1 chapter 1 lecture vnajib

8/10/2019 1 Chapter 1 Lecture Vnajib

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THERMODYNAMICS

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SEM I 2013/2014


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Chapter 1BASIC CONCEPTS AND

DEFINITIONS


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Chapter 1.0 Basic Concepts And Definitions(8 hours)

1.1 Introduction;1.2 Laws of thermodynamics;1.3 System, boundary and surrounding;1.4 Closed s stems and o en s stems;

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1.5 Process, state and thermodynamic properties;1.6 The working fluid, the types of properties,

properties as a function of the state, a state diagram, thetypes of processes;1.7 The pressure, volume and temperature, andmeasurement units.


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1.1 INTRODUCTION

Thermodynamics : The science of energy.

Energy : The ability to cause changes.

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The name thermodynamics stems from Greekwords therme (heat) and dynamis (power).


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Air Craft Propulsion

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Offshore wind farm Steam Engines


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Physics that deals with the mechanical action or relationship betweenheat and work

Example 1 : Heat to work.

Heat Q from flame provides energyto do work

Example 2 : Work to heat.

Work done by person isconverted to heat energy viafriction.

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Refrigerator Turbojet engine

Electrical Power Plant Automotive engine


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1.2 Laws of thermodynamics

The 1 st Law of thermodynamics :

- The conservation of energy principle.

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Energy cannot be created or destroyed.

The study of thermodynamics is concerned with theways energy is stored within a body and how energy

transformations, which involve heat and work may takeplace.


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THERMODYNAMICS RESEARCHAPPROACH1. Classical thermodynamics : A macroscopic approach

to the study of thermodynamics that does not require aknowledge of the behavior of individual particles. E.g Only use Pressure Gauge to know level of pressure inside

Cylinder (ease engineering analysis)

2. Statistical thermodynamics : A microscopicapproach, based on the average behavior of

large groups of individual particles. E.g Conduct detail study of atom/particles behavior to identify total

pressure inside Cylinder (more complex)

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Conservation ofenergy principle :During an interaction,

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from one form toanother but the total

amount of energyremains constant.


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1ST LAW 2ND LAW

CONCEPT of

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Conservation of energy

principle for the human body.

Heat flows in the direction of

decreasing temperature.


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IMPORTANCE OF DIMENSIONS AND UNITS

Any physical quantity can becharacterized by dimensions .

The magnitudes assigned to thedimensions are called units .

Dimensions & units divided into 2 main

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1. Primary or fundamental dimensions ,

e.g mass m, length L, time t , and temperature T

2. Secondary dimensions , or derived

dimensions from primary dimensionse.g velocity V , energy E , and


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Metric SI system : A simple andlogical system based on adecimal relationship betweenthe various units.

E.g. m, km, kg etc.

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English system : It has noapparent systematic numericalbase, and various units in thissystem are related to each otherrather arbitrarily.E.g. inch, ft, oz etc.


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W weight

m massg gravitationalacceleration

Some relatioship (SI and English Units)

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A body weighing 60 kgfon earth will weight only10 kgf on the moon.


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W weightm massg gravitationalacceleration

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The relative magnitudes of the forceunits newton (N), kilogram-force

(kgf), and pound-force (lbf).

The weight of a unitmass at sea level.


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The definition of the FORCE units.

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Work = Force Distance

1 J = 1 Nm

The definition of the WORK units.


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All non-primary units (secondary units) can be formed by combinations of primary units .Force units, for example, can be expressed as

Dimensional homogeneity All equations must be dimensionally homogeneous .

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Unity Conversion RatiosThey can also be expressed more conveniently as unity conversion ratios as

Unity conversion ratios are identically equal to 1 and are unit-less,and thus such ratios (or their inverses) can be inserted convenientlyinto any calculation to properly convert units.


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1.3 SYSTEM, BOUNDARY AND SURROUNDING

1. System : A quantity of matter (jirim)or a region (kawasan) in spacechosen for study.

Systems may be considered to beclosed or open .

2. Boundary (sempadan) : The real or

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mag nary sur ace t at separatesthe system from its surroundings.

The boundary of a system can befixed or movable .

3. Surroundings (sekitaran) : Thephysical space, or mass or regionoutside the systems boundary.


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1.4 Closed, Open, and Isolated Systems

Systems in thermodynamic study divided into 3 main category which are:

1. Closed system2. Open system3. Isolated System

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Closed system Open system


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Isolated System Boundary

Mass

System Surr 3Mass

Work

Surr 1

Heat = 0Work = 0Mass = 0

AcrossIsolated

Surr4

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Boundary HeatSurr 2


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1. Closed system

-consists of a fixed amount of mass and no mass may cross the system boundary.

The closed system boundary may move.

- Examples of closed systems are sealed tanks and piston cylinder devices (note thevolume does not have to be fixed). However, energy in the form of heat and work

may cross the boundaries of a closed system.

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Mass cannot cross the boundariesof a closed system, but energy can

A closed system with a moving

boundary


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A solid is a closed system(no mass transfer acrossboundary)

Expanding or compressing a gas

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can also be a closed system ifthere is no flow or majorleakages.


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2. Open system ,

-Also called control volume ,

-has mass as well as energy crossing the boundary , called a control surface.Examples of open systems are pumps, nozzles, diffusers, compressors, turbines,throttling valves, mixing chambers, pipe, duct flow and heat exchangers.

27 An open system (a control volume) with one inlet and one exit


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OPEN SYSTEMS

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Open system (controlvolume) : A properlyselected region in space.

It usually encloses a devicethat involves mass flowsuch as a compressor,

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, . Both mass and energy can

cross the boundary of acontrol volume.

An open system (a control volume) with oneinlet and one exit.


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Control surface : The boundaries of a control

volume. It can be real or imaginary.

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3. Isolated system

-is a general system of fixed mass where no heat or work may cross the boundaries .

- Also no energy crossing the boundaries and is normally a collection of a mainsystem and its surroundings that are exchanging mass and energy amongthemselves and no other system.

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Isolated System Boundary

Mass

System Surr 3Mass

Work

Surr 1

Heat = 0Work = 0Mass = 0

Across

IsolatedBoundary HeatSurr 2

Surr 4


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1.5 Process, state andthermodynamic

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properties;


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PROPERTIES OF A SYSTEM Properties:

Any characteristic of a system.

Some familiar properties are pressure P , temperature T , volume V ,

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.

Properties considered either to be extensive or intensive .


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Extensive propertiesare those whose values depend on the

size or extent of the system.

Some Extensive Properties:a. mass

b. volumec. total energy

Intensive propertiesare those that are independent of size.

Some intensive properties:

a. temperature b. pressurec. densityc. age

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. co or


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Example, the specific volume, v= V/m and,specific energy e=E/m

Specific properties:Extensive properties per unit mass.

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==

kg m

mV

massVolume

v3


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Not all properties are independent/single . Some are defined in termsof other ones.

E.g density is defines as mass per unit volume

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Sometimes, the density of a substance is given relative to thedensity of a well know substance.

Specific gravity : The ratio ofthe density of a substance tothe density of some standard

substance at a specifiedtemperature (usually water at

4C).


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Also, some familiar used property is:

Specific volume Specific weight :The weight of a

unit volume of asubstance.

Specific gravity

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STATE AND EQUILIBRIUM State of the system:

Set of properties that completely describes the condition.

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A system at two different states.


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Equilibrium : A state of balance.

Thermodynamics deals with equilibrium states.

In an equilibrium state there are no unbalanced potentials(or driving forces) within the system.

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A closed system reaching thermal equilibrium.


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1. Thermal equilibrium : If the temperature is the samethroughout the entire system.

2. Mechanical equilibrium: If there is no change in pressure atany point of the system with time.

TYPES OF EQUILIBRIUM

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3. Phase equilibrium: If a system involves two phases andwhen the mass of each phase reaches an equilibrium leveland stays there .

4. Chemical equilibrium: If the chemical composition of asystem does not change with time, that is, no chemicalreactions occur .


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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 . Simple compressible system: If a system involves no

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, , , ,

effects.

The state of nitrogen is fixed by two

independent, intensive properties:1. Temperature, T2. Specific volume, v

PROCESSES AND CYCLES


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PROCESSES AND CYCLESProcess :

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

Path :The series of states through which a system passes during a process.

To describe a process completely, one should specify the initial and finalstates, as well as the path it follows, and the interactions with the surroundings.

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Quasistatic or quasi-equilibrium process:

When a process proceeds in such a manner that the system remainsinfinitesimally close to an equilibrium state at all times.

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P di


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Process diagramsplotted by employing thermodynamic properties as coordinates arevery useful in visualizing the processes.

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

47The P -V diagram of a compression process.

Th fi i i ft d t d ig t f hi h


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

1. Isothermal process : A process during which the temperature T remainsconstant.

2. Isobaric process : A process during which the pressure P remains constant.3. Isochoric (or isometric) process : A process during which the specific

volume v remains constant.

Cycle : A process during which the initial and final states are identical.

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1.6 The working fluid,the types of properties,properties as a function

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,diagram, the types of

processes;

The Steady Flow Process


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The Steady-Flow Process

The term steady implies nochange with time . Theopposite of steady isunsteady , or transient .

A large number of

engineering devices operatefor long periods of timeunder the same conditions,and they are classified assteady-flow devices .

During a steady-flow process, fluid

properties withinthe control

volume maychange with

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Steady-flow process : A

process during which a fluidflows through a controlvolume steadily.

Steady-flow conditions canbe closely approximated bydevices that are intended forcontinuous operation suchas turbines, pumps, boilers,condensers, and heatexchangers or power plants

or refrigeration systems.

position but notwith time.

Under steady-flow conditions, the massand energy contents of a control volumeremain constant.


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1.7 The pressure,

volume and

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measurement units.

Temperature Scales


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Temperature Scales All temperature scales are based on

some easily reproducible states such asthe freezing and boiling points of water:the ice point and the steam point.

Ice point : A mixture of ice and waterthat is in equilibrium with air saturatedwith vapor at 1 atm pressure (0C or

32F). Steam point : A mixture of liquid water

and water vapor (with no air) inequilibrium at 1 atm pressure (100C or212F).

P versus T plotsof the

experimentaldata obtained

from a constant-volume gas

thermometer

using fourdifferent gasesat different (butlow) pressures.

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Celsius scale : in SI unit system Fahrenheit scale : in English unit

system Thermodynamic temperature scale : A

temperature scale that is independent ofthe properties of any substance.

Kelvin scale (SI) Rankine scale (E) A temperature scale nearly identical to

the Kelvin scale is the ideal-gastemperature scale . The temperatureson this scale are measured using aconstant-volume gas thermometer. A constant-volume gas thermometer would

read 273.15C at absolute zero pressure.

C i f


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Comparison oftemperature

scales.

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The reference temperature in the original Kelvin scale was the ice point ,273.15 K, which is the temperature at which water freezes (or ice melts).

The reference point was changed to a much more precisely reproduciblepoint, the triple point of water (the state at which all three phases of water

coexist in equilibrium), which is assigned the value 273.16 K.

Comparison ofmagnitudes ofvarioustemperatureunits.

PRESSURE


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PRESSURE

Pressure : A normal force exertedby a fluid per unit area

68 kg 136 kg

A feet =300cm2

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The normal stress (or pressure) on thefeet of a chubby person is much greater

than on the feet of a slim person.

Somebasicpressure

gages.

. g cm . g cm

P =68/300=0.23 kgf/cm2

Absolute pressure : The actual pressure at a given position. It is


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p p g pmeasured relative to absolute vacuum (i.e., absolute zero pressure).

Gage pressure : The difference between the absolute pressure andthe local atmospheric pressure. Most pressure-measuring devices arecalibrated to read zero in the atmosphere, and so they indicate gagepressure.

Vacuum pressures : Pressures below atmospheric pressure.

Throughoutthis text, the

ressure P

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will denoteabsolute

pressureunlessspecifiedotherwise.

Variation of Pressure with Depth


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Variation of Pressure with Depth

When the variation of densitywith elevation is known

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Free-body diagram of a rectangular fluid element in equilibrium.

The pressure of a fluid at restincreases with depth (as a

result of added weight).

In a room filled with


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In a room filled witha gas, the variation

of pressure withheight is negligible.

Pressure in a liquidat rest increaseslinearly withdistance from the

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.

The pressure is thesame at all points ona horizontal plane ina given fluidregardless ofgeometry, providedthat the points areinterconnected bythe same fluid.

Pascals law : The pressure applied to a


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p ppconfined fluid increases the pressurethroughout by the same amount.

The area ratio A2/ A1 iscalled the ideal mechanicaladvantage of the hydraulic

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Lifting of a large weightby a small force by theapplication of Pascals

law.

.

The Manometer


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Measuring thepressure drop across

a flow section or a flowdevice by a differential

manometer.

It is commonly used to measure small andmoderate pressure differences. A manometercontains one or more fluids such as mercury, water,alcohol, or oil.

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In stacked-up fluid layers, thepressure change across a fluid layerof density and height h is gh .

The basicmanometer.

Other Pressure Measurement Devices


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Bourdon tube : Consists of a hollow metal tubebent like a hook whose end is closed andconnected to a dial indicator needle.

Pressure transducers : Use various techniquesto convert the pressure effect to an electricaleffect such as a change in voltage, resistance,or capacitance.

Pressure transducers are smaller and faster,and they can be more sensitive, reliable, and

60Various types of Bourdon tubes used

to measure pressure.

prec se an e r mec an ca coun erpar s.

Strain-gage pressure transducers: Work byhaving a diaphragm deflect between twochambers open to the pressure inputs.

Piezoelectric transducers : Also called solid-state pressure transducers , work on theprinciple that an electric potential is generated ina crystalline substance when it is subjected tomechanical pressure.

THE BAROMETER AND ATMOSPHERIC PRESSURE


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Atmospheric pressure is measured by a device called a barometer ; thus, theatmospheric pressure is often referred to as the barometric pressure .

A frequently used pressure unit is the standard atmosphere , which is defined asthe pressure produced by a column of mercury 760 mm in height at 0C ( Hg =13,595 kg/m 3) under standard gravitational acceleration ( g = 9.807 m/s 2).

The length or the

61The basic barometer.

-of the tube has no

effect on the heightof the fluid column of

a barometer,provided that the

tube diameter islarge enough toavoid surface tension

(capillary) effects.

1 chapter 1 lecture vnajib

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