Unformatted text preview: enterline). Do you agree? Explain.
8–32C Consider fully developed laminar flow in a circular
pipe. If the diameter of the pipe is reduced by half while the
flow rate and the pipe length are held constant, the pressure
drop will (a) double, (b) triple, (c) quadruple, (d) increase by a
factor of 8, or (e) increase by a factor of 16.
8–33C Consider fully developed laminar flow in a circular
pipe. If the viscosity of the fluid is reduced by half by heating
while the flow rate is held constant, how will the pressure drop
change?
8–34C How does surface roughness affect the heat transfer
in a tube if the fluid flow is turbulent? What would your response be if the flow in the tube were laminar?
8–35 Water at 15°C (
999.1 kg/m3 and
1.138 10 3
kg/m s) is flowing in a 4cmdiameter and 30m long horizontal pipe made of stainless steel steadily at a rate of 5 L/s.
Determine (a) the pressure drop and (b) the pumping power
requirement to overcome this pressure drop.
5 L/s 8–38 Answers: (a) 188 kPa,(b) 0.71 W 8–40 Water is to be heated from 10°C to 80°C as it flows
through a 2cminternaldiameter, 7mlong tube. The tube is
equipped with an electric resistance heater, which provides
uniform heating throughout the surface of the tube. The outer
surface of the heater is well insulated, so that in steady operation all the heat generated in the heater is transferred to
the water in the tube. If the system is to provide hot water at a
rate of 8 L/min, determine the power rating of the resistance
heater. Also, estimate the inner surface temperature of the pipe
at the exit.
8–41 Hot air at atmospheric pressure and 85°C enters a
10mlong uninsulated square duct of cross section 0.15 m
0.15 m that passes through the attic of a house at a rate of
0.10 m3/s. The duct is observed to be nearly isothermal at
70°C. Determine the exit temperature of the air and the rate of
heat loss from the duct to the air space in the attic.
Answers: 75.7°C, 941 W
Attic
space Air
85°C 70°C
0.1 m3/s 4 cm 30 m FIGURE P8–35
8–36 In fully developed laminar flow in a circular pipe, the
velocity at R/2 (midway between the wall surface and the cen Repeat Problem 8–37 for a pipe of inner radius 5 cm. FIGURE P8–41
8–42 Reconsider Problem 8–41. Using EES (or other)
software, investigate the effect of the volume
flow rate of air on the exit temperature of air and the rate of
heat loss. Let the flow rate vary from 0.05 m3/s to 0.15 m3/s. cen58933_ch08.qxd 9/4/2002 11:29 AM Page 454 454
HEAT TRANSFER Plot the exit temperature and the rate of heat loss as a function
of flow rate, and discuss the results. Parabolic
solar collector 8–43 Consider an air solar collector that is 1 m wide and 5 m
long and has a constant spacing of 3 cm between the glass
cover and the collector plate. Air enters the collector at 30°C at
a rate of 0.15 m3/s through the 1mwide edge and flows along
the 5mlong passage way. If the average temperatures of the
glass cover and the collector plate are 20°C and 60°C, respectively, determine (a) the net rate of heat transfer to the air in the
collector and (b) the temperature rise of air as it flows through
the collector. Air
30°C
0.15 m3/s 5m Insulation Collector
plate, 60°C Glass
cover
20°C FIGURE P8–43
8–44 Consider the flow of oil at 10°C in a 40cmdiameter
pipeline at an average velocity of 0.5 m/s. A 300mlong section of the pipeline passes through icy waters of a lake at 0°C.
Measurements indicate that the surface temperature of the pipe
is very nearly 0°C. Disregarding the thermal resistance of the
pipe material, determine (a) the temperature of the oil when the
pipe leaves the lake, (b) the rate of heat transfer from the oil,
and (c) the pumping power required to overcome the pressure
losses and to maintain the flow oil in the pipe.
8–45 Consider laminar flow of a fluid through a square channel maintained at a constant temperature. Now the mean velocity of the fluid is doubled. Determine the change in the
pressure drop and the change in the rate of heat transfer between the fluid and the walls of the channel. Assume the flow
regime remains unchanged.
8–46 Repeat Problem 8–45 for turbulent flow. 8–47E The hot water needs of a household are to be met by
heating water at 55°F to 200°F by a parabolic solar collector at
a rate of 4 lbm/s. Water flows through a 1.25in.diameter thin
aluminum tube whose outer surface is blackanodized in order
to maximize its solar absorption ability. The centerline of the
tube coincides with the focal line of the collector, and a glass Water
200°F
4 lbm/s Glass tube
Water tube FIGURE P8–47E
sleeve is placed outside the tube to minimize the heat losses. If
solar energy is transferred to water at a net rate of 350 Btu/h
per ft length of the tube, determine the required length of the
parabolic collector to meet the hot water requirements of this
house. Also, determine the surface temperature of the tube at
the exit.
8–48 A 15cm 20cm printed circuit board whose components are not allowed to come into direct contact with air for
reliability reasons is to be...
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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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