Principles of Heat Transfer (Activate Learning with these NEW titles from Engineering!)
8th Edition
ISBN: 9781305387102
Author: Kreith, Frank; Manglik, Raj M.
Publisher: Cengage Learning
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A long steel rod 0.305 m in diameter is initially at a temperature of 588 K. It is immersed in an oil bath maintained at 311 K. The surface convective coefficient is 125 W/m2 -K. Calculate the temperature at the center of the rod after 1 h. The average physical properties of the steel are k = 38 W/m-K and α = 0.0381 m2/h.
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.
To warm up some milk for a baby, a mother pours milk into a thin-walled cylindrical container whose diameter is 6 cm. The height of the milk in the container is 7 cm. She then places the container into a large pan filled with hot water at 70°C. The milk is stirred constantly, so that its temperature is uniform at all times. If the heat transfer coefficient between the water and the container is 120 W/m2·K, determine how long it will take for the milk to warm up from 3°C to 38°C. Assume the entire surface area of the cylindrical container (including the top and bottom) is in thermal contact with the hot water. Take the properties of the milk to be the same as those of water. Can the milk in this case be treated as a lumped system? Why?
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- 2.38 The addition of aluminum fins has been suggested to increase the rate of heat dissipation from one side of an electronic device 1 m wide and 1 m tall. The fins are to be rectangular in cross section, 2.5 cm long and 0.25 cm thick, as shown in the figure. There are to be 100 fins per meter. The convection heat transfer coefficient, both for the wall and the fins, is estimated to be K. With this information determine the percent increase in the rate of heat transfer of the finned wall compared to the bare wall.arrow_forward1.60 Two electric resistance heaters with a 20 cm length and a 2 cm diameter are inserted into a well-insulated 40-L tank of water that is initially at 300 K. If each heater dissipates 500 W, what is the time required for bringing the water temperature in the tank to 340 K? State your assumption for your analysis.arrow_forwardCitrus fruits are very susceptible to cold weather, and extended exposure to subfreezing temperatures can destroy them. Consider an 8-cm-diameter orange that is initially at 15°C. A cold front moves in one night, and the ambient temperature suddenly drops to –6°C, with a heat transfer coefficient of 15 W/m2·K. Using the properties of water for the orange and assuming the ambient conditions to remain constant for 20400 seconds before the cold front moves out, determine the surface temperature of orange that night. Solve this problem using analytical one-term approximation method. The properties of the orange are approximated by those of water at the average temperature of about 5°C, k = 0.571 W/m·°C and α = 0.136 × 10–6 m2/s. The surface temperature of orange that night isarrow_forward
- The author and his then 6-year-old son have conducted the following experiment to determine the thermal conductivity of a hot dog. They first boiled water in a large pan and measured the temperature of the boiling water to be 94°C, which is not surprising, since they live at an elevation of about 1650 m in Reno, Nevada. They then took a hot dog that is 12.5 cm long and 2.2 cm in diameter and inserted a thermocouple into the midpoint of the hot dog and another thermocouple just under the skin. They waited until both thermocouples read 20°C, which is the ambient temperature. They then dropped the hot dog into boiling water and observed the changes in both temperatures. Exactly 2 min after the hot dog was dropped into the boiling water, they recorded the center and the surface temperatures to be 59°C and 88°C, respectively. The density of the hot dog can be taken to be 980 kg/m3, which is slightly less than the density of water, since the hot dog was observed to be floating in water while…arrow_forwardA 2.5-cm-thick sheet of plastic initially at 20oC is placed between two heated steel plates that are maintained at 140oC. The plastic is to be heated just long enough for its mid-plane temperature to reach 130oC. If the thermal conductivity of the plastic is 1.2 x 10 -3 W/m/K, the thermal diffusivity is 2.7 x 10 -6 m2/s, and the thermal resistance at the interface between the plates and the plastic is negligible, determine a) the required heating time, b) the temperature at a plane 0.6 cm from the steel plate at the moment the heating is discontinued, and c) the time required of the plastic to reach the temperature of 130oC at 0.6 cm from the steel plate.arrow_forwardA person puts a few apples into the freezer at 15°C cool them quickly for guestswho are about to arrive. Initially, the apples are at a uniform temperature of 20°C,and the heat transfer coefficient on the surfaces is 8 W/m2·K. Treating the apples as9-cm-diameter spheres and taking their properties to be 840 kg/m3, Cp 3.81 kJ/kg·K, k = 0.418 W/m·K, and α =10-7 m2/s, determine the center and surface temperatures of the apples in 1 h. Also, determine the amount of heat transfer from each apple. Solve this problem using analytical one-term approximation method (notthe Heisler charts). Answer: Center: 11.2 ℃, Surface: 2.7 ℃, heat transfer: 17.2 kJarrow_forward
- During a picnic on a hot summer day, the only available drinks were those at the ambient temperature of 90°F. In an effort to cool a 12-fluid-oz drink in a can, which is 5 in high and has a diameter of 2.5 in, a person grabs the can and starts shaking it in the iced water of the chest at 32°F. The temperature of the drink can be assumed to be uniform at all times, and the heat transfer coefficient between the iced water and the aluminum can is 30 Btu/h·ft2·°F. Using the properties of water for the drink, estimate how long it will take for the canned drink to cool to 40°F.arrow_forwardhow is a three-dimensional heat equation reduced if heat transfer occurs in an insulated long, thin rod or wire of a specific length? Explain what will be the new equation?arrow_forwardA steel plate having a thickness of 100 mm is suddenly exposed to a hot gas at 1000 oC in a furnace. One surface of the plate is heated while the other surface of the plate can be approximated to be adiabatic. The initial temperature of the steel plate is 20 oC. The thermal conductivity and thermal diffusivity of the steel plate are 34.8 W/m K and 0.555 x 10-5 m2 /s respectively. The convective heat-transfer coefficient is 174 W/m2 K. a) Determine the time necessary to raise the surface temperature of the steel plate to 500 oC. b) Determine the maximum temperature difference in the cross-section of the steel plate at the time evaluated in part a) above. c) Determine the heat energy transferred to the steel plate per unit wall surface area by the time evaluated in part a) above. Use the conduction chartsarrow_forward
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