ME330_S07_HWSet5B - ME 330 Fluid Mechanics Department of...

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ME 330 Fluid Mechanics Spring 2007 Department of Engineering Western Kentucky University CHAPTER 5B HW - Homework Solutions Problem #’s 5-14, 5-15C, 5-16C, 5-19, 5-21 and 5-24, Due March 2, 2007 5-14 Solution The minimum fresh air requirements of a residential building is specified to be 0.35 air changes per hour. The size of the fan that needs to be installed and the diameter of the duct are to be determined. Analysis The volume of the building and the required minimum volume flow rate of fresh air are L/min 3150 = = = = × = = = L/h 189,000 h / m 189 ) /h 35 . 0 )( m 540 ( ACH m 540 ) m m)(200 7 . 2 ( 3 3 room 3 2 room V V V & The volume flow rate of fresh air can be expressed as ) 4 / ( 2 D V VA π = = V & Solving for the diameter D and substituting, m 0.106 = = = m/s) (6 ) /s m 3600 / 189 ( 4 4 3 V D V & Therefore, the diameter of the fresh air duct should be at least 10.6 cm if the velocity of air is not to exceed 6 m/s. Discussion Fresh air requirements in buildings must be taken seriously to avoid health problems. 5-15C Solution We are to discuss mechanical energy and how it differs from thermal energy. Analysis Mechanical energy is the form of energy that can be converted to mechanical work completely and directly by a mechanical device such as a propeller . It differs from thermal energy in that thermal energy cannot be converted to work directly and completely . The forms of mechanical energy of a fluid stream are kinetic, potential, and flow energies. Discussion It would be nice if we could convert thermal energy completely into work. However, this would violate the second law of thermodynamics. House 200 m 2 0.35 ACH
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ME 330 Fluid Mechanics Spring 2007 Department of Engineering Western Kentucky University 5-16C Solution We are to define and discuss mechanical efficiency. Analysis Mechanical efficiency is defined as the ratio of the mechanical energy output to the mechanical energy input . A mechanical efficiency of 100% for a
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ME330_S07_HWSet5B - ME 330 Fluid Mechanics Department of...

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