9.5 Measurement of the Wall Heat Transfer
219
Fig. 9.25 Platinum films for measuring surface temperature
a large number of gauges or thermocouples to determine the distribution of heat flux
on a complex shape (several hundred can be installed on certain hypersonic models).
Methods such as temperature sensitive paint (TSP) or infrared thermography (IR)
do not have this disadvantage because they capture the distribution of heat flux
over a larger portion of the model and they are almost un-intrusive when surface
heating is not required. The IR-thermography has been significantly developed and
takes advantage of the spectacular developments of infrared cameras, to the point
that the accuracy obtained now outperforms that of local sensors. IR-thermography is
becoming the reference method as sensors are only used in areas that are not optically
accessible.
9.5.2 Thermo-Sensitive Paints
In thermo-sensitive paint, the model made of a thermally insulating material, is coated
with a paint and depending on the temperature its colour ranges from green to blue,
yellow, brown and black as the temperature increases, with colour changes occurring
at well-defined temperature levels (see Fig. 9.26). The heat flux is determined by
recording the variation of the colour during the test. A frame-by-frame analysis of
the live recording determines the temperature history on the surface of the model by
differentiating between the instants of the colour change.
219
Fig. 9.25 Platinum films for measuring surface temperature
a large number of gauges or thermocouples to determine the distribution of heat flux
on a complex shape (several hundred can be installed on certain hypersonic models).
Methods such as temperature sensitive paint (TSP) or infrared thermography (IR)
do not have this disadvantage because they capture the distribution of heat flux
over a larger portion of the model and they are almost un-intrusive when surface
heating is not required. The IR-thermography has been significantly developed and
takes advantage of the spectacular developments of infrared cameras, to the point
that the accuracy obtained now outperforms that of local sensors. IR-thermography is
becoming the reference method as sensors are only used in areas that are not optically
accessible.
9.5.2 Thermo-Sensitive Paints
In thermo-sensitive paint, the model made of a thermally insulating material, is coated
with a paint and depending on the temperature its colour ranges from green to blue,
yellow, brown and black as the temperature increases, with colour changes occurring
at well-defined temperature levels (see Fig. 9.26). The heat flux is determined by
recording the variation of the colour during the test. A frame-by-frame analysis of
the live recording determines the temperature history on the surface of the model by
differentiating between the instants of the colour change.