition
(see below and Sect. 9.1.7). By processing a series of images taken at known time
intervals, it is possible to build the temporal history of the temperature of the model
and to deduce the distribution of the heat flux on its surface from the heat equation.
Figure 9.28 shows the distribution of heat flux on a hemisphere in a flow at Mach 5,
in false colours, the hottest regions being indicated by “cold” hues, and vice versa.
The method is very powerful because it gives a complete picture of the distribution
of heat flux over the model. In addition, the process is very sensitive and reversible,
the model does not have to be changed or modified between each test. However, as
seen above, the method requires a window which allows infrared radiation (unless
the camera can be installed inside the test section). This is an important constraint,
the choice in optics being limited. A minor disadvantage is the reduced size of the
sensor: 640 × 512 pixels, which is small compared to what is achieved in the visible
221
9.5.3 Infrared Thermography
All bodies emit radiation whose intensity is a function of the temperature T, distinction has to be made between the total intensity radiated over the entire spectrum and
that radiated at a particular wavelength λ. For the black body, the radiated energy is
given by Planck’s law which states that the energy per unit of time (power) radiated
in a certain direction OX by a unit surface of the black body—or radiative flux—is
given by:
L
0
λ =
2h c
2
λ
−5
exp
hc
kλ T
− 1
where h is the Planck constant, k is the Boltzmann constant, and c is the speed of
light. The radiative property of a real body is characterised by its emissivity defined
as the ratio between the intensity of the light emitted by this body and the intensity
of the light emitted by the black body at the same temperature.
In infrared thermography (IR), the model is observed by an infrared camera containing a detector element sensible to infrared radiation at a certain wavelength (the
band 3–5 μm is the most used). Since the signal delivered by the camera is proportional to the radiative flux, this information must be converted into temperature in
order to build the temperature map of the model. This can be done by calibrating
the system by observing a black body whose temperature and emissivity are known.
The calibration can also be done by placing thermocouples on the model that provide
the temperature at selected points. The test section must be equipped with windows
that allow infrared radiation, such as germanium for the 8–12 μm band, silicon for
the 3–5 μm band or zinc sulphide. This becomes a serious constraint for infrared
thermography.
Figure 9.27 shows an infrared camera mounted outside the test section of a supersonic wind tunnel. A thermal image of the model is obtained where the regions at
different temperatures are represented by grey or false colour scales. This qualitative
aspect of thermography is very useful for detecting the laminar to turbulent transition
(see below and Sect. 9.1.7). By processing a series of images taken at known time
intervals, it is possible to build the temporal history of the temperature of the model
and to deduce the distribution of the heat flux on its surface from the heat equation.
Figure 9.28 shows the distribution of heat flux on a hemisphere in a flow at Mach 5,
in false colours, the hottest regions being indicated by “cold” hues, and vice versa.
The method is very powerful because it gives a complete picture of the distribution
of heat flux over the model. In addition, the process is very sensitive and reversible,
the model does not have to be changed or modified between each test. However, as
seen above, the method requires a window which allows infrared radiation (unless
the camera can be installed inside the test section). This is an important constraint,
the choice in optics being limited. A minor disadvantage is the reduced size of the
sensor: 640 × 512 pixels, which is small compared to what is achieved in the visible