chapter 1 su

8/2/2019 Chapter 1 Su

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Chapter 1

INTRODUCTION ANDBASIC CONCEPTS

Mehmet Kanoglu

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Thermodynamics: An Engineering Approach, 6th EditionYunus A. Cengel, Michael A. Boles

McGraw-Hill, 2008


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Application Areas of Thermodynamics

SOLAR HOT WATERSYSTEM

AIR CONDITIONING SYSTEM

REFRIGERATION SYSTEM

THE HUMAN BODY

AIRPLANES

POWER PLANTS


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THERMODYNAMICS AND ENERGY

The name thermodynamicsstemsfrom the Greek words therme(heat)and dynamis(power-movement-work).

HEAT + WORK = ENERGY

Energy: The ability to cause changes

Thermodynamics: The science ofenergy.

Energy cannot be created ordestroyed.

Conservation of energy principle:During an interaction, energy canchange from one form to another but

the total amount of energy remainsconstant.

Energy cannot be createdor destroyed; it can onlychange forms.

55

010

PE =

KE =

z


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A person who has a greater energy input(food) than energy output (exercise) willgain weight (store energy in the form offat)

CONSERVATION OF ENERGY

Heat flows in the direction ofdecreasing temperature.


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Classical thermodynamics: A macroscopicapproach to the study of thermodynamics that doesnot require a knowledge of the behavior of individualparticles.

Analisis kelakuan sistem secara kasar & keseluruhan

Cth. Pengukuran suhu air dalam sebuah balang

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

Lebih tepat tetapi kompleks

THERMODYNAMICS APPROACH


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DIMENSIONS AND UNITS

Any physical quantity can becharacterized by dimensions.

The magnitudes assigned to thedimensions are called units.

PRIMARY DIMENSION SECONDARY DIMENSION

Mass (kg)

Length (m)

Time (s)

Temperature (K)

Velocity (m/s)

Energy (N.m) or J

Acceleration (m/s2)

Density (kg/m3)

50 kJ = 50 x 103 J

40 cm = 40 x 10-2 m


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TWO COMMON UNITS

Metric SI system: A simple and logical system based on adecimal relationship between the various units.

English system: It has no apparent systematic numericalbase, and various units in this system are related to eachother rather arbitrarily.

The SI unit prefixes are used in all branches of engineering.

SI UNIT English Unit

Mass (kg)

Length (m)

Time (s)

Pound-mass (lbm)

Foot (ft)

Second (s)


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Some SI and English Units

The definition of the force units.

Work = Force Distance1 J = 1 Nm

1 cal = 4.1868 J1 Btu = 1.0551 kJ

SI English


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The term weight is oftenincorrectly used to express mass(kg)

weight Wis a force(gravitationalforce applied to a body)

weight, however, changes with achange in gravitationalacceleration

At sea level a mass of 1 kg

weighs 9.807 N

W weightm massg gravitational

acceleration

A body weighing60 kgf on earthwill weigh only 10kgf on the moon.

Some SI and English Units


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CLOSED AND OPEN SYSTEM System: A quantity of matter or a

region in space chosen for study.

Surroundings: The mass orregion outside the system

Boundary: The real or imaginary

surface that separates the systemfrom its surroundings.

The boundary of a system can befixedor movable.

Systems may be considered to beclosedor open.


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CLOSED SYSTEM OPEN SYSTEM

Also known as a control mass

No mass can cross its boundary

But energy, in the form of heat or

work, can cross the boundary

The volume does not have to be fixed

Also known as a control volume

Involve mass flow in and out of a

system

Both energy, in the form of heat or

work, can cross the boundary

Most control volumes have fixedboundaries

Control Surface

OPENSYSTEM

control

volume


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PROPERTIESOF A SYSTEM

Property:Any characteristic of asystem.

Some familiar properties arepressure P, temperature T, volume

V, and mass m. Properties are considered to be

either intensiveor extensive.

An easy way to determinewhether a property is intensive or

extensive is to divide the systeminto two equal parts with animaginary partition

Criterion to differentiate intensiveand extensive properties.


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Extensive properties Intensive properties

Those whose values depend on the

size of the system and can mixed.volume, mass and internal energy, u

Those that are independent of the

size of a system,The value can change but cannot bemixed

Temperature, pressure, and density.

1 kg 2 kg 3 kg+ =

PROPERTIES OF A SYSTEM

50 80 130+


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Density ismass per unitvolume;

Density

Specific volume

PROPERTIES OF A SYSTEM

Specific properties: Extensiveproperties per unit mass.

(m3/kg)

specific volumeis volume perunit mass.


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STATE AND EQUILIBRIUM

A closed system reaching thermal

equilibrium.

A system at two different states.

Thermodynamics deals withequilibriumstates.

Equilibrium:A state of balance. In an equilibrium state there are no

unbalanced potentials (or drivingforces) within the system.

Thermal equilibrium:If thetemperature is the same throughoutthe entire system.

Mechanical equilibrium:If there isno change in pressure at any pointof the system with time.

Phase equilibrium:If a systeminvolves two phases and when themass of each phase reaches anequilibrium level and stays there.

Chemical equilibrium:If thechemical composition of a systemdoes not change with time, that is,no chemical reactions occur.


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The State Postulate/Kenyataan Aturan Dua Sifat

The number of properties requiredto fix the state of a system is givenby the state postulate:

The state of a simplecompressible system iscompletely specified by twoindependent, intensiveproperties.

Simple compressible system:Ifa system involves no electrical,magnetic, gravitational, motion,and surface tension effects.

The state of nitrogen isfixed by two independent,intensive properties.


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PROCESSES AND CYCLESProcess: Any change that a system undergoes from one equilibrium state to

another.

Perubahan suatu sistem dari keadaan keseimbangan awal kepadakeseimbangan akhir.

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

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

Process diagrams plotted by employingthermodynamic properties as coordinates arevery useful in visualizing the processes.


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The prefix iso- is often used to

designate a process for which aparticular property remainsconstant.

Isothermal process: A processduring which the temperature Tremains constant.

Isobaric process:A processduring which the pressure Premains constant.

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

Kitar: Beberapa proses yangdigabungkan membentuk satu litartertutup

Keadaannya kembali semula kekeadaan awalnya pada akhiranproses

Perubahan bersih sifat-sifatnya

adalah malar

Constantproperty Name of the process

Temperature

Pressure

Volume

Entropy

Entalphy

Isothermal (sesuhu)

Isobaric (setekanan)

Isometrik (seisipadu)

Isentropic (seentropi)

Isentalphic (seentalpi)

PROCESSES AND CYCLES

1

2

3 4Property A

Property B


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Quasi-equilibrium process/Proses MiripStatik

Quasistatic or quasi-equilibrium process: When a process proceeds in such

a manner that the system remains infinitesimally close to an equilibrium

state at all times.Proses yang berlaku tidak terhinggaperlahannya

Semua sifat berubah secara seragam

Pantas:Molekul gas yang berdekatanpermukaan omboh tidak sempat tersebarke tempat lainBerkumpul di permukaan omboh

Hasilkan kawasan tekanan tinggiTekanan gas tidak seragam, sistemtidak seimbang

Perlahan: Molekul mempunyai masauntuk tersebar


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The Steady-Flow Process The term steadyimplies no

change with time. Theopposite of steady isunsteady, or transient.

A large number ofengineering devices operatefor long periods of timeunder the same conditions,and they are classified as

steady-flow devices.

Steady-flow process: Aprocess 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.

During a steady-flow process, fluid

properties withinthe control

volume may

change withposition but not

with time.

Under steady-flow conditions, the mass

and energy contents of a control volumeremain constant.


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TEMPERATURE

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 at lower temperature

until both bodies attain the same temperature

reached thermal equilibrium

Both bodies contain same physical property

called temperature

freezing cold, cold, warm, hot?

A metal chair, for example, will feel much colder than

a wooden one even when both are at the same

temperature.

Heat flows in the direction ofdecreasing temperature.


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THE ZEROTH LAW OF THERMODYNAMICS

The zeroth law of thermodynamics: If two bodies are in thermal

equilibrium with a third body, they are also in thermal equilibrium witheach other.

By replacing the third body with a thermometer, the zeroth law canbe restated as two bodies are in thermal equilibrium if both have thesame temperature reading even if they are not in contact.

Two bodies reaching thermal equilibrium after being

brought into contact in an isolated enclosure.


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TEMPERATURE SCALES

All temperature scales are based on some easily reproducible

states such as the freezing and boiling points of water: the icepointand the steam point.

Ice point: A mixture of ice and water that is in equilibrium withair saturated with vapor at 1 atm pressure (0C or 32F).

Steam point: A mixture of liquid water and water vapor (with

no air) in equilibrium at 1 atm pressure (100C or 212F). The temperature scales used in the system today are the

Celsius scaleandthe Fahrenheit scale

Celsius scale:in SI unit system (C)

Fahrenheit scale: in English unit system (F)


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Temperature scale that is

independent of the

properties of any substance

CalledKelvin scale (SI)

Rankine scale (English)

The lowest temperature on

the Kelvin scale is absolute

zero, or 0 Kideal-gas temperature scale aremeasured using a constant-volume

gas thermometer thatwould read273.15C at absolute zero pressure.

THERMODYNAMICTEMPERATURE SCALE


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On the Celsius scale, the ice and steampoints were originally assigned the values

of 0 and 100C,

The corresponding values on theFahrenheit scale are 32 and 212F.

magnitudes of each division of 1 K and1C are identical

when we are dealing with temperaturedifferences T, the temperature interval onboth scales is the same

Raising the temperature of a substance by10C is the same as raising it by 10 K.

COMPARISON OF TEMPERATURESCALES

100.00 373.15 212.00 671.67 Steampoint

T (C) = T (K) - 273


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EXAMPLE 14Expressing Temperature Rise in Different UnitsDuring a heating process, the temperature of a system rises by 10C. Expressthis rise in temperature in K, F, and R.

Solution

T(K) = T(C) = 10 K

T( R ) = 1.8 T(K) = (1.8)(10) = 18 R

T(F) = T(R) = 18F

DiscussionNote that the units C and K are interchangeable when dealingwith temperature difference


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PRESSURE

The normal stress (or pressure)

on the feet of a chubby person ismuch greater than on the feet of a

slim person.

Pressure: A normal force exerted by a fluidper unit area

P = F/A

The pressure unit pascal is too small forpressures encountered in practice.

Therefore, its multiples kilopascal(1 kPa 103 Pa)and megapascal(1MPa 106 Pa) are commonly

used.

68 kg 136 kg

Afeet=300cm2

0.23 kgf/cm2 0.46 kgf/cm2

P= W/Afeet = 68/300=0.23 kgf/cm2

136 /300 = 0.46 kgf/cm3


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Absolute pressure: The actual pressure at a given position. It ismeasured 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 are

calibrated to read zero in the atmosphere, and so they indicate gagepressure.

Vacuum pressures: Pressures below atmospheric pressure.

Somebasicpressuregages.


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EXAMPLE 15

A vacuum gage connected to a chamber reads 40 kPa at a location where

the atmospheric pressure is 100 kPa. Determine the absolute pressure inthe chamber.

SolutionThe absolute pressure is easily determined from Eq. 116 to bePabs = Patm - Pvac = 100 40 = 60 kPa

Note that the local value of the atmospheric pressure is used whendetermining the absolute pressure.


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Variation of Pressure with Depth

Pressure in a fluid at rest does not change in the horizontal direction

Pressure in a fluid increases with depth because more fluid rests ondeeper layers, and the effect of this extra weight on a deeper layer isbalanced by an increase in pressure

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

result of added weight).

P = Patm + gh or Pgage = gh


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Note that the pressures at points A, B, C, D, E, F, and Gare the samesince they are at the same depth, and they are interconnected by the samestatic fluid.

pressures at points Hand Iare not the same since these two points cannot

be interconnected by the same fluid

Variation of Pressure with Depth


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Pascals law: The pressure applied to aconfined fluid increases the pressurethroughout by the same amount.

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

law.

The area ratio A2/A1 iscalled the ideal mechanicaladvantageof the hydrauliclift.


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The Manometer

It is commonly used to measure small and moderate pressure differences. Amanometer contains one or more fluids such as mercury, water, alcohol, or oil.

Since the gravitational effects of gases are negligible, the pressure anywhere inthe tank and at position 1 has the same value

pressure in a fluid does not vary in the horizontal direction, P2 = P1

The differential fluid column of height his instatic equilibrium, and it is open to theatmosphere


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In stacked-up fluid layers, thepressure change across afluid layer of density andheight his gh.

Many engineering problems and some manometers involve multiple immisciblefluids of different densities stacked on top of each other

(1) the pressure change across afluid column of height his P =gh,

(2) pressure increases downward in a given fluid

and decreases upward (i.e., Pbottom >Ptop),

(3) two points at the same elevation in a

continuous fluid at rest are at the same pressure

the pressure at the bottom of the tank in Fig. 147

can be determined by starting at the freesurface where the pressure is Patm, movingdownward until we reach point 1at the bottom, and setting the result equal to P1


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Manometers are particularly well-suited to measure pressure drops acrossa horizontal flow section between two specified points

The working fluid can be either a gas or a liquid whose

density is 1. The density of the manometer fluid is 2, and the differentialfluid height is h

Measuring thepressure drop across

a flow section or a flowdevice by a differential

manometer.

A relation for the pressure differenceP1 -P2 can be obtained by starting

at point 1 with P1, moving along thetube by adding or subtracting the ghterms until we reach point 2, andsetting the result equal to P2:


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EXAMPLE 17The water in a tank is pressurized by air, and the pressure is measured by amultifluid manometer. The tank is located on a mountain at an altitude of 1400 m where theatmospheric pressure is 85.6 kPa. Determine the air pressure in the tank if h1= 0.1 m, h2 = 0.2m, and h3 = 0.35 m. Take the densities of water, oil, and mercury to be 1000 kg/m3, 850

kg/m3, and 13,600 kg/m3, respectively.

Starting with the pressure at point 1 at the airwaterinterface, moving along the tube by adding orsubtracting the ghterms until we reach point 2, andsetting the result equal to Patm since the tube is open tothe atmosphere gives

P1 + watergh1 + oilgh2 - mercurygh3 =Patm

P1=Patm + g(mercurygh3 -watergh1 - oilgh2)

22/100

1

/.1

12.0850

1.0100035.01360081.96.85

mN

kPa

smkg

N

kPa

P1= 130 kPa


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Other Pressure Measurement Devices

Various types of Bourdon tubes used

to measure pressure.

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, andprecise than their mechanical counterparts.

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.


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THE BAROMETER AND ATMOSPHERIC PRESSURE

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

The basic barometer.

The length or thecross-sectional areaof the tube has no

effect on the heightof the fluid column of

a barometer,

provided that thetube diameter islarge enough to

avoid surface tension(capillary) effects.


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chapter 1 su

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