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Unformatted text preview: y and emissivity, and discuss the results.
8–55 The components of an electronic system dissipating
90 W are located in a 1mlong horizontal duct whose cross
section is 16 cm
16 cm. The components in the duct are
cooled by forced air, which enters at 32°C at a rate of
0.65 m3/min. Assuming 85 percent of the heat generated inside
is transferred to air flowing through the duct and the remaining
15 percent is lost through the outer surfaces of the duct, deter 18 cm Air
inlet FIGURE P8–60 PCB, 10 W cen58933_ch08.qxd 9/4/2002 11:29 AM Page 456 456
HEAT TRANSFER components is not to exceed 70°C anywhere in the system. Use
air properties at 25°C. Air, 0.27 m3/s
10°C, 95 kPa Review Problems
8–61 A geothermal district heating system involves the transport of geothermal water at 110°C from a geothermal well to a
city at about the same elevation for a distance of 12 km at a rate
of 1.5 m3/s in 60cmdiameter stainless steel pipes. The fluid
pressures at the wellhead and the arrival point in the city are to
be the same. The minor losses are negligible because of the
large lengthtodiameter ratio and the relatively small number
of components that cause minor losses. (a) Assuming the
pumpmotor efficiency to be 65 percent, determine the electric
power consumption of the system for pumping. (b) Determine
the daily cost of power consumption of the system if the unit
cost of electricity is $0.06/kWh. (c) The temperature of geothermal water is estimated to drop 0.5°C during this long flow.
Determine if the frictional heating during flow can make up for
this drop in temperature.
8–62 Repeat Problem 8–61 for cast iron pipes of the same
diameter. 20
cm Air
Compressor
150 hp FIGURE P8–65
water, determine the outlet temperature of air as it leaves the
underwater portion of the duct. Also, for an overall fan efficiency of 55 percent, determine the fan power input needed to
overcome the flow resistance in this section of the duct. 8–63 The velocity profile in fully developed laminar flow in
a circular pipe, in m/s, is given by (r) 6(1 100r2) where
r is the radial distance from the centerline of the pipe in m. Determine (a) the radius of the pipe, (b) the mean velocity
through the pipe, and (c) the maximum velocity in the pipe. Air
25°C, 3 m/s 8–64E The velocity profile in fully developed laminar flow
of water at 40°F in a 80ftlong horizontal circular pipe, in ft/s,
is given by (r) 0.8(1 625r2) where r is the radial distance from the centerline of the pipe in ft. Determine (a) the
volume flow rate of water through the pipe, (b) the pressure
drop across the pipe, and (c) the useful pumping power required to overcome this pressure drop.
8–65 The compressed air requirements of a manufacturing
facility are met by a 150hp compressor located in a room that
is maintained at 20°C. In order to minimize the compressor
work, the intake port of the compressor is connected to the
outside through an 11mlong, 20cmdiameter duct made of
thin aluminum sheet. The compressor takes in air at a rate of
0.27 m3/s at the outdoor conditions of 10°C and 95 kPa. Disregarding the thermal resistance of the duct and taking the heat
transfer coefficient on the outer surface of the duct to be 10
W/m2 °C, determine (a) the power used by the compressor to
overcome the pressure drop in this duct, (b) the rate of heat
transfer to the incoming cooler air, and (c) the temperature rise
of air as it flows through the duct.
8–66 A house built on a riverside is to be cooled in summer
by utilizing the cool water of the river, which flows at an average temperature of 15°C. A 15mlong section of a circular
duct of 20cm diameter passes through the water. Air enters the
underwater section of the duct at 25°C at a velocity of 3 m/s.
Assuming the surface of the duct to be at the temperature of the 11 m 15°C
Air
River, 15°C FIGURE P8–66
8–67 Repeat Problem 8–66 assuming that a 0.15mmthick
layer of mineral deposit (k 3 W/m °C) formed on the inner
surface of the pipe.
8–68E The exhaust gases of an automotive engine
leave the combustion chamber and enter a
8ftlong and 3.5in.diameter thinwalled steel exhaust pipe at
800°F and 15.5 psia at a rate of 0.2 lbm/s. The surrounding
ambient air is at a temperature of 80°F, and the heat transfer
coefficient on the outer surface of the exhaust pipe is
3 Btu/h ft2 °F. Assuming the exhaust gases to have the properties of air, determine (a) the velocity of the exhaust gases at
the inlet of the exhaust pipe and (b) the temperature at which
the exhaust gases will leave the pipe and enter the air.
8–69 Hot water at 90°C enters a 15m section of a cast iron
pipe (k 52 W/m °C) whose inner and outer diameters are 4
and 4.6 cm, respectively, at an average velocity of 0.8 m/s. The
outer surface of the pipe, whose emissivity is 0.7, is exposed to
the cold air at 10°C in a basement, with a convection heat cen58933_ch08.qxd 9/4/2002 11:29 AM Page 457 457
CHAPTER 8
Tambient = 10°C 8–72 Liquidcooled systems have high heat transfer coefficients associated with them,...
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This note was uploaded on 01/28/2010 for the course HEAT ENG taught by Professor Ghaz during the Spring '10 term at University of Guelph.
 Spring '10
 Ghaz

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