Engineering Thermodynamics

UNIT-1

Learning Concept 1: Thermodynamic System and Control Volume

Definition: A thermodynamic system is a specific quantity of matter or a defined region in space

chosen for analysis. The region outside the system is called the surroundings, and the boundary
separates the system from its surroundings. A control volume is a fixed region in space with a control
surface across which mass and energy can transfer. A closed system (control mass) has a fixed mass,
no mass transfer across its boundary, but energy can be exchanged. An open system (control volume)
allows both mass and energy transfer across its boundary. An isolated system exchanges neither mass
nor energy with its surroundings.

Fig: Thermodynamic system

Types of Thermodynamic Systems

A. Open System (Control Volume)

 Definition: Exchanges both mass and energy with surroundings.

 Example: Boiler, turbine, compressor, human body

B. Closed System (Control Mass)

 Definition: Exchanges only energy, not mass, with surroundings.

 Example: Gas in a piston-cylinder, pressure cooker, sealed battery

C. Isolated System

 Definition: No exchange of mass or energy with surroundings.

 Example: Ideal thermos flask, perfectly insulated system

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 Different Types of Thermodynamic Systems

Open System Examples

1. Steam flowing through a turbine

2. expander
3. Air compressor
4. Pump
5. Fan
6. Nozzles and diffusers
7. Boiler
8. Condenser
9. Air preheater
10. Super heater
11. Economiser
12. Heaters and coolers
13. Expansion valve
14. Mixing chamber
15. Heat Exchanger
16. Human lungs
17. Car radiator
18. Open cup of tea
19. Kitchen exhaust fan
20. Water flowing in a pipe
21. Jet engine

Closed System Examples

1. Gas in a sealed piston-cylinder

2. Pressure cooker (lid closed)
3. Refrigerant in a sealed refrigeration loop
4. Inflated football
5. Battery during charging

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 6. Hot coffee in a sealed cup
7. A gas tank (with closed valve)
8. Sealed water bottle heating up
9. Submarine cabin (for short duration)
10. Car tire

Isolated System Examples

1. Ideal thermos flask

2. Universe (approximation)
3. Perfectly insulated box
4. Vacuum bottle with no heat transfer
5. Theoretical calorimeter (adiabatic + rigid)

Key Terms

Term Meaning
System boundary Separates system from surroundings
Surroundings Everything outside the system
Control volume Open system boundary with flow (e.g., turbines, nozzles)
Control surface The outer boundary of the control volume

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Learning Concept 2: Thermodynamic Equilibrium, State, Property, Process, Cycle

• Definition: Thermodynamic equilibrium is a condition where a system's macroscopic

properties do not change with time, and the system exhibits thermal, mechanical, phase, and
chemical equilibrium. The state of a system is its condition defined by the values of its
macroscopic properties. A property is any measurable characteristic of a system (e.g.,
temperature, pressure, volume, energy). Properties can be intensive (independent of mass,
e.g., temperature, pressure) or extensive (dependent on mass, e.g., volume, total energy).
• Properties are considered to be either intensive or extensive. Intensive properties are those
that are independent of the mass of a system, such as temperature, pressure, and density.
Extensive properties are those whose values depend on the size—or extent—of the system.
Total mass, total volume, and total momentum are some examples of extensive properties. An
easy way to determine whether a property is intensive or extensive is to divide the system into
two equal parts with an imaginary partition, as shown in Fig. 1–23. Each part will have the
same value of intensive properties as the original system, but half the value of the extensive
properties
• Any change that a system undergoes from one equilibrium state to another is called a process,
and the series of states through which a system passes during a process is called the path of
the process
• A process is a transformation of a system from one equilibrium state to another.
• A cycle is a sequence of processes that returns the system to its initial state.
• Cycle: A closed loop on a P-V diagram, showing a series of processes returning to the starting
point.
• Examples:
Process: Heating a gas at constant pressure, causing its volume and temperature to
change.
Cycle: The refrigeration cycle in a refrigerator.

Cyclic process
Steady flow process: A large number of engineering devices operate for long periods of time under
the same conditions, and they are classified as steady-flow devices. the steady-flow process is defined
as a process during which a fluid flows through a control volume steadily or there is no change in
properties with time.

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Learning Concept 3 & 4: Work and Heat for Constant Pressure, Constant Volume, and
Isothermal Process, Adiabatic Process, and Polytropic Process, Zeroth law of Thermodynamics
Definition:
 Heat is energy transferred due to a temperature difference. Heat is defined as the form of
energy that is transferred between two systems (or a system and its surroundings) by virtue of
a temperature difference.
 Heat is energy in transition. It is recognized only as it crosses the boundary of a system.
 A process during which there is no heat transfer is called an adiabatic process
 As a form of energy, heat has energy units, kJ
 A constant pressure (isobaric) process occurs at a constant pressure.
 A constant volume (isochoric/isometric) process occurs at a constant volume.
 An isothermal process occurs at a constant temperature.
 An adiabatic process occurs without any heat transfer (Q = 0).
 A polytropic process follows the relation PVⁿ = constant, where n is the polytropic index.
 Work is the energy transfer associated with a force acting through a distance. A rising piston,
a rotating shaft, and an electric wire crossing the system boundaries are all associated with
work interactions.
 Work, like heat, is an energy interaction between a system and its surroundings. As mentioned
earlier, energy can cross the boundary of a closed system in the form of heat or work.
Therefore, if the energy crossing the boundary of a closed system is not heat, it must be work.
Heat is easy to recognize: Its driving force is a temperature difference between the system and
its surroundings. The work done per unit time is called power and is denoted W ·. The unit of
power is kJ/s, or kW.
 The generally accepted formal sign convention for heat and work interactions is as follows:
 heat transfer to a system is positive and work done by a system is positive;
 heat transfer from a system/heat rejected is negative and work done on a system are negative.

Heat transfer vs Work transfer:

 Heat and work are energy transfer mechanisms between a system and its surroundings, and
there are many similarities between them:
 Both are recognized at the boundaries of a system as they cross the boundar ies. That is, both
heat and work are boundary phenomena.
 Systems possess energy, but not heat or work. Both are associated with a process, not a state.
Unlike properties, heat or work has no meaning at a state.
 Both are path functions (i.e., their magnitudes depend on the path followed during a process
as well as the end states).
 Examples:
o Constant Pressure: Expansion of steam in a turbine stage with constant inlet pressure.
o Constant Volume: Heating air in a rigid, sealed container.
o Isothermal: Very slow expansion of an ideal gas while in contact with a heat reservoir.

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The Zeroth Law of Thermodynamics states that if two systems are each in thermal equilibrium with
a third system, then they are also in thermal equilibrium with each other.

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 •

The Zeroth Law establishes the concept of temperature as a measurable and comparable property.
It allows us to:

 Define a temperature scale.

 Use thermometers to measure temperature reliably.

Suppose we have three systems:

 System A
 System B
 System C

If:

 A is in thermal equilibrium with C, and

 B is also in thermal equilibrium with C,

Then:

 A and B are in thermal equilibrium with each other (i.e., they are at the same temperature).

Implication:

 Enables the use of a thermometric device (system C) to compare temperatures of other sys-
tems.
 Forms the foundation for defining temperature scales such as Celsius, Kelvin, Fahrenheit,
etc.

Real-Life example:

If a thermometer (system C) shows the same reading when placed in contact with a cup of water
(system A) and a metal block (system B), we conclude that both the water and the metal block are at
the same temperature — even though they are not in contact with each other.

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Learning Concept 5: First Law of Thermodynamics, Joule's Experiment
Definition: The First Law of Thermodynamics (Conservation of Energy) states that energy cannot
be created or destroyed, only transformed from one form to another.

Joule's experiment involved a well-insulated container filled with water and a paddle wheel driven
by falling weights. It demonstrated that mechanical work done on the system (water) increased its
temperature, equivalent to heat transfer, thus establishing the equivalence of work and heat and the
concept of internal energy as a state function.

Fig: Heat and Work (Joules Experiment)

 Work, W1-2 done on the system can be measured by the fall of the weight.
 The system temperature rises as work is done on the system.
 Let the insulation now be removed. The system reaches its initial state by heat transfer across
the system boundaries.
 Therefore, the work done is proportional to the heat transfer.
 The constant of proportionality is the Joule’s equivalent.
The cycle consists of two processes, one an adiabatic work transfer followed by heat transfer

Examples of First Law of Thermodynamics

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Energy balance
• The net change (increase or decrease) in the total energy of a system during a process is equal to
the difference between the total energy entering and total energy leaving the system.

Energy change = Energy at the final state – Energy at the initial state
In the absence of electrical, magnetic or surface tension effects,
∆E = ∆U + ∆KE + ∆PE
• Thus, for stationary systems, ∆E = ∆U

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Concept 6: First Law of TD Applied for Cycle
Definition: For a thermodynamic cycle, the system returns to its initial state after a series of
processes. Therefore, the change in internal energy over a cycle is zero (ΔU_cycle = 0). Applying the
First Law to a cycle yields 0 = Qnet - W_net, which means the net heat transfer to the system during a
cycle is equal to the net work done by the system during the cycle: Q_net = W_net.

Examples:

A heat engine takes in heat and produces work in a cycle. The net work output equals the net heat
input.
A refrigerator uses work input to transfer heat from a cold to a hot region in a cycle. The network
input equals the net heat rejected minus the heat absorbed.
There are various types of cycles in thermodynamics, and some of those important cycles are listed
as follows:
 Carnot Cycle
 Rankine Cycle
 Otto Cycle
 Diesel Cycle
 Brayton Cycle
 Stirling Cycle
 Rankine cycle
 Vapour compression refrigeration cycle
 Air refrigeration cycle
 Reversed Carnot cycle

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Learning Concept 7: Application of the First Law to Non-Flow Processes (Constant Pressure,
Constant Volume, and Isothermal Process)-Closed Systems
Constant Volume Process (Isochoric Process)
A constant volume process (also called an isochoric process) is a thermodynamic process in which
the volume remains constant. Since there is no change in volume, no boundary work (PdV work) is
done by or on the system.
Examples: Combustion in a Closed Rigid Container (Bomb Calorimeter), Spark-Ignition Engine
(Ideal Otto Cycle), Heating or Cooling in Rigid Tanks, Pressure Cookers, Heat applied to sealed glass
bulbs containing gas.

Relationship between pressure, volume, and temperature for an ideal gas

Work Done (PdV Work): The work done in any quasi-static process is given by:

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Change in internal energy

Heat Transfer

Constant Pressure Process (Isobaric Process)

A constant pressure process is a thermodynamic process in which the pressure remains constant
throughout the transformation from one state to another. The system may change its volume and
temperature, but pressure stays unchanged.

Applications: Boiling of Water, Heat addition in piston-cylinder engine, Air heating in open
containers, Isobaric Expansion in Ideal Brayton Cycle.
Relationship Between Properties (Using PV = mRT)

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First Law of Thermodynamics Applied to Constant Pressure Process

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Constant Temperature Process (Isothermal Process)
An isothermal process is a thermodynamic process that occurs at a constant temperature. Since
temperature is constant, internal energy (ΔU) of an ideal gas remains unchanged.

Applications: Gas compression/expansion in heat baths, Air pump with slow compression, Phase
change processes.
Property Relationships (Using PV = mRT)

First Law of Thermodynamics for Isothermal Process

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Work Done in Isothermal Process

Summary Table

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Learning Concept 8. Application of the First Law to Non-Flow Processes (Adiabatic and
Polytropic Processes)-Closed Systems
Adiabatic Process (Isentropic Process)
Adiabatic process is a process in which there is no exchange of heat takes place from the working
substance to the surrounding during its expansion or compression. This is possible when the working
system is made thermally insulated, that is no heat can leave or enters it during the process. So we
can say that during adiabatic or isentropic process.
Heat does not leave or enters the working substance (usually a gas).
The temperature of the gas changes as the work is done on the cost of internal energy.
The work done is equal to the change in internal energy.

Applications: Rapid compression in IC engines, Idealized expansion and compression processes in

Gas turbines, Expansion in steam nozzles, etc.
The various relationship of the reversible adiabatic process is given below:

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Polytropic Process
A polytropic process is a generalized thermodynamic process that follows the equation:
Where:
 P: pressure
 V: volume
 n: polytropic index (a constant that defines the nature of the process)
Applications:
Compressors and turbines- Real gas compression/expansion often follows a polytropic path
(not truly adiabatic or isothermal).
Refrigeration and air-conditioning cycles- The expansion and compression in the cycles are
modeled as polytropic.
Gas flow in nozzles and diffusers- For variable area flow passages, polytropic models are
often used.

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Fig: Pressure-volume diagram for different polytrophic processes

Special cases of n:

Relationship between P, V, and T (for ideal gas):

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