In analyzing the performance of a thermal system, the engineer must be able to
identify the relevant heat transfer processes. Only then can the system
behavior be properly quantified. For the following systems, identify the
pertinent processes, designating them by appropriately labeled arrows on a
sketch of the system. Answer additional questions that appear in the problem
statement.
(a) Identify the heat transfer processes that determine the temperature of an
asphalt pavement on a summer day. Write an energy balance for the surface of
the pavement.
(b) Microwave radiation is known to be transmitted by plastics, glass, and
ceramics but to be absorbed by materials having polar molecules such as water.
Water molecules exposed to microwave radiation align and reverse alignment
with the microwave radiation at frequencies up to \(10^{9} \mathrm{~s}^{-1}\),
causing heat to be generated. Contrast cooking in a microwave oven with
cooking in a conventional radiant or convection oven. In each case, what is
the physical mechanism responsible for heating the food? Which oven has the
greater energy utilization efficiency? Why? Microwave heating is being
considered for drying clothes. How would the operation of a microwave clothes
dryer differ from a conventional dryer? Which is likely to have the greater
energy utilization efficiency? Why?
(c) To prevent freezing of the liquid water inside the fuel cell of an
automobile, the water is drained to an onboard storage tank when the
automobile is not in use. (The water is transferred from the tank back to the
fuel cell when the automobile is turned on.) Consider a fuel cell-powered
automobile that is parked outside on a very cold evening with
\(T_{\infty}=-20^{\circ} \mathrm{C}\). The storage tank is initially empty at
\(T_{i, t}=-20^{\circ} \mathrm{C}\), when liquid water, at atmospheric pressure
and temperature \(T_{i, w}=50^{\circ} \mathrm{C}\), is introduced into the tank.
The tank has a wall thickness \(t_{t}\) and is blanketed with insulation of
thickness \(t_{\text {ins }}\). Identify the heat transfer processes that will
promote freezing of the water. Will the likelihood of freezing change as the
insulation thickness is modified? Will the likelihood of freezing depend on
the tank wall's thickness and material? Would freezing of the water be more
likely if plastic (low thermal conductivity) or stainless steel (moderate
thermal conductivity) tubing is used to transfer the water to and from the
tank? Is there an optimal tank shape that would minimize the probability of
the water freezing? Would freezing be more likely or less likely to occur if a
thin sheet of aluminum foil (high thermal conductivity, low emissivity) is
applied to the outside of the insulation?
(d) Your grandmother is concerned about reducing her winter heating bills. Her
strategy is to loosely fit rigid polystyrene sheets of insulation over her
double-pane windows right after the first freezing weather arrives in the
autumn. Identify the relevant heat transfer processes on a cold winter night
when the foamed insulation sheet is placed (i) on the inner surface and (ii)
on the outer surface of her window. To avoid condensation damage, which
configuration is preferred? Condensation on the window pane does not occur
when the foamed insulation is not in place.
(e) There is considerable interest in developing building materials with
improved insulating qualities. The development of such materials would do much
to enhance energy conservation by reducing space heating requirements. It has
been suggested that superior structural and insulating qualities could be
obtained by using the composite shown. The material consists of a honeycomb,
with cells of square cross section, sandwiched between solid slabs. The cells
are filled with air, and the slabs, as well as the honeycomb matrix, are
fabricated from plastics of low thermal conductivity. For heat transfer normal
to the slabs, identify all heat transfer processes pertinent to the
performance of the composite. Suggest ways in which this performance could be
enhanced.
(f) A thermocouple junction (bead) is used to measure the temperature of a hot
gas stream flowing through a channel by inserting the junction into the
mainstream of the gas. The surface of the channel is cooled such that its
temperature is well below that of the gas. Identify the heat transfer
processes associated with the junction surface. Will the junction sense a
temperature that is less than, equal to, or greater than the gas temperature?
A radiation shield is a small, openended tube that encloses the thermocouple
junction, yet allows for passage of the gas through the tube. How does use of
such a shield improve the accuracy of the temperature measurement?
(g) A double-glazed, glass fire screen is inserted between a wood-burning
fireplace and the interior of a room. The screen consists of two vertical
glass plates that are separated by a space through which room air may flow
(the space is open at the top and bottom). Identify the heat transfer
processes associated with the fire screen.
(h) A thermocouple junction is used to measure the temperature of a solid
material. The junction is inserted into a small circular hole and is held in
place by epoxy. Identify the heat transfer processes associated with the
junction. Will the junction sense a temperature less than, equal to, or
greater than the solid temperature? How will the thermal conductivity of the
epoxy affect the junction temperature?
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