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April 19, 2015

Homework Help: Heat Transfer

Posted by Rangen on Friday, August 3, 2012 at 10:16am.

A hot-wire anemometer is a temperature-based instrument for measuring velocities in a flowing gas. The key component of the device is a very fine wire (diameter = 0.0004 in., length = 0.20 in.) that is electrically heated. The wire is positioned so that the air stream passes over it in cross flow. The heating of the wire is accomplished by
Ohmic dissipation of electric energy, in the same manner that the wires of a toaster are heated. The Ohmic heating of the wire is 0.0253 W, and the wire temperature is 147 F. In the steady state, this heat is transferred to the air flow. The air that passes over the wire has a temperature of 73 F and a pressure of 726 mm Hg.

Required results:
(a) The velocity of the air flow.
(b) Calculate the Reynolds number based on the wire diameter.
(c) If the velocity were to be doubled, and the wire and air stream temperatures were maintained as stated, what is the required Ohmic dissipation?

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