Thermodynamics 4'1 key points

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.

2
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.

3
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.

4
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).

5
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.

6
Thermodynamics 4.1Learning summary

• By the end of this section you will
• 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.

7
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.

8
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.

9
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.

10
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.

11
Thermodynamics 4.2Learning 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...)

12
Thermodynamics 4.2Learning 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,
• and, when applied to a closed system undergoing a
  process 12,

13
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.

14
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
• where is the Carnot efficiency, and
  temperatures T1 and T2 are the absolute
  temperatures, in kelvin, of the heat reservoirs.

15
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.

16
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.

17
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.

18
Thermodynamics 4.3Learning 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...)

19
Thermodynamics 4.3Learning 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
• (continued...)

20
Thermodynamics 4.3Learning 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
• 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...)

21
Thermodynamics 4.3Learning summary

• By the end of this section you will know
• 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.

22
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

23
Thermodynamics 4.4 key points

• the work transferred per unit mass (w) is equal
  to

24
Thermodynamics 4.4Learning 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
• and the SFEE
• These equations apply to both reversible and
  irreversible flow processes.

25
Thermodynamics 4.4Learning 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

26
Thermodynamics 4.4Learning 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

27
Thermodynamics 4.5 key points

• is the universal gas constant (8.3145 x 103
  J kmol-1 K-1). is the molar mass (kg kmol-1)
  of the gas this is numerically equal to the
  molecular weight of the gas.

28
Thermodynamics 4.5Learning 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...)

29
Thermodynamics 4.5Learning summary

• By the end of this section you will know
• 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).

30
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

31
Thermodynamics 4.6Learning 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
• 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.

32
Thermodynamics 4.7Learning summary

• By the end of this section you will know
• 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 (gt103 K).
• Heat conduction in a solid is governed by
  Fouriers law and the thermal conductivity of the
  material. Convective heat transfer to or from a
  surface is governed by Newtons 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.

33
Thermodynamics 4.7Learning summary

• By the end of this section you will know
• 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.

34
Thermodynamics 4.8Learning summary

• By the end of this section you will know
• 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.

35
Thermodynamics 4.8Learning summary

• By the end of this section you will know
• 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.