On the Interpretation of the Output of Hot-Film Anemometers and a Scheme of Dynamic Compensation for Water Temperature Variation

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Author(s): Tan-atichat, J., Nagib, H. M., Pluister, J. W.
Corporate Author(s): Mechanics and Mechanical and Aersopace Engineering Department, Illinois Institute of Technology
Laboratory: Air Force Office of Scientific Research
Corporate Report Number: IIT Fluids & Heat Transfer Report R73-7
Date of Publication: 1973-08
Pages: 55
Contract: Grant AFOSR 73-2509
DoD Task:
Identifier: AD0784912

Using a special calibration tunnel developed during the course of this study, the static and dynamic response of several kinds of commercially available hot-film probes with single and multiple sensors of the cylindrical-fiber type are examined. The effects of different parameters, including those of the anemometer bridge, on the output and performance of the probes are evaluated. In particular, the consequences of variations in water temperature on the hot-film anemometer output are determined. The low turbulence level (less than 0.1%) calibration tunnel is equipped with a heater capable of raising the water temperature to any desired value up to 120°F and controlling it within 0.2°F over a range of calibrated mean velocities from 0.01 to 1.4 ft/sec (better than .±_0.2% accuracy except for U < 0.1 ft/sec where the accuracy is approximately +O. 5%). The results reveal a large effect of the water temperature on the calibration curves (in an extreme case a change in temperature of only 5.5°F can result in a 100% error in the mean velocity reading). A scheme which utilizes a temperature sensing probe immersed in the working fluid is used to compensate for the water temperature variation . Several possible circuit configurations for this scheme, including an optimum circuit design, are investigated and the results from some of them are presented and discussed. The circuit has a frequency response to temperature variations which depends on the thermal time constant of the temperature probe (up to several cycles per second can be obtained using commercially available probes) and can be used to compensate for temperature variations of more than 20°F with an accuracy better than +0.2%. By using an effective value (much smaller than E0 ) instead of the zero velocity bridge voltage (Eo) in exponential-type linearizers, a constant exponent is found useful in linearizing the anemometer output over a wider range of velocities.

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