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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