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Unformatted text preview: struments are available for
most liquid flows but are relatively costly. Electromagnetic flowmeters are treated in
Hot-wire anemometer. A very fine wire (d 0.01 mm or less) heated between two
small probes, as in Fig. 6.29e, is ideally suited to measure rapidly fluctuating flows
such as the turbulent boundary layer. The idea dates back to work by L. V. King in
1914 on heat loss from long thin cylinders. If electric power is supplied to heat the
cylinder, the loss varies with flow velocity across the cylinder according to King’s law
q I2R a b( V)n 1
2 where n
at very low Reynolds numbers and equals at high Reynolds numbers.
The hot wire normally operates in the high-Reynolds-number range but should be calibrated in each situation to find the best-fit a, b, and n. The wire can be operated either at constant current I, so that resistance R is a measure of V, or at constant resistance R (constant temperature), with I a measure of velocity. In either case, the output
is a nonlinear function of V, and the equipment should contain a lin...
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- Spring '08