Thermodynamics: An Engineering Approach, 6 Edition

Thermodynamics: An Engineering Approach, 6th EditionYunus A. Cengel, Michael A. BolesMcGraw-Hill, 2008Chapter 2ENERGY, ENERGYTRANSFER, AND GENERALENERGY ANALYSISSUMMARYDR. MUNZER EBAIDDr. Munzer Ebaid1

FORMS OF ENERGY Internal energy, U:The sum of all the microscopic forms of energy. Kinetic energy, KE:The energy that a system possesses as a result of itsmotion relative to some reference frame. Potential energy, PE:The energy that a system possesses as a result of itselevation in a gravitational field.Dr. Munzer Ebaid2

Kinetic energyKinetic energy per unitmassPotential energyPotential energy perunit massTotal energy of asystemEnergy of a systemper unit massDr. Munzer Ebaid3

Mass flow rateEnergy flow rateTotal energy per unit massDr. Munzer Ebaid4

Mechanical EnergyMechanical energy: The form of energy that can beconverted to mechanical work completely and directly by anideal mechanical device such as an ideal turbine.Kinetic and potential energies: The familiar forms ofmechanical energy.Mechanical energy of a flowing fluid per unit massRate of mechanical energy of a flowing fluidDr. Munzer Ebaid5

Mechanical EnergyMechanical energy change of a fluid during incompressibleflow per unit massRate of mechanical energy change of a fluid duringincompressible flowExample: (Wind Energy) V 2&Wind energy Power E m& 2 V 2 Wind energy per unit mass e 2 Dr. Munzer Ebaid 6

Energy is recognized as heattransfer only as it crosses thesystem boundary.During an adiabatic process, asystem exchanges no heat with itssurroundings.Dr. Munzer Ebaid7

Historical Background on Heat (Cont’d)Heat transfer mechanisms: Conduction: The transfer of energy from the moreenergetic particles of a substance to the adjacent lessenergetic ones as a result of interaction between particles. Convection: The transfer of energy between a solidsurface and the adjacent fluid that is in motion, and itinvolves the combined effects of conduction and fluidmotion. Radiation: The transfer of energy due to the emission ofelectromagnetic waves (or photons).Dr. Munzer Ebaid8

ENERGY TRANSFER BY WORKWork: The energy transfer associated with a force actingthrough a distance.A rising piston, a rotating shaft, and an electric wirecrossing the system boundaries are all associated withwork interactionsWork done per unit massPower is the work done perunit time (kW)Dr. Munzer Ebaid9

Heat vs. Work Both are recognized at the boundaries of a system as theycross the boundaries. That is, both heat and work areboundary phenomena.Systems possess energy, but not heat or work.Both are associated with a process, not a state.Unlike properties, heat or work has no meaning at a state.Both are path functions (i.e., their magnitudes dependon the path followed during a process as well as the endstates).Path functions haveinexact differentials (δ )Dr. Munzer Ebaid10

Electrical WorkElectrical workElectrical powerWork Force DistanceWhen potential differenceand current change with timeWhen force is not constantWhen potential differenceand current remain constantDr. Munzer Ebaid11

MECHANICAL FORMS OF WORK There are two requirements for a work interaction between asystem and its surroundings to exist:9 there must be a force acting on the boundary.9 the boundary must move.Work Force DistanceWhen force is not constantThe work done is proportional to the force applied (F) and the distancetraveled (s).Dr. Munzer Ebaid12

Shaft WorkA force F acting through a moment arm r generates a torque TThis force acts through a distance (s)Shaft workThe power transmitted through the shaft is the shaft work doneper unit timeDr. Munzer Ebaid13

Shaft work is proportional to thetorque applied and the number ofrevolutions of the shaft.Energy transmission throughrotating shafts is commonlyencountered in practice.Dr. Munzer Ebaid14

Spring WorkFor linear elastic springs, the displacement (x) is proportional to the forceappliedk: spring constant (kN/m)When the length of the spring changes by a differential amount dx underthe influence of a force F, the work done isSubstituting and integrating yieldx1 and x2: the initial and the finaldisplacementsDr. Munzer Ebaid15

Work Done on Elastic Solid BarsSolid bars behave assprings under theinfluence of a force.Dr. Munzer Ebaid16

Work Associated with the Stretching of a Liquid FilmStretching a liquidfilm with a movablewire.Dr. Munzer Ebaid17

Work done to raise or to Accelarate a BodyExamples:1. Power needs of a car to climb a hill at constant velocityP.E W& g mg ΔZ Δt mgVvertical mg (V sin α )2. Work needs of a car to accelerateK .E W a (1m V22 V122)3. Power needs of a car to accelerate()1K .E W& a m V22 V12 / Δt2Dr. Munzer Ebaid18

Non-mechanical Forms of WorkElectrical work: The generalized force is the voltage (theelectrical potential) and the generalized displacement is theelectrical charge.Magnetic work: The generalized force is the magnetic fieldstrength and the generalized displacement is the totalmagnetic dipole moment.Electrical polarization work: The generalized force is theelectric field strength and the generalized displacement isthe polarization of the medium.Dr. Munzer Ebaid19

THE FIRST LAW OF THERMODYNAMICS The first law of thermodynamics (the conservation of energyprinciple) provides a sound basis for studying the relationships amongthe various 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.Energy cannot be created ordestroyed; it can only changeforms.Dr. Munzer Ebaid20

Energy BalanceThe net change (increase or decrease) in the total energy of the systemduring 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 onan adiabatic system is equal to the increase in theenergy of the system.Dr. Munzer Ebaid21

Energy BalanceThe energy change of a system during aprocess is equal to the net work and heattransfer between the system and itssurroundings.Dr. Munzer Ebaid22

Energy Change of a System, ΔEsystemInternal, kinetic, andpotential energy changesDr. Munzer Ebaid23

Mechanisms of Energy Transfer, Ein and Eout Heat transfer Work transfer Mass flowConstant Rate(kJ)Dr. Munzer Ebaid24

Mechanisms of Energy Transfer, Ein and EoutExamples:1. Accelerate of Air by a FandE 0E& in E& out dtW& elec,in2. Heating Effect of a Fan2Vout m& air (keout ) m& air2dEE& in E& out 0dtW& elec,in Q& out )3. Annual Lighting Cost of a Class RoomLighting Power Power consumed per Lamp X No. Of LampsLighting Energy Lighting Power X Operating HoursLighting Cost Lighting Energy X Unit costDr. Munzer Ebaid25

Heating value of the fuel: The amount of heat released when a unitamount 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.Dr. Munzer Ebaid26

The definition of the heating value of gasoline.The efficiency of space heating systems of residential andcommercial buildings is usually expressed in terms of theannual fuel utilization efficiency (AFUE), which accounts forthe combustion efficiency as well as other losses such as heatlosses to unheated areas and start-up and cool down losses.Dr. Munzer Ebaid27

Generator: A device that converts mechanical energy toelectrical energy. Generator efficiency: The ratio of the electrical poweroutput to the mechanical power input. Thermal efficiency of a power plant: The ratio of the netelectrical power output to the rate of fuel energy input.Overall efficiency of a power plantDr. Munzer Ebaid28

Efficiencies of Mechanical and Electrical DevicesMechanical efficiencyThe mechanical efficiency of a fan is theratio of the kinetic energy of air at thefan exit to the mechanical power input.Dr. Munzer Ebaid29

Efficiencies of Mechanical and Electrical DevicesThe effectiveness of the conversion process between themechanical work supplied or extracted and the mechanicalenergy of the fluid is expressed by the pump efficiency andturbine efficiency,Dr. Munzer Ebaid30

Pump efficiencyGenerator efficiencyPump-Motor overall efficiencyTurbine-Generator overall efficiencyDr. Munzer Ebaid31

The overall efficiency of a turbine–generator is the product of theefficiency of the turbine and the efficiency of the generator, andrepresents the fraction of the mechanical energy of the fluid convertedto electric energy.Dr. Munzer Ebaid32

Ozone and Smog Smog: Made up mostly of ground-level ozone (O3), but italso contains numerous other chemicals, including carbonmonoxide (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 presenceof sunlight on hot calm days to form ground-level ozone.Dr. Munzer Ebaid33

Ozone and SmogOzone irritates eyes and damages the air sacs in the lungs where oxygen andcarbon dioxide are exchanged, causing eventual hardening of this soft andspongy tissue.It also causes shortness of breath, wheezing, fatigue, headaches, and nausea,and aggravates respiratory problems such as asthma The other serious pollutant in smog is carbon monoxide, which is acolorless, odorless, poisonous gas.It is mostly emitted by motor vehicles.It deprives the body’s organs from getting enough oxygen by bindingwith the red blood cells that would otherwise carry oxygen. It is fatal athigh levels.Suspended particulate matter such as dust and soot are emitted byvehicles and industrial facilities. Such particles irritate the eyes and thelungs.Dr. Munzer Ebaid34

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-sulfurcoal.Motor vehicles also contribute to SO2 emissions since gasoline anddiesel fuel also contain small amounts of sulfur. The sulfur oxides and nitric oxides react with water vapor and otherchemicals high in the atmosphere in the presence of sunlight to formsulfuric and nitric acids . The acids formed usually dissolve in the suspended water droplets inclouds or fog. These acid-laden droplets, which can be as acidic as lemon juice, arewashed from the air on to the soil by rain or snow. This is known asAcid Rain.Dr. Munzer Ebaid35

The Greenhouse Effect: Global Warming Greenhouse effect: Glass allows the solar radiation to enter freely butblocks the infrared radiation emitted by the interior surfaces. This causes arise in the interior temperature as a result of the thermal energy buildup in aspace (i.e., car).The surface of the earth, which warms up during the day as a result of theabsorption of solar energy, cools down at night by radiating part of itsenergy into deep space as infrared radiation.Carbon dioxide (CO2), water vapor, and trace amounts of some othergases such as methane and nitrogen oxides act like a blanket and keep theearth warm at night by blocking the heat radiated from the earth. The resultis global warming.These gases are called “Greenhouse Gases,” with CO2 being the primarycomponent. CO2 is produced by the burning of fossil fuels such as coal, oil, andnatural gas.Dr. Munzer Ebaid36

The Greenhouse Effect: Global WarmingGases causes the greenHouse Effect are1.2.3.4.Carbon dioxide (CO2)water vaporMethanenitrogen oxidesThe greenhouse effect on earth.Dr. Munzer Ebaid37

THE ENDDr. Munzer Ebaid38

Thermodynamics: An Engineering Approach, 6th Edition Yunus A. Cengel, Michael A. Boles McGraw-Hill, 2008 SUMMARY . Mechanical energy: The form of energy that can be . The first law of thermodynamics (the conservation of energy