Abstract
This paper reports a heat-wake method for measuring extremely low velocity. The advantage of this method is that the measurement is not affected by the variation of air temperature. The probe consists of three wires. The upstream wire is heated periodically by an electric current. A temperature fluctuation is produced behind the wire, convected downstream and used as a tracer of the velocity. The two downstream wires are sensor-wires which are operated at a constant temperature. The fluctuation is detected by the second and the third wire with a time delay corresponding to the time of flight from the second wire to the third wire. The two signals from the wires are fed into the circuit, amplified and converted to square-waves. After subtracting and integrating, the final output is obtained as d.c. voltage proportional to the time delay. The velocity of the flow is inversely proportional to the output voltage. The performance of the instrument is as follows. Velocities from 0.1m/s to 1m/s can be measured with good resolution. The relation between the velocity and the output does not vary when the temperature of air varies from 15℃ to 50℃. The quick response of the instrument enables continuous measurement of velocity by traversing a probe. This method is useful at extremely low velocities of the flow, such as natural convection, which includes temperature fluctuations.
