ME/AER 325: Elements of Heat Transfer
Homework 2
1. 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 appropriate labeled arrows on a sketch
of the system. Answer additional questions as they appear in the problem statements.
(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 not 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 109 s−1 , causing heat to be generated. Contrast cooking in a microwave oven with cooking
in a conventional radiant or convective oven. I/n each case, what is the physical mechanism
responsible for heating the food? Which oven has the greater energy utilization efficiency?
Why? Microwave heating is 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?
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(c) 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. See figure.
figure for part c.
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(d) 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. See figure.
figure for part d.
(e) 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,
open-ended 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? See figure.
figure for part e.
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(f) 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.
figure for part f.
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