heat transfer and thermodynamics

1. DR. MUHAMMAD GHAFFAR DOGGAR
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2nd LAW OF THERMODYNAMICS
HEAT TRANSFER AND
THERMODYNAMICS

2. BACKGROUND
The first law of thermodynamics asserts that ”energy must be conserved in
any process involving the exchange of heat and work between a system
and its surroundings”. A machine that violated the first law would be called
a perpetual motion machine of the first kind because it would manufacture its
own energy out of nothing and thereby run forever. Such a machine would
be impossible even in theory.
However, this impossibility would not prevent the construction of a machine that
could extract essentially limitless amounts of heat from its surroundings (earth,
air, and sea) and convert it entirely into work. Although such a hypothetical
machine would not violate conservation of energy, the total failure of inventors
to build such a machine, known as a perpetual motion machine of the second
kind, led to the discovery of the second law of thermodynamics.
The second law of thermodynamics can be precisely stated in two forms, as
originally formulated in the 19th century by the Scottish physicist William
Thomson (Lord Kelvin) and the German physicist Rudolf Clausius respectively:
“A cyclic transformation whose only final result is to transform heat extracted
from a source which is at the same temperature throughout into work is
impossible”.
“A cyclic transformation whose only final result is to transfer heat from a body at
a given temperature to a body at a higher temperature is impossible”.

4. Kelvin-Planck Statement
Heat Engines
“It is impossible to extract an amount of heat QH from a hot
reservoir and use it all to do work W”. Some amount of heat
QC must be exhausted to a cold reservoir. This pre-cludes a perfect heat
engine. This is sometimes called the "first form" of the second law, and
is referred to as the Kelvin-Planck statement of the second law.

5. Clausius Statement
Refrigerator
It is not possible for heat to flow from a colder body to a
warmer body without any work having been done to accomplish
this flow. Energy will not flow spontaneously from a
low temperature object to a higher temperature object. This
precludes a perfect refrigerator.
The statements about refrigerators apply to air conditioners
and heat pumps, which embody the same principles. This is the
"second form" or the Clausius statement of the second law.

6. Clausius Statement
It is important to note that when it is stated that
energy will not spontaneously flow from a cold object
to a hot object, that statement is referring to net
transfer of energy.
Energy can transfer from the cold object to the hot
object either by transfer of energetic particles or
electromagnetic radiation, but the net transfer will be
from the hot object to the cold object in any
spontaneous process.
Work is required to transfer net energy to the hot
object.

7. ENTROPY
Entropy
In any cyclic process the entropy will either increase or remain the same.
Entropy
a state variable whose change is defined for a
reversible process at T where Q is the heat
absorbed.
a measure of the amount of energy which is
unavailable to do work.
a measure of the disorder of a system.
a measure of the multiplicity of a system.
Since entropy gives information about the evolution (slow development) of an isolated
system with time, it is said to give us the direction of "time's arrow" . If snapshots of a
system at two different times shows one state which is more disordered, then it could be
implied that this state came later in time. For an isolated system, the natural course of events
takes the system to a more disordered (higher entropy) state.

8. 2nd Law of Thermodynamics
Work and Energy
One thing the Second Law explains is that it is impossible to
convert heat energy to mechanical energy with 100% efficiency.
After the process of heating a gas to increase its pressure to
drive a piston, there is always some left over heat in the gas
that cannot be used to do any additional work. This waste heat
must be discarded by transferring it to a heat sink source.
In the case of a car engine, this is done by exhausting the spent
fuel and air mixture to the atmosphere. Additionally, any device
with movable parts produces friction that converts mechanical
energy to heat that is generally un-usable and must be removed
from the system by transferring it to a heat sink. This is why
claims for perpetual motion machines are summarily rejected by
the US Patent Office.

9. 2nd Law of Thermodynamics
Work and Energy
When a hot and a cold body are brought into contact with
each other, heat energy will flow from the hot body to the
cold body until they reach thermal equilibrium, i.e., the same
temperature. However, the heat will never move back the
other way; the difference in the temperatures of the two
bodies will never spontaneously increase.
Moving heat from a cold body to a hot body requires work to
be done by an external energy source such as a heat pump.
The most efficient engines we build right now are large gas
turbines, said David McKee, a professor of physics at
Missouri State University. “They burn natural gas or other
gaseous fuels at very high temperature, over 2,000 degree
C, and the exhaust coming out is just a warm breeze.
Nobody tries to extract energy from the waste heat, because
there’s just not that much there.”

10. 2nd Law of Thermodynamics
The arrow of time
The Second Law indicates that thermodynamic processes
that involve the transfer or conversion of heat energy, are
irreversible because they all result in an increase in entropy.
An irreversible process is a thermodynamic process that departs
from equilibrium. In terms of pressure and volume, it occurs when
the pressure, or the volume of a system changes dramatically and
instantaneously that the volume or the pressure, do not have the
time to reach equilibrium. When we tear a page from our notebooks,
we cannot change this and ‘un-tear’. This is an irreversible process.
One of the most consequential implications of the Second
Law, according to Mitra, is that it gives us the
thermodynamic arrow of time. In theory, some
interactions, such as collisions of rigid bodies or certain
chemical reactions, look the same whether they run forward
or backward.

11. 2nd Law of Thermodynamics
The arrow of time
In practice, however, all exchanges of energy are
subject to in-efficiencies, such as friction and
radiative heat loss, which increase the entropy
of the system being observed.
Therefore, because there is no such thing as a
perfectly reversible process, if someone asks what
is the direction of time, we can answer with
confidence that time always flows in the
direction of increasing entropy.

12. 2nd Law of Thermodynamics
The Fate of the Universe
The Second Law also predicts the end of the universe,
according to Boston University. “The universe will end
in a ‘heat death’ in which everything is at the same
temperature. This is the ultimate level of disorder; if
everything is at the same temperature, no work can be
done, and all the energy will end up as the random
motion of atoms and molecules.”
In the far distant future, stars will have used up all of
their nuclear fuel ending up as stellar remnants (a
number of the objects we see in the sky are not stars
but the remains of stars that have died) such as white
dwarfs, neutron stars or black holes,

13. 2nd Law of Thermodynamics
The Fate of the Universe
According to Margaret Murray Hanson, a physics
professor at the University of Cincinnati, they will
eventually evaporate into protons, electrons,
photons and neutrinos, ultimately reaching
thermal equilibrium with the rest of the Universe.
Fortunately, John Baez, a mathematical physicist
at the University of California Riverside, predicts
that this process of cooling down could take long,
millions of years with the temperature dropping to
around 10−30 Kelvin.

14. Entropy and Heat Death
The example of a heat engine illustrates one of the many
ways in which the 2nd law of thermodynamics can be
applied. One way to generalize the example is to consider
the heat engine and its heat reservoir as parts of an
isolated, closed, system i.e., one that does not exchange
heat or work with its surroundings.
For example, the heat engine and reservoir could be
encased in a rigid container with insulating walls. In this
case the second law of thermodynamics says that no matter
whatever process takes place inside the container,
its entropy must increase or remain the same in the limits
of a reversible process.

15. Entropy and Heat Death
Similarly, if the universe is an isolated system, then its
entropy too must increase with time. Indeed,
the implication is that the universe must ultimately suffer a
“heat death” as its entropy progressively increases toward a
maximum value and all parts come into
thermal equilibrium at a uniform temperature. After that
point, no further changes involving the conversion of heat
into useful work would be possible.
In general, the equilibrium state for an isolated system is
precisely that state of maximum entropy. This is equivalent
to an alternate definition for the term entropy as a measure
of the disorder of a system, such that a completely random
dispersion of elements corresponds to maximum entropy,
or minimum information.

16. Examples …
How Energy Flows from Useful to Not-So Useful
The Unstoppable Tendency of Energy.
Everything that happens is caused by an energy change. Energy
changes form, or moves from place to place. Energy
changes are the driving force of the universe.
“The driving force of all energy change is the unstoppable
tendency of energy to flow from high concentrations of energy
to lower concentrations of energy”.
“When it comes to doing useful work like running or powering a car, highly
concentrated energy is easier to use and more efficient than low
concentrations of energy.”
Foods like carbohydrates and fats, and liquid fuels like gasoline, have
highly concentrated potential energy stored in their chemical bonds. They
have a lot of energy in a fairly small space and it is efficient for us to
convert that concentrated energy into useful energy to keep our bodies
and our machines going.

17. Our Machines Convert Concentrated Energy to Less Useful
"Spread-Out" Energy.
When a diesel engine turns a generator, the engine's mechanical
energy is converted into electricity. The electricity is still pretty
concentrated, but not all of the mechanical energy is converted
to electricity. Some of the energy "leaks" away through friction
and heat.
The generator wires are heated up by internal friction as
electrons flow through them. The generator cooling fan heats up
more air by blowing it over the generator to keep it cool. All of
this heat "spreads out" into the air around the generator. The
energy is still there, but is no longer useful to us
A typical generator converts about 90 to 98 percent of the
mechanical energy put into it into concentrated electricity. The
other 2 to 10 percent leaks away into less useful low grade
energy.

18. As the electrical energy flows through the transmission lines
to our houses, the wires are heated by the flowing electrons
and more energy is lost as it heats up the air around the
wires.
Finally the electrical energy reaches our houses where it is
converted to heat or mechanical energy. The mechanical
energy is also converted to thermal energy through friction.
All of the concentrated rotating mechanical energy put into
the generator has been converted to less concentrated low-
grade thermal energy. Ultimately, it will be radiated to space
(the ultimate dilution), gone forever.

19. When you exercise, some of the food energy gets converted into
muscle work, but most of it gets converted to what we engineers call
low-grade thermal energy. That's why you get all hot and sweaty. In
fact, more than 60% of the food energy is converted to body-warming
sweat-making thermal energy during metabolism of food energy. That
leaves only 40% to do useful work in the cells. If you also figure in the
energy required to digest the food and to pump it around in blood to all
the cells, the final number can be significantly less than 40%. That's
about the same as many of our human-made engines.
This athlete is converting concentrated food
energy through work processes into
mechanical muscle movement, kinetic energy
of motion, and thermal energy (heat) - mostly
thermal energy. All of the mechanical energy
will be converted to low-grade thermal energy
by the end of the race.

20. As with human-made machines and devices, all of the
mechanical work done by the body cells also ends up as
low-grade heat (thermal energy), lost to us forever. The
total amount of energy hasn't changed (1st law), but we
can't use it anymore (2nd Law).
This diesel engine will convert all of the concentrated fuel
energy into work and thermal energy - mostly thermal
energy. Eventually, the mechanical energy will also become
low-grade thermal energy. The total amount of energy
hasn't changed, but it is now so "spread-out" that it cannot
be re-used.

21. An Example of Energy's Downhill Flow to Uselessness
Let's build a dam.
The dam makes a big lake, with very deep high-pressure
water at the base of the dam. Now we can make a big tube
through the dam to guide the high-pressure water through a
big turbine as it flows to the low-pressure side, where the
energy is less concentrated.
The turbine converts the high-pressure energy in the water to
mechanical energy, which drives a big generator. The big
generator converts most of the mechanical energy of the
turbine into electrical energy. Some, as we learned above,
leaks away to less concentrated low-grade thermal energy.
The electrical energy is still pretty concentrated as it is driven
by high voltage through power lines to our house (it is flowing
from higher voltage potential to lower voltage potential).
Along the way a little more concentrated energy "leaks" into
the air around the power lines, becoming low-grade thermal
energy. Engineers call this "transmission losses". It is gone
forever.

22. When the remaining electrical energy reaches our homes, we
each take a small portion and convert it to mechanical energy in
a washing machine, or maybe to thermal energy in an electrical
clothes dryer, as well as mechanical energy to tumble the
clothes. All of the mechanical energy in the washer and dryer is
eventually also converted to low-grade thermal energy through
friction.

23. When the remaining electrical energy reaches our homes,
we each take a small portion But all of the formerly fresh
concentrated solar energy will once again end up as low-
grade spread out energy that can't be re-used. It too will
radiate out into the frigid cold of space, nevermore to be
used.
We need continuous fresh doses of solar energy to keep
the cycle going. That's why we call solar energy
renewable. Hydroelectric energy, like wind energy, is a
form of indirect solar energy. It is replenished by the sun.
Petroleum is a fossil fuel. It is not replenished. It is not
renewable. When all of our fossil fuel has been converted
to low-grade thermal energy and radiated into space, that
will be the end of it.

24. The final destination of all the energy from the hydroelectric
power plant is thousands of light bulbs, electric heaters, toasters,
washing machines, electric dryers, water heaters, stereos,
televisions, and other electrical devices in which it is always and
finally converted to low-grade thermal energy that heats up the
air around us a little bit. All that formerly concentrated energy is
much less concentrated now, all spread out in a form we can't re-
use. Eventually it radiates out into cold space as electromagnetic
radiation.
What about the water in the dam? That can be replaced right?
Sure. All we need is more fresh (emphasis on fresh)
concentrated solar energy to evaporate the water from the
oceans and lakes. Then we need some solar powered wind
energy to blow the clouds over the land where they can form into
rain droplets. Now we need gravity to pull the raindrops to the
ground and make the water in streams and rivers flow downhill
to the dam. Finally, we still need gravity to make the weight of
deep water create highly concentrated pressure energy to drive
the turbine.

25. Summary of Examples..
To our human bodies and to our machines, the Second Law
describes how energy eventually and always "runs down" until it
can't be re-used for anything except warming the environment (air,
rocks, ground, water) around us. It means, in the big picture, that
the world of living things powered by the sun, would quickly run
down to a cold condition of "not-living", if we didn't get a fresh dose
of concentrated solar energy every day.
And what about non-renewable fuels like coal, petroleum, and
natural gas?
Once the potential chemical energy stored in that stuff is converted
to mechanical and thermal energy, it is lost, lost, lost, never to be
re-used - forever.
And if you use 250 horsepower more than you need to drive
yourself to work, you have wasted a lot of irreplaceable
concentrated energy - turned all of it into waste heat and flushed it
right down the metaphorical toilet.