The schematic diagram of a hot-wire/hot-film probe is shown in Fig. 7.3.3. A thermal anemometer is called a hot-wire anemometer when is the sensing element is a wire. It is called as a hot-film anemometer if the sensor is a thin metallic film. For a hot-wire anemometer, the sensing element has a diameter of few micron and length of 2mm. In the case of hot-film anemometer , the sensing element is of 0.1µm thick and mounted on a ceramic support. The sensing element is usually made out of platinum, tungsten or platinum-iridium alloy.
Fig. 7.3.4: (a) Schematic representation of anemometer measurement; (b) Anemometer feedback controlled circuit.
Both hot-wire and hot-film probes are operated using a feed-back controlled bridge (Fig. 7.3.4) that controls the input power to the probe to maintain constant temperature when there is a change in fluid velocity. The higher is the flow velocity, the more will be heat transfer from the sensor and more voltage/power will be required for the sensor. When the sensor is maintained at constant temperature, the thermal energy remains constant. So, the electrical heating 
of the sensor is equal to the rate of heat loss through convection 
and is often governed by King's law .
 |
|
where, i is the electric current in the circuit, V is the flow velocity, 
is the temperature coefficient of resistance, 
are the calibration constants, 
are the wire temperature, free stream fluid temperature and reference temperature, respectively. With appropriate calibration, Eq. (7.3.6) can be expressed through a close correlation between flow velocity 
and voltage 
.
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(7.3.7) |
One of the important applications includes the turbulence measurements where the velocity fluctuations are important. Two or more wires at one point in the flow can make simultaneous measurements of the fluctuating components. The thermal anemometers have distinct advantages of measuring very high velocities (~1000m/s) with excellent spatial and temperature resolution for liquids as well as gases. The hot film probes are extremely sensitive to fluctuations in the fluid velocity and have been used for measurements involving frequencies as high as 50 kHz. The time constants of the order of 1ms can be obtained with hot-wire probes operating in air. Moreover, simultaneous measurement of velocity components (three-dimensional) can be done by aligning three sensors on a single probe.