Abstract

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 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).

In general, the Fourier components are inclined to the wall - the lower frequencies making smaller angles with the wall than the higher frequencies. The higher frequency disturbances became more nearly perpendicular to the wall in the central region of the pipe. For points very near the wall the disturbances appear to be very obliquely inclined.

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 EQ) instead of the zero- velocity bridge voltage (E0) in exponential-type linearizers, a constant exponent is found useful in linearizing the anemometer output over a wider range of velocities, especially the very low ones.

Finally, a linearized hot-film anemometer compensated for temperature variation by utilizing the present scheme is successfully used to obtain precision measurements in a standard laminar flow- field where the water temperature varied. The results compare favorably with classical theory which is quite encouraging in view of the low overheat ratio used with hot-films and the large effects of temperature on water density and viscosity.

Meeting Name

3rd Biennial Symposium on Turbulence in Liquids (1973: Sep., Rolla, MO)

Department(s)

Chemical and Biochemical Engineering

Comments

This research is supported by U.S. Air Force, Office of Scientific Research under Grant No. 73-2509, which is monitored by Capt. W. H. Smith, and by the National Science Foundation under Grant No. GK-16980 from the Division of Engineering, Fluid Mechanics Program.

Document Type

Article - Conference proceedings

Presentation Type

Contributed Paper

Session

Hot-Film Anemometry

Document Version

Final Version

File Type

text

Language(s)

English

Rights

© 1973 University of Missouri--Rolla, All rights reserved.

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