工程热力学(英语).docVIP

1-11 A barometer is used to measure the height of a building by recording reading at the bottom and at the top of the building. The height of the building is to be determined. Assumptions The variation of air density with altitude is negligible. Properties The density of air is given to be ( = 1.18 kg/m3. The density of mercury is 13,600 kg/m3. Analysis Atmospheric pressures at the top and at the bottom of the building are Taking an air column between the top and the bottom of the building and writing a force balance per unit base area, we obtain It yields h = 288.6 m which is also the height of the building. 1-21 The air pressure in a duct is measured by an inclined manometer. For a given vertical level difference, the gage pressure in the duct and the length of the differential fluid column are to be determined. Assumptions The manometer fluid is an incompressible substance. Properties The density of the liquid is given to be ( = 0.81 kg/L = 810 kg/m3. Analysis The gage pressure in the duct is determined from The length of the differential fluid column is Discussion Note that the length of the differential fluid column is extended considerably by inclining the manometer arm for better readability. 2-4 No. This is the case for adiabatic systems only. 2-6 A classroom is to be air-conditioned using window air-conditioning units. The cooling load is due to people, lights, and heat transfer through the walls and the windows. The number of 5-kW window air conditioning units required is to be determined. Assumptions There are no heat dissipating equipment (such as computers, TVs, or ranges) in the room. Analysis The total cooling load of the room is determined from where Substituting, Thus the number of air-conditioning units required is 2-18 The flow of air through a flow channel is considered. The diameter of the wind channel downstream from the rotor and the power produced by the windmill are to be determined. Analysis The specific v