Temperature measurements

Abstract: This chapter highlights both contact temperature sensors (thermocouples and temperature sensitive paints) and devices allowing measurements of temperature at a distance: infrared thermo-cameras. Devices and techniques used to measure tempera­ture recovery factor, film coefficient and stagnation temperature of an air stream are described.

Key words: infrared thermo-cameras, temperature sensitive paints, thermocouples.

5.1 Sensors

Temperature measurements are of great importance in problems of heat transfer between a body and a fluid stream. If the flow is incompressible, the only relevant temperatures are the wall temperature, Tw, and the static temperature of the stream, TM. In supersonic and hypersonic flows aerodynamic heating is present and it is necessary to consider also the stagnation temperature of the stream, T0m, and the adiabatic wall temperature, TaW.

Temperature sensors can be classified into two main groups: contact sensors and remote sensors. Among the contact sensors thermocouples are dominant in point measures, temperature-sensitive paints (TPS), are used with success in the mapping of large areas.

In order to obtain reasonably accurate measures of temperature with contact sensors, some basic requirements must be fulfilled:

■ as every sensor measures its own temperature, it is necessary that the sensor be in thermal equilibrium with the body;

■ the sensor must not conduct heat to or from the point of measurement;

■ the probe should not interfere with the way in which heat transfer occurs between the stream and the body: conduction, convection, radiation and mass transfer (evaporation or condensation); for example, if there is radiation, the emissivity of the wall should not be modified by the presence of the sensor.

The remote sensors deduce the temperature by measuring the radiation, infrared or visible, emitted by the body: the measure is not limited to high temperatures (pyrometry in the visible), efficient methods have been developed (thermographic cameras) which allow the measure in the infrared range, even at temperatures below ambient, with an accuracy comparable to that of a thermocouple.

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