Concept explainers
Consider a large
Both sides of the plate are exposed to an environment at
Explain where in the plate the highest and the lowest temperatures will occur, and determine their values.
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Heat And Mass Transfer
- 1.60 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_forward2.38 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_forward1.19 A cryogenic fluid is stored in a 0.3-m-diameter spherical container is still air. If the convection heat transfer coefficient between the outer surface of the container and the air is 6.8 , the temperature of the air is 27°C, and the temperature of the surface of the sphere is –183°C, determine the rate of heat transfer by convection.arrow_forward
- 3.16 A large, 2.54-cm.-thick copper plate is placed between two air streams. The heat transfer coefficient on one side is and on the other side is . If the temperature of both streams is suddenly changed from 38°C to 93°C, determine how long it takes for the copper plate to reach a temperature of 82°C.arrow_forwardA 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 harrow_forwardQuestion 2 Consider a 1.5-m-high electric hot-water heater that has a diameter of 40 cm and maintains the hot water at 60 °C. The tank is located in a small room whose average temperature is 27 °C, and the heat transfer coefficients on the inner and outer surfaces of the heater are 50 and 12 W/m2⋅K, respectively. The tank is placed in another 46-cm-diameter sheet metal tank of negligible thickness, and the space between the two tanks is filled with foam insulation (k = 0.03 W/m⋅K). The thermal resistances of the water tank and the outer thin sheet metal shell are very small and can be neglected. The price of electricity is $0.08/kWh, and the homeowner pays $280 a year for water heating. Determine the fraction of the hot-water energy cost (in %) of this household that is due to the heat loss from the tank. Heat transfers through the top and bottom of the tank are negligible. In continuation of Question 2, hot-water tank insulation kits consisting of 3-cm-thick fiberglass insulation…arrow_forward
- Consider a stainless steel spoon (k = 15 W/m·K) partially immersed in boiling water at 95°C in a kitchen at 25°C. The handle of the spoon has a cross section of 0.2cm x 1.2 cm, and extends 18 cm in the air from the free surface of the water. If the heat transfer coefficient at the exposed surfaces of the spoon handle is 15 W/m2·°C, determine the temperature difference across the exposed surface of the spoon handle. State your assumptions.arrow_forwardA 3-mm-diameter and 5-m-long electric wire is tightly wrapped with a 2-mm thick plastic cover whose thermal conductivity is k = 0.15 W/m·°C. Electrical measurements indicate that a current of 10 A passes through the wire and there is a voltage drop of 8 V along the wire. If the insulated wire is exposed to a medium at T = 30°C with a heat transfer coefficient of h = 12 W/m2·°C, determine the temperature at the interface of the wire and the plastic cover in steady operation. Also determine whether doubling the thickness of the plastic cover will increase or decrease this interface temperature.arrow_forwardConsider a 1.2-m-high and 2-m-wide double-pane window consisting of two 0.0023-m-thick layers of glass (k = 0.78 W/m·K) separated by a 12-mm-wide vacuum space. Take the convection heat transfer coefficients on the inner and outer surfaces of the window to be h1 = 10 W/m2·K and h2 = 25 W/m2·K, and disregard any heat transfer by radiation. Assume that the space between the two glass layers is evacuated.Determine the steady rate of heat transfer (in W) through the glass window. The room is maintained at 24°C while the temperature of the outdoors is –5°C. (Radiation in outer side of the double-pane window should be disregarded but in the inner part, the only mechanism of heat transfer in vacuum is by radiation. Emissivity for glass is around 1, and the temperature of inner surfaces of the double-pane window should be assumed to be 5 and 15 'C.)arrow_forward
- A 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_forwardA 0.083-in-diameter electrical wire at 90°F is covered by 0.02-in-thick plastic insulation (k = 0.075 Btu/ h·ft·°F). The wire is exposed to a medium at 50°F, with a combined convection and radiation heat transfer coefficient of 2.5 Btu/h·ft2·°F. Determine if the plastic insulation on the wire will increase or decrease heat transfer from the wire.arrow_forwardA 3-mm-diameter and 5-m-long electric wire is tightly wrapped with a 2-mm- thick plastic cover whose thermal conductivity is k=0.15 W/m · °C. Electrical measurements indicate that a current of 10 A passes through the wire and there is a voltage drop of 8 V along the wire. If the insulated wire is exposed to a medium at T=30°C with a heat transfer coefficient of h=12 W/m2 · °C, determine the temperature at the interface of the wire and the plastic cover in steady operation.arrow_forward
- Principles of Heat Transfer (Activate Learning wi...Mechanical EngineeringISBN:9781305387102Author:Kreith, Frank; Manglik, Raj M.Publisher:Cengage Learning