Hotwire Anemometry

High temporal resolution is possible with a hotwire anemometer, though the attendant disadvantages of intrusive effects and low spatial resolution are equally significant. The principle of operation of a hotwire anemometer is briefly described below.
The output of a hotwire anemometer depends on convective heat transfer from a very fine (micron-sized) heated wire to the flowing fluid. The thermal equilibrium of the wire placed in a fluid medium gives the following energy balance relationship

	(2)

Here and is the change in internal energy of the wire. Further, A is the surface area of hotwire per unit length in units of m, is the convective heat transfer coefficient, is the temprature of wire, is the temperature of fluid, is the current flowing through wire, Amp, and is the resistance of wire One can write

where m is the mass of wire per unit length, and is the specific heat of the wire material The differential equation governing time-dependent convective heat transfer is now expressed as

	(4)

The quantity of interest in the above formulation is the convective heat transfer coefficient h. The rate of change of internal energy is practically zero for the CTA (constant temperature anemometer) mode of the hotwire. This approximation leads to the equation

	(5)

where is the Nusselt number, is the sensor diameter, is the thermal conductivity of fluid, and is the length of the sensitive area of the hotwire probe.