unit1 td by sathiyan

8/3/2019 Unit1 Td by Sathiyan

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Cycles .............................................................................................................................. 12

Point and path function ................................................................................................... 12

Temperature ................................................................................................................... 12

Thermal equilibrium experimental understanding ........................................................... 13

Zeroth law of thermodynamics ........................................................................................ 13



Unit 1

Thermodynamics

The science relating heat and work transfer and relative changes in the properties of

the working substance is called thermodynamics It deals with the energies processed by gases and vapours, conversion of energy in

terms of heat and work by relating the properties within the system

Applications of Thermodynamics

1) I.C.Engines

2) Refrigeration and AC.

3) Turbines

4) Air compressor

5) Steam and nuclear power plant6) Gas turbine

7) Jet propulsion

Energy conversion

The conversion of one form of energy into another form of energy is known as

energy conversion

The energy conversion is taken place in two types they are

1) Direct energy conversion

2) Indirect energy conversion

Direct energy conversion

The required output energy is gotten by converting the source energy directly

Example: Electric generator, photovoltaic cells

In the above said example the conversion of mechanical energy into electrical energy

made directly.

The devices which are all performs direct energy conversion process is called direct

energy conversion device.

Indirect energy conversion

The input energy is first converted into various forms of energy then finally required

form of energy output is made.

Example: Steam power plant.

In the above said example the heat energy is first converted into mechanical energy

then it is converted to electrical energy.

The devices which are all performs indirect energy conversion process is called

indirect energy conversion device.



Energy conversion devices

Common energy conversion devices

ENERGY CONVERSION DEVICES ENERGY I/P USEFUL ENERGY O/P

Electric heater Electrical Thermal

Hair drier Electrical Thermal

Air conditioner Electrical ThermalI.C.Engine Chemical energy Mechanical energy

Energy conversion efficiency:

The ratio between output energy and the input energy is called efficiency

=

X 100

Note: normally the efficiency always expressed in percentage (%)

Types of efficiency:

I) Thermal efficiency (ηth)

II) Mechanical efficiency (ηmech)

III) Electrical efficiency (ηelec)

IV) Air standard efficiency (ηair)

V) Volumetric efficiency (ηvol)

VI) Relative efficiency (ηrel)

PROBLEMS

An electric motor consumes 100w and produces 90w of mechanical powder.

Determine its efficiency.

Solution:

Given: input energy=100w, output energy =90w

Energy

conversion

deviceEnergy input

in one form

Energy output in

another form



(%) =

X 100

(%) =90

100

X 100

η = 90%

Thermodynamic system

It is defined as a proper space or area or a collection of matter on which the study of

transfer and energy conservation is made.

Surroundings

The matters or the particles which are external to the system as called surroundings.

It may be affected by the system and may affect the system.

Boundary

A layer which is separating the system and surrounding is called boundary. It may

physical or imaginary and fixed or movable. It has negligible thickness.

Universe

The entire system and surrounding forms together universe.

Classification of thermodynamic system

Thermodynamic system may be classified into three types

i) Closed system

ii) Open system

iii) Isolated system

Closed system

A system which does not permit any mass transfer but energy transfers takes place.

A closed system is also known as control mass

Consider a piston cylinder arrangement which is heated at the cylinder side.

Consider the cylinder is filled with gas so that the gas has not any provisions to



escapes from the cylinder. So here the gas is said to be a system, cylinder wall is said

to be boundary. If the system (gas) is heated which, expands due to thermal changes

to give useful work done to the piston.

This process is not allowing any mass transfer perhaps, the heat transfer is take place

and so mass of the system is constant.

Open system

A system where both heat and mass transfer takes place is known as open system

It is often called by control volume. ( example air compressor)

Consider an air compressor system, here low pressure air is compressed to high

pressure air and leaves the system continuously Certain energy is supplied to the system externally to run the compressor. Hence

some of the energy input converted to useful energy output.

When compressing air into high pressure, so that heat is produced inside the system,

which dissipates to the atmosphere.

So mass transfer, work transfer and heat transfer all takes place in this system.

CONTROL VOLUME

Any specified thermal devices had mass flow in and out of the system to transfer any

energy is called control volume. Both mass and energy can cross the boundary or the control volume is called control

surface.



Isolated system

A system which is not affected by the surroundings. Here, there is no heat, work and

mass transfer takes place. It is an imaginary system.

Example: universe

Homogenous system and heterogeneous system

Every substance can exist in any one of following three phases, viz solid, liquid and

gas.

Any quantity of substance exist in single physical and chemical structure is called

phase.

A system consisting of a single phase is called homogeneous system, while a system

consisting of more than one phase is known as heterogeneous system.

Properties of Thermodynamic system

Any measurable or observable characteristics of the substance when the system

remains in equilibrium state.

Example: Temperature, volume, entropy, enthalpy, pressure, etc.,

Properties are classified into two types

1) Intensive or intrinsic properties

2) Extensive or extrinsic properties

Intensive properties

Intensive properties are those that are independent of the mass of a system, such as

temperature, pressure, and density.

The properties like pressure, temperature and density are always same on the entire

parts of the system.



Extensive properties

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.

The properties like mass, volume weight are not always same if we take some part of the system.

How to identify the system

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.

Each part will have the same value of intensive properties as the original state of the

system, but half the value of the original state is called extensive properties.

State of the system

The status of the system at any instance of time or any moment is known as state of a system.

Some of thermodynamic state variables decide the state of the system. Like

pressure, volume, temperature, etc., are named as state variables.

Consider a thermodynamic system having two states, viz., compressed state and

expanded state.



The mass (m1) of gas is compressed to the pressure of p1 for the volume of v1 say asthe fig (a) shows compressed state.

The same mass (m1) of gas is expanded to the pressure of p2 for the volume of v2 say

as the fig (b) shows expanded state.

Here the fact noticed is the mass is safe but other properties like pressure,

temperature, volume have changed which describes some of the state altered from

each other.

To specify the state at least two properties are plotted in graph to study the state of

the system.

Pressure

Volume

Thermodynamic equilibrium

The macroscopic property should not change, if the system is isolated from its

surroundings then the system is said to be in thermodynamic equilibrium. (stable)

Therefore, there should not any changes in macroscopic properties if the system

exists in an equilibrium state.

The study of the properties of the system is made when only at the system in

equilibrium. We can say the system is in equilibrium only by satisfying the following three

conditions



1) Mechanical equilibrium.

2) Chemical equilibrium.

3) Thermal equilibrium.

When the system is free of unbalanced forces within the system and also between

the system and surrounding then it is said to be in a state of mechanical equilibrium. If there is no chemical reaction or transfer of matter from one part of the system to

another, such as diffusion or solution, the system is said to exist in a state of

chemical equilibrium.

When a system existing in mechanical and chemical equilibrium is separated from its

surroundings by a diathermic wall (Diathermic means ‘which allows heat to flow’)

and if there is no spontaneous change in any property of the system, the system is

said to exist in a state of thermal equilibrium.

Process Any change that a system undergoes from one equilibrium state to another is called

a process.

The series of states through which a system passes during a process is called the

path of the process.

To describe a process completely, one should specify the initial and final states of the

process, as well as the path it follows, and the interactions with the surroundings.

(i) Quasi-static process

In thermodynamics, a quasi-static process is a thermodynamic process that happens

infinitely slowly.

However, it is very important of note that no real process is quasi-static.

Therefore in practice, such processes can only be approximated by performing them

infinitesimally slowly.

A quasi-static process ensures that the system will go through a sequence of states

that are infinitesimally close to equilibrium (so the system remains in quasi-static

equilibrium), in which case the process is typically reversible.



This is illustrated in Fig. When a gas in a piston-cylinder device is compressedsuddenly, the molecules near the face of the piston will not have enough time to

escape and they will have to pile up in a small region in front of the piston,

Thus creating a high-pressure region there. Because of this pressure difference, the

system can no longer be said to be in equilibrium, and this makes the entire process

nonquasi-equilibrium.

However, if the piston is moved slowly, the molecules will have sufficient time to

redistribute and there will not be a molecule pileup in front of the piston.

As a result, the pressure inside the cylinder will always be nearly uniform and will

rise at the same rate at all locations. Since equilibrium is maintained at all times, this

is a quasi-equilibrium process.

(ii) Reversible and irreversible process

The process is said to be an irreversible process if it cannot return the system and

the surroundings to their original conditions when the process is reversed. The

irreversible process is not at equilibrium throughout the process.

For example, when we are driving the car uphill, it consumes a lot of fuel and this

fuel is not returned when we are driving down hill. Many factors contribute in

making any process irreversible. The most common of these are Friction,Unrestrained expansion of a fluid, Heat transfer through a finite temperature

difference, mixing of two different substances.

The basic concept is that most of the thermodynamic processes have a preferred

direction just as Heat always flows from hotter object to colder object. Once a gas is

released in a room, it expands in room and never contracts without indulgence of

any external force etc.

But in some systems, the reverse occurs. Normally it happens when that system is

close to thermal equilibrium. This equilibrium has to be inside the system itself and

also within the system and its surroundings. When this stage is reached, even a small



change can change the direction of the process and therefore such a reversible

process is also known as an equilibrium process.

A very simple example can be of two metal jars A and B which are at a thermal

equilibrium and are in contact with each other. Now when we heat jar A slightly,

heat starts to flow from Jar A to Jar B. This is the direction of this process. Now thisprocess can be reversed just by cooling Jar A slightly. When Jar A is cooled, heat

flows from Jar B to Jar A till thermal equilibrium is reached.

(iii) Flow and non flow process

The working fluid enters the system and leaves after doing work, this process is

called flow process.

Here both mass and energy flows across the boundary.

Example: turbine, boiler, compressor.

In some process the working fluid is recircuited again and again to do the work, thisprocess is called non-flow process.

Here only energy crosses the boundary and leaving the mass within the boundary.

Example: constant volume and constant pressure process

Cycles

A thermodynamic cycle consists of a series of thermodynamic processes transferring

heat and work, while varying pressure, temperature, and other state variables, and

also it returning a system to its initial state.

Here are some of the well known thermodynamic cycles Ideal cycle, Diesel cycle, Otto cycle, Brayton cycle, etc.,

Point and path function

Path function: Their magnitudes depend on the path followed during a process as

well as the end states.

The work transfer and the heat transfer are depends upon the path of the process

takes place.

Point Function: They depend only on the state and not on how a system reaches

that state. All properties are point functions. The cyclic integral of a path function isnon-zero. Work and heat are path functions.

properties are point functions, (i.e. pressure, volume, temperature and entropy)

Temperature

Although we are familiar with temperature as a measure of “hotness” or “coldness,”

it is not easy to give an exact definition for it.

Based on our physiological sensations, we express the level of temperature

qualitatively with words like freezing cold, cold, warm, hot, and red-hot.

However, we cannot assign numerical values to temperatures based on our

sensations alone. Furthermore, our senses may be misleading.



A metal chair, for example, will feel much colder than a wooden one even when both

are at the same temperature.

Fortunately, several properties of materials change with temperature in a repeatable

and predictable way, and this forms the basis for accurate temperature

measurement. The commonly used mercury-in-glass thermometer, for example, is based on the

expansion of mercury with temperature. Temperature is also measured by using

several other temperature-dependent properties.

Thermal equilibrium experimental understanding

It is a common experience that a cup of hot coffee left on the table eventually cools

off and a cold drink eventually warms up.

That is, 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 atlower temperature until both bodies attain the same temperature.

At that point, the heat transfer stops, and the two bodies are said to have reached

thermal equilibrium. The equality of temperature is the only requirement for

thermal equilibrium.

Another example: if we have two different materials in the two different

temperatures kept in contact, after some time both material come to same

temperature if it has isolated surrounding as illustrated in following fig.

Zeroth law of thermodynamics

The zeroth law was first formulated and labelled by R. H. Fowler in 1931. As the

name suggests, its value as a fundamental physical principle was recognized more

than half a century after the formulation of the first and the second laws of thermodynamics.

It states “when two systems are in thermal equilibrium with the third system

separately, then they themselves are in thermal equilibrium with each other”.

Consider three systems, viz., A,B,C. If the system A and system B is in contact and

also the system C is in contact with system B then we can say the temperature of all

the three system will be same due to thermal equilibrium.

unit1 td by sathiyan

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