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BASICS OF HEAT TRANSFER TEST 3
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Question 1 of 20
1. Question
1 pointsFor heat transfer purposes, a standing man can be modeled as a 30cmdiameter, 170cmlong vertical cylinder with both the top and bottom surfaces insulated and with the side surface at an average temperature of 34°C. For a convection heat transfer coefficient of 15 W/m² · °C, determine the rate of heat loss from this man by convection in an environment at
Correct
336 W
Incorrect
336 W

Question 2 of 20
2. Question
1 pointsHot air at 80°C is blown over a 2m x 4m flat surface at 30°C. If the average convection heat transfer coefficient is 55 W/m² · °C, determine the rate of heat transfer from the air to the plate, in kW
Correct
22 KW
Incorrect
22 KW

Question 3 of 20
3. Question
1 pointsA 5cmexternaldiameter, 10mlong hot water pipe at 80°C is losing heat to the surrounding air at 5°C by natural convection with a heat transfer coefficient of 25 W/m² · °C. Determine the rate of heat loss from the pipe by natural convection, in W.
Correct
2945 W
Incorrect
2945 W

Question 4 of 20
4. Question
1 pointsThe inner and outer glasses of a 6ft x 6ft doublepane window are at 60°F and 42°F, respectively. If the 0.25in. space between the two glasses is filled with still air, determine the rate of heat transfer through the window.
Correct
439 Btu/h
Incorrect
439 Btu/h

Question 5 of 20
5. Question
1 pointsAn ice chest whose outer dimensions are 30 cm x 40 cm x 40 cm is made of 3cmthick Styrofoam (k 0.033 W/m · °C). Initially, the chest is filled with 40 kg of ice at 0°C, and the inner surface temperature of the ice chest can be taken to be 0°C at all times. The heat of fusion of ice at 0°C is 333.7 kJ/kg, and the surrounding ambient air is at 30°C. Disregarding any heat transfer from the 40cm x 40cm base of the ice chest, determine how long it will take for the ice in the chest to melt completely if the outer surfaces of the ice chest are at 8°C
Correct
32.7 days
Incorrect
32.7 days

Question 6 of 20
6. Question
1 pointsA 200ftlong section of a steam pipe whose outer diameter is 4 inches passes through an open space at 50°F. The average temperature of the outer surface of the pipe is measured to be 280°F, and the average heat transfer coefficient on that surface is determined to be 6 Btu/h · ft² · °F. Determine the rate of heat loss from the steam pipe and
Correct
289,000 Btu/h
Incorrect
289,000 Btu/h

Question 7 of 20
7. Question
1 pointsA 200ftlong section of a steam pipe whose outer diameter is 4 inches passes through an open space at 50°F. The average temperature of the outer surface of the pipe is measured to be 280°F, and the average heat transfer coefficient on that surface is determined to be 6 Btu/h · ft²· the annual cost of this energy loss if steam is generated in a natural gas furnace having an efficiency of 86 percent, and the price of natural gas is $0.58/therm (1 therm 100,000 Btu).
Correct
$17,074/yr
Incorrect
$17,074/yr

Question 8 of 20
8. Question
1 pointsConsider a person whose exposed surface area is 1.7 m² , emissivity is 0.7, and surface temperature is 32°C. Determine the rate of heat loss from that person by radiation in a large room having walls at a temperature of 300 K( answer upto one decimal )
Correct
37.4 W
Incorrect
37.4 W

Question 9 of 20
9. Question
1 pointsConsider a person whose exposed surface area is1.7 m² , emissivity is 0.7, and surface temperature is 32°C. Determine the rate of heat loss from that person by radiation in a large room having walls at a temperature of 280 K.(answer upto one decimal)
Correct
169.2 W
Incorrect
169.2 W

Question 10 of 20
10. Question
1 pointsA 0.3cmthick, 12cmhigh, and 18cmlong circuit board houses 80 closely spaced logic chips on one side, each dissipating 0.06 W. The board is impregnated with copper fillings and has an effective thermal conductivity of 16 W/m · °C. All the heat generated in the chips is conducted across the circuit board and is dissipated from the back side of the board to the ambient air. Determine the temperature difference between the two sides of the circuit board.(answer upto three decimal )
Correct
0.042°C
Incorrect
0.042°C

Question 11 of 20
11. Question
1 pointsAn engineer who is working on the heat transfer analysis of a house in English units needs the convection heat transfer coefficient on the outer surface of the house. But the only value he can find from his handbooks is 20 W/m² · °C, which is in SI units. The engineer does not have a direct conversion factor between the two unit systems for the convection heat transfer coefficient. Using the conversion factors between W and Btu/h, m and ft, and °C and °F, express the given convection heat transfer coefficient in Btu/h · ft² · °F.(answer upto two decimal)
Correct
3.52 Btu/h · ft² · °F
Incorrect
3.52 Btu/h · ft² · °F

Question 12 of 20
12. Question
1 pointsConsider a person standing in a room at 23°C. Determine the total rate of heat transfer from this person if the exposed surface area and the skin temperature of the person are 1.7 m² and 32°C, respectively, and the convection heat transfer coefficient is 5 W/m² · °C. Take the emissivity of the skin and the clothes to be 0.9, and assume the temperature of the inner surfaces of the room to be the same as the air temperature.
Correct
161 W
Incorrect
161 W

Question 13 of 20
13. Question
1 pointsA 1.4mlong, 0.2cmdiameter electrical wire extends across a room that is maintained at 20°C. Heat is generated in the wire as a result of resistance heating, and the surface temperature of the wire is measured to be 240°C in steady operation. Also, the voltage drop and electric current through the wire are measured to be 110 V and 3 A, respectively. Disregarding any heat transfer by radiation, determine the convection heat transfer coefficient for heat transfer between the outer surface of the wire and the air in the room.
Correct
170.5 W/m² · °C
Incorrect
170.5 W/m² · °C

Question 14 of 20
14. Question
1 pointsA 1000W iron is left on the iron board with its base exposed to the air at 20°C. The convection heat transfer coefficient between the base surface and the surrounding air is 35 W/m² · °C. If the base has an emissivity of 0.6 and a surface area of 0.02 m² , determine the temperature of the base of the iron.
Correct
674°C
Incorrect
674°C

Question 15 of 20
15. Question
1 pointsA 4mlong section of an air heating system of a house passes through an unheated space in the attic. The inner diameter of the circular duct of the heating system is 20 cm. Hot air enters the duct at 100 kPa and 65°C at an average velocity of 3 m/s. The temperature of the air in the duct drops to 60°C as a result of heat loss to the cool space in the attic. Determine the rate of heat loss from the air in the duct to the attic under steady conditions. Also, determine the cost of this heat loss per hour if the house is heated by a natural gas furnace having an efficiency of 82 percent, and the cost of the natural gas in that area is $0.58/therm (1 therm= 105,500 kJ).
Correct
0.488 kJ/s, $0.012/h
Incorrect
0.488 kJ/s, $0.012/h

Question 16 of 20
16. Question
1 pointsA passive solar house that is losing heat to the outdoors at an average rate of 50,000 kJ/h is maintained at 22°C at all times during a winter night for 10 hours. The house is to be heated by 50 glass containers each containing 20 L of water heated to 80°C during the day by absorbing solar energy. A thermostatcontrolled 15kW backup electric resistance heater turns on whenever necessary to keep the house at 22°C.
How long did the electric heating system run that night?(answer upto two decimal)
Correct
4.77 h
Incorrect
4.77 h

Question 17 of 20
17. Question
1 pointsA passive solar house that is losing heat to the outdoorsat an average rate of 50,000 kJ/h is maintained at 22°C atall times during a winter night for 10 hours. The house is to beheated by 50 glass containers each containing 20 L of waterheated to 80°C during the day by absorbing solar energy.A thermostatcontrolled 15kW backup electric resistanceheater turns on whenever necessary to keep the house at 22°C.
How long would the electric heater have run that night ifthe house incorporated no solar heating?(answer upto two decimal )
Correct
9.26 h
Incorrect
9.26 h

Question 18 of 20
18. Question
1 pointsA 4m x 5m x 6m room is to be heated by one ton(1000 kg) of liquid water contained in a tank placed in the room. The room is losing heat to the outside at an average rate of 10,000 kJ/h. The room is initially at 20°C and 100 kPa, and is maintained at an average temperature of 20°C at all times. If the hot water is to meet the heating requirements of this room for a 24hour period, determine the minimum temperature of the water when it is first brought into the room. Assume constant specific heats for both air and water at room temperature
Correct
77.4°C
Incorrect
77.4°C

Question 19 of 20
19. Question
1 pointsConsider a 3m x 3m x 3m cubical furnace whose top and side surfaces closely approximate black surfaces at a temperature of 1200 K. The base surface has an emissivity of 0.7, and is maintained at 800 K. Determine the net rate of radiation heat transfer to the base surface from the top and side surfaces
Correct
594,400 W
Incorrect
594,400 W

Question 20 of 20
20. Question
1 pointsIn order to cool 1 short ton (2000 lbm) of water at 70°F in a tank, a person pours 160 lbm of ice at 25°F into the water. Determine the final equilibrium temperature in the tank. The melting temperature and the heat of fusion of ice at atmospheric pressure are 32°F and 143.5 Btu/lbm, respectively.
Correct
56.3°F
Incorrect
56.3°F