Dr. Chuck projects a ball horizontally from a lab bench. The ball lands on a bullseye marked on the floor a horizontal distance equal to the ball's initial speed
A) multiplied by its time in the air.
B) coupled with its speed of fall.
C) squared plus its downward speed squared when hitting the floor.
D) all of the above
E) none of the above
Answer: A
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Water flowing in a long aluminum tube is to be heated by air flowing perpendicular to the exterior of the tube. The ID of the tube is 1.85 cm and its OD is 2.3 cm. The mass flow rate of the water through the tube is 0.65 kg/s and the temperature of the water in the tube averages 30°C. The free stream velocity and ambient temperature of the air are 10 m/s and 120°C, respectively. Estimate the overall heat transfer coefficient for the heat exchanger using appropriate correlations from previous chapters. State all your assumptions.
GIVEN
• Air flowing perpendicular to the exterior of an aluminum tube with water flowing within the tube
• Tube diameters
? Inside (Di) = 1.85 cm = 0.0185 m
? Outside (Do) = 2.3 cm = 0.023 m
• Mass flow rate of water (mw) = 0.65 kg/s
• Average temperature of the water (Tw) = 30°C
• Air free stream velocity (Va) = 10 m/s
• Air temperature (Ta) = 120°C FIND
• The overall heat transfer coefficient (U)
ASSUMPTIONS
• Steady state
• The variation of the Prandtl number of air with temperature is negligible
• The aluminum is pure
SKETCH
PROPERTIES AND CONSTANTS
thermal conductivity of aluminum (kal) = 238 W/(m K) at 75°C
for water at 30°C
Density (?w) = 996 kg/m3 Thermal conductivity (kw) = 0.615 W/(m K)
Absolute viscosity (?w) = 792 × 10–6 (Ns)/m2
Prandtl number (Prw) = 5.4
for dry air at 120°C
Thermal conductivity (ka) = 0.0320 W/(m K)
Kinematic viscosity (?) = 26.0 × 10–6 m2/s
Prandtl number (Pr) = 0.71
Which kind of object is the best standard candle for measuring distances to extremely distant galaxies?
A. a white dwarf. B. a Cepheid variable star. C. a white dwarf supernova.
Suppose we represent Earth with a basketball. On this scale, most of the air in Earth's atmosphere would fit in a layer that is
A) about the thickness of a sheet of paper. B) about an inch thick. C) about 6 inches thick. D) about a half-inch thick.
A girl is sitting on the edge of a pier with her legs dangling over the water. Her soles are 80.0 cm above the surface of the water. A boy in the water looks up at her feet and wants to touch them with a reed. (nwater = 1.333.) He will see her soles as being
A. right at the water surface. B. 53.3 cm above the water surface. C. exactly 80.0 cm above the water surface. D. 107 cm above the water surface. E. an infinite distance above the water surface.