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Dynamic response of forced convective heat transfer from hot-film sensors to mercury. Part 2. Experiment

Published online by Cambridge University Press:  20 April 2006

D. G. Malcolm  and
V. Verma
D. G. Malcolm
Affiliation:
Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, Canada D. G. Malcolm & Associates Incorporated, 303–336 Fifth Avenue North, Saskatoon, Canada S7K 2P4
V. Verma
Affiliation:
Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, Canada Montreal Engineering Company Ltd., 125 Ninth Avenue Southeast, Calgary, Canada T2G OP6
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Abstract

The thermal response of hot-film anemometers to fluid velocity fluctuations is investigated for low-Prandtl-number fluids. A quartz-coated hot-film probe with a diameter of 0[sdot ]002 inches and an aspect ratio of 20 was oscillated in a horizontal plane while immersed in a steadily rotating tank of mercury. The probe was oscillated sinusoidally from 2 to 1200 Hz with a vibrator. The amplitude of velocity fluctuation was regulated to about 20 % of the mean flow within a Peclet-number range of 0[sdot ]1–1[sdot ]0.

The findings concur with the theoretical results obtained numerically and published earlier by Malcolm & Verma (1973) and compare well with some results of other researchers. The results confirm that the sensitivity of the hot-film probes is inhibited at low Peclet numbers, even at quite low frequencies in liquid metals owing to their very low Prandtl numbers.

Two important effects are noticed: (a)The amplitude of fluctuation is attenuated and the degree of attenuation depends upon a non-dimentional quantity αf/$\macr{U}$2, for the range of Peclet numbers considered, whereL α is the thermal diffusivity, f is the frequency of the fluctuations and $\macr{U}$ is the free-stream velocity, in compatible units. The amplitude is attenuated by 10 % and 90 % at αf/$\macr{U}$2 values of 0[sdot ]02 and 4[sdot ]0 respectively. (b)There is a phase lag in the hot-film probe signal with respect to the true velocity of the fluctuation which is somewhat the same as that in potential flow at low frequencies, but is considerably higher than that in potential flow at higher frequencies. The measured lag does not level off asympotically at high frequencies as noted in the numerically obtained results for potential flow.

Corrections may be made to unsteady velocity measurements in low Prandtl number fluids to account for the above effects with some confidence depending upon the value of the αf/$\macr{U}$2 quantity. The results of the investigation are of more general interest, in that the hot-film sensor can be considered as a model of a long circular cylinder in a flow at low to moderate Peclet numbers.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 112 , November 1981 , pp. 475 - 485
Copyright
© 1981 Cambridge University Press

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References

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