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EXTERNAL FORCED CONVECTION TEST 1
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Question 1 of 20
1. Question
1 pointsEngine oil at 60°C flows over the upper surface of a 5mlong flat plate whose temperature is 20°C with a velocity of 2 m/s (Fig. ). Determine the total drag force and the rate of heat transfer per unit width of the entire plate.
Assumptions 1 The flow is steady and incompressible. 2 The critical Reynolds
number is Recr = 5 x 10^5Correct
SOLUTION Engine oil flows over a flat plate. The total drag force and the rate of heat transfer per unit width of the plate are to be determined.
Analysis Noting that L =5 m, the Reynolds number at the end of the plate is
The total drag force acting on the entire plate can be determined by multiplying the value obtained above by the width of the plate. This force per unit width corresponds to the weight of a mass of about 18 kg. Therefore, a person who applies an equal and opposite force to the plate to keep it from moving will feel like he or she is using as much force as is necessary to hold a 18kg mass from dropping. Similarly, the Nusselt number is determined using the laminar flow relations for a flat plate,
Incorrect
SOLUTION Engine oil flows over a flat plate. The total drag force and the rate of heat transfer per unit width of the plate are to be determined.
Analysis Noting that L =5 m, the Reynolds number at the end of the plate is
The total drag force acting on the entire plate can be determined by multiplying the value obtained above by the width of the plate. This force per unit width corresponds to the weight of a mass of about 18 kg. Therefore, a person who applies an equal and opposite force to the plate to keep it from moving will feel like he or she is using as much force as is necessary to hold a 18kg mass from dropping. Similarly, the Nusselt number is determined using the laminar flow relations for a flat plate,

Question 2 of 20
2. Question
1 pointsThe local atmospheric pressure in Denver, Colorado (elevation 1610 m), is 83.4 kPa. Air at this pressure and 20°C flows with a velocity of 8 m/s over a 1.5 m x 6 m flat plate whose temperature is 140°C (Fig. ). Determine the rate of heat transfer from the plate if the air flows parallel to the 6mlong side
Assumptions 1 Steady operating conditions exist. 2 The critical Reynolds number is Recr = 5 x 10^5 . 3 Radiation effects are negligible. 4 Air is an ideal gas.
Properties The properties k, u, Cp, and Pr of ideal gases are independent of pressure, while the properties V and ∝ are inversely proportional to density and thus pressure. The properties of air at the film temperature of Tf =(Ts + T∞)/2 = (140 +20)/2 = 80°C and 1 atm pressure are
K=0.02953 W/m · °C PR=0.7154
[email protected] 1 atm=2.097 x10^5 m²/s
The atmospheric pressure in Denver is P =(83.4 kPa)/(101.325 kPa/atm) = 0.823 atm. Then the kinematic viscosity of air in Denver becomes
[email protected] atm= IP=(2.097 x 10^5 m²/s)/0.823=2.548 x 10^5 m²/s
Correct
SOLUTION The top surface of a hot block is to be cooled by forced air. Therate of heat transfer is to be determined for two cases
Analysis When air flow is parallel to the long side, we have L = 6 m, and the Reynolds number at the end of the plate becomes
ReL=VL/v=(8 m/s)(6 m)/ 2.548 x 10^5m²/s=1.884x 10^6
which is greater than the critical Reynolds number. Thus, we have combined laminar and turbulent flow, and the average Nusselt number for the entire plate is determined to be
Incorrect
SOLUTION The top surface of a hot block is to be cooled by forced air. Therate of heat transfer is to be determined for two cases
Analysis When air flow is parallel to the long side, we have L = 6 m, and the Reynolds number at the end of the plate becomes
ReL=VL/v=(8 m/s)(6 m)/ 2.548 x 10^5m²/s=1.884x 10^6
which is greater than the critical Reynolds number. Thus, we have combined laminar and turbulent flow, and the average Nusselt number for the entire plate is determined to be

Question 3 of 20
3. Question
1 pointsThe local atmospheric pressure in Denver, Colorado (elevation 1610 m), is 83.4 kPa. Air at this pressure and 20°C flows with a velocity of 8 m/s over a 1.5 m x 6 m flat plate whose temperature is 140°C (Fig. ). Determine the rate of heat transfer from the plate if the air flows parallel to the 1.5m side
Assumptions 1 Steady operating conditions exist. 2 The critical Reynolds number is Recr = 5 x 10^5 . 3 Radiation effects are negligible. 4 Air is an ideal gas.
Properties The properties k, u, Cp, and Pr of ideal gases are independent of pressure, while the properties V and ∝ are inversely proportional to density and thus pressure. The properties of air at the film temperature of Tf =(Ts + T∞)/2 = (140 +20)/2 = 80°C and 1 atm pressure are
K=0.02953 W/m · °C PR=0.7154
[email protected] 1 atm=2.097 x10^5 m²/s
The atmospheric pressure in Denver is P =(83.4 kPa)/(101.325 kPa/atm) = 0.823 atm. Then the kinematic viscosity of air in Denver becomes
[email protected] atm= IP=(2.097 x 10^5 m²/s)/0.823=2.548 x 10^5 m²/s
Correct
SOLUTION The top surface of a hot block is to be cooled by forced air. Therate of heat transfer is to be determined for two cases
Incorrect
SOLUTION The top surface of a hot block is to be cooled by forced air. Therate of heat transfer is to be determined for two cases

Question 4 of 20
4. Question
1 pointsA 2.2cmouterdiameter pipe is to cross a river at a 30mwide section while being completely immersed in water (Fig. ). The average flow velocity of water is 4 m/s and the water temperature is 15C. Determine the drag force exerted on the pipe by the river.
Assumptions 1 The outer surface of the pipe is smooth so that( Figure) can be used to determine the drag coefficient. 2 Water flow in the river is steady. 3 The direction of water flow is normal to the pipe. 4 Turbulence in river flow is not considered.
Correct
Incorrect

Question 5 of 20
5. Question
1 pointsThe forming section of a plastics plant puts out a continuous sheet of plastic that is 4 ft wide and 0.04 in. thick at a velocity of 30 ft/min. The temperature of the plastic sheet is 200°F when it is exposed to the surrounding air, and a 2ftlong section of the plastic sheet is subjected to air flow at 80°F at a velocity of 10 ft/s on both sides along its surfaces normal to the direction of motion of the sheet, as shown in (Figure ). Determine the rate of heat transfer from the plastic sheet to air by forced convection and radiation
Assumptions 1 Steady operating conditions exist. 2 The critical Reynolds number is Recr = 5 x 10^5 . 3 Air is an ideal gas. 4 The local atmospheric pressure is 1 atm. 5 The surrounding surfaces are at the temperature of the room air.
Properties The properties of the plastic sheet are given in the problem statement. The properties of air at the film temperature of Tf = (Ts T∞)/2 = (200 + 80)/2 = 140°F and 1 atm pressure are
k=0.01623 Btu/h · ft · °F Pr=0.7202
v=0.7344 ft²/h=0.204 x 10^3 ft² /s
Correct
SOLUTION Plastic sheets are cooled as they leave the forming section of a plastics plant. The rate of heat loss from the plastic sheet by convection and radiation and the exit temperature of the plastic sheet are to be determined.
Analysis : We expect the temperature of the plastic sheet to drop somewhat as it flows through the 2ftlong cooling section, but at this point we do not know the magnitude of that drop. Therefore, we assume the plastic sheet to be isothermal at 200°F to get started. We will repeat the calculations if necessary to account for the temperature drop of the plastic sheet. Noting that L = 4 ft, the Reynolds number at the end of the air flow across the plastic sheet is
Therefore, the rate of cooling of the plastic sheet by combined convection and radiation is
Qtotal=Qconv+Qrad=2054 + 2584=4638 Btu/h
Incorrect
SOLUTION Plastic sheets are cooled as they leave the forming section of a plastics plant. The rate of heat loss from the plastic sheet by convection and radiation and the exit temperature of the plastic sheet are to be determined.
Analysis : We expect the temperature of the plastic sheet to drop somewhat as it flows through the 2ftlong cooling section, but at this point we do not know the magnitude of that drop. Therefore, we assume the plastic sheet to be isothermal at 200°F to get started. We will repeat the calculations if necessary to account for the temperature drop of the plastic sheet. Noting that L = 4 ft, the Reynolds number at the end of the air flow across the plastic sheet is
Therefore, the rate of cooling of the plastic sheet by combined convection and radiation is
Qtotal=Qconv+Qrad=2054 + 2584=4638 Btu/h

Question 6 of 20
6. Question
1 pointsThe forming section of a plastics plant puts out a continuous sheet of plastic that is 4 ft wide and 0.04 in. thick at a velocity of 30 ft/min. The temperature of the plastic sheet is 200°F when it is exposed to the surrounding air, and a 2ftlong section of the plastic sheet is subjected to air flow at 80°F at a velocity of 10 ft/s on both sides along its surfaces normal to the direction of motion of the sheet, as shown in (Figure ). Determine the temperature
of the plastic sheet at the end of the cooling section. Take the density, specific heat, and emissivity of the plastic sheet to be p= 75 lbm/ft³, Cp =0.4 Btu/lbm · °F, and ε= 0.9.Assumptions 1 Steady operating conditions exist. 2 The critical Reynolds number is Recr = 5 x 10^5 . 3 Air is an ideal gas. 4 The local atmospheric pressure is 1 atm. 5 The surrounding surfaces are at the temperature of the room air.
Properties The properties of the plastic sheet are given in the problem statement. The properties of air at the film temperature of Tf = (Ts T∞)/2 = (200 + 80)/2 = 140°F and 1 atm pressure are
k=0.01623 Btu/h · ft · °F Pr=0.7202
v=0.7344 ft²/h=0.204 x 10^3 ft² /s
Correct
SOLUTION Plastic sheets are cooled as they leave the forming section of a plastics plant. The rate of heat loss from the plastic sheet by convection and radiation and the exit temperature of the plastic sheet are to be determined
ANALYSIS: To find the temperature of the plastic sheet at the end of the cooling section, we need to know the mass of the plastic rolling out per unit time (or the mass flow rate), which is determined from
Incorrect
SOLUTION Plastic sheets are cooled as they leave the forming section of a plastics plant. The rate of heat loss from the plastic sheet by convection and radiation and the exit temperature of the plastic sheet are to be determined
ANALYSIS: To find the temperature of the plastic sheet at the end of the cooling section, we need to know the mass of the plastic rolling out per unit time (or the mass flow rate), which is determined from

Question 7 of 20
7. Question
1 pointsA long 10cmdiameter steam pipe whose external surface temperature is 110°C passes through some open area that is not protected against the winds (Fig.). Determine the rate of heat loss from the pipe per unit of its length when the air is at 1 atm pressure and 10°C and the wind is blowing across the pipe at a velocity of 8 m/s.
Assumptions :1 Steady operating conditions exist. 2 Radiation effects are negligible. 3 Air is an ideal gas.
Properties :The properties of air at the average film temperature of Tf = (Ts +T∞)/2 =(110 + 10)/2 = 60°C and 1 atm pressure
K=0.02808 W/m · °C Pr=0.7202
v=1.896 x 10^5 m²/s
Correct
SOLUTION :A steam pipe is exposed to windy air. The rate of heat loss from the steam is to be determined
Analysis :The Reynolds number is
The rate of heat loss from the entire pipe can be obtained by multiplying the value above by the length of the pipe in m.
Incorrect
SOLUTION :A steam pipe is exposed to windy air. The rate of heat loss from the steam is to be determined
Analysis :The Reynolds number is
The rate of heat loss from the entire pipe can be obtained by multiplying the value above by the length of the pipe in m.

Question 8 of 20
8. Question
1 pointsA 25cmdiameter stainless steel ball (P= 8055 kg/m³, Cp =480 J/kg · °C) is removed from the oven at a uniform temperature of 300°C (Fig.). The ball is then subjected to the flow of air at 1 atm pressure and 25°C with a velocity of 3 m/s. The surface temperature of the ball eventually drops to 200°C. Determine the average convection heat transfer coefficient during this cooling
process and estimate how long the process will take.Assumptions 1 Steady operating conditions exist. 2 Radiation effects are negligible. 3 Air is an ideal gas. 4 The outer surface temperature of the ball is uniform at all times. 5 The surface temperature of the ball during cooling is changing. Therefore, the convection heat transfer coefficient between the ball and the air will also change. To avoid this complexity, we take the surface temperature of the ball to be constant at the average temperature of (300 + 200)/2 =250°C in the evaluation of the heat transfer coefficient and use the value obtained for the entire cooling process.
Properties The dynamic viscosity of air at the average surface temperature is μs =μ@250°C =2.76 x 10^5 kg/m · s. The properties of air at the freestream temperature of 25°C and 1 atm are
K=0.02551 W/m · °C V=1.562 x 10^5 m²/s
μ=1.849 x 10^ 5 kg/m · s Pr=0.7296
Correct
SOLUTION A hot stainless steel ball is cooled by forced air. The average convection heat transfer coefficient and the cooling time are to be determined.
Analysis The Reynolds number is determined from
In this calculation, we assumed that the entire ball is at 200°C, which is not necessarily true. The inner region of the ball will probably be at a higher temperature than its surface. With this assumption, the time of cooling is determined to be
Δt=Q/Qave=3,163,000 J/ 610 J/s=5185 s=1 h 26 min
Incorrect
SOLUTION A hot stainless steel ball is cooled by forced air. The average convection heat transfer coefficient and the cooling time are to be determined.
Analysis The Reynolds number is determined from
In this calculation, we assumed that the entire ball is at 200°C, which is not necessarily true. The inner region of the ball will probably be at a higher temperature than its surface. With this assumption, the time of cooling is determined to be
Δt=Q/Qave=3,163,000 J/ 610 J/s=5185 s=1 h 26 min

Question 9 of 20
9. Question
1 pointsHot water at Ti = 120°C flows in a stainless steel pipe (k =15 W/m · °C) whose inner diameter is 1.6 cm and thickness is 0.2 cm. The pipe is to be covered with adequate insulation so that the temperature of the outer surface of the insulation does not exceed 40°C when the ambient temperature is To = 25°C. Taking the heat transfer coefficients inside and outside the pipe to be hi =70 W/m² · °C and ho = 20 W/m² · °C, respectively, determine the thickness of fiberglass insulation (k = 0.038 W/m · °C) that needs to be installed on the pipe.
Assumptions 1 Heat transfer is steady since there is no indication of any change with time. 2 Heat transfer is onedimensional since there is thermal symmetry about the centerline and no variation in the axial direction. 3 Thermal conductivities are constant. 4 The thermal contact resistance at the interface is negligible.
Properties The thermal conductivities are given to be k = 15 W/m · °C for the steel pipe and k = 0.038 W/m · °C for fiberglass insulation.(answer upto two decimal)
Correct
SOLUTION A steam pipe is to be covered with enough insulation to reduce the exposed surface temperature. The thickness of insulation that needs to be installed is to be determined.
Analysis The thermal resistance network for this problem involves four resistances in series and is given in (Figure ) The inner radius of the pipe is r1 = 0.8 cm and the outer radius of the pipe and thus the inner radius of the insulation is r2 = 1.0 cm. Letting r3 represent the outer radius of the insulation, the areas of the surfaces exposed to convection for an L = 1mlong section of the pipe become
A1=2πr1L=2π(0.008 m)(1 m) =0.0503 m²
A3=2πr3L=2πr3(1 m) =6.28r3 m²
Then the individual thermal resistances are determined to be
Incorrect
SOLUTION A steam pipe is to be covered with enough insulation to reduce the exposed surface temperature. The thickness of insulation that needs to be installed is to be determined.
Analysis The thermal resistance network for this problem involves four resistances in series and is given in (Figure ) The inner radius of the pipe is r1 = 0.8 cm and the outer radius of the pipe and thus the inner radius of the insulation is r2 = 1.0 cm. Letting r3 represent the outer radius of the insulation, the areas of the surfaces exposed to convection for an L = 1mlong section of the pipe become
A1=2πr1L=2π(0.008 m)(1 m) =0.0503 m²
A3=2πr3L=2πr3(1 m) =6.28r3 m²
Then the individual thermal resistances are determined to be

Question 10 of 20
10. Question
1 pointsDuring a cold winter day, wind at 55 km/h is blowing parallel to a 4mhigh and 10mlong wall of a house. If the air outside is at 5°C and the surface temperature of the wall is 12°C, determine the rate of heat loss from that wall by convection.
Correct
9081 W
Incorrect
9081 W

Question 11 of 20
11. Question
1 pointsDuring a cold winter day, wind at 55 km/h is blowing parallel to a 4mhigh and 10mlong wall of a house. If the air outside is at 5°C and the surface temperature of the wall is 12°C, What would your answer be if the wind velocity was doubled?
Correct
16,200 W
Incorrect
16,200 W

Question 12 of 20
12. Question
1 pointsThe top surface of the passenger car of a train moving at a velocity of 70 km/h is 2.8 m wide and 8 m long. The top surface is absorbing solar radiation at a rate of 200 W/m² , and the temperature of the ambient air is 30°C. Assuming the roof of the car to be perfectly insulated and the radiation heat exchange with the surroundings to be small relative to convection, determine the equilibrium temperature of the top surface of the car. A
Correct
35.1°C
Incorrect
35.1°C

Question 13 of 20
13. Question
1 pointsRepeat Problem 7–30 for a location at an elevation of 1610 m where the atmospheric pressure is 83.4 kPa.
Correct
4
Incorrect
4

Question 14 of 20
14. Question
1 pointsAn average person generates heat at a rate of 84 W while resting. Assuming onequarter of this heat is lost from the head and disregarding radiation, determine the average surface temperature of the head when it is not covered and is subjected to winds at 10°C and 35 km/h. The head can be approximated as a 30cmdiameter sphere.(answer upto one decimal)
Correct
12.7°C
Incorrect
12.7°C

Question 15 of 20
15. Question
1 pointsConsider a person who is trying to keep cool on a hot summer day by turning a fan on and exposing his entire body to air flow. The air temperature is 85°F and the fan is blowing air at a velocity of 6 ft/s. If the person is doing light work and generating sensible heat at a rate of 300 Btu/h, determine the average temperature of the outer surface (skin or clothing) of the person.
Correct
95.1°F
Incorrect
95.1°F

Question 16 of 20
16. Question
1 pointsConsider a person who is trying to keep cool on a hot summer day by turning a fan on and exposing his entire body to air flow. The air temperature is 85°F and the fan is blowing air at a velocity of 6 ft/s. If the person is doing light work and generating sensible heat at a rate of 300 Btu/h. The average human body can be treated as a 1ft diameter cylinder with an exposed surface area of 18 ft². Disregard any heat transfer by radiation. What would your answer be if the air velocity were doubled?(answer upto one decimal)
Correct
91.6°F
Incorrect
91.6°F

Question 17 of 20
17. Question
1 pointsHot water at 110°C flows in a cast iron pipe (k = 52 W/m · °C) whose inner radius is 2.0 cm and thickness is 0.3 cm. The pipe is to be covered with adequate insulation so that the temperature of the outer surface of the insulation does not exceed 30°C when the ambient temperature is 22°C. Taking the heat transfer coefficients inside and outside the pipe to be hi = 80 W/m² · °C and ho = 22 W/m² · °C, respectively, determine the thickness of fiber glass insulation (k = 0.038 W/m · °C) that needs to be installed on the pipe.(answer upto two decimal)
Correct
1.32 cm
Incorrect
1.32 cm

Question 18 of 20
18. Question
1 pointsConsider a furnace whose average outer surface temperature is measured to be 90°C when the average surrounding air temperature is 27°C. The furnace is 6 m long and 3 m in diameter. The plant operates 80 h per week for 52 weeks per year. You are to insulate the furnace using fiber glass insulation (kins = 0.038 W/m · °C) whose cost is $10/m2 per cm of thickness for materials, plus $30/m2 for labor regardless of thickness. The combined heat transfer coefficient on the outer surface is estimated to be ho = 30 W/m² · °C. The furnace uses natural gas whose unit cost is $0.50/therm input (1 therm =105,500 kJ), and the efficiency of the furnace is 78 percent. The management is willing to authorize the installation of the thickest insulation (in whole cm) that will pay for itself (materials and labor) in one year. That is, the total cost of insulation should be roughly equal to the drop in the fuel cost of the furnace for one year. Determine the thickness of insulation to be used and the money saved per year. Assume the surface temperature of the furnace and the heat transfer coefficient are to remain constant.
Correct
14 cm
Incorrect
14 cm

Question 19 of 20
19. Question
1 pointsConsider a house that is maintained at 22°C at all times. The walls of the house have R3.38 insulation in SI units (i.e., an L/k value or a thermal resistance of 3.38 m² · °C/W). During a cold winter night, the outside air temperature is 4°C and wind at 50 km/h is blowing parallel to a 3mhigh and 8mlong wall of the house. If the heat transfer coefficient on the interior surface of the wall is 8 W/m²· °C, determine the rate of heat loss from that wall of the house. Draw the thermal resistance network and disregard radiation heat transfer.
Correct
122 W
Incorrect
122 W

Question 20 of 20
20. Question
1 pointsConsider a person who is trying to keep cool on a hot summer day by turning a fan on and exposing his body to air flow. The air temperature is 32°C, and the fan is blowing air at a velocity of 5 m/s. The surrounding surfaces are at 40°C, and the emissivity of the person can be taken to be 0.9. If the person is doing light work and generating sensible heat at a rate of 90 W, determine the average temperature of the outer surface (skin or clothing) of the person. The average human body can be treated as a 30cmdiameter cylinder with an exposed surface area of 1.7 m²(answer upto one decimal)
Correct
36.2°C
Incorrect
36.2°C