One stone is dropped from the side of a bridge, and a second is dropped from the same place a short time later. What is true about the distance between the stones as a function of time after both have been dropped? Ignore air resistance
A. Increases, finally reaching a constant value
B. Always increases
C. Is constant
D. Decreases, and then reaches a constant value
E. Always decreases
B. Always increases
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Exhaust gases from a power plant are used to preheat air in a cross-flow heat exchanger. The exhaust gases enter the heat exchanger at 450°C and leave at 200°C. The air enters the heat exchanger at 70°C, leaves at 250°C, and has a mass flow rate of 10 kg/s. Assume the properties of the exhaust gases can be approximated by those of air. The overall heat transfer coefficient of the heat exchanger is 154 W/(m2 K). Calculate the heat exchanger surface area required if (a) the air is unmixed and the exhaust gases are mixed and (b) both fluids are unmixed.
GIVEN
• Cross-flow heat exchanger – exhaust gas to air
• Exhaust temperatures
? Te,in = 450°C
? Te,out = 200°C
• Air temperatures
? Ta,in = 70°C
? Ta,out = 250°C
• Air flow rate m a= 10 kg/s
• Overall heat transfer coefficient (U) = 154 W/(m2 K)
FIND
The heat exchanger surface area (A) if
(a) Air is unmixed, exhaust is mixed
(b) Both are unmixed
ASSUMPTIONS
• The properties of the exhaust gases can be approximated by those of air
• The air is in the tube
SKETCH
PROPERTIES AND CONSTANTS
the specific heat of air at the mean temperature of 160°C (cpa)
When a projectile is launched, the velocity may be described as:
a. increasing in the vertical direction. b. decreasing in the horizontal direction. c. constant in the vertical direction. d. constant in the horizontal direction.
Elliptical galaxies are more likely to be found in clusters than are spiral galaxies
Indicate whether the statement is true or false
Which of the following molecules is the most polar?
A) HCl B) BrF C) CO D) Image