1.
The heat transfer coefficient for air flowing over a sphere is to be determined by observing the
temperaturetime history of a sphere fabricated from pure copper. The sphere, which is 12.7 mm
in diameter, is at 66ºC before it is inserted into an airstream having a temperature of 27ºC. A
thermocouple on the outer surface of the sphere indicates 55ºC, 69 seconds after the sphere is
inserted in the airstream. Assume, and then justify, that the sphere behaves as a spacewise
isothermal object and calculate the heat transfer coefficient.
ASSUMPTIONS:
(1) Temperature of sphere is spatially uniform,
(2) Negligible radiation exchange,
(3) Constant properties.
PROPERTIES:
Table A1
, for pure copper (333K):
ρ = 8933 kg/m
3
, c
p
= 389 J/kg K, k = 398 W/m K.
ANALYSIS:
Assuming the uniform temperature over the sphere, we use the Lumped Capacitance Method.
t
cL
ρ
h
t
cV
ρ
hA
T
T
T
T
c
i
exp
exp
where
002117
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 Spring '11
 koraykadirsafak
 Heat, Heat Transfer, heat transfer coefficient, uniform temperature, temperature difference, Ti T L

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