14ME2014

ENGINEERING THERMODYNAMICS

C.LINDON ROBERT LEE

SCHOOL OF MECHANICAL SCIENCES

KARUNYA UNIVERSITY

Email-lindon@karunya.edu

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Application Areas of Thermodynamics

Objectives

.To expose the fundamentals of thermodynamics

To integrate the basic concepts into various applications

To introduce the basic terms in thermodynamics

Basic Concepts

Thermodynamic systems

Boundary

Control volume

System and surroundings

Universe

Properties

BASICCONCEPTS

State

Process

Cycle

Equilibrium

Work and Heat Transfer

Point and Path functions

Comments on Thermodynamics

Thermodynamics is a funny subject. The first

time you go through it, you don't understand

it at all. The second time you go through it,

you think you understand it, except for one or

two small points. The third time you go

through it, you know you don't understand it,

but by that time you are so used to it, it

doesn't bother you any more. Arnold Sommerfeld

Introduction

The name Thermodynamics is derived from the Greek words therme meaning heat and dynamics meaning

power.

Thus Thermodynamics is the science of energy transfer and its effect on the

physical properties of substance.

Introduction-Defnition

Thermodynamics is the science of energy transfer and its effect on the physical properties of the substance.

All activities in nature involve interaction between energy and matter.

.

Ma

rsh

al

THERMODYNAMIC SYSTEM&TYPES

It is defined as a quantity of matter or a region in space upon which the attention is concentrated in

the analysis of a problem.

SYSTEM

Surrondings

Everything external to the system is called the surroundings or the environment.

Surrounding..cont

Identify the system

Identify the Boundary

Identify the Surrounding-Does it influence

BOUNDARY

Anything outside the system is termed as surroundings

Boundaries are of two types

i)Fixed or rigid ii) Movable or Deformable

Closed System

Closed System: No mass transfer across the boundary, There may be energy transfer into or out of the system. Eg Air in a

cylinder bounded by the piston.

Q

P,V,T

CLOSED SYSTEM

Energy in form of heat enters can Enter and Leave Mass cannot enter and leave m=constant

Open System: The matter crosses the boundary, eg an air- compressor in which air

enters at low pressure and leaves at high

pressure. Here the system is certain volume

(covering the compressor) called as control

volume,bounded by a surface called the

control surface.

Define the system or control volume

and draw a sketch

The sketch should identify

the system boundaries and properties

the mass and energy fluxes

the various states which properties needed

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Open system (control volume): A properly selected region in space.

It usually encloses a device that involves mass flow such as a compressor, turbine, or nozzle.

Both mass and energy can cross the boundary of a control volume.

Control surface: The boundaries of a control volume. It can be real or imaginary.

An open system (a

control volume) with one

inlet and one exit.

Open system

Mass can enter and leave

Energy can also enter and leave

OPEN SYSTEM

Energy Transfer

Closed systems:

Heat Work

Open systems:

Heat Work Mass flow

ISOLATED SYSTEM

Isolated system is one in which there is

no interaction between the systemn and

surroundings.

m=constant and E=constant

Define the system or control volume

and draw a sketch

The sketch should

i) identify the system

ii) the system boundaries

iii) the mass and energy fluxes

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PROCESSES

Process: Any change that a system undergoes from one equilibrium state to another.

Examples:Compression and Expansion Process

.

Some common properties that are used as coordinates are temperature T, pressure P, and volume V (or specific volume v).

The prefix iso- is often used to designate a process for which a particularproperty remains constant

. Isothermal process: A process during

which the temperature T remains constant.

Isobaric process: A process during which the pressure P remains constant.

Isochoric (or isometric) process: A process during which the specific volume v remains constant.

The P-V diagram of

a compression

process.

CYCLE

A thermodynamic cycle is a series of state changes such that the final stste is identical with the initial state.

Minimum two process is required for a cycle. What is the change in property for a cycle.

PATH A Succession of states passed through during a

change of state is called the path of the change of state.

QUASISTATIC PROCESS

A quasistatic process is one that takes place so slowly that the system may be considered as passing through a succession of equilibrium states.

Example: Gas contained in a piston cylinder assembly,rep by P1,T1,V1.The weight of piston balances the upward force. If weight is removed there ill be unbalanced force and the piston hits the stopper.Now if the weight is replaced by number of smaller weights and removed one by one very slowly from top of piston at any instant.Then every state passed will be an equilibrium state.

Quasi- static process P

roce

ss

Quasi-staticprocess

Every State passed through by the system will be an equilibrium state

Such a process which is but a locus of all equilibrium points passed

through by the system, is known as Quasistatic process.Quasi means

almost. Infinite slowness is the characteristic feature of a quasistatic.

PROPERTIES

OF A SYSTEM Property: Any measurable or

observable characteristic of a system.

Some familiar properties are pressure P, temperature T, volume V, and mass m.

Properties are considered to be either intensive or extensive.

Intensive properties: Those that are independent of the mass of a system, such as Pressure,temperature.

Extensive properties: Those whose values are dependendent on the mass such as volume.

EXTENSIVE PROPERTIES

Consider a mass of liquid in a container ,divide the container in two halves.

V=V1+V2.

INTENSIVE PROPERTIES Consider a hot liquid in a vessel,measure the

temperature let it be T

Divide the vessel into two halves,then measure the temperatures the temperature still reads T.

T= T1+T2 T NOT EQUAL TO T1+T2

The State Postulate

The number of properties required to fix the state of a system is given by the state postulate:

The state of a simple compressible system is completely specified by two independent, intensive properties.

.

The state of nitrogen is fixed by two independent, intensive properties.

THERMODYNAMIC EQUILIBRIUM

Thermodynamic equilibrium is the state of a system when no change in any of its macroscopic

properties is noticed on its isolation from its

surroundings. It requires three types of equilibrium to

be satisfied.

a) Mechanical: No unbalanced force inside and

surroundings.

b) Chemical: No chemical reaction, transfer of

matter.

c) Thermal : A system in equilibria (a) and (b) above,

and surrounded by diathermic (allowing heat flow)

wall with no change in any property

TEMPERATURE AND THE ZEROTH LAW OF

THERMODYNAMICS

R.H Fowler in 1931 invented after 1st and 2nd law of Thermodynamics

Zeroth law of thermodynamics

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ZEROTH LAW

If two thermodynamic systems A and B

are in thermal equilibrium, and B and C

are also in thermal equilibrium, then A

and C are in thermal equilibrium.

Work interaction M

ars

ha

l

Work is done by a force as it acts upon a body moving in the direction of the force.

The action of force through a distance is called mechanical work.

W = F x d

Works is said to be done by the system if the sole effect on things external to the system can be reduced to the raising of weight.

The weight may not actually be raised, but the net effect external to the system would be the raising of the weight.

Work

Work

Case 1:Consider the Battery and motor as a system.The motor is driving the fan.The system is doing work upon the

surrounding.

CAN THIS BE CALLED WORK DONE?

Case 2:Replace the fan by a pulley and weight as shown in second figure in next slide.The weight may be raised with the

pulley driven by the motor

THE SOLE EFFECT ON THINGS EXTERNAL TO THE SYSTEM IS THEN THE RAISING OF A WEIGHT.

Work

Battery- Motor System

Work transfer a system

Pdv Work or Displacement Work

Let the gas in the cylinder be a system initially at P1,V1.The system is in TD equilibrium. The piston is the only boundary

which moves due to gas pressure.Let the piston move out to a

new final position 2,which is alo in TD equilibrium specified by

P2 and V2.

Consider the piston moving out a small distance dl and if a is the area of the piston.

FORCE F=p.a (We know that Pressure=Force/Area) Small work done is dw=ForcexDistance dw=F.dl : dw=p.a.dl :dw=pdv where dv=a.dl Work done for whole path is summing up. Work done is area under the pv diagram

Work

PdV or displacement Work

Work is a path Function

The change in volume thus depends on the end states of the system irrespective of the path the system follows

For a cyclic process, the initial and final states of the system are the same and hence , the change in any property is zero.

Pdv work in various quasi static processes

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Work Done V=C

For a constant Volume Process

dv=0

w=

W=ZERO

Work Done in P=C

W=

W=P dv

W=P(V2-V1)

W=mR(T2-T1)

Work Done in T=C i.e PV=C

W=

PV=C P=C/V

W=C

W=PV

Work in various quasi-static process

(Reversible process)

Polytropic Process(pVn= C)

Free Expansion

Work done in free expansion is Zero

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FREE EXPANSION

Consider the gas and vacuum as a system, which is

separated by a partition.

Remove the partition, the gas rushes and fills the

entire volume.

Neglecting the work associated with the removal of

partition.

There is no work transfer involved as no work crosses

the boundary.

=0 Even though Not equal to Zero

If vacuum space is divided into a large number of

small volumes by partition.

If the partition is removed one by one slowly then

every state passed is in equilibrium and work done

can be estimated from relation.

Yet in free expansion there is no resistance to the

fluid at system boundary as the volume increases.

Since vacuum does not offer any resistance,there is

no work transfer involved in free expansion.

Work from closed cycles

Consider cycle A -> B -> A

WA->B

-WB->A

Work from closed cycles

Consider cycle A -> B -> A

WA->B->A= Area

Example a) What amount of work

is performed by the gas

in the cycle IAFI?

P is in N/m2 V is in m3

W=AREA OF TRIANGLE

W=1/2XBASEXHEIGHT

W=1/2X2m3x3N/m2=3J

area enclosedW

V (m3)

Solution

a) Find WAB

V (m3)

P (N/m2)

25

50

75

0.2 0.4 0.6

A

B

C

WAB = Area = -----J

Concept of Temperature and Heat

Temperature Measure of hotness or coldness Heat is defined as form of energy that is transferred by virtue of

a temperature difference.

HEAT INTERACTION

Point function

Thermodynamic properties are point functions. Since for a given state, there is a definite value for each property.

Example: P, T, Energy

Path function

Its not a function of end states of the process and it depends on the path of the system follows. Example : Work, Heat

Isobaric

P = constant

Isovolumetric

V = constant

Isothermal

T = constant

Adiabatic

P

P

P

P

V

V

V

V