Download Chapter 7 Thermal and Energy Systems

Chapter 7
Thermal and Energy Systems
is prepared by:
Dr. Roozbeh Vaziri
MENG&MECT190
Purpose of This Chapter
Calculate various energy, heat, work, and
power quantities that are encountered in
mechanical engineering, and express their
numerical values in the SI and USCS.
Describe how heat is transferred from one
location to another by the processes of
conduction, convection, and radiation.
Apply the principle of energy conservation for
a mechanical system.
Explain how heat engines operate and
understand the limitations on their efficiency.
• Energy is needed to accelerate an object, stretch it,
heat it, and elevate it.
• In the internal-combustion engine of Figure, diesel fuel
is burned to release thermal energy.
• The engine converts thermal energy into the rotation
of its crankshaft and ultimately into the motion of a
vehicle.
• Gravitational Potential Energy
Near the surface of the Earth, the acceleration
of gravity is given by the standard acceleration
values
Gravitational potential energy is associated
with changing the elevation of an object
within a gravitational field, and it is measured
relative to a reference height, for instance the
ground or the top of a workbench.
• Elastic Potential Energy
Elastic potential energy is stored by an object
when it is stretched or bent in the manner
described by Hooke’s law
Hooke's law is a principle of physics that states
that the force F needed to extend or compress a
spring by some distance X is proportional to that
distance.
That is: F = kX, where k is a constant factor
characteristic of the spring: its hardness, and X is
small compared to the total possible deformation
of the spring.
For a spring having stiffness k, the elastic
potential energy stored within it is given by
• DL is the spring’s elongation, defined as the
distance that it has been stretched or
compressed.
• Kinetic Energy
• Kinetic energy is associated with an object’s
motion.
• As forces or moments act on a machine, they
cause its components to move and store kinetic
energy by virtue of velocity.
• The motion can be in the form of
Vibration (the cone of a stereo speaker)
Rotation (the flywheel attached to an engine’s
crankshaft)
Translation (the straight-line motion of the piston in
an engine or compressor).
• Work of a Force
The work of a force is illustrated in Figure in
the context of a piston that slides horizontally
in its cylinder.
The force F is applied to the piston to
compress the gas in the cylinder while the
piston moves to the right
• If the gas already has been compressed to a
high pressure and the piston moves to the left
the force F can be applied to resist that
expansion.
• The work W of the force as the piston moves
through the distance is defined by
• Power
• When a force performs work during the
interval of time , the average power is
• As work is performed more rapidly
becomes smaller, and the average power
increases accordingly
• Heating Value
• When a fuel is burned, the chemical reactions
that take place release thermal energy and
waste products including water vapor, carbon
monoxide, and particulate material.
• Mechanical engineers design machinery that
manages the release of energy stored in
chemical form, and that energy is
subsequently converted to more useful forms.
• In a furnace, power plant, or gasoline engine,
the energy output of the combustion process
is measured by a quantity called the heating
value H.
• Heating values are the amount of energy
released per unit mass of fuel burned.
• In calculations involving the combustion of a
fuel, the heat Q that is released by burning
mass m is given by
• Specific Heat
• As heat flows into an object, its temperature
rises from an initial value T0 to T according to
• The specific heat, the parameter c is a
property that captures how materials differ
with respect to the amount of heat they must
absorb to raise their temperature.
• The physical property that quantifies the
amount of heat that must flow into or out of a
material to produce a phase change is called
the latent heat.
• Transfer of Heat
• heat as energy that is being transferred from
one location to another because of a
temperature difference.
• The three mechanisms for heat transfer are
known as conduction, convection, and
radiation
• Heat Conduction
The quantity of heat that flows along the rod
during a time interval Δt is given by
This principle is known as Fourier’s law of heat
conduction
• Thermal conduction is the transfer of heat
(internal energy) by microscopic collisions of
particles and movement of electrons within a
body.
• The rate at which energy is conducted as heat
between two bodies is a function of the
temperature difference (temperature gradient)
between the two bodies and the properties of
the conductive medium through which the heat is
transferred.
• Thermal conduction was originally called
diffusion.
• Heat convection
• Heat can also be transferred by a fluid that is in
motion; that process is known as convection.
• The cooling system of an automobile engine, for
instance, operates by pumping a mixture of water
and antifreeze through passageways inside the
engine’s block.
• Excess heat is removed from the engine,
transferred temporarily to the coolant by
convection, and ultimately released into the air
by the vehicle’s radiator.
• Because a pump circulates the coolant, heat
transfer is said to occur by forced convection.
• A liquid or gas can circulate on its own,
without the help of a pump or fan, because of
the buoyancy forces created by temperature
variations within the fluid.
• As air is heated, it becomes less dense, and
buoyancy forces cause it to rise and circulate.
The rising flow of warm fluid(and the falling
flow of cooler fluid to fill its place) is called
natural convection.
• Heat radiation
• The third mechanism of heat transfer is
radiation, which refers to the emission and
absorption of heat without direct physical
contact.
• Radiation occurs when heat is transmitted by
the long infrared waves of the
electromagnetic spectrum.
ENERGY CONSERVATION AND
CONVERSION
• With the concepts of energy, work, and heat
in mind, we will now explore the conversion of
energy from one form to another.
• The chemical energy stored in the fuel (be it
gasoline, jet fuel, or natural gas) is released as
heat, which in turn is converted into
mechanical work.
• The principles of energy conservation and
conversion are built on the concept of a
system, which is a collection of materials and
components that are grouped together with
respect to their thermal and energy behavior.
• System is imagined to be isolated
• All forces that crossed an imaginary boundary
drawn around the body were included on the
diagram, and other effects were disregarded.
• The first law of thermodynamics states that these
three quantities balance according to
• Heat Q is positive when it flows into the system;
W is positive when the system performs work on
its surroundings; and DU is positive when the
system’s internal energy increases.
HEAT ENGINES AND EFFICIENCY
• The heat engine sketched schematically in Figure
represents any machine that is capable of converting
the heat supplied to it into mechanical work.
• The sources of energy, or heat reservoirs, are large
enough so that their temperatures do not change as
heat is removed or added.
• In the context of an automotive engine,
 Qh represents the heat released by burning fuel
in the engine’s combustion chamber;
W is the mechanical work associated with the
rotation and torque of the engine’s crankshaft;
Ql is the heat that warms the engine block and is
expelled from the exhaust
• The second law of thermodynamics:
No machine can operate in a cycle and only
transform the heat supplied to it into work
without also rejecting part of the supplied heat.
• The ideal Carnot efficiency of the heat engine
is given by