Transcript Slide 1

Thermodynamics 4.1 – key points
• No matter can cross the boundary of a closed
system.
• In an open system, matter can flow into or out of
the system across parts of the boundary which
are imaginary or permeable.
• The matter contained within or flowing through a
system can be referred to as the working fluid
regardless of whether it is a gas, liquid or vapour.
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.1 – key points
• A property of a system can be a characteristic
defining something particular to the system, such
as its volume, or it can be a property of the
working fluid.
• Pressure is defined as the force exerted per unit
area at a boundary.
• Usually, absolute pressure values are required
for calculations and this is referred to simply as
pressure, without the absolute.
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.1 – key points
• Values of temperature (T) may be presented in
more than one unit, either in degrees Celsius or
degrees Kelvin, but remember that the SI unit of
temperature is the kelvin.
• If the values of enough properties of a system can
be determined through measurements or
calculation to allow the values of all others to be
found, the state of the system is defined.
• When the state of a system or the working fluid
passing through a system changes, it is said to
undergo a process.
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.1 – key points
• During some processes, the states in between will
also be states of thermodynamic equilibrium. This
type of process is reversible: the changes in the
system state can be defined exactly and reversed
to restore the initial conditions in the system and
the surroundings.
• All other processes are irreversible (a common
cause of irreversibility is the generation of kinetic
energy in fluids and gases).
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.1 – key points
• If a closed system undergoes a series of
processes such that the initial and final states of
the system are the same, the system has
undergone a cycle.
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.1
Learning summary
By the end of this section you will:
•
•
•
Unit 4
be familiar with and understand the key terms and definitions given
in this section. They are shown in bold. The terminology is used
throughout the following sections in the presentation of subjects.
be familiar with the properties, nomenclature and units introduced
here. Some or all of these properties feature strongly in any
thermodynamic analysis. Take care to identify the correct units to
use in calculations: if in doubt, use SI units and absolute values of
pressures and temperatures.
have learned to sketch processes on process or state diagrams to
aid understanding, and note given values of properties or other
defining information on these. This summarizes information in a
concise and useful form.
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.2 – key points
• According to the convention adopted, transfers of
energy to the system from the surroundings are
positive.
• When a closed system is taken through a cycle,
the sum of the net work transfer and the net heat
transfer is zero.
V
W    p dV
V1
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.2 – key points
• When using the following equation, if p is taken to
be system pressure, the process must be
reversible, because otherwise the pressure
cannot be defined.
V
W    p dV
V1
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.2 – key points
• The change in internal energy of a closed system
is equal to the sum of the heat transferred and the
work done during any change of state.
• The internal energy of a closed system remains
unchanged if the system is thermally isolated from
its surroundings.
• Work W and heat transfer Q are not properties,
but it is sometimes convenient to consider
quantities of work and heat transfer per unit mass
of matter in the system. These are described as
specific work w and specific heat transfer q and
have units of J kg-1.
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.2 – key points
• The change in internal energy of a closed system
is equal to the sum of the heat transferred and the
work done during any change of state.
• The internal energy of a closed system remains
unchanged if the system is thermally isolated from
its surroundings.
• Work W and heat transfer Q are not properties,
but it is sometimes convenient to consider
quantities of work and heat transfer per unit mass
of matter in the system. These are described as
specific work w and specific heat transfer q and
have units of J kg-1.
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.2
Learning summary
By the end of this section you will know:
•
Work and heat transfer are the means by which energy can be
transferred across the boundary of a closed system. These are not
properties of the system. Our convention is that work or heat
transfer to the system will be positive.
• The first law of thermodynamics embodies the principle of
conservation of energy. When applied to a closed system
undergoing a cycle, there is no net transfer of energy into or out of
the system over the cycle, nor is there any net change in the
energy stored in the system.
• When a closed system undergoes a process that changes its state
from state 1 to state 2, energy transfer by work or heat transfer will
raise or lower the internal energy of the system.Changes in other
forms of energy will usually be negligible.
(continued...)
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.2
Learning summary
By the end of this section you will know:
•
Two important results from the application of the first law of
thermodynamics to a closed system are that, when applied to a
closed system undergoing a cycle,
Wnet  Qnet  0
•
and, when applied to a closed system undergoing a process 1–2,
W12  Q12  U 2  U 1
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.3 – key points
• No heat engine can produce a net amount of work
output while exchanging heat with a single
reservoir only.
• A heat engine is any device or system designed to
convert heat into work output through a cycle of
processes; it must have at least one prime mover,
one source of heat transfer to the heat engine and
one sink of heat transfer from the heat engine.
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.3 – key points
• The first Carnot principle is that all reversible heat
engines operating on any cycle between the same
two reservoirs will have efficiency equal to:
carnot  1 
T2
T1
where is the Carnot efficiency, and
temperatures T1 and T2 are the absolute
temperatures, in kelvin, of the heat reservoirs.
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.3 – key points
• The second Carnot principle is that the efficiency
of a heat engine operating between two reservoirs
will be less than the Carnot efficiency if the heat
engine is irreversible.
• The efficiency of any heat engine, reversible or
irreversible, will be less than the Carnot efficiency
if it operates between more than two heat
reservoirs.
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.3 – key points
• The Clausius inequality states that for any
reversible heat engine (or closed system
undergoing a reversible cycle), the integral around
the cycle of dQ/T vwill be zero; for all irreversible
heat engines (or closed systems undergoing an
irreversible cycle), this integral will be negative.

Unit 4
dQ
0
T
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.3 – key points
• Entropy is created during an irreversible process.
• It is impossible to construct a system which will
operate in a cycle, extract heat from a reservoir,
and do an equivalent amount of work on the
surroundings.
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.3
Learning summary
By the end of this section you will know:
•
that the second law of thermodynamics distinguishes between
work and heat transfer and recognizes that work transfer is the
more valuable of these. This does not contradict the first law;
energy transferred by one is indistinguishable from energy
transferred by the other, and the principle of energy conservation is
not violated. There are, however, limits on how efficiently heat can
be drawn from a source and converted into work output using a
system which operates in a cycle.
(continued...)
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.3
Learning summary
By the end of this section you will know:
•
As a consequence of the second law, a system operating in a cycle
and producing work output must be exchanging heat with at least
two reservoirs at different temperatures. If a system is operating
in a cycle while exchanging heat with only one reservoir, if a net
transfer of work occurs it must be to the system. Work can be
converted continuously and completely into heat, but heat cannot
be converted completely and continuously into work. The efficiency
of a heat engine designed to produce a net work output is defined
as

net work output W

heat supplied
Q1
(continued...)
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.3
Learning summary
By the end of this section you will know:
•
•
Note that it is the heat supplied that we are trying to convert to
work output, not the net heat transfer.
Efficiency is the measure of success in achieving this. The highest
possible efficiency that can be achieved is the Carnot efficiency:
carnot  1 
•
T2
T1
The existence of entropy is a corollary of the second law. It is
important to remember that entropy is a property, like pressure or
temperature, but also that it provides a measure of order and
irreversibility. It is not conserved, like mass of energy, and the
entropy of the Universe is increasing continuously as the result of
the myriad irreversible processes taking place: an implication of
the Clausius inequality is that entropy is created during an
irreversible process; this may result in an increase in entropy of the
system and/or of the surroundings.
(continued...)
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.3
Learning summary
By the end of this section you will know:
•
Unit 4
Entropy can only be transferred across the boundary of a closed
system with heat transfer, not work transfer. If no heat transfer
takes place, the entropy within a closed system remains constant
during reversible processes and increases during irreversible
processes.
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.4 – key points
• The processes undergone in open systems are
flow processes.
• For steady flow through an open system which
has fixed boundaries, the quantities of matter and
energy within the system boundaries are each
constant and do not change with time.
• Under steady flow conditions, there will be mass
flow continuity.
• The heat transferred per unit mass (q) is equal to:
Q m
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.4 – key points
• the work transferred per unit mass (w) is equal to:
W m
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.4
Learning summary
By the end of this section you will know:
•
•
Open systems have parts of their boundary which matter can
cross. The matter passing through an open system undergoes a
flow process. Under steady flow conditions, matter enters and
leaves the system at the same mass flowrate and the mass of
matter in the system remains constant.
The analysis of steady flows through open systems is based on the
mass flow continuity equation:
m  m
inlets
Unit 4
outlets


c22 c12
Q  W  m  h2  h1    gz2  gz1 
2
2


•
and the SFEE
•
These equations apply to both reversible and irreversible flow
processes.
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.4
Learning summary
By the end of this section you will know:
•
•
Specific enthalpy h is a property defined as the combination of
properties (u + pv). This combination appears as a natural
grouping in the steady flow equation, and others, including results
which apply to closed system problems. Enthalpy and specific
enthalpy have the units of energy (J) or specific energy (J kg-1)
respectively, but these have no independent physical meaning and
enthalpy can be considered to have been invented as a
convenience rather than discovered.
The specific work done during any, reversible or irreversible,
steady flow process can be determined using the SFEE if the
remaining terms have known values. In the restricted case of a
reversible flow process in which kinetic energy and potential
energy changes can be neglected, the specific work can also be
determined from:
2
w  v dp
1
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.4
Learning summary
By the end of this section you will know:
•
Students must be careful not to confuse this with the
corresponding result for specific work done during a reversible
process on a closed system:
2
w    p dv
1
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.5 – key points
• R is the universal gas constant (=8.3145 x 103 J
kmol-1 K-1). m is the molar mass (kg kmol-1) of the
gas; this is numerically equal to the molecular
weight of the gas.
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.5
Learning summary
By the end of this section you will know:
•
The working fluids commonly used in thermodynamic systems are
gases and condensable vapours which may change phase at
conditions of interest. The behaviour of working fluids must be
understood as part of the analysis of system behaviour. This
requires knowledge of the properties which distinguish one working
fluid from another, and models of behaviour which define how the
working fluid will respond to changes in state.
• The behaviour of air and many other gases used in engineering
thermodynamic systems can be modelled as that of a perfect gas.
A perfect gas obeys the perfect gas equation pv = mRT
• This is the equation of state of the gas. In addition, the specific
heats of a perfect gas are constants which do not change as
temperature or pressure changes.
(Continued...)
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.5
Learning summary
By the end of this section you will know:
•
Unit 4
Water is the example of a condensable vapour covered in this
section. The equation of state is more complex than the perfect
gas equation and usually evaluated using a computer. Results are
presented in tables in (Rogers and Mayhew, 1995).
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.6 – key points
• A polytropic process is one which obeys the
polytropic law , in which n is a constant called the
polytropic index.
• An adiabatic process is one during which no heat
transfer occurs. A reversible adiabatic process will
be isentropic: no entropy is created if the process
is reversible and none can be transferred to or
from the system if no heat transfer occurs. For a
perfect gas, reversible adiabatic and isentropic
processes will obey the same polytropic law with
n
Unit 4
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.6
Learning summary
By the end of this section you will know:
•
•
the common types of process described in this section and the
conditions which apply in each case. The names of the processes
are independent of the working fluid: perfect gases and steam can
undergo an isothermal process or an isentropic process, etc.
For a polytropic process the relationship between pressure and
volume changes is fixed by the definition as
p1v1n  p2 v2n
•
•
Unit 4
In general, however, changes in property values which occur when
a working fluid under goes a process depend on the type of
process and the characteristics of the working fluid; these are
different for a perfect gas and steam.
The work done during reversible processes is different for closed
and open systems. The results for a perfect gas are summarized in
Table 4.8.
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.7
Learning summary
By the end of this section you will know:
•
•
Unit 4
The modes and processes that control rates of heat transfer have
been introduced in this section. The three fundamental modes of
heat transfer are conduction, convection and radiation. Heat
conduction is the prime mode of heat transfer in solids; convection
is usually the dominant mode of heat transport in liquids and
gases. Radiation is the only mode which transmits energy through
a vacuum and is likely to be the dominant mode of heat transfer
from surfaces at high temperatures (>103 K).
Heat conduction in a solid is governed by Fourier’s law and the
thermal conductivity of the material. Convective heat transfer to or
from a surface is governed by Newton’s law of cooling. Heat
transfer coefficient is often taken to be a constant for a particular
problem although its value may depend on fluid properties, flow
conditions and surface geometry. At moderate temperatures and
temperature differences between bodies, the effective radiative
heat transfer coefficient can be added to the convective heat
transfer coefficient.
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.7
Learning summary
By the end of this section you will know:
•
Unit 4
Solids and surfaces offer a thermal resistance to heat transfer.
Under conditions of steady heat transfer through several layers
and surfaces in series, the overall thermal resistance can be
determined from the thermal resistances of each layer and surface.
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.8
Learning summary
By the end of this section you will know:
•
•
•
Unit 4
The cycles analysed in this section are ideal, thermodynamic cycles.
These provide insights to the types of cycle used to generate
mechanical power.
No cycle can have a higher efficiency than the Carnot efficiency, but
the ideal Stirling and Ericsson cycles achieve an efficiency value equal
to this. In these cycles, heat transfer at temperatures between the
maximum and minimum available takes place internally, through
regeneration. External heat transfer across system boundaries occurs
isothermally and only at these maximum and minimum temperatures,
meeting conditions for the Carnot efficiency to be achieved.
The Rankine cycle with superheat is the basis for a practical cycle for
the generation of power output using steam as the working fluid. The
cycle is less efficient than the Carnot cycle because heat supply takes
place over a range of temperatures rather than the maximum possible.
An Introduction to Mechanical
Engineering: Part One
Thermodynamics 4.8
Learning summary
By the end of this section you will know:
•
•
Unit 4
The efficiencies of the Brayton, Otto and Diesel cycles depend on the
ratio of specific heats and the pressure ratio (Brayton cycle),
compression ratio (Otto cycle) or compression ratio and cut-off ratio
(Diesel cycle). There is no need to remember the particular results for
these cycles, but students should understand how these are derived.
There are similarities between the ideal thermodynamic cycles and
real engine and power plant cycles. Plant and engines operating on
the Stirling, Ericsson and Rankine cycles have external heat supply
and heat rejection and the working fluids do undergo continuous
thermodynamic cycles. Differences between the ideal and the real
cycles are more marked for internal combustion engines. In these, fuel
is burned within the working fluid, changing its composition as well as
releasing chemical energy. The working fluid is replaced during
successive cycles of the machine, so internal combustion engines
such as gas turbines and reciprocating internal combustion engines do
not operate on true thermodynamic cycles, but on a machine cycles.
An Introduction to Mechanical
Engineering: Part One