Development of instruments for fluid velocity measurement using heated thermistors

Abstract

In this work, two instruments for fluid velocity measurement are presenteei. The systems use heated commercial thermistors and two distinct techniques for heating the probes are used: the heat pulse and the constant temperature methods. Each of these corresponds to specific instruments. In the first method of operation, the probe's excitation is periodically switched between low-power and high-power. At low-power, the probe works in temperature sensing mode and at high-power, in velocity sensing mode. The thermal transient behaviour due to the cooling period after the application of a heat pulse is correlated to the fluid velocity around the thermistor. In the constant temperature principie, the probe temperature is maintained constant by varying the dissipated power through the thermistor, and the steady-state form of heat transfer is correlated to the fluid velocity. For each method, the employed theoretical model is described as well as the hardware used. The probes are previously calibrated in terms of the temperature variation, by means of a constant temperature bath, for determining their resistance-temperature curves and estimating thermal properties and time constants. After this, the probes are calibrated for air speed varying from 0.01 to 12 m/s, using a commercial calibrator. Suitability for Reynolds numbers up to 32,000 (based on the test section average width) is verified by a wind tunnel test. Analysis of instruments performance and limitations is also given. The automation of the data acquisition is performed using a Personal Computer and a programmable data acquisition board, so, in the first method, the heat pulse is under the control of a PC.