The force on the walls of a vessel of a contained gas is due to:
A. the repulsive force between gas molecules
B. a slight loss in the speed of a gas molecule during a collision with the wall
C. a change in momentum of a gas molecule during a collision with the wall
D. elastic collisions between gas molecules
E. inelastic collisions between gas molecules
Answer: C. a change in momentum of a gas molecule during a collision with the wall
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A pressure of 10.0 mm Hg is measured using a constant-volume gas thermometer at a temperature of 50.0°C. What is the pressure (in mm Hg) at the zero-point temperature?
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Water at 80°C is flowing through a thin copper tube (15.2-cm-ID) at a velocity of 7.6 m/s. The duct is located in a room at 15°C and the heat transfer coefficient at the outer surface of the duct is 14.1 W/(m2 K). (a) Determine the heat transfer coefficient at the inner surface. (b) Estimate the length of duct in which the water temperature drops 1°C.
GIVEN
• Water flowing through a thin copper tube in a room
• Water temperature (Tb) = 80°C
• Inside diameter of tube (D) = 15.2 cm = 0.152 cm
• Water velocity (V) = 7.6 m/s
• Room air temperature (T?) = 15°C
• Outer surface heat transfer coefficient ( h co ) = 14.1 W/(m2 K)
FIND
(a) The heat transfer coefficient at the inner surface ( h ci )
(b) Length of duct (L) for temperature drop of 1°C
ASSUMPTIONS
• Steady state
• Thermal resistance of the copper tube is negligible
• Fully developed flow
SKETCH
PROPERTIES AND CONSTANTS
for water at 80°C
Density (?) 971.6 kg/m3
Thermal conductivity (k) = 0.673 W/(m K)
Absolute viscosity (?) = 356.7 × 10–6 (Ns)/m2
Prandtl number (Pr) = 2.13
Specific heat (c) = 4194 J/(kg K)