chapter 2 lecture s12

8/2/2019 Chapter 2 Lecture S12

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Chapter 2ENERGY, ENERGY

TRANSFER, AND GENERALENERGY ANALYSIS

Tom Krupenkin, ME361, S2012

Thermodynamics: An Engineering Approach, 7th EditionYunus A. Cengel, Michael A. Boles

McGraw-Hill, 2011


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Objectives Introduce the concept of energy and define its various forms.

Discuss the nature of internal energy.

Define the concept of heat and the terminology associated with energytransfer by heat.

Discuss the three mechanisms of heat transfer: conduction,convection, and radiation.

Define the concept of work, including electrical work and several formsof mechanical work.

Introduce the first law of thermodynamics, energy balances, andmechanisms of energy transfer to or from a system.

Determine that a fluid flowing across a control surface of a controlvolume carries energy across the control surface in addition to anyenergy transfer across the control surface that may be in the form ofheat and/or work.

Define energy conversion efficiencies.

Discuss the implications of energy conversion on the environment.


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INTRODUCTION If we take the entire roomincluding the air and the refrigerator (or

fan)as the system, which is an adiabatic closed system since theroom is well-sealed and well-insulated, the only energy interactioninvolved is the electrical energy crossing the system boundary andentering the room.

As a result of the conversion of electric energy consumed by thedevice to heat, the room temperature will rise.

A refrigeratoroperating with itsdoor open in a well-sealed and well-

insulated room

A fan running in awell-sealed and

well-insulated roomwill raise the

temperature of air inthe room.


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FORMS OF ENERGY Energy can exist in numerous forms such as thermal, mechanical,

kinetic, potential, electric, magnetic, chemical, and nuclear, and theirsum constitutes the total energy, Eof a system.

Thermodynamics deals only with the changeof the total energy.

Macroscopic forms of energy: Those a system possesses as a wholewith respect to some outside reference frame, such as kinetic andpotential energies.

Microscopic forms of energy: Those related to the molecular

structure of a system and the degree of the molecular activity. Internal energy, U:The sum of all the microscopic forms of energy.

The macroscopic energy of anobject changes with velocity and

elevation.

Kinetic energy, KE: The energythat a system possesses as a result

of its motion relative to somereference frame.

Potential energy, PE: The energythat a system possesses as a resultof its elevation in a gravitational

field.


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Total energyof a system

Energy of a systemper unit mass

Potential energy

per unit mass

Kinetic energyper unit mass

Potential energy

Total energy

per unit mass

Kinetic energy

Mass flow rate

Energy flow rate


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Some Physical Insight to Internal Energy

The internal energy of asystem is the sum of all formsof the microscopic energies.

The various forms ofmicroscopicenergies that makeup sensibleenergy.

Sensible energy:The portionof the internal energy of asystem associated with thekinetic energies of themolecules.

Latent energy: The internalenergy associated with the

phase of a system.Chemical energy: The internalenergy associated with theatomic bonds in a molecule.

Nuclear energy: Thetremendous amount of energy

associated with the strongbonds within the nucleus of theatom itself.

Internal = Sensible + Latent + Chemical + Nuclear

Thermal = Sensible + Latent


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The macroscopickinetic energy is anorganized form of energy and is muchmore useful than the disorganizedmicroscopickinetic energies of themolecules.

The total energy of a system, canbe containedor storedin a system,and thus can be viewed as thestaticforms of energy.

The forms of energy not stored in asystem can be viewed as thedynamicforms of energy or asenergy interactions.

The dynamic forms of energy arerecognized at the system boundaryas they cross it, and they representthe energy gained or lost by asystem during a process.

The only two forms of energy

interactions associated with aclosed system are heat transferand work.

The difference between heat transfer and work: An energy interaction isheat transfer if its driving force is a temperature difference. Otherwise it is

work.


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More on Nuclear Energy

The fission of uranium and the fusion ofhydrogen during nuclear reactions, and

the release of nuclear energy.

The best known fissionreactioninvolves the split of the uranium atom

(the U-235 isotope) into other elementsand is commonly used to generateelectricity in nuclear power plants (440of them in 2004, generating 363,000MW worldwide), to power nuclearsubmarines and aircraft carriers, and

even to power spacecraft as well asbuilding nuclear bombs.

Nuclear energy by fusionis releasedwhen two small nuclei combine into alarger one.

The uncontrolled fusion reaction wasachieved in the early 1950s, but all theefforts since then to achieve controlledfusion by massive lasers, powerfulmagnetic fields, and electric currents togenerate power have failed.


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Mechanical EnergyMechanical energy:The form of energy that can be converted tomechanical work completely and directly by an ideal mechanical device such

as an ideal turbine.Kinetic and potential energies: The familiar forms of mechanical energy.

Mechanical energy of aflowing fluid per unit mass

Rate of mechanicalenergy of a flowing fluid

Mechanical energy change of a fluid during incompressible flow per unit mass

Rate of mechanical energy change of a fluid during incompressible flow


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MECHANICAL WORK There are two requirements for a work to exist:

there must be a forceacting on the object.

the object must move.

The work done is proportional to the force

applied (F) and the distance traveled (s).

Work = Force Distance

When force is not constant

If there is no movement,no work is done.


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Mechanical Energy

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V V

V = 0

211 2

E mV2

0 ?!E


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Mechanical Energy

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V V

V = 0

211 2

E mV2

0 ?!E ( )W F S


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Mechanical Energy

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gW F h g bW F F h g bW F F h E mgh E mgh

h h h

E W

?!E W

empty filled drained


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Mechanical Energy

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1h

2h

l l

A A

aW P Al aW P Al

1 1E mgh 2 2E mgh


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ENERGY TRANSFER BY HEAT

Energy can cross theboundaries of a closed systemin the form of heat and work.

Temperature difference is the drivingforce for heat transfer. The larger thetemperature difference, the higher is therate of heat transfer.

Heat:The form of energy that istransferred between two

systems (or a system and itssurroundings) by virtue of atemperature difference.


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Energy isrecognizedas heattransfer onlyas it crossesthe system

boundary.

During an adiabatic process, a systemexchanges no heat with its surroundings.

Heat transferper unit mass

Amount of heat transferwhen heat transfer ratechanges with time

Amount of heat transferwhen heat transfer rate

is constant


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Historical Background on Heat

Kinetic theory: Treats moleculesas tiny balls that are in motion and

thus possess kinetic energy. Heat: The energy associated with

the random motion of atoms andmolecules.

Heat transfer mechanisms:

Conduction:The transfer of energy

from the more energetic particles ofa substance to the adjacent lessenergetic ones as a result ofinteraction between particles.

Convection:The transfer of energybetween a solid surface and the

adjacent fluid that is in motion, andit involves the combined effects ofconduction and fluid motion.

Radiation:The transfer of energydue to the emission ofelectromagnetic waves (or

photons).

In the early nineteenth century, heat was

thought to be an invisible fluid called thecaloricthat flowed from warmer bodies tothe cooler ones.


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ENERGY TRANSFER BY WORK Work:The energy transfer associated with a force acting through a distance.

A rising piston, a rotating shaft, and an electric wire crossing thesystem boundaries are all associated with work interactions

Formal sign convention:Heat transfer to a system and work done by asystem are positive; heat transfer from a system and work done on a systemare negative.

Alternative to sign convention is to use the subscripts inand outto indicatedirection. This is the primary approach in this text.

Specifying the directionsof heat and work.

Work doneper unit mass

Power is thework done per

unit time (kW)


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Heat vs. Work Both are recognized at the boundaries

of a system as they cross theboundaries. That is, both heat and work

are boundaryphenomena. Systems possess energy, but not heator work.

Both are associated with a process, nota state.

Unlike properties, heat or work has no

meaning at a state. Both are path functions(i.e., their

magnitudes depend on the path followedduring a process as well as the endstates). Properties are point functions; but

heat and work are path functions(their magnitudes depend on the

path followed).Properties are point functionshave exact differentials (d).

Path functionshave inexact

differentials ()


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Electrical Work

Electrical power in terms of resistanceR, current I, and potential difference V.

Electrical work

Electrical power

When potential differenceand current change with time

When potential differenceand current remain constant


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MECHANICAL FORMS OF WORK There are two requirements for a work interaction between a

system and its surroundings to exist:

there must be a forceacting on the boundary. the boundary must move.

The work done is proportional to the force

applied (F) and the distance traveled (s).

Work = Force Distance

When force is not constant

If there is no movement,no work is done.


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ShaftWork

Energy transmission through rotating shafts

is commonly encountered in practice.

Shaft work is proportional to thetorque applied and the number

of revolutions of the shaft.

A force Facting througha moment arm r

generates a torque T

This force acts through a distance s

The power transmitted through the shaft

is the shaft work done per unit time

Shaftwork


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Spring Work

Elongationof a springunder the

influence ofa force.

When the length of the spring changes bya differential amount dxunder the influenceof a force F, the work done is

For linear elastic springs, the displacementxis proportional to the force applied

k:spring constant (kN/m)

Substituting and integrating yield

x1 and x2: the initial and the finaldisplacements

Thedisplacementof a linearspring doubleswhen the force

is doubled.


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Work Done on Elastic Solid Bars

Solid barsbehave asspringsunder theinfluence of

a force.

Stretchinga liquid film

with amovable

wire.

Work Associated with the Stretching of a Liquid Film


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Work Done to Raise or to Accelerate a Body

1. The work transfer needed to raise a body is equalto the change in the potential energy of the body.

2. The work transfer needed to accelerate a body isequal to the change in the kinetic energy of thebody.

The energytransferred to

a body whilebeing raisedis equal tothe change inits potentialenergy.

Nonmechanical Forms of Work

Electrical work: The generalized force is thevoltage(the electrical potential) and thegeneralized displacement is the electrical charge.

Magnetic work: The generalized force is themagnetic field strengthand the generalized

displacement is the total magnetic dipole moment.

Electrical polarization work: The generalizedforce is the electric field strengthand thegeneralized displacement is the polarization of themedium.


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THE FIRST LAW OF THERMODYNAMICS The first law of thermodynamics(the conservation of energy

principle) provides a sound basis for studying the relationships among thevarious forms of energy and energy interactions.

The first law states that energy can be neither created nor destroyedduring a process; it can only change forms.

The First Law:For all adiabatic processes between two specified states ofa closed system, the net work done is the same regardless of the nature ofthe closed system and the details of the process.

Energycannot becreated ordestroyed;it can onlychange

forms.

The increase in the energy of apotato in an oven is equal to theamount of heat transferred to it.


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In the absence of anywork interactions, theenergy change of asystem is equal to thenet heat transfer.

The work

(electrical) doneon an adiabaticsystem is equalto the increasein the energy ofthe system.

The work (shaft)

done on anadiabatic systemis equal to theincrease in theenergy of thesystem.


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Energy BalanceThe net change (increase or decrease) in the total energy of the system

during a process is equal to the difference between the total energyentering and the total energy leaving the system during that process.

The work (boundary) done on anadiabatic system is equal to the

increase in the energy of the system.

The energy changeof a system duringa process is equal

to the network andheat transferbetween the

system and itssurroundings.


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Energy Change of a System, Esystem

Internal, kinetic, and

potential energy changes


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Energy Balance

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2E kx

acid

?


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Energy Balance

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h

E mgh

m

m


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Mechanisms of Energy Transfer, Ein and Eout

Heat transfer

Work transfer

Mass flow

The energycontent of acontrol volumecan be changedby mass flow aswell as heat and

work interactions.

(kJ)

A closed massinvolves onlyheat transfer

and work.

For a cycle E =0,thus Q =W.


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ENERGY CONVERSION EFFICIENCIES

Efficiencyis one of the most frequently used terms in thermodynamics, and itindicates how well an energy conversion or transfer process is accomplished.

Efficiency of a water

heater:The ratio of theenergy delivered to thehouse by hot water tothe energy supplied tothe water heater.

The definition ofperformance is not limited

to thermodynamics only.


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Heating value of the fuel: The amount of heat released when a unit

amount of fuel at room temperature is completely burned and thecombustion products are cooled to the room temperature.

Lower heating value (LHV): When the water leaves as a vapor.

Higher heating value (HHV): When the water in the combustion gases iscompletely condensed and thus the heat of vaporization is also recovered.

The definition of the heating value ofgasoline.

The efficiency of space heatingsystems of residential andcommercial buildings is usuallyexpressed in terms of the annualfuel utilization efficiency

(AFUE), which accounts for thecombustion efficiency as well asother losses such as heat lossesto unheated areas and start-upand cooldown losses.

Generator A de ice that con erts mechanical energ to electrical


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Generator: A device that converts mechanical energy to electricalenergy.

Generator efficiency: The ratio of the electrical power output to themechanical power input.

Thermal efficiencyof a power plant: The ratio of the net electricalpower output to the rate of fuel energy input.

A 15-Wcompact

fluorescentlamp providesas much light

as a 60-Wincandescent

lamp.

Lighting efficacy:

The amount of lightoutput in lumensper W of electricityconsumed.

Overall efficiencyof a power plant


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The efficiency of a cooking

appliance represents thefraction of the energysupplied to the appliance thatis transferred to the food.

Using energy-efficient appliances conserveenergy.

It helps the environmentby reducing theamount of pollutants emitted to theatmosphere during the combustion of fuel.

The combustion of fuel produces carbon dioxide, causes global warming

nitrogen oxides and hydrocarbons,cause smog

carbon monoxide, toxic

sulfur dioxide, causes acid rain.


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Efficiencies of Mechanical and Electrical Devices

The mechanicalefficiency of a fan is theratio of the kineticenergy of air at the fanexit to the mechanicalpower input.

The effectiveness of the conversion process betweenthe mechanical work supplied or extracted and themechanical energy of the fluid is expressed by the

pump efficiencyand turbine efficiency,

Mechanical efficiency


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Generatorefficiency

Pump-Motoroverall efficiency

Turbine-Generator

overall efficiency

The overall efficiency of aturbinegenerator is the productof the efficiency of the turbine andthe efficiency of the generator,and represents the fraction of themechanical energy of the fluid

converted to electric energy.

Pumpefficiency

ENERGY AND ENVIRONMENT


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ENERGY AND ENVIRONMENT The conversion of energy from one form to another often affects the

environment and the air we breathe in many ways, and thus the study of energyis not complete without considering its impact on the environment.

Pollutants emitted during the combustion of fossil fuels are responsible forsmog, acid rain, and global warming.

The environmental pollution has reached such high levels that it became aserious threat to vegetation, wild life, and human health.

Energy conversion processes are often

accompanied by environmental pollution.

Motor vehicles are the largest source of airpollution.

O d S


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Ozone and Smog Smog: Made up mostly of ground-level ozone (O3), but it also contains numerous other

chemicals, including carbon monoxide (CO), particulate matter such as soot and dust,volatile organic compounds (VOCs) such as benzene, butane, and other hydrocarbons.

Hydrocarbons and nitrogen oxides react in the presence of sunlight on hot calm days toform ground-level ozone.

Ozoneirritates eyes and damages the air sacs in the lungs where oxygen and carbondioxide are exchanged, causing eventual hardening of this soft and spongy tissue.

It also causes shortness of breath, wheezing, fatigue, headaches, and nausea, andaggravates respiratory problems such as asthma.

Ground-level ozone, which is the primary componentof smog, forms when HC and NOxreact in the

presence of sunlight in hot calm days.

The other serious pollutant in smog is carbonmonoxide, which is a colorless, odorless, poisonousgas.

It is mostly emitted by motor vehicles.

It deprives the bodys organs from getting enoughoxygen by binding with the red blood cells that would

otherwise carry oxygen. It is fatal at high levels. Suspended particulate matter such as dust and sootare emitted by vehicles and industrial facilities. Suchparticles irritate the eyes and the lungs.

Acid Rain


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Acid Rain The sulfur in the fuel reacts with oxygen to form sulfur dioxide (SO2), which is an

air pollutant.

The main source of SO2 is the electric power plants that burn high-sulfur coal.

Motor vehicles also contribute to SO2 emissions since gasoline and diesel fuelalso contain small amounts of sulfur.

Sulfuric acid and nitric acid are formedwhen sulfur oxides and nitric oxides react withwater vapor and other chemicals high in the

atmosphere in the presence of sunlight.

The sulfur oxides and nitric oxides reactwith water vapor and other chemicals highin the atmosphere in the presence ofsunlight to form sulfuric and nitric acids.

The acids formed usually dissolve in thesuspended water droplets in clouds orfog.

These acid-laden droplets, which can beas acidic as lemon juice, are washed from

the air on to the soil by rain or snow. Thisis known as acid rain.

Th G h


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The GreenhouseEffect: GlobalWarming

Greenhouse effect: Glass allows the solarradiation to enter freely but blocks theinfrared radiation emitted by the interiorsurfaces. This causes a rise in the interior

temperature as a result of the thermalenergy buildup in a space (i.e., car).

The surface of the earth, which warms upduring the day as a result of the absorptionof solar energy, cools down at night byradiating part of its energy into deep space

as infrared radiation. Carbon dioxide (CO2), water vapor, and

trace amounts of some other gases suchas methane and nitrogen oxides act like ablanket and keep the earth warm at nightby blocking the heat radiated from theearth. The result is global warming.

These gases are called greenhousegases, with CO2 being the primarycomponent.

CO2 is produced by the burning of fossil

fuels such as coal, oil, and natural gas.

The greenhouse effect on earth.

A 1995 report: The earth has already warmed about 0 5C during the last


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A 1995 report: The earth has already warmed about0.5 C during the lastcentury, and they estimate that the earths temperature will rise another2C bythe year 2100.

A rise of this magnitude can cause severe changes in weather patterns withstorms and heavy rains and flooding at some parts and drought in others, major

floods due to the melting of ice at the poles, loss of wetlands and coastal areasdue to rising sea levels, and other negative results.

Improved energy efficiency, energy conservation, and using renewableenergy sources help minimize global warming.

The average car produces several times itsweight in CO2 every year (it is driven 20,000km a year, consumes 2300 liters of gasoline,and produces 2.5 kg of CO2 per liter).

Renewable energies such as wind arecalled green energy since they emit no

pollutants or greenhouse gases.

S


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Summary Forms of energy

Macroscopic = kinetic + potential

Microscopic = Internal energy (sensible + latent + chemical + nuclear) Energy transfer by heat

Energy transfer by work

Mechanical forms of work

The first law of thermodynamics

Energy balance

Energy change of a system

Mechanisms of energy transfer (heat, work, mass flow)

Energy conversion efficiencies

Efficiencies of mechanical and electrical devices (turbines, pumps) Energy and environment

Ozone and smog

Acid rain

The Greenhouse effect: Global warming

chapter 2 lecture s12

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