In analyzing the performance of a thermal system, the engineer must be able to i

Question

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 109 s1, 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 operation of a
microwave clothes dryer differ from a conventional
dryer? Which is likely to have the greater energy
utilization efficiency and why?

Explanation / Answer

a)

Heat transfer occurs via conduction and radiation

Conduction occurs via the air and asphalt pavement interface

radiation accurs through the sunlight

These two processes determine the heat flow and thus the temperature of the pavement

b)

For microwave oven, the electomagnetic radiation is used for heating.

The radiation causes the rotation of polar molecules of the food and thus causes its heating

In a conventional oven, the method is both radation and convection

A microwave has a greater efficiency because of the uniform heating of the food as opposed to the heating from bottom to top of the food in convenctional oven

A microwave dryer will use heat to dry the clothes but a conventional dryer uses the rotational energy to pull out the water particles in the clothes

Related Questions

Navigate

• Browse (All)
• Browse I
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.