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The water in a 40-L tank is to be heated from 15°C to 45°C by a...

The water in a 40-L tank is to be heated from 15°C to 45°C by a 21-cm-diameter spherical heater whose surface temperature is maintained at 85°C. Determine how long the heater should be kept on.

Given: The properties of air at 1 atm and the film temperature of 57.5°C.

Fluid temperature = Average temperature for water (15+45)/2=30°C

k = 0.6515 W/m.°C, v = 0.493 x 10-6 m2/s, Pr = 3.12, and ? = 0.501 x 10-3 K-1

The properties of water at 30°C.
? = 996 kg/m3 and cp = 4178 J/kg.°C, and volume, V = 0.040 m3

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