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University of Nairobi Thermodynamics and Heat Transfer Exercise

Question 1:

Children asked their parent for an ice cream after dinner. They eat their ice cream as long as it does

not melt and its structure is solid. The parent drove to buy an ice cream in the grocery store. The

ice cream is sold in family sized package. The store keeps the ice cream packages in the freezer

until it is sold. The packages are made from polystyrene material and their size is 0.2´0.1´0.07

m3. The store freezer temperature is kept at -10oC. The ice cream has a solid structure (not melting)

as long as its temperature remains below 0oC. The time it takes to reach back from the store is 20

minutes Will the kids eat the ice cream at home?

Ice cream properties: k = 2.2 W/moC, C = 1800 J/kgoC, r = 900 kg/m3 and h = 10 W/m2oC.

Question 2:

Air surrounding a light bulb is moving at 25 m/sec. The surrounding conditions are kept at

atmospheric pressure and at temperature 25oC. The light bulb external surface is kept at 140oC.

What is the heat transfer rate from the light bulb to the surrounding? Assume the light bulb as a

sphere with a diameter of 50 mm.

Question 3:

An underwater sonar that maps the ocean bathymetry is encapsulated in a sphere with a diameter

of 85 mm. During operation, the sonar generates heat at a rate of 300W.

What is the sonar surface temperature when it’s located in a water column where the temperature

is 15oC and the water current is 1 m/sec?

The sonar was pulled out of the water without turning it off, thus, it was still working. The air

temperature was 15oC and the air speed was 3 m/sec. What was the sonar surface temperature?

Was there any reason for concern?

Question 4:

In order to cool down a hot steel sphere (its diameter is 5 cm), CO2 gas is blown over it through a

pipe with a diameter of 10 cm. The CO2 gas is kept at atmospheric pressure while moving through

a smooth pipe at a speed of 6 m/sec. The gas temperature entering the pipe is 300K and exiting the

pipe is 340 K. The pipe temperature at the entrance is 350K and at the exit is 550K. The sphere is

located just about the pipe exit.

Find the convective heat transfer coefficient of the gas moving in the pipe, the heat transfer rate at

the pipe and the pipe length.

What is the surface temperature of the sphere if the heat transfer rate between the sphere and the

gas is 7W and its surface temperature is higher than the gas?


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