Hot-wire Anemometry

A hot-wire anemometer measures local instantaneous velocity based on principles of heat transfer. However, it requires that the fluid itself be at a uniform temperature. It can be used to measure three components of velocity and velocity fluctuations arising in turbulent flow. This is possible because of the high speed of response of the hot-wire probe and the associated feedback circuit. A hot-wire probe is used in gas flows, while a hot-film is used for liquid flow. The hot-wire has a limitation that it is insensitive to the flow direction. Further, it has a non-linear input-output relationship which makes its sensitivity non-uniform over any velocity range. In particular, the sensitivity decreases with increasing velocity.

The hot-wire probe is a platinum-coated tungsten wire, typically of diameter and about mm length, supported between highly conducting prongs (Figure 2.10). Tungsten has high temperature coefficient of resistance (i.e., resistance increases rapidly with temperature) and the platinum coating affords strength as well as protection against corrosion of the thin wire.