Example
A hot-wire anemometer is a flow device used to measure flow velocity based on the principle of convective heat transfer. Electric current is passing through a thin cylindrical wire to heat it up to a high temperature, that is why it is called “hot-wire”. Heat is dissipated to the fluid flowing the wire by convection heat transfer such that the wire can be maintained at a constant temperature. Determine the velocity of the airstream (it is known to be higher than 40 m/s and has a temperature of 25°C), if a wire of 0.02 mm diameter achieved a constant temperature of 150°C while dissipating 50 W per meter of electric energy.
Constant temperature 150°C
Air (87.5°C), Pr=0.707, n=15?10-6 m2/s, k=0.026 W/m.K