3. An electric immersion heater, 10 mm in diameter and 300 mm long, is rated at 550 W. The emissivity of the heater is ε = 0.85. a) If the heater is horizontally positioned in a large tank of water at 20°C, estimate its surface temperature. b) If the heater is accidentally operated in air at 20°C, estimate the surface temperature. Hint: Radiation heat transfer can be neglected in liquids, but not in gases like air.

2. A heater is a thin vertical panel 1.0m long and 0.7m high and is used in a warehouse to keep workers warm. The heater has air circulating on each side. Assume the maximum temperature of the panel is 60°C (already above the board line that is safe for human hands to touch briefly without getting hurt). Assume the room air temperature is 18°C but the warehouse wall temperature is 5°C. Model the surface with an emittance of 0.9 and Vair = 1.57x105 m²/s. a. Determine the maximum power rating for the heater. b. Now if you run the heat by standing on its side (it will be 1.0 m high and 0.7 m long), determine the surface temperature. c. Compare case a and b and explain any differences you see.

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