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example 7-1 - Example 7-1 Reconsider Example 5-1 this time...

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Example 7-1 Reconsider Example 5-1, this time including radiation heat transfer. In Example 5-1, heat leaves a toaster oven through a glass viewing window that is 1.5 cm thick, 8 cm high and 12 cm wide. The inside surface of the glass is at 200°C and the room is at 25°C. Heat is transferred by conduction through the glass and by both natural convection and radiation to the surroundings. If the convection heat transfer coefficient is 4 W/m 2 ·K, and the emissivity of the glass is 0.92, find the rate of heat loss through the window. The radiative heat transfer coefficient depends on the outer surface temperature of the glass, which is an unknown. To simplify the problem, assume the outer surface of the glass is at the temperature calculated in Example 5-1, or 193°C. Solution: cond conv rad Q Q Q = + ± ± ± ( ) 3 2 2 3 rad s s surr s surr surr h T T T T T T εσ = + + + From Example 5-1, o 193 C 466K s T =

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