The roof of a house consists of a 0.70-ft-thick concrete slab (k = 3 W/m · °C) that is 18 m wide and 23 m long. The emissivity of the outer surface of the roof is 0.92, and the convection heat transfer coefficient on that surface is estimated to be 18 W/m2 · °C. The inner surface of the roof is maintained at 295 K. On a clear winter night, the ambient air is reported to be at 282.15 K while the night sky temperature for radiation heat transfer is 260 K. Considering both radiation and convection heat transfer, determine the outer surface temperature and the rate of heat transfer through the roof.

The roof of a house consists of a 0.70-ft-thick concrete slab (k = 3 W/m · °C) that is 18 m wide and 23 m long. The emissivity of the outer surface of the roof is 0.92, and the convection heat transfer coefficient on that surface is estimated to be 18 W/m2 · °C. The inner surface of the roof is maintained at 295 K. On a clear winter night, the ambient air is reported to be at 282.15 K while the night sky temperature for radiation heat transfer is 260 K. Considering both radiation and convection heat transfer, determine the outer surface temperature and the rate of heat transfer through the roof.

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.

Chapter1: Basic Modes Of Heat Transfer

Section: Chapter Questions

Problem 1.19P: 1.19 A cryogenic fluid is stored in a 0.3-m-diameter spherical container is still air. If the...

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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.
A cooling system is to be designed for a food storage warehouse for keeping perishable foods cool prior to transportation to grocery stores. The warehouse has an effective surface area of 1860 m2 exposed to an ambient air temperature of 32C. The warehouse wall insulation (k=0.17W/(mK)) is 7.5 cm thick. Determine the rate at which heat must be removed (W) from the warehouse to maintain the food at 4C.
1.19 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.
1. A 1000-W 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. 2. The inner and outer surfaces of a 5-m x 6-m brick wall of thickness 30 cm and thermal conductivity 0.69 W/m °C are maintained at temperatures of 20°C and 5°C, respectively. Determine the rate of heat transfer through the wall, in W.
Consider a flat-plate solar collector placed horizontally on the flat roof of a house. The collector is 5 ft wide and 15 ft long, and the average temperature of the exposed surface of the collector is 94°F. The emissivity of the exposed surface of the collector Is 0.9. Determine the rate of heat loss from the collector by convection and radiation on a calm day when the ambient air temperature is 70 °F and the effective sky temperature for radiation exchange is 50°F. Take the convection heat transfer coefficient on the exposed surface as 2.5 Btu/h-ft2.°F. Tuky = 50°F 70°F Solar collector Given: a=0.1714 x 10-8 Btu/h-ft2.R4 4500 Btu/h
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In a bread-making factory, a 10-kW oven has a hearth area of 4 m2 and operates at 215 ºC.It is loaded with two batches of bread dough in baking tins; 140 loaves on the first batchand 112 loaves on the second batch. The surface of each loaf measures 14 cm x 25 cm.Assuming that the emissivity of dough is 0.85, that the dough bakes at 110 ℃, and that92% of the heat is transmitted in the form of radiant energy, calculate the efficiency ofenergy use (as the percentage of the supplied radiant energy which is absorbed by the food)for each batch.
A 3-m2 hot black surface at 80°C is losing heat to the surrounding air at 25°C by convection with a convection heat transfer coefficient of 12 W/m2.°C, and by radiation to the surrounding surfaces at 15°C. Compute for the total rate of heat loss from the surface.
A 1000-W iron is left on the ironing 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.
Consider a sealed 20-cm-high electronic box whose base dimensions are 40 cm x 40 cm placed in a vacuum chamber. The emissivity of the outer surface of the box is 0.95. If the electronic components in the box dissipate a total of 100 W of power and the outer surface temperature of the box is not to exceed 55°C, determine the temperature at which the surrounding surfaces must be kept if this box is to be cooled by radiation alone. Assume the heat transfer from the bottom surface of the box to the stand to be negligible.
An electric device made of copper stands on an insulating carpet in a breezy room at 20°C. A fan is set in the room to ensure a continuous air circulation. Determine the total rate of heat transfer from the device if the average values of its exposed surface area and outer surface temperature are 2.5 m² and 30°C, respectively.
A 50-cm * 50-cm circuit board that contains 121 square chips on one side is to be cooled by combined natural convection and radiation by mounting it on a vertical surface in a room at 25°C. Each chip dissipates 0.18 W of power, and the emissivity of the chip surfaces is 0.7. Assuming the heat transfer from the back side of the circuit board to be negligible, and the temperature of the surrounding surfaces to be the same as the air temperature of the room, determine the surface temperature of the chips. Evaluate air properties at a film temperature of 30°C and 1 atm pressure. Is this a good assumption?