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Unformatted text preview: cooled by passing cool air through a
20cmlong channel of rectangular cross section 0.2 cm 14
cm drilled into the board. The heat generated by the electronic
components is conducted across the thin layer of the board to
the channel, where it is removed by air that enters the channel
at 15°C. The heat flux at the top surface of the channel can be
considered to be uniform, and heat transfer through other surfaces is negligible. If the velocity of the air at the inlet of the
channel is not to exceed 4 m/s and the surface temperature of
the channel is to remain under 50°C, determine the maximum
total power of the electronic components that can safely be
mounted on this circuit board. Air
15°C
Air channel
0.2 cm × 14 cm Electronic
components FIGURE P8–48
8–49 Repeat Problem 8–48 by replacing air with helium,
which has six times the thermal conductivity of air.
8–50 Reconsider Problem 8–48. Using EES (or other)
software, investigate the effects of air velocity at
the inlet of the channel and the maximum surface temperature
on the maximum total power dissipation of electronic components. Let the air velocity vary from 1 m/s to 10 m/s and the
surface temperature from 30°C to 90°C. Plot the power dissipation as functions of air velocity and surface temperature, and
discuss the results. cen58933_ch08.qxd 9/4/2002 11:29 AM Page 455 455
CHAPTER 8 8–51 Air enters a 7mlong section of a rectangular duct of
cross section 15 cm 20 cm at 50°C at an average velocity of
7 m/s. If the walls of the duct are maintained at 10°C, determine (a) the outlet temperature of the air, (b) the rate of heat
transfer from the air, and (c) the fan power needed to overcome
the pressure losses in this section of the duct.
Answers: (a) 32.8°C, (b) 3674 W, (c) 4.2 W 8–52 Reconsider Problem 8–51. Using EES (or other)
software, investigate the effect of air velocity on
the exit temperature of air, the rate of heat transfer, and the fan
power. Let the air velocity vary from 1 m/s to 10 m/s. Plot the
exit temperature, the rate of heat transfer, and the fan power as
a function of the air velocity, and discuss the results.
8–53 Hot air at 60°C leaving the furnace of a house enters a
12mlong section of a sheet metal duct of rectangular cross
section 20 cm 20 cm at an average velocity of 4 m/s. The
thermal resistance of the duct is negligible, and the outer surface of the duct, whose emissivity is 0.3, is exposed to the cold
air at 10°C in the basement, with a convection heat transfer coefficient of 10 W/m2 °C. Taking the walls of the basement to
be at 10°C also, determine (a) the temperature at which the hot
air will leave the basement and (b) the rate of heat loss from the
hot air in the duct to the basement.
10°C
ho = 10 W/ m2·°C
12 m Hot air
60°C
4 m /s Air duct
20 cm × 20 cm
ε = 0.3 mine (a) the exit temperature of air and (b) the highest component surface temperature in the duct.
8–56 Repeat Problem 8–55 for a circular horizontal duct of
15cm diameter.
8–57 Consider a hollowcore printed circuit board 12 cm
high and 18 cm long, dissipating a total of 20 W. The width of
the air gap in the middle of the PCB is 0.25 cm. The cooling air
enters the 12cmwide core at 32°C at a rate of 0.8 L/s. Assuming the heat generated to be uniformly distributed over the two
side surfaces of the PCB, determine (a) the temperature at
which the air leaves the hollow core and (b) the highest temperature on the inner surface of the core.
Answers: (a) 54.0°C, (b) 72.8°C 8–58 Repeat Problem 8–57 for a hollowcore PCB dissipating 35 W.
8–59E Water at 54°F is heated by passing it through 0.75in.internaldiameter thinwalled copper tubes. Heat is supplied to
the water by steam that condenses outside the copper tubes at
250°F. If water is to be heated to 140°F at a rate of 0.7 lbm/s,
determine (a) the length of the copper tube that needs to be
used and (b) the pumping power required to overcome pressure
losses. Assume the entire copper tube to be at the steam temperature of 250°F.
8–60 A computer cooled by a fan contains eight PCBs, each
dissipating 10 W of power. The height of the PCBs is 12 cm
and the length is 18 cm. The clearance between the tips of the
components on the PCB and the back surface of the adjacent
PCB is 0.3 cm. The cooling air is supplied by a 10W fan
mounted at the inlet. If the temperature rise of air as it flows
through the case of the computer is not to exceed 10°C, determine (a) the flow rate of the air that the fan needs to deliver,
(b) the fraction of the temperature rise of air that is due to the
heat generated by the fan and its motor, and (c) the highest
allowable inlet air temperature if the surface temperature of the
Air
outlet FIGURE P8–53
0.3 cm 8–54 Reconsider Problem 8–53. Using EES (or other)
software, investigate the effects of air velocity
and the surface emissivity on the exit temperature of air and the
rate of heat loss. Let the air velocity vary from 1 m/s to 10 m/s
and the emissivity from 0.1 to 1.0. Plot the exit temperature
and the rate of heat loss as functions of air velocit...
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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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