1. (See Fluids in the News Article titled "Hi-tech Ceiling Fans," Section 12.7.) Explain why reversing the direction of rotation of a ceiling fan results in airflow in the opposite direction.
2. For the fan of both Examples 5.19 and 5.28 discuss what fluid flow properties you would need to measure to estimate fan efficiency.
Example 5.19
Moment-of-Momentum-Power
GIVEN An air fan has a bladed rotor of 12-in. outside diameter and 10-in. inside diameter as illustrated in Fig. E5.19a. The height of each rotor blade is constant at 1 in. from blade inlet to outlet. The flowrate is steady, on a timeaverage basis, at 230 ft3 /min and the absolute velocity of the air at blade inlet, V1, is radial. The blade discharge angle is 30° from the tangential direction. The rotor rotates at a constant speed of 1725 rpm.
FIND Estimate the power required to run the fan.
Examples 5.28
Energy-Fan Performance
GIVEN Consider the fan of Example 5.19
FIND Show that only some of the shaft power into the air is converted into useful effects. Develop a meaningful efficiency equation and a practical means for estimating lost shaft energy