The development of a pulsed-wire probe for measuring flow velocity with a wide bandwidth

The velocity range of a “time-of-flight” thermal flowmeter for slowly time varying flows can be increased by using wires (or other heating/sensing elements) with large thermal inertia (time constant) and heating the sending wire with a continuous periodic current, instead of discrete square-wave pulses as in the usual pulsed-wire anemometer. Because the time constants increase as the flow speed decreases, the time lag due to thermal inertia supplements the time lag due to the true time of flight, thus increasing the speed range/sensitivity of the device, especially at the low-speed end. The “output” is the phase shift between the current to the sending wire and the temperature of the receiving wire. The latter basically acts as a resistance thermometer. The device described here is for mainly unidirectional internal flows, and uses two parallel wires of 12.5 μm diameter. The usable speed range is 0.05–25 m s−1 and the frequency response with 30 Hz excitation current is nominally 15 Hz.