A composite refrigerator wall is composed of 5 cm of corkboard sandwiched between a 1.2 cm thick layer of oak and a 0.8 mm thick layer of aluminum lining on the inner surface. The average convective heat transfer coefficients at the interior and exterior wall are 11 and 8 W/(m2 K), respectively. (a) Draw the thermal circuit. (b) Calculate the individual resistances of the components of this composite wall and the resistances at the surfaces. (c) Calculate the overall heat transfer coefficient through the wall. (d) For an air temperature of -1 0C inside the refrigerator and 32°C outside, calculate the rate of heat transfer per unit area through the wall.
GIVEN
• Refrigerator wall: oak, corkboard, and aluminum
• Thicknesses
? Oak (Lo) = 1.2 cm= 1.2*10-2 m
? Corkboard (Lc) = 5 cm= 5*10-2 m
? Aluminum (La) = 0.8 mm = 8*10-4 m
• Convective heat transfer coefficients
? Interior ( ci h ) = 12 W/(m2 K)
? Exterior ( co h ) = 8 W/(m2 K)
• Air temperature
? Inside (Ti) = -1°C
? Outside (To) = 32°C
FIND
(a) Draw the thermal circuit (b) The individual resistances (c) Overall heat transfer coefficient (U) (d) Rate of heat transfer per unit area (q/A)
ASSUMPTIONS
• One dimensional, steady state heat transfer
• Constant thermal conductivities
• Contact resistance between the different materials is negligible
SKETCH
PROPERTIES AND CONSTANTS
Oak (ko)= 0.19 W/(m K) at 20°C
Corkboard (kc) = 0.0415 W/(m K) at 20°C
Aluminum (ka) = 236 W/(m K) at 0°C
(a) The thermal circuit for the refrigerator wall is shown below
(b) The resistances to convection from the inner and outer surfaces is given
The resistances to conduction through the components of the wall is given
(c) The overall heat transfer coefficient satisfies
(d) The rate of heat transfer through the wall is given
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Fill in the blank(s) with the appropriate word(s).
The Moon is about one-quarter the diameter of Earth. How does its mass compare?
A. The Moon is 1/81 as massive as Earth. B. The Moon is 1/4 as massive as Earth. C. The Moon is 1/6 as massive as Earth. D. The Moon is 1/32 as massive as Earth.
A long 0.6-m-OD 347 stainless steel (k = 14 W/(m K)) cylindrical billet at 16°C room temperature is placed in an oven where the temperature is 260°C. If the average heat transfer coefficient is 170 W/(m2 K), (a) estimate the time required for the center temperature to increase to 232°C by using the appropriate chart and (b) determine the instantaneous surface heat flux when the center temperature is 232°C. but assume that the billet is only 1.2-m-long and the average heat transfer coefficient at both ends is 136 W/(m2 K).
GIVEN
• A cylindrical billet placed in an over
• Billet outside diameter = 0.6 m
• Thermal conductivity (k) = 14 W/(m K)
• Initial temperature (To) = 16°C
• Oven temperature (T?) = 260°C
• The average heat transfer coefficient h cs = 170 W/(m2 K)
• Increase of the center temperature is 232°C
• Billet length (2L) = 1.2 m
• Heat transfer coefficient at the ends h ce = 136 W/(m2 K)
FIND
• The time required using the appropriate charts
ASSUMPTIONS
• Two dimensional conduction within the billet
• Constant and uniform thermal properties
• Constant oven temperature
SKETCH
The speed of light changes when it goes from ethyl alcohol (n = 1.36) to carbon tetrachloride (n = 1.46). The ratio of the speed in carbon tetrachloride to the speed in ethyl alcohol, v2/v1, is
a. 1.99. b. 1.07. c. 0.932. d. 0.511. e. 0.760.