Introduction to thermodynamics

Introduction to Engineering Thermodynamics

Dr.RohitSinghLather,AssociateProfessor

Science blasts many doubts, foresees what is not obvious It is the eye of everyone, one who hasn't got it, is like blind ||

Thermodynamics – A Philosophy • Thermodynamics is the science that primarily deals with energy• In its origins, thermodynamics was the study of engines• First century AD - Heron of Alexandria, first recognized thermal engineer

• 1593 - Galileo develops a water thermoscope

AeolipileHero of Alexandria

Reaction engine First recorded steam engine

thermoscope

Source: www.wikipedia.com

Aristotle“Nature abhors a vacuum”

Empty or unfilled spaces are unnatural as they go against the laws of nature and physics

Otto von GuerickeDesigned and built the world's first vacuum pump

• 1650 - Otto von Guericke designed and built the world's first vacuum pump and created theworld's first ever vacuum known as the Magdeburg hemispheres, a precursor of the engine

Magdeburg hemispheres : Large copper hemispheres, with mating rims, were used to demonstrate the power of atmospheric pressure. The rims were sealed with grease and the air

was pumped outSource: www.wikipedia.com

Source: www.wikipedia.com; www.google.com

• 1656 - English scientist Robert Hooke, built an air pump- Using this pump, Boyle and Hooke noticed a correlation between pressure, temperature,

and volume(Boyles’s Law - pressure and volume are inversely proportional)

• 1679 – Denis Papin conceived of the idea of a piston and a cylinder engine after watching steamrelease valve of steam digester rhythmically move up and kept the machine fromexploding, which was a closed vessel with a tightly

Steam Digester Air PumpSource: www.wikipedia.com; www.google.com

• 1697 – Thomas Savery an engineer built the first engine (based on Papin's designs)

• 1700’s – Industrial Revolution

• 1712 - Thomas Newcomen built another engine

- Early engines were crude and inefficient, but attracted the attention of the leading scientists of the time

• 1760s - Joseph Black Professor at the University of Glasgow develops calorimetry- Developed the fundamental concepts of heat capacity and latent heat- Joseph Black with James Watt (employed as an instrument maker), performed

experiments together, but it was Watt who conceived the idea of the externalcondenser which resulted in a large increase in steam engine efficiency

• 1780s - James Watt improves the steam engine

• 1824 – Sadi Carnot, the "father of thermodynamics", published ”Reflections on the motive powerof fire”, a discourse on heat, power, energy and engine efficiency

- The paper outlined the basic energetic relations between the Carnot engine, the Carnotcycle and motive power. Discusses idealized heat engines

- Marked the start of thermodynamics as a modern scienceSource: www.wikipedia.com; www.google.com

• 1849 -Lord Kelvin coined the word “thermodynamics”

• 1850 - Rudolf Clausius came up with the term “entropy”

• 1850s - The first and second laws of thermodynamics emerged simultaneously in the, primarily outof the works of William Rankine, Rudolf Clausius, and William Thomson (Lord Kelvin)

• 1859 – William Rankine - first thermodynamic textbook

• 1871 - James Maxwell formulated the Statistical Mechanical branch of thermodynamics

• 1875 - Ludwig Boltzmann precisely connected entropy and molecular motion


Thermodynamics and its branches

• Description of the states of thermodynamical systems at near-equilibrium, using macroscopic, empiricalproperties directly measurable in the laboratory

• Deals with exchanges of energy, work and heat based on the laws of thermodynamics

• Emerged with the development of atomic and molecular theories• Relates the microscopic properties of individual atoms and molecules to the macroscopic, bulk properties of

materials that can be observed on the human scale, thereby explaining thermodynamics at the microscopiclevel

• Equilibrium thermodynamics is the systematic study of transformations of matter and energy in systems asthey approach equilibrium

• Non-equilibrium thermodynamics is a branch of thermodynamics that deals with systems that are notin thermodynamic equilibrium

• Study of the interrelation of energy with chemical reactions or with a physical change of state within theconfines of the laws of thermodynamics

Classical Thermodynamics

Statistical Mechanics (Statistical Thermodynamics)

Chemical Thermodynamics

Treatment of equilibrium


Applications of Thermodynamics

Power plants

The human body

Air-conditioningsystems

Airplanes

Car radiators Refrigeration systems

Source: Yunus A. Cengel and Michael A. Boles Thermodynamics: An Engineering Approach, McGraw Hill, 8th Edition

Introduction We introduce here classical thermodynamics

• “Thermodynamics” is of Greek origin, and is translated as the combination of therme: heat and dynamis: power

• Thermodynamics is based on empirical observation• The word “thermo-dynamic,” used first by Lord Kelvin• Study of the relationship between heat, work, and other forms of

energy• Describes what is possible and what is impossible during energy

conversion processes• Describes the "direction" of a process• Studies the effects of temperature on physical systems at the

macroscopic scale• All of these things accurately describe thermodynamics• Thermodynamics is the study of energy conversion, most typically

through terms of heat and work

Sir William Thomson a.k.aLord Kelvin (1824 – 1907) Source: www.britannica.com/biography/William-Thomson-Baron-Kelvin

Forms of Energy

Energy

Macroscopic Microscopic Kinetic Potential

Sensible(translational + rotational + vibrational)

Latent(inter molecular phase change)

Chemical(Atomic Bonds)

Atomic(bonds within nucleolus of atoms)

SummationofallthemicroscopicenergiesiscalledInternalEnergy

E=U+KE+PE(kJ)

LowGrade HighGradeHeat Work

Macroscopic vs. Microscopic • The behavior of a system may be investigated from either a microscopic or macroscopic point of

view

Statistical Approach

Macroscopic Approach

• On the basis of statistical considerations and probabilitytheory

• we deal with average values for all particles underconsideration

- Reducing the number of variables to a few that can behandled

• Concerned with the gross or average effects of manymolecules

• These effects can be perceived by our senses and measuredby instruments

Understanding microscopic point of view

Cube with 1m3Air

2.4 × 1025

Molecules

Due to collision the position velocity and energy changes for each molecules

The behavior of the gas is described by summing up the behavior of each molecule

To specify position and velocity, we need three coordinates x, y and z

1.4 × 1026 equations

Container

Gas exerts pressure on the walls due to

change in momentum of the molecules as they collide with the wall

• From a macroscopic point of view, we are concerned not with the action of the individual

molecules but with the time-averaged force on a given area, which can be measured by a

pressure gauge

• Macroscopic observations are completely independent of our assumptions regarding the nature

of matter

• We are always concerned with volumes that are very large compared to molecular dimensions

and, therefore, with systems that contain many molecules

• Because we are not concerned with the behavior of individual molecules, we can treat the

substance as being continuous, disregarding the action of individual molecules

Continuum• The limit in which discrete changes from molecule to molecule can be

ignored and distances and times over which we are concerned aremuch larger than those of the molecular scale

• This will enable the use of calculus in our continuum thermodynamics

Thermodynamic System

A quantity of fixed mass under

investigation

UniverseCombination of system and surroundings

SurroundingsEverything external to the system

System BoundaryInterface separating system and surroundings (fixed or moving)

Heat In

WorkOut

Definitions

Open Systema system in which mass

crosses boundary, energy transfer in and out

Closed Systema system with fixed

mass, no mass transfer, energy may transfer in

and out

Isolated SystemA system in which there

are no interactions between system and

surroundings, no mass and energy transfer

system → fixed mass

constant mass, but possible variable volume


Control Volume

Control Volume

• Control Volume: fixed volume over which mass can pass in and out of its boundary

• The mass within a control volume may or may not be constant- If there is fluid flow in and out there may or may not be accumulation of mass within the

control volume

control volume → potentially variable mass, openSource: Yunus A. Cengel and Michael A. Boles Thermodynamics: An Engineering Approach, McGraw Hill, 8th Edition

A control volume can involve fixed, moving, real and imaginary boundaries


Thermodynamic PropertiesIf you can’t measure it, you can’t improve it – Lord Kelvin

Thermodynamic properties can be divided into two general classes, intensive and extensive properties

When all the properties of a system have definite values, the system is said to exist at a definite state

Property - A property of a system is any observable (macroscopic) characteristics of thesystem. The properties we shall deal with are measurable in terms of numbers and units ofmeasurements (eg. Pressure, density, temperature etc.)

Extensive Property The value of an extensive property

varies directly with the mass, examples are: Mass and total volume

Intensive PropertyIntensive property is independent of the amount of mass, examples

are: Temperature, pressure, specific volume, and density

Thus, if a quantity of matter in a given state is divided into two equal parts, each part will have the same value of intensive property as the original and half the value of the extensive property

• An Extensive variable depends on the size of the system.

• Examples of extensive variables are internal energy, enthalpy, heat capacity at constant pressure, heat capacity at constant volume, entropy, Helmholtz energy, Gibbs energy, volume

• For a system consisting of several parts, an extensive property of the ensemble of the parts is the sum of the corresponding extensive property of each of the parts

• Extensive properties of a system containing a pure species are proportional to the number of moles of the species present

• An Intensive variable has a uniform value in different subdivisions of a system

• Examples of intensive variables are pressure, temperature, identical in all points of the system, Molar variables or partial molar variables, specific mass, mole fractions, molar heat capacity at constant pressure, have the same values in all points of one phase of the system.

• They may differ from one phase† to another.

State• State is the condition of the system at an instance of time as described or measured by the

propertiesOR

Each unique condition of a system is called a state

• At a particular state, all properties have fixed values

State 1 State 2P1T1V1 P2T2V2

State functions the endpoints of your definite integral are all that matter: you could parameterize any path you want between the endpoints and the

resulting integral is the same

Property and Non Property

An infinitesimal change in a state function is represented by an exact differential

Change of a State Variable as the Result of a Thermodynamic Process

General ProcessFor a state variable, X , (XF – XI) is

independent of the path used for the process. The intermediate states of the

system are irrelevant

Cyclic ProcessConsider a thermodynamic change of a system to some intermediate state via

path 1. Then along path 2, bring the system back to its initial state. This

process is a cyclic process

I F I Int.

(XF – XI)path 1 = (XF – XI)path 2

Path 1

Path 2

Path 1

Path 2

The change of X is zero for a cyclic process

Intermediate StateInitial StateFinal StateInitial State

Source: Introductory Thermodynamics,Pierre Infelta Swiss Federal Institute of Technology Lausanne, Switzerland

• Example of a cyclic process: the initial state and finalstate is identical

• There is no volume change• The change of any state variable is zero for any

cyclic process

• A variable X is a state variable (or state function) if itschange for a cyclic process is zero

• X is also a state variable if its change for a generalprocess depends only on the initial and final states ofthe system and not on the way the change is achieved.

• The differential form dX is then called an exactdifferential

• The line integral of an exact differential isindependent of the path of integration

1

2

3

x

Y

Source: Introductory Thermodynamics,Pierre Infelta Swiss Federal Institute of Technology Lausanne, Switzerland

Processes and Cycles • Change of State: implies one or more properties of the system has changed

- the changes are slow relative to the underlying molecular time scales• Process: a succession of changes of state

- We assume our processes are all sufficiently slow such that each stage of the process is near equilibrium

- isothermal: constant temperature- isobaric: constant pressure- isochoric: constant volume

• Path: The succession of states passed through during a change of state is called path of the change of state

• Cycle: series of processes which returns to the original state. (A thermodynamic cycle is defined as a series of state changes such that the final state is identical with the initial state)

- The cycle is a thermodynamic “round trip.”

Volume

Pres

sure

a

b

1

2

Cycle1-2-1

Processa - b

Cyclea series of state changes such that the

final state is identical with the initial state

Reversible ProcessWhen a system undergoes changes in such a manner it is able to retain its original condition by following

the same thermodynamic path in the reverse direction, it is then said to have undergone a

reversible process

Irreversible processWhen the system is unable to reach the original

condition by retracing its path or attain the original conditions along other thermodynamic

paths, then the process is said to be an irreversible process

A process becomes irreversible due to the friction

ProcessChange of state, when the path is

completely specified A process is completely specified by the end states, the path, and the interactions that take place at

the boundary

Quasi-Static Process • Arbitrarily slow process such that system always stays stays arbitrarily close to thermodynamic

equilibrium• Infinite slowness is the characteristics of a quasi-static process• It is a succession of equilibrium states• A quasi-static process is also reversible process

Dots indicate equilibrium states

Pres

sure

1

2

VolumeEvery state passed through by the system will be an equilibrium state

Such a process is locus of all the equilibrium points passed through by

the system

SystemBoundary

PistonWeight

FinalState

InitialState

MultipleWeights

FinalState

InitialState

Piston

dv

dp

Thermodynamic Equilibrium

• Thermodynamic Equilibrium: state in which no spontaneous changes (macroscopic properties) are observed with respect to time- We actually never totally achieve equilibrium, we only approximate it- It takes infinite time to achieve final equilibrium

Non-equilibrium thermodynamics: branch of thermodynamics which considers systems often far from equilibrium and the time-dynamics of their path to equilibrium

Chemical EquilibriumCharacterized by equal

chemical potentials

Thermal EquilibriumCharacterized by equal temperature

Mechanical EquilibriumCharacterized by equal

forces (pressure)