Thermodynamics & Heat

Engines

Basic Concepts

Thermodynamics• Thermodynamics= therme + dynamis

• Latin word therme means = heat

• Dynamis means = power or forces causing motion

so, overall meaning of thermodynamics is heat–power or force

interaction between system and surrounding.

for example

It is based upon general observation and those may be formulated

in form of thermodynamic law as –

• Zeroth law of thermodynamics

• First law of thermodynamics

• Second law of thermodynamics

• Application areas of thermodynamics• Steam power plant

• I.C.Engine

• Refrigerator and air conditioning

• Gas turbine

• Compressor etc.

Schematic of a Carnot refrigerator

• Microscopic and macroscopic view of thermodynamics• Macro- Large scale

• Micro – small scale

Macroscopic Microscopic

Attention is focused on certain quantity of

matter without considering the activity

occurred at molecular level

Matter consituting the system is

considered to comprise a large no. of

discrete particles called molecules.

A few properties are required to describe the

system such as P,V ,T etc and these can be

perceived by senses and measured by

available instruments. Example Expansion of

gases in a I.C. engine

Large no. of variables are required to

describe the system such as position, KE,

Velocity, P,V, T etc. It is very difficult to

measure these quantities with help of

available instruments.Example KTG

Requires Simple mathematical formulae to

analyse the system

Requires Advanced statistical and

mathematical formulae to analyse the

system

Known as statistical thermodynamics Known as Classical thermodynamics

System Mass Energy Example

Closed System × √ gas filled in a cylinder

Open System √ √ compressor, turbine, or nozzle

Isolated System × ×

gas filled in a cylinder but with

insulation

TYPES OF THERMODYNAMIC SYSTEM

SYSTEM- Definite region in space on which attention isconcentrated for investigation of the thermodynamicproblems i.e. heat, work transfer, etc. It may be classified onthe basis of transfer of mass & energy as indicated in table-

• Homogeneous System• Quantity of matter is homogeneous throughout in

chemical and physical structure i.e. system in a

single phase

• Pure substance• Substance that is homogeneous and invariable in

chemical composition i.e. combustion product,

atmospheric air

Thermodynamic Properties, Processes and Cycles

• Properties

• Characteristics by which physical condition of any system

can easily be defined , is known as property.

• Two types-

• Intensive ( Independent of mass example pressure,

temperature, density, composition, viscosity, thermal

conductivity)

• Extensive ( depends on mass examples- energy, enthalpy ,

entropy, volume etc.)

• Check for a property-

dP= Mdx + Ndy would be a thermodynamic property if its

differential is exact i.e.

• Specific quantity = Absolute / Mass and denoted by small

letters.Applicable for quantities depending upon the mass

like, internal energy, enthalpy, heat, work, volume etc.

• State• If any system have definite values of properties , it is known as

definite state . Properties are the state variables of any system

• Change in state• Any change in property will lead to change in state.

• Path• Locus of all change of states is known as path.

• Process• When path is completely defined , it becomes one process

• Process may be reversible or irreversible in nature.

• Reversible: it is possible to attain the initial states by eliminating the

effects. For example quasi static process ( reversible process)

• Cycle• Final state of any process is identical with the initial state , it

becomes one cycle.

• State, change in states, path, process, and cycles can be described

on a diagram that is drawn between property vs property as shown

Quasi Static Vs Non Quasi StaticQuasi- Almost slow, or infinitely slow

Quasi static Non Quasi Static1. Infinitely slowness is the characteristic

of process and all the intermediate

change in states are equilibrium with each

other.

1. Nature of process is very fast and

there is no equilibrium with intermediate

change of states.

2. Path (1-2) of process can easily be

defined due to all the change in states

are in equilibrium , hence process can

be drawn on graph paper with firm line.

2. Path of process (1-2) can not be

easily defined due to existence of non

equilibrium change in states, hence can

be drawn on graph paper with dotted

line.

3. Processes are reversible in nature. It

means it is possible to attain the initial

states by eliminating the effect.

3. Processes are irreversible in natute.

It means it is not possible to attain the

initial states by eliminating the effect

4. Example: Expansion of gases behind

the pistion against infinitely small

weigthts.

4. Example: Expansion of gases

behind the pistion against a single

weigtht.

Example : Compression process in piston –cylinder arrangement

Reversible & Irreversible process

Reversible Process Irreversible Process1. It is possible to attain the initial states

after eliminating the effects introduced to

obtain the final state.

1. It is not possible to attain the initial

states after eliminating the effects

introduced to obtain the final state.

Initial state will always be different in

reverse process

2. All the quasi static processes are

reversible in nature .

2. All the non quasi static processes

are reversible in nature .

3.Process will become reversible by

eliminating the causes of irrversibility i.e.

resisted expansion of gases, no internal

molecular friction or external friction

3. Causes of irrversibility: (a) Internal

friction between molecules (b) Free

expansion of gases ( c) Paddle wheel

work- Braking action causing the

conversion of mechanical work in form

of heat., it is not possible to aobtain the

motion of wheel by supplying the same

amount of heat to wheel.

4. Clausius inequality

∮

dQ/T=0 for cyclic

process or no change in entropy for

reversible process( ds =0)

4. Clausius inequality

∮

dQ/T< 0 for

cyclic process or change in entropy for

irreversible process( ds ≠0)

• No spontaneous change in macroscopic property (i.e. isolated system)

• Conditions for thermodynamic equilibrium

• Mechanical equilibrium ( No pressure gradient withinthe system and also between system & surroundingsi.e.δΡ=0, or no unbalance force)

• Chemical equilibrium (No transfer of mass by anychemical process across the boundary of system i.e.diffusion and no unbalanced chemical reaction withinthe system)

• Thermal equilibrium ( No transfer of heat across theboundary of system when it is separated from universeby means of Diathermic wall- that allows the heat orδT=0)

Thermodynamics Equilibrium

Concept of Continuum

• In concept of continuum matter within the system isassumed to be continuous and distributed uniformly.

• Importance- Used for defining the pressure and density

Pressure• Definition : P = Normal Force / Cross sectional area

• Units: Height of liquid ( 760 mm of Hg), N/m2, Pascal, Bar, Torr etc.

• One atmospheric pressure= 1.01325 N/m2

Pascal’s LawThe pressure is the same at all points on a horizontal plane in a given fluidregardless of geometry, provided that the points are interconnected by thesame fluid. (see figure below)

Measurement- Pressure• U tube manometer- Used for measurement of pressure.

• For same liquid equation of pressure can be written very

easily as: take +ve sign if it is desired to obtain the

pressure at lower level as shown in diagram below.

Example

Solution:

Review Questions & Problems

• Book Engineering thermodynamics by P K Nag , (Ed.Third )P. No. 15 Review questions section Q. No.1.1 ,1.4 to 1.17

• Problems( P.No. 16, Q.No. 1.5, 1.6, 1.8, 1.9)• Questions/ Problems given in assignment no. 1