Transcript Unit 61: Engineering Thermodynamics

Unit 61: Engineering
Thermodynamics
Lesson 1: Setting the Scene for the
Course
Objective
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• The purpose of this lesson is to introduce
some of the basic concepts relating to
thermodynamics
What is
Thermodynamics?
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• So what exactly is Thermodynamics?
• Thermodynamics is a science in which the
storage, the transformation and the transfer
of energy are studied.
How is Energy Stored?
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• Energy is stored as…
– internal energy (associated with temperature),
– kinetic energy (due to motion),
– potential (due to elevation) and
– chemical (due to chemical composition)
– etc
How is Energy
Transformed?
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• Energy is transformed from one form
(potential to kinetic; chemical to internal
energy; etc) to another.
How is Energy
Transferred?
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• Energy is transferred across a boundary as
either heat or work.
Relating Energy to
Macroscopic (large scale) properties
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• Thermodynamics uses mathematical equations to
describe the transformations (changes) and transfer of
energy to material properties such as temperature,
density, pressure, volume or enthalpy.
– Note: these macroscopic properties of matter are capable
of being measured and are often capable of being
perceived by our senses.
– Note: macroscopic properties of matter contrasts markedly
with the microscopic properties of matter such as masses,
speeds, energies , etc of the constituent atoms / molecules
that cannot be perceived directly by our senses.
Macroscopic properties v
Microscopic properties
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• If the atomic model of matter is valid then it
should be possible to explain the macroscopic
properties of matter in terms of the
microscopic and visa versa, since the same
thing is being considered but from two
different perspectives
Thermodynamic Laws
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• Much of what we know about
thermodynamics has been based upon
experimental observations, the results of
which have been organised into mathematical
statements which we term laws (zeroth law,
first law, second law and third law)
An Engineer’s Perspective
on Thermodynamics
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• An engineer’s objective in studying
thermodynamics is most often the analysis or
design of a large scale system – anything from an
air-conditioning unit to a nuclear power station.
• A system may be regarded as a continuum in
which the activity of the constituent molecules is
averaged into measureable quantities such as
pressure, temperature, velocity etc.
Thermodynamic
Systems
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• A thermodynamic system is a definite quantity
of matter contained within some closed
surface. This might be an imagined boundary
like the deforming boundary of a certain
amount of mass as it flows through a pump or
it could be an obvious one like that enclosing
the gas in a cylinder.
• All matter and space external to a system is
collectively called its surroundings.
Thermodynamic Systems
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Weight
Piston
System under
consideration
Compressed gas
(the system)
Cylinder
System boundary
or wall
System
boundary or wall
Thermodynamic
Systems
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• Thermodynamics is concerned with with the
interactions of a system and its surroundings
or with one system interacting with another.
– A system interacts with its surroundings by
transferring energy across its boundary.
– Material may or may not cross the boundary of a
given system
– If the system does not exchange energy with the
surroundings, it is called an isolated system
Control Volume
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• In many cases, an analysis is simplified if
attention is focused on a volume in space into
which or from which a substance (material)
flows. Such a volume is called a control
volume.
– A pump, a turbine, an inflating balloon are
examples of control volumes.
– The surface that completely surrounds the control
volume is called a control surface
Control Volume
Liquid in
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Liquid out
Pump
Control
Volume
Control surface
Energy in
Thermodynamic System v
Control Volume
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• In trying to resolve a particular problem, we
deal with it by either considering using a
control volume or using a systems approach.
• If there is a mass flux across a boundary of a
region then a control volume is required
otherwise a systems approach should be used.
Properties and State
of a System
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• The matter in a system may exist in several
phases: a solid, liquid or a gas.
• A phase is a quantity of matter that has the
same chemical composition throughout i.e. it
is homogeneous (the same throughout in all
directions).
• Phase boundaries separate the phases in what
when taken as a whole is called a mixture.
Properties and State
of a System
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• A property is any quantity which serves to
describe a system (mass, volume, pressure,
position, velocity, temperature, etc).
• Sometimes shape is important when surface
effects are considered or colour when
radiation heat transfer is being investigated
• The state of a system is its condition as
described by giving values to its properties at
a particular instant in time.
Properties and State
of a System
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• The essential feature of a property is that it
has a unique value when a system is in a
particular state and this value does not
depend on the previous states that the
system passed through that is it is not a path
function.
• Since a property is not dependent on the path
any change depends only on the initial and
final states of the system.
Properties and State
of a System
Property 1
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Path 2
B
Path 1
Note – this is path
dependent!
A
Property 2
Properties and State
of a System
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Thus using the symbol φ to represent a property…
φ2
dφ = φ2 – φ1
φ1
This requires that dφ be an exact differential; φ2 – φ1
represents the change in the property as the system changes
from state 1 to state 2 along the path.
Note path 1 does not equal path 2!
Properties and State
of a System
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• A relatively small number of independent
properties suffice to fix all other properties
and thus the state of the system.
• If the system is composed of a single phase,
free from magnetic, electrical and surface
effects, the state is fixed when any two
properties are fixed!
Properties and State
of a System
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• Thermodynamic properties are divided into
two general types, intensive and extensive.
• An intensive property is one which does not
depend on the mass (or size) of the system
i.e. temperature, pressure, density, etc. They
are the same for the entire system. For
example if you have 1-g of water or 1-kg; the
density is the same!
Properties and State
of a System
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• If we bring two systems together, intensive
properties are not summed i.e. if we have 1kg
of water at 50oC and a hot metal spoon at
150oC is allowed to be dropped into the water
the resulting temperature is not 50oc + 150oC,
its somewhere in-between the two values
Properties and State
of a System
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• An extensive property is one that depends
upon the mass (or size) of the system i.e.
volume, momentum, kinetic energy.
• If two systems are brought together the
extensive property of the new system is the
sum of the extensive properties of the original
two systems.
Properties and State
of a System
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• If we divide an extensive property by the mass
a specific property results.
• Thus specific volume is defined as…
v = V
m
Thermodynamic
Equilibrium
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• When the temperature or the pressure of a
system is referred to it is assumed that all
points of the system have the same or
essentially the same temperature or pressure.
• When properties are assumed constant from
point to point and when there is no tendency
for change with time, a condition of
thermodynamic equilibrium exists.
Thermodynamic
Equilibrium
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• If the temperature say is suddenly increased
at some part of the system boundary
spontaneous redistribution is assumed to
occur until all parts of the system are at the
same temperature.
• If a system undergoes a large change in its
properties when subjected to some small
disturbance it is said to be in metastable
equilibrium
Process
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• When a system changes from one equilibrium
state to another, the path of successive states
through which the system passes is called a
process.
• If when passing from one state to the next,
the deviation from equilibrium is infinitesimal
a quasi-equilibrium process occurs and each
state in the process may be idealised as an
equilibrium state.
Process
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• Many processes such as the compression and
expansion of gases in an internal combustion
engine, can be approximated by quasiequilibrium processes with no significant loss
of accuracy. If a system undergoes a quasiequilibrium process (such as the
thermodynamically slow compression of air in
a cylinder) it may be sketched on appropriate
coordinates by using a solid line.
Process
A
P1
P2
B
V1
V2
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Process
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• If a system however goes from one
equilibrium state to another through a series
of non-equilibrium states (as in combustion) a
non-equilibrium process occurs. A dashed line
represents such a process: between (P1, V1)
and (P2, V2) properties are not uniform
throughout the system and thus the state of
the system cannot be well defined!
Process
A
A
P1
P2
B
V1
V2
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Process
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Weight
Piston
Compressed gas
(the system)
Whether a particular
process may be considered
quasi-equilibrium or nonequilibrium depends on how
the process is carried out.
Consider a weight W added
to the piston. How would
you add W in an nonequilibrium manner and a
equilibrium manner?
Process Cycle
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• When a system in a given initial state
experiences a series of quasi-equilibrium
processes and returns to the initial state, the
system undergoes a cycle.
• At the end of the cycle the properties of the
system have the same values they had at the
beginning.
Process Cycle
P
2
1
4
3
V
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Process Cycle
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• Iso.. Is derived from a Greek work meaning
‘the same’.
– Thus isothermal process is one in which the
temperature is constant i.e. the same temperature
– An isobaric process is one in which the pressure
remains constant
– An isometric process is one in which the volume
remains constant.
Process Cycle
P
2
isometric
isobaric
1
4
3
V
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Units
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• Handout 1 – pg 6 Schaum, ‘Thermodynamics
for Engineers’.