UNIT V

Thermodynamics: Thermodynamics system (open, closed, isolated), Thermodynamic

Properties: Definition and Units of – Temperature, Pressure (atmospheric, absolute,

gauge), Volume, Internal Energy, Enthalpy, Concept of Mechanical Work,

Thermodynamics Laws with examples – Zeroth, First, Limitation of First Law,

Concept of Heat Sink, Source, Heat Engine, Heat Pump, Refrigeration Engine, 2nd law

of thermodynamics statement (Kelvin Plank, Claussius), Numerical of 2nd law only

Measurements: Measurement of Temperature (Thermocouple – Type according to

temperature range and application), Measurement of Pressure (Barometer, Bourdon

Pressure Gauge, Simple U Tube Manometer with numerical)

Thermal Engineering

IntroductionDefinition of Thermodynamics: Branch of physical science which deals with

study of energy transfer and its effects on properties of system and surrounding

Application of thermodynamics:

Thermodynamic System, Surrounding and Boundary

Thermodynamic System• A quantity of matter or region in

space chosen for study

Surroundings• Mass or region outside the system

Boundary• The real or imaginary surface that

separates the system from the surroundings

Types of Thermodynamics Systems

Closed System (control mass)

• Consists of a fixed amount of mass

• Mass can NOT cross the boundary

• Energy can cross the boundary

• Volume does not have to be fixed

Isolated System

• Special case of a closed system

• No energy is allowed to cross the boundary

mass = constantVolume = constantEnergy = constant

Open System (control volume)• Any arbitrary region in space• Usually involves a device through which

mass flows• Mass and energy can cross the boundary• Can be fixed in space or have a moving

boundary• Control volumes can change in actual

volume with time

Properties of a SystemProperty

• Any characteristic of a system• E.g. temperature, pressure, volume

Intensive Property• Independent of the size of the system• E.g. temperature, pressure, density

Extensive Property• Value depends on the size (or extent) of

the system• E.g. mass, volume, total energy

Specific Properties• Extensive properties per unit mass• E.g. specific volume, specific energy

mVv mEe

Property Unit ConversionTemperature Kelvin ( K),

Celcius1 = 273 K

Pressure N/m2, bar, Pascal(pa), mm of Hg

1 bar = 10 5 Pa

1 atm = 1.01325 bars

Volume m3, Litre 1 litre = 10-3 m3

Internal energy

Joule (J), N.m 1J = 1 N.m

Enthalpy Joule (J)

Units & Conversion

Zeroth Law of Thermodynamics Observation

• When a body is brought into contact with another body that is at a different temperature, heat is transferred from the body at higher temperature to the one at lower temperature until both bodies attain the same temperature (thermal equilibrium)

Zeroth Law of Thermodynamics• If two bodies are in thermal equilibrium with a

third body, they are also in equilibrium with each other

• Cannot be concluded from the other laws of thermodynamics

• Serves as a validity of temperature measurement• Restated: Two bodies are in thermal equilibrium if

both have the same temperature reading, even if they are not in contact

When closed system executes a cyclic process the algebric sum of work transfers is propertional to the algebraic sum of heat transfer

CYCLECYCLEQW

Example: Automobile Engine

Chemical Kinetic

Heater/Furnace Chemical HeatHydroelectric Gravitational

ElectricalSolar Optical ElectricalNuclear Nuclear Heat,

Kinetic, Optical

Battery Chemical ElectricalFood Chemical Heat,

KineticPhotosynthesis Optical Chemical

Ist Law of a Thermodynamics

Limitation of First Law of Thermodynamics

A cup of hot coffee left in a cooler room eventually cools off. The reverse of this process- coffee getting hotter as a result of heat transfer from a cooler room does not take place.Consider heating of a room by passage of electric current through an electric resistor. Transferring of heat from room will not cause electrical energy to be generated through the wire. Consider a paddle-wheel mechanism operated by fall of mass. Potential energy of mass decreases and internal energy of the fluid increases. Reverse process does not happen, although this would not violate first law. Water flows down hill where by potential energy is converted into K.E. Reverse of this process does not occur in nature.

Processes proceed in a certain direction and not in the reverse direction. The first law places no restriction on direction.

A process will not occur unless it satisfies both the first and second laws of thermodynamics.

Second law not only identifies the direction of process, it also asserts that energy has quality as well as quantity.

Need Of Second Law of Thermodynamics

Source and SinkA reservoir that supplies energy in the form of heat is called a source and one that absorbs energy in the form of heat is called a sink. For example, atmospheric air is a source for heat pumps and a sink for air conditioners.

Thermal ReservoirA thermal reservoir is a large system (very high mass x specific heat value) from which a quantity of energy can be absorbed or added as heat without changing its temperature. The atmosphere and sea are examples of thermal reservoirs.

W= Q1-Q2

Heat Engine

Q1

Q2

Refrigerator and Heat Pump

Statement of Second law of thermodynamics

The Kelvin-Planck Statement:It is impossible for any device that

operates on a cycle to receive heat from single reservior and produce net amount of work.

Thermal energy reservoir

Engine

Q1= 100 kw

Q2=0

W = 100kw

Heat engine violetes the Kelvin – Planck statement

Clausius Statement

It is impossible to construct a device that operates in a cycle and produces no effect other than the transfer of heat from a low temperature body to higher temperature body.

Warm environment

REFRIGERATOR

Cold refrigerated space

Q1 = 5 KJ

Q2 = 5 kJ

W = 0

Temperature Measurement

What is a Thermocouple?A thermocouple consists of two dissimilar metals, joined together at one end, which produce a small voltage when heated (or cooled). This voltage is measured and used to determine the temperature of the heated metals. The voltage for any one temperature is unique to the combination of metals used.

Summary of Different Types of Thermocouple

Pressure Measurement Devices

Absolute pressure: The actual pressure at a

given position

Gage pressure: Difference between absolute

and atmospheric pressure.

Vacuum pressure: Pressure below

atmospheric pressure.

Barometer

Bourdon Pressure Gauge

Simple U Tube Manometer