A 2.5 cm sphere is maintained at 50°C in either an air stream or a water stream, both at 20°C and 2 m/s velocity. Compare the rate of heat transfer and the drag on the sphere for both fluids.
GIVEN
• A sphere in an air stream or a water stream
• Sphere diameter (D) = 2.5 cm = 0.025 m
• Sphere temperature (Ts) = 50°C
• Fluid temperature (Tf) = 20°C
• Fluid velocity (U?) = 2 m/s FIND
• The rate of heat transfer (q) and the drag force
ASSUMPTIONS
• Radiation is negligible
SKETCH
PROPERTIES AND CONSTANTS
The Reynolds number is
For air
For water
can be applied to both cases
For air
For water
The rate of heat transfer is
For air
For water
The total drag coefficient can be read and is defined in
For air, CD = 0.4
COMMENTS
Note that the heat transfer increases by a factor of 324 in water while the drag force increases by a
factor of 1065.
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