A team of students tests a material for its thermal conductivity (k). After 20 minutes in a heat box, the temperature is 48° C inside the box and 28° C on top of the material. The following data is true about this test: Area of material = .0225 m2 Thickness of material = .0127 m Light bulb = 25 W What is the thermal conductivity constant for the material? Calculate the amount of energy transferred through the material. Determine the R-value of the material. Based on your calculations, would the material be a reasonable choice for home insulation? Yes of No

A team of students tests a material for its thermal conductivity (k). After 20 minutes in a heat box, the temperature is 48° C inside the box and 28° C on top of the material. The following data is true about this test: Area of material = .0225 m2 Thickness of material = .0127 m Light bulb = 25 W What is the thermal conductivity constant for the material? Calculate the amount of energy transferred through the material. Determine the R-value of the material. Based on your calculations, would the material be a reasonable choice for home insulation? Yes of No

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.

Chapter2: Steady Heat Conduction

Section: Chapter Questions

Problem 2.26P

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As a designer working for a major electric appliance manufacturer, you are required to estimate the amount of fiberglass insulation packing (k = 0.035 W/m K) that is needed for a kitchen oven shown in the figure below. The fiberglass layer is to be sandwiched between a 2-mm-thick aluminum cladding plate on the outside and a 5-mm-thick stainless steel plate on the inside that forms the core of the oven. The insulation thickness is such that the outside cladding temperature does not exceed 40C when the temperature at the inside surface of the oven is 300C. Also, the air temperature in the kitchen varies from 15Cto33C, and the average heat transfer coefficient between the outer surface of the oven and air is estimated to be 12.0W/m2K. Determine the thickness of the fiberglass insulation that is required for these conditions. What would be the outer surface temperature when the inside surface of the oven is at 475C?
To determine the thermal conductivity of a structural material, a large 15-cm-thick slab of the material is subjected to a uniform heat flux of 2500 W/m2 while thermocouples embedded in the wall at 2.5 cm. intervals are read over a period of time. After the system had reached equilibrium, an operator recorded the thermocouple readings shown below for two different environmental conditions: Distance from the Surface (cm) Temperature (C) Test 1 0 40 5 65 10 97 15 132 Test 2 0 95 5 130 10 168 15 208 From these data, determine an approximate expression for the thermal conductivity as a function of temperature between 40 and 208C.
Using the information in Problem 1.22, estimate the ambient air temperature that could cause frostbite on a calm day on the ski slopes. 1.22 In order to prevent frostbite to skiers on chair lifts, the weather report at most ski areas gives both an air temperature and the wind-chill temperature. The air temperature is measured with a thermometer that is not affected by the wind. However, the rate of heat loss from the skier increases with wind velocity, and the wind-chill temperature is the temperature that would result in the same rate of heat loss in still air as occurs at the measured air temperature with the existing wind. Suppose that the inner temperature of a 3-mm-thick layer of skin with a thermal conductivity of 0.35W/mKis35C and the air temperature is 20C. Under calm ambient conditions the heat transfer coefficient at the outer skin surface is about 20W/m2K (see Table 1.4), but in a 40-mph wind it increases to 75W/m2K. If frostbite occurs when the skin temperature drops to about 10C, do you advise the skier to wear a face mask? What is the skin temperature drop due to the wind?
1.77 Explain each in your own words. (a) What is the mode of heat transfer through a large steel plate that has its surfaces at specified temperatures? (b) What are the modes when the temperature on one surface of the steel plate is not specified, but the surface is exposed to a fluid at a specified temperature?
A plane wall 15 cm thick has a thermal conductivity given by the relation k=2.0+0.0005T[W/mK] where T is in kelvin. If one surface of this wall is maintained at 150C and the other at 50C, determine the rate of heat transfer per square meter. Sketch the temperature distribution through the wall.
1.3 A furnace wall is to be constructed of brick having standard dimensions of Two kinds of material are available. One has a maximum usable temperature of 1040°C and a thermal conductivity of 1.7 W/(m K), and the other has a maximum temperature limit of 870°C and a thermal conductivity of 0.85 W/(m K). The bricks have the same cost and are laid in any manner, but we wish to design the most economical wall for a furnace with a temperature of 1040°C on the hot side and 200°C on the cold side. If the maximum amount of heat transfer permissible is 950 , determine the most economical arrangement using the available bricks.
1.22 In order to prevent frostbite to skiers on chair lifts, the weather report at most ski areas gives both an air temperature and the wind-chill temperature. The air temperature is measured with a thermometer that is not affected by the wind. However, the rate of heat loss from the skier increases with wind velocity, and the wind-chill temperature is the temperature that would result in the same rate of heat loss in still air as occurs at the measured air temperature with the existing wind. Suppose that the inner temperature of a 3-mm-thick layer of skin with a thermal conductivity of 0.35 W/m K is and the air temperature is . Under calm ambient conditions the heat transfer coefficient at the outer skin surface is about (see Table 1.4), but in a 40-mph wind it increases to . (a) If frostbite occurs when the skin temperature drops to about , do you advise the skier to wear a face mask? (b) What is the skin temperature drop due to the wind?
Discuss the modes of heat transfer that determine the equilibrium temperature of the space shuttle Endeavour when it is in orbit. What happens when it reenters the earths atmosphere?
1.71 The thermal conductivity of silver at 212°F is 238 Btu/h ft °F. What is the conductivity in SI units?
A rod with cross-sectional area of 100mm2 is stretched between two fixed points such that the initial tensile load is 10kN. The rod has a modulus of elasticity equal to 180 GPa and a coefficient of linear expansion of 0.00001m/m-C. The initial tension is designed such that the rod will have zero stress when there is a certain change n temperature of the rod environment. What is the temperature (in C, 2 decimal places)?
2. The coefficient of linear expansion of aluminum is 0.000023/C°. An aluminum rod 5 meters long is heated so that its temperature rises from 25 °C to 50 °C. What is the change in its length? 3. A rectangular block of concrete has a volume of 5 cubic meters and initial temperature of 30°C. By how much will it expand if the temperature rises by 5 C°? The coefficient of thermal expansion of concrete is 10/C". 4. Cylindrical bars of steel and brass with the same diameter and length are joined at one end. The tenperature of the steel bar is 350 K; brass, 500 K. What is the temperature at the junction? The thermal conductivity of steel is 45 W/mol K; brass, 116.
What will be the rise in temperature in 30 minutes of a block of copper of 500-gram mass if it is joined to a cylindrical copper rod 20 cm long and 3.0 mm in diameter when a temperature difference of 80 degree Celsius is maintained between ends of the rod? The thermal conductivity of copper is 1.02 cal/cm2-sec-°C/cm (neglect heat losses). Please include FBD