The instruction manual for a hot-wire anemometer states that ‘roughly speaking, the current varies as the one-fourth power of the average velocity at a fixed wire resistance’. Check this statement, using the heat transfer characteristics of thin wire in air and water.
GIVEN
• A thin current carrying wire in an air or water stream
FIND
• Show that the current (I) varies as the one-fourth power of the fluid velocity (V?) at a fixed resistance (ReI) ASSUMPTIONS
• Radiative heat transfer is negligible
This solution follows the same procedure as the solution to Problem 6.17
Holding the wire resistance constant has the effect of holding the wire temperature (Ts) and therefore,
the fluid properties constant. Ts, Tb, At, and ReI are constant.
According to
for 40 < Re < 1000 m = 0.5 ? I a U?1/4 Since the wire diameter is typically a few microns, we expect the Reynolds number to be very low. Therefore, m = 0.4 to 0.5 and m/2 = 0.2 to 0.25
You might also like to view...
Molecular Speeds: An oxygen molecule, O2, falls in a vacuum. From what height must it fall so that its translational kinetic energy at the bottom of its fall equals the average translational kinetic energy of an oxygen molecule at 920 K? The mass of one O2 molecule is 5.312 × 10-26 kg, and the Boltzmann constant is 1.38 × 10-23 J/K. Neglect air resistance and assume that g remains constant at 9.8 m/s2 throughout the fall of the molecule.
A. 49 km B. 12 km C. 24 km D. 37 km E. 5.2 km
Consider each of the following processes. Is the process consistent (C) or inconsistent (D) with the limitations on equation T8.12 
(that N be fixed and that the process not involve any energy transfer that is neither heat nor work due to a quasistatic volume change)? A glass of milk is spilled on the floor.
C. Consistent D. Inconsistent
A pitcher claims he can throw a baseball with as much momentum as a 3.00-g bullet moving with a speed of 1500 m/s. A baseball has a mass of 0.145 kg. What must be its speed if the pitcher's claim is valid?
What will be an ideal response?
At t = 0, a wheel rotating about a fixed axis at a constant angular acceleration has an angular velocity of 2.0 rad/s. Two seconds later it has turned through 5.0 complete revolutions. What is the angular acceleration of this wheel?
a. 17 rad/s2 b. 14 rad/s2 c. 20 rad/s2 d. 23 rad/s2 e. 13 rad/s2