A substance that heats up relatively slowly has a
A) high specific heat capacity.
B) low specific heat capacity.
C) either of these
D) neither of these
Answer: A
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Air at an average temperature of 150°C flows through a short square duct 10 × 10 × 2.25 cm at a rate of 15 kg/h. The duct wall temperature is 430°C. Determine the average heat transfer coefficient, using the duct equation with appropriate L/D correction. Compare your results with flow-over-flat-plate relations.
GIVEN
• Air flowing through a short square duct
• Average air temperature (Ta) = 150°C
• Duct dimensions = 10 × 10 × 2.25 cm = 0.1 × 0.1× 0.0225 m
• Duct wall surface temperature (Ts) = 430°C
• Mass flow rate (m ) = 15 kg/h
FIND
The average heat transfer coefficient ( h c) using (a) The duct equation with appropriate L/D correction (b) The flow-over-flat-plate relation
ASSUMPTIONS
• Constant and uniform duct wall temperature
SKETCH
PROPERTIES AND CONSTANTS
for dry air at the average temperature of 150°C
Thermal conductivity (k) = 0.0339 W/(m K)
Absolute viscosity (?b) = 23.683 × 10–6 (Ns)/m2
Prandtl number (Pr) = 0.71 At the surface temperature of 430°C,
the absolute viscosity (?s) = 33.66 × 10–6