Homework Heat Transfer

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HW 7



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3- Solution

1. You are driving down a highway on a sunny 7oC, fall day. The sun heats the 1m2, flat, steel hood of your car to 93oC. Just as you enter a tunnel, your engine makes an odd noise. You slow to a constant speed of 30 miles per hour through the 1 mile long tunnel. At the exit of the tunnel, you pull over to look under the hood. If the hood must be below 60oC to prevent burning your hand, can you safely touch the hood with your bare hand? (Note: The thickness of the hood is 2.5 mm. Neglect any outside sources of heating, such as the engine that may still be warm.)

2. A steel radiant tube, that is 8 inches in diameter and 8 feet long, is placed inside of a convective oven. An electric element inside of the tube produces 33 kw of energy. If 5000 ft3/minute of air, at 500oC, is passed over the tube (flowing perpendicular to the length of the tube). What will be the surface temperature of the tube and the temperature of the air after passing over the tube?

3. If the incoming Nitrogen is at a pressure of 25 psi and temperature of 20oC, and I wish to flow 1200ft3/hr. of Nitrogen through a smooth black carbon steel pipe; what diameter of pipe should I use if I want fully developed laminar flow and a pressure of 1 psi at the exit of the pipe, 300 ft. away?

4. After pressure spraying, there is a thin layer of water (at 70oF) left inside of a 24 inch diameter retort that is 20 ft. long. A jet heater is place in front of the entrance to the tube. The jet heater is turned on and produces 2000 ft3/minute of 120oF air. What is the mass transfer convection coefficient of the water into the air?

5. The surface of a furnace may never be hotter than 140oF. If the furnace is sitting inside of a 70oF, small room, where the air movement is minimal, what is the maximum heat flux that is permissible through the insulation and side steel? (note: The side of the furnace is 8 ft. high.)

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