|
Hotwire anemometry
A two channel hotwire anemometer (DANTEC) was used for the present investigation.
Hotwire anemometry (HWA) is based on the principle of compensation of the rate of heat
loss of a small heated metallic wire exposed to flow. The probe responds primarily to
the magnitude of the velocity vector. The anemometer output voltage then undergoes
signal conditioning to filter out noise and improve the signal-to-noise ratio. The operating temperature of the hotwire is usually much higher than the room temperature, typically 150 in air flow measurements. In the present experiments, the probe operated at
a temperature of around ; this minimized mixed convection and radiation errors,
without appreciable loss of sensitivity. Higher temperatures enhance the sensitivity of
wire but make the wire fragile. Additionally, to avoid oxidization it is essential that the
wire temperature at any point along the wire element is kept well below 3500C.To illustrate these points, consider a typical hotwire probe (DANTEC 55P11) for
which the electrical properties are For an overheat ratio of 1.5, the operational resistance of the wire will be The corresponding
mean wire temperature can be evaluated from the equation
as being equal to about which is well below the oxidation temperature. On the
front panel
of the anemometer, the parameter fixed is the operating resistance of the
wire.
|