Exceedingly high temperatures are needed in a fusion reactor because _______
a. electrostatic barriers must be overcome
b. the fuel must be ionized
c. combustion needs heat to occur
d. water must be boiled and pressurized.
a
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Atmospheric pressure air is heated in a long annulus (25-cm-ID, 38-cm-OD) by steam condensing at 149°C on the inner surface. If the velocity of the air is 6 m/s and its bulk temperature is 38°C, calculate the heat transfer coefficient.
GIVEN
Atmospheric flow through an annulus with steam condensing in inner tube
Inside diameter (Di) = 25 cm = 0.25 m
Outside diameter (Do) = 38 cm = 0.38 m
Steam temperature (Ts) = 149°C
Air velocity (V) = 6 m/s
Air bulk temperature (Tb) = 38°C
FIND
The heat transfer coefficient (hc)
ASSUMPTIONS
Steady state
Steam temperature is constant and uniform
Heat transfer to the outer surface is negligible
Air temperature given is the average air temperature
Thermal resistance of inner tube wall and condensing steam is negligible (Inner tube wall surface
temperature = Ts)
SKETCH
PROPERTIES AND CONSTANTS
for dry air at 38°C
One method of storing solar energy for use during cloudy days, or at night, is to store it in the form of sensible heat in a rock bed. Suppose such a rock bed has been heated to 70°C and it is desired to heat a stream of air by blowing it through the bed. If the air inlet temperature is 10°C and the mass velocity of the air in the bed is 0.5 kg/(s m2), how long must the bed be in order for the initial outlet air temperature to be 65°C? Assume that the rocks are spherical, 2 cm in diameter, and that the bed void fraction is 0.5. (Hint: The surface area of the rocks per unit volume of the bed is (6/Dp)(1 –(?).)
GIVEN
Packed bed of rocks with air blowing through it
Initial temperature of rocks (Tr) = 70°C
Inlet air temperature (Ta,in) = 10°C
Mass velocity of air (m/A) = 0.5 kg/(s m2)
Outlet air temperature (Ta,out) = 65°C
Surface area per unit volume (As/V) = (6/Dp)(1 –(?) )
FIND
Length of bed required (L)
ASSUMPTIONS
Rocks are spherical with diameter (D) = 2 cm = 0.02 m
Void fraction of the bed (?) = 0.5
Rock temperature remains practically constant
SKETCH
If the magnetic field points in the +z direction but is decreasing in magnitude, which way will current flow?
A. Counterclockwise B. Clockwise C. No current will flow.
Which one of the following quantities is at a maximum when an object in simple harmonic motion is at its maximum displacement?
a. speed b. acceleration c. kinetic energy d. frequency