03-07-2012, 03:04 PM
A Heat Transfer Textbook
heat transfer.pdf (Size: 6.15 MB / Downloads: 203)
Introduction
The radiation of the sun in which the planet is incessantly plunged, penetrates
the air, the earth, and the waters; its elements are divided, change
direction in every way, and, penetrating the mass of the globe, would raise
its temperature more and more, if the heat acquired were not exactly
balanced by that which escapes in rays from all points of the surface and
expands through the sky. The Analytical Theory of Heat, J. Fourier
Heat transfer
People have always understood that something flows from hot objects to
cold ones. We call that flow heat. In the eighteenth and early nineteenth
centuries, scientists imagined that all bodies contained an invisible fluid
which they called caloric. Caloric was assigned a variety of properties,
some of which proved to be inconsistent with nature (e.g., it had weight
and it could not be created nor destroyed). But its most important feature
was that it flowed from hot bodies into cold ones. It was a very useful
way to think about heat. Later we shall explain the flow of heat in terms
more satisfactory to the modern ear; however, it will seldom be wrong to
imagine caloric flowing from a hot body to a cold one.
The flow of heat is all-pervasive. It is active to some degree or another
in everything. Heat flows constantly from your bloodstream to the air
around you. The warmed air buoys off your body to warm the room you
are in. If you leave the room, some small buoyancy-driven (or convective)
motion of the air will continue because the walls can never be perfectly
isothermal. Such processes go on in all plant and animal life and in the
air around us. They occur throughout the earth, which is hot at its core
and cooled around its surface. The only conceivable domain free from
heat flow would have to be isothermal and totally isolated from any other
region. It would be “dead” in the fullest sense of the word — devoid of
Introduction §
any process of any kind.
The overall driving force for these heat flow processes is the cooling
(or leveling) of the thermal gradients within our universe. The heat flows
that result from the cooling of the sun are the primary processes that we
experience naturally. The conductive cooling of Earth’s center and the radiative
cooling of the other stars are processes of secondary importance
in our lives.
The life forms on our planet have necessarily evolved to match the
magnitude of these energy flows. But while “natural man” is in balance
with these heat flows, “technological man”1 has used his mind, his back,
and his will to harness and control energy flows that are far more intense
than those we experience naturally. To emphasize this point we suggest
that the reader make an experiment.
Generate as much power as you can, in some way that permits you to
measure your own work output. You might lift a weight, or run your own
weight up a stairwell, against a stopwatch. Express the result in watts (W).
Perhaps you might collect the results in your class. They should generally
be less than 1 kW or even 1 horsepower (746 W). How much less might
be surprising.
Thus, when we do so small a thing as turning on a 150 W light bulb,
we are manipulating a quantity of energy substantially greater than a
human being could produce in sustained effort. The energy consumed
by an oven, toaster, or hot water heater is an order of magnitude beyond
our capacity. The energy consumed by an automobile can easily be three
orders of magnitude greater. If all the people in the United States worked
continuously like galley slaves, they could barely equal the power output
of even a single city power plant.
Our voracious appetite for energy has steadily driven the intensity
of actual heat transfer processes upward until they are far greater than
those normally involved with life forms on earth. Until the middle of the
thirteenth century, the energy we use was drawn indirectly from the sun
1Some anthropologists think that the term Homo technologicus (technological man)
serves to define human beings, as apart from animals, better than the older term Homo
sapiens (man, the wise). We may not be as much wiser than the animals as we think we
are, but only we do serious sustained tool making. Heat transfer 5
using comparatively gentle processes — animal power, wind and water
power, and the combustion of wood. Then population growth and deforestation
drove the English to using coal. By the end of the seventeenth
century, England had almost completely converted to coal in place of
wood. At the turn of the eighteenth century, the first commercial steam
engines were developed, and that set the stage for enormously increased
consumption of coal. Europe and America followed England in these
developments.
The development of fossil energy sources has been a bit like Jules
Verne’s description in Around the World in Eighty Days in which, to win
a race, a crew burns the inside of a ship to power the steam engine. The
combustion of nonrenewable fossil energy sources (and, more recently,
the fission of uranium) has led to remarkably intense energy releases in
power-generating equipment. The energy transferred as heat in a nuclear
reactor is on the order of one million watts per square meter.
A complex system of heat and work transfer processes is invariably
needed to bring these concentrations of energy back down to human proportions.
We must understand and control the processes that divide and
diffuse intense heat flows down to the level on which we can interact with
them. To see how this works, consider a specific situation. Suppose we
live in a town where coal is processed into fuel-gas and coke. Such power
supplies used to be common, and they may return if natural gas supplies
ever dwindle. Let us list a few of the process heat transfer problems that
must be solved before we can drink a glass of iced tea.
• A variety of high-intensity heat transfer processes are involved with
combustion and chemical reaction in the gasifier unit itself.
• The gas goes through various cleanup and pipe-delivery processes
to get to our stoves. The heat transfer processes involved in these
stages are generally less intense.
• The gas is burned in the stove. Heat is transferred from the flame to
the bottom of the teakettle. While this process is small, it is intense
because boiling is a very efficient way to remove heat.
• The coke is burned in a steam power plant. The heat transfer rates
from the combustion chamber to the boiler, and from the wall of
the boiler to the water inside, are very intense.
• The steam passes through a turbine where it is involved with many
heat transfer processes, including some condensation in the last
6 Introduction §1.2
stages. The spent steam is then condensed in any of a variety of
heat transfer devices.
• Cooling must be provided in each stage of the electrical supply system:
the winding and bearings of the generator, the transformers,
the switches, the power lines, and the wiring in our houses.
• The ice cubes for our tea are made in an electrical refrigerator. It
involves three major heat exchange processes and several lesser
ones. The major ones are the condensation of refrigerant at room
temperature to reject heat, the absorption of heat from within the
refrigerator by evaporating the refrigerant, and the balancing heat
leakage from the room to the inside.
• Let’s drink our iced tea quickly because heat transfer from the room
to the water and from the water to the ice will first dilute, and then
warm, our tea if we linger.
A society based on power technology teems with heat transfer problems.
Our aim is to learn the principles of heat transfer so we can solve
these problems and design the equipment needed to transfer thermal
energy from one substance to another. In a broad sense, all these problems
resolve themselves into collecting and focusing large quantities of
energy for the use of people, and then distributing and interfacing this
energy with people in such a way that they can use it on their own puny
level.
We begin our study by recollecting how heat transfer was treated in
the study of thermodynamics and by seeing why thermodynamics is not
adequate to the task of solving heat transfer problems.
Relation of heat transfer to thermodynamics
The First Law with work equal to zero
The subject of thermodynamics, as taught in engineering programs, makes
constant reference to the heat transfer between systems. The First Law
of Thermodynamics for a closed system takes the following