4-38 Carbon steel balls (p = 7833 kg/m2, k = 54 W/m K, c = 0.465 kJ/kg°C, and a 1.474 - 10-6 m2/s) 8 mm in diameter are annealed by heating them first to 900°C in a furnace and then page 300allowing them to cool slowly to 100°C in ambient air at 35°C. If the average heat transfer coefficient is 75 W/m2-K, determine how long the annealing process will take. If 2500 balls are to be annealed perhour, determine the total rate of heat transfer from the balls to the ambient air.FIGURE P4-38Air, 35°CFurnaceSteel ball 100C900°C

Assumptions:The balls have constant thermal properties.The shape of the balls is spherical having…

Answered: 4-38 Carbon steel balls (p = 7833…

Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)

8th Edition

ISBN:9781305387102

Author:Kreith, Frank; Manglik, Raj M.

Publisher:Kreith, Frank; Manglik, Raj M.

Chapter3: Transient Heat Conduction

Section: Chapter Questions

Problem 3.12P

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An ordinary egg can be approximated as a 5.5-cm-diameter sphere whose properties are roughly k=0.6 W/m.oC and a=0.14 x 10-6 m2/s. The egg is initially at a uniform temperature of 8 oC and is dropped into boiling water at 97 oC. Taking the convection heat transfer coefficient to be h=1400 W/m2.oC, determine how long it will take for the center of the egg to reach 70oC.
A long 35-cm-diameter cylindrical shaft made of stainless steel 304 (k = 14.9 W/m·K, r = 7900 kg/m3, cp = 477 J/kg·K, and a 5 3.95 * 10-6 m2/s) comes out of an oven at a uniform temperature of 400°C. The shaft is then allowed to cool slowly in a chamber at 150°C with an average convection heat transfer coefficient of h = 60 W/m2·K. Determine the temperature at the center of the shaft 20 min after the start of the cooling process. Also, determine the heat transfer per unit length of the shaft during this time period. Solve this problem using analytical one-term approximation method (not the Heisler charts).
Consider water (at a temperature of 276 K ) flowing inside a cylindrical pipe, having a diameter of 0.1 m and a thin wall. Under specific circumstances, temperature at the wall of the pipe reaches 258 K that helps form a layer of ice on the inner side of the wall of the pipe. Temperature and convection heat transfer coefficient at the inner surface of the ice layer are 273 K and 1500m2⋅Kw, respectively. Determine how thick the ice layer should be?
In a power plant, pipes transporting superheated vapor are very common. In a certain plant, superheated vapor is flowing at a rate of 0.3 kg/s inside a 5 cm diameter pipe of 10 m length. The air in the plant is maintained at 20oC while the surface temperature of the pipe is 100oC. If the temperature drop between the inlet and exit of the pipe is 30oC and the specific heat capacity of the vapor is 2190 kJ/kg.K, determine the convective heat transfer coefficient at the pipe surface. If necessary, how could this coefficient be increased?
In a manufacturing facility, 2-in-diameter brassballs (k = 64.1 Btu/h·ft·°F, r = 532 lbm/ft3, and cp =0.092 Btu/lbm·°F) initially at 250°F are quenched in a waterbath at 120°F for a period of 2 min at a rate of 120 ballsper minute. If the convection heat transfer coefficient is42 Btu/h·ft2·°F, determine (a) the temperature of the balls afterquenching and (b) the rate at which heat needs to be removedfrom the water in order to keep its temperature constant at120°F.
The exhaust gases of an automotive engine leave the combustion chamber and enter a 8-ft-long and 3.5-in-diameter thin-walled steel exhaust pipe at 800°F and 15.5 psia at a rate of 0.05 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.
Consider hotdog being cooked in boiling water in a pan. Would the heat transfer be modeled as one-dimensional or two-dimensional? Would the heat transfer be steady or transient? Explain.
Layers of 23-cm-thick meat slabs (k =0.47 W/m · °C) and α= 0.13x 10-6 m2/s) initially at a uniform temperature of 7°C are to be frozen by refrigerated air at -30°C flowing at a velocity of 1.4 m/s. The average heat transfer coefficient between the meat and the air is 20 W/m2· °C. Assuming the size of the meat slabs to be largerelative to their thickness, determine how long it will take for the center temperature of the slabs to drop to 18°C. Also, determine the surface temperature of the meat slab at that time. Answer: 22.1 h, -26.9 ℃
A person puts a few apples into the freezer at -13°C to cool them quickly for guests who are about to arrive. Initially, the apples are at a uniform temperature of 37°C, and the heat transfer coefficient on the surfaces is 6.3 W/m2 · °C. Treating the apples as 8.2-cm-diameter spheres and taking their properties to be ρ= 836 kg/m3, Cp = 4.05 kJ/kg · °C, k= 0.426 W/m · °C, and α= 1.43 ×10-7 m2/s, determine the center and surface temperatures of the apples in 1.9 h
In a production facility, 3-cm-thick large brass plates (k =110 W/m·K, ρ= 8530kg/m3, Cp = 380 J/kg·K, and α= 33.9 x10-6 m2/s) that are initially at a uniform temperature of 25°C are heated by passing them through an oven maintained at 700°C. The plates remain in the oven for a period of 10 min. Taking the convection heat transfer coefficient to be h = 80 W/m2·K, determine the surface temperature ofthe plates when they come out of the oven. Solve this problem using analytical oneterm approximation method (not the Heisler charts). Can this problem be solved using lumped system analysis? Justify your answer. Answer: 445 ℃
A person puts a few apples into the freezer at 215°C tocool them quickly for guests who are about to arrive. Initially,the apples are at a uniform temperature of 20°C, and the heattransfer coefficient on the surfaces is 8 W/m2·K. Treating theapples as 9-cm-diameter spheres and taking their properties tobe r = 840 kg/m3, cp = 3.81 kJ/kg·K, k = 0.418 W/m·K, anda = 1.3 * 1027 m2/s, determine the center and surface temperaturesof the apples in 1 h. Also, determine the amount ofheat transfer from each apple. Solve this problem using analyticalone-term approximation method (not the Heisler charts).
A pipe in a manufacturing plant is transporting superheated vapor at a mass flow rate of 0.3 kg/s. The pipe is 10 m long and has an inner diameter of 5 cm and a wall thickness of 6 mm. The pipe has a thermal conductivity of 17 W/m·K, and the inner pipe surface is at a uniform temperature of 120°C. The temperature drop between the inlet and exit of the pipe is 7°C, and the constant pressure specific heat of vapor is 2190 J/kg·°C. If the air temperature in the manufacturing plant is 25°C, determine the heat transfer coefficient as a result of convection between the outer pipe surface and the surrounding air.

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