A 4.0-kg mass, initially at rest on a horizontal frictionless surface, is struck by a 2.0-kg mass moving along the x axis with a speed of 8.0 m/s. After the collision, the 2.0-kg mass has a speed of 4.0 m/s at an angle of 37° from the positive x axis. What is the speed of the 4.0-kg mass after the collision?
A. 2.0 m/s
B. 2.7 m/s
C. 4.9 m/s
D. 2.4 m/s
E. 3.6 m/s
Answer: B
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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