Heat Chap01 Lecture Note1

7/24/2019 Heat Chap01 Lecture Note1

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Chapter 1INTRODUCTION AND

BASIC CONCEPTS

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Objectives

Understand how thermodynamics and heat transfer are related

to each other

Distinguish thermal energy from other forms of energy, and heat

transfer from other forms of energy transfer Perform general energy balances as well as surface energy

balances

Understand the basic mechanisms of heat transfer, which are

conduction, convection, and radiation, and Fourier's law of heatconduction, Newton's law of cooling, and the StefanBoltzmannlaw of radiation

Identify the mechanisms of heat transfer that occur

simultaneously in practice

Develop an awareness of the cost associated with heat losses

Solve various heat transfer problems encountered in practice

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THERMODYNAMICS AND HEAT TRANSFER

Heat: The form of energy that can be transferred from one

system to another as a result of temperature difference.

Thermodynamics is concerned with the amountof heat

transfer as a system undergoes a process from one

equilibrium state to another.

Heat Transfer deals with the determination of the ratesof

such energy transfers as well as variation of temperature.

The transfer of energy as heat is always from the higher-

temperature medium to the lower-temperature one.

Heat transfer stops when the two mediums reach the same

temperature.

Heat can be transferred in three different modes:

conduction,convection,radiation

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Application Areas of Heat Transfer

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Historical Background Kinetic theory: Treats molecules astiny balls that are in motion and thus

possess kinetic energy.

Heat: The energy associated with therandom motion of atoms and

molecules.

Caloric theory: Heat is a fluidlike

substance called the caloric that is amassless, colorless, odorless, and

tasteless substance that can bepoured from one body into another

It was only in the middle of thenineteenth century that we had a truephysical understanding of the natureof heat.

Careful experiments of theEnglishman James P. Joule publishedin 1843 convinced the skeptics thatheat was not a substance after all, and

thus put the caloric theory to rest.


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ENGINEERING HEAT TRANSFER

Heat transfer equipment such as heat exchangers, boilers, condensers, radiators,heaters, furnaces, refrigerators, and solar collectors are designed primarily on the

basis of heat transfer analysis.

The heat transfer problems encountered inpractice can be considered in two

groups: (1) ratingand (2) sizingproblems.

The rating problems deal with the determination of the heat transfer rate for anexisting system at a specified temperature difference.

The sizing problems deal with the determination of the size of a system in order totransfer heat at a specified rate for a specified temperature difference.

An engineering device or process can be studied either experimentally(testing andtaking measurements) or analytically(by analysis or calculations).

The experimental approachhas the advantage that we deal with the actual physicalsystem, and the desired quantity is determined by measurement, within the limits ofexperimental error. However, this approach is expensive, timeconsuming, and oftenimpractical.

The analytical approach (including the numerical approach) has the advantage that itis fast and inexpensive, but the results obtained are subject to the accuracy of theassumptions, approximations, and idealizations made in the analysis.

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Modeling in Engineering

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Energy can exist in numerous forms such as:

thermal,

mechanical,

kinetic,

potential,

electrical,

magnetic,

chemical,

nuclear.

Their sum constitutes the total energy E(or e on a unit massbasis) of a system.

The sum of all microscopic forms of energy is called theinternal energy of a system.

HEAT AND OTHER FORMS OF ENERGY

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Internal energy: May be vieed as the sum of the kinetic and potentialenergies of the molecules.

Sensible heat: The kinetic energy of the molecules.

Latent heat: The internal energy associated ith the phase of a system.

Chemical (bond) energy: The internal energy associated ith theatomic bonds in a molecule.

Nuclear energy: The internal energy associated ith the bonds ithinthe nucleus of t he atom itself.

What is thermal energy?

What is the difference between thermalenergy and heat?

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Internal Energy and Enthalpy

!n the analysis of systems thatinvolve fluid flo, e fre"uentlyencounter the combination ofproperties u and Pv.

The combination is defined asenthalpy (h # u $ Pv).

The term Pvrepresents the floenergy of the fluid (also calledthe flo ork).


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Specific Heats of Gases,Liquids, and Solids

Specific heat: The energy required to raise thetemperature of a unit mass of a substance byone degree.

To kinds of specific heats: specific heat at constant volume cv specific heat at constant pressure cp

The specific heats of a substance, in general,depend on to independent properties suchas temperature and pressure.

%t lo pressures all real gases approach idealgas behavior, and therefore their specific heats

depend on temperature only.

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Incompressible substance: % substancehose specific volume (or density) doesnot change ith temperature orpressure.

The constant&volume and constant&pressure specific heats are identical forincompressible substances.

The specific heats ofincompressiblesubstances depend on temperature only.

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Energy Transfer

Energy can be transferred to or from a givenmass by two mechanisms:

heat transfer and work.

Heat transfer rate: The amount of heattransferred per unit time.

Heat flux: The rate of heat transfer per unit

area normal to the direction of heat transfer.

when is constant:

Power: The workdone per unit time.

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THE FIST LA! "F THE#"$%&A#ICS

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The energy balance for anysystem undergoing any processin the rate form

The first law of thermodynamics (conservation of energyprinciple) states that energy can neither be created nor destroyed

during a process; it can only change forms.

The net change (increase ordecrease) in the total energy ofthe system during a process isequal to the difference between

the total energy entering and thetotal energy leaving the system

during that process.

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In heat transfer problems it is convenient towrite a heat balance and to treat theconversion of nuclear, chemical,mechanical, and electrical energies intothermal energy as heat generation.

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Energy Balance for Closed Systems (Fixed Mass)

A closed system consists of a fixed mass.

The total energy E for most systemsencountered in practice consists of the

internal energy U.

This is especially the case for stationarysystems since they dont involve anychanges in their velocity or elevation during

a process.


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Energy Balance forSteady-Flow Systems

A large number of engineering devices such aswater heaters and car radiators involve mass flowin and out of a system, and are modeled ascontrol volumes.

Most control volumes are analyzed under steadyoperating conditions.

The term steadymeans no change with timeat aspecified location.

Mass flow rate: The amount of mass flowingthrough a cross section of a flow device per unittime.

Volume flow rate: The volume of a fluid flowingthrough a pipe or duct per unit time.

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Surface Energy Balance

This relation is valid for both steady andtransient conditions, and the surface

energy balance does not involve heatgeneration since a surface does not

have a volume.

A surface contains no volume or mass,and thus no energy. Theoretically, a

surface can be viewed as a fictitioussystem whose energy content remains

constant during a process.

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HEAT TA&SFE #ECHA&IS#S

Heatas the form of energy that can be transferred from one system toanother as a result of temperature difference.

% thermodynamic analysis is concerned ith the amountof heat transferas a system undergoes a process from one e"uilibrium state to another.

The science that deals ith the determination of the rates of suchenergy transfers is the heat transfer.

The transfer of energy as heat is alays from the higher&temperaturemedium to the loer&temperature one, and heat transfer stops henthe to mediums reach the same temperature.

'eat can be transferred in three basic modes:

conduction

convection

radiation

%ll modes of heat transfer re"uire the existence of a temperaturedifference.

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Heat conductionthrough a large plane

wall of thickness xand area A.

CONDUCTIONConduction: The transfer of energy fromthe moreenergetic particles of a substance to the adjacent lessenergetic ones as a result of interactions between theparticles.

In gases and liquids, conduction is due to thecollisionsand diffusionof the molecules during theirrandom motion.

In solids, it is due to the combination of vibrationsofthe molecules in a lattice and the energy transport byfree electrons.

The rate of heat conduction through a plane layer isproportional to the temperature difference across thelayer and the heat transfer area, but is inverselyproportional to the thickness of the layer.

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When x0 Fouriers law ofheat conduction

Thermal conductivity, k: A measure of the ability ofa material to conduct heat.

Temperature gradientdT/dx: The slope of thetemperature curve on a T-x diagram.

Heat is conducted in the direction of decreasing

temperature, and the temperature gradient becomesnegative when temperature decreases with

increasing x. The negative signin the equationensures that heat transfer in the positive x direction

is a positive quantity.

The rate of heat conductionthrough a solid is directlyproportional toits thermalconductivity.

In heat conductionanalysis, A represents

the area normal to thedirection of heat

transfer. 24


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ThermalConductivityThermal conductivity:The rate ofheat transferthrough a unit thicknessof the material per unitarea per unit

temperature difference.

The thermal conductivityof a material is a

measure of the ability ofthe material to conduct

heat.

A high value for thermalconductivity indicates

that the material is agood heat conductor,

and a low value indicatesthat the material is apoor heat conductor orinsulator.

A simple experimental setupto determine the thermal

conductivity of a material.

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The range ofthermal

conductivity ofvarious

materials atroomtemperature.

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The mechanisms of heat

conduction in differentphases of a substance.

The thermal conductivities of gases suchas air vary by a factor of 104 from those

of pure metals such as copper.

Pure crystals and metals have thehighest thermal conductivities, and gasesand insulating materials the lowest.

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The variation ofthe thermal

conductivity ofvarious solids,

liquids, and gases

with temperature.

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Thermal Diffusivity

cp Specific heat, J/kgC: Heat capacity perunit mass

cp Heat capacity, J/m3C: Heat capacity

per unit volume Thermal diffusivity, m2/s: Represents

how fast heat diffuses through a material

A material that has a high thermalconductivity or a low heat capacity will

obviously have a large thermal diffusivity.

The larger the thermal diffusivity, the fasterthe propagation of heat into the medium.

A small value of thermal diffusivity meansthat heat is mostly absorbed by the

material and a small amount of heat isconducted further.

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CONVECTION

Convection: The mode ofenergy transfer between asolid surface and the

adjacent liquid or gas that is

in motion, and it involvesthe combined effects of

conductionand fluid motion.

The faster the fluid motion,

the greater the convection

heat transfer.

In the absence of any bulkfluid motion, heat transferbetween a solid surface and

the adjacent fluid is by pure

conduction.

Heat transfer from a hot surface to air

by convection.


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Forced convection: If

the fluid is forced to flowover the surface by

external means such as

a fan, pump, or the wind.

Natural (or free)convection: If the fluidmotion is caused bybuoyancy forces that are

induced by densitydifferences due to the

variation of temperature

in the fluid.

The cooling of a boiled egg by

forced and natural convection.

Heat transfer processes that involve change of phaseof a fluid are alsoconsidered to be convection because of the fluid motion induced duringthe process, such as the rise of the vapor bubbles during boiling or the

fall of the liquid droplets during condensation.

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Newtons law of cooling

h convection heat transfer coefficient, W/m2 CAs the surface area through which convection heat transfer takes placeTs the surface temperatureT the temperature of the fluid sufficientlyfar from the surface.

The convection heat transfercoefficient his not a propertyof the fluid.

It is an experimentally

determined parameter

whose value depends on allthe variables influencing

convection such as

- the surface geometry

- the nature of fluid motion

- the properties of the fluid

- the bulk fluid velocity

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RADIATION

Radiation: The energy emitted by matter in the form of electromagnetic

waves(or photons) as a result of the changes in the electronicconfigurations of the atoms or molecules.

Unlike conduction and convection, the transfer of heat by radiation doesnot require the presence of an intervening medium.

In fact, heat transfer by radiation is fastest (at the speed of light) and itsuffers no attenuation in a vacuum. This is how the energy of the sun

reaches the earth.

In heat transfer studies we are interested in thermal radiation, which is

the form of radiation emitted by bodies because of their temperature.

All bodies at a temperature above absolute zero emit thermal radiation.

Radiation is a volumetric phenomenon, and all solids, liquids, andgases emit, absorb, or transmit radiation to varying degrees.

However, radiation is usually considered to be a surface phenomenonfor solids.

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StefanBoltzmann law

= 5.670 108 W/m2 K4 StefanBoltzmann constant

Blackbody: The idealized surface that emits radiation at the maximum rate.

Blackbody radiation represents the maximum

amount of radiation that can be emitted froma surface at a specified temperature.

Emissivity : A measure of how closelya surface approximates a blackbody forwhich = 1 of the surface. 0 1.

Radiation emittedby real surfaces

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Absorptivity : The fraction of the radiation energy incident on a

surface that is absorbed by the surface. 0 1

A blackbody absorbs the entire radiation incident on it (= 1).

Kirchhoffs law: The emissivity and the absorptivity of a surface at

a given temperature and wavelength are equal.

The absorption of radiation incident on

an opaque surface of absorptivity .


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Radiation heat transfer between asurface and the surfaces surrounding it.

Net radiation heat transfer:The difference between the

rates of radiation emitted by thesurface andthe radiation

absorbed.

The determination of the netrate of heat transfer by radiation

between two surfaces is acomplicated matter since itdepends on

the properties of the surfaces

their orientation relative to

each other

the interaction of the mediumbetween the surfaces with

radiation

Radiation is usuallysignificant relative to

conduction or naturalconvection, but negligible

relative to forced convection.

When a surface is completely enclosed by amuch larger (or black) surface at temperatureTsurr separated by a gas (such as air) thatdoes not intervene with radiation, the net rate

of radiation heat transfer between thesetwo surfaces is given by

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Combined heat transfer coefficienthcombined

Includes the effects of both convection and radiation

When radiation and convection occur

simultaneously between a surface and a gas:

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SIMULTANEOUS HEATTRANSFER MECHANISMS

Although there are three mechanisms ofheat transfer, a medium may involve

only two of them simultaneously.

Heat transfer is only by conduction in opaque solids,but by conduction and radiation in semitransparent

solids.

A solid may involve conduction and radiation but notconvection. A solid may involve convection and/or

radiation on its surfaces exposed to a fluid or othersurfaces.

Heat transfer is by conduction and possibly byradiation in a still fluid (no bulk fluid motion) and byconvection and radiation in a flowing fluid.

In the absence of radiation, heat transfer through a

fluid is either by conduction or convection, dependingon the presence of any bulk fluid motion.

Convection = Conduction + Fluid motion

Heat transfer through a vacuum is by radiation.

Most gases between two solid surfaces

do not interfere with radiation.Liquids are usually strong absorbers of

radiation.

'E(E&TI"& TH")GH $ESIG&

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The first of the fundamental canons of ethics for engineers is to holdparamount the safety, health, and welfare of the public when fulfillingtheir professional duties.

In 2007, the National Institute for Occupational Safety and Health (NIOSH)launched the National Prevention through Design (PtD) initiative, with the

mission to prevent or reduce work-related injuries, illnesses, andfatalities by including prevention considerations in all circumstances that

impact individuals in the work places.

As such, the concept of PtD involves applying the means of reducing

risks and preventing hazards in the design of equipment, tools,processes, and work facilities.

The concepts of PtD can also be rationally applied to preventingfailures and damages of devices, products, and systems. Since suchfailures and damages are often led to negative impacts on the

environment, profitability, and ultimately the society at large.

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Within the context of heat and masstransfer, the PtD concepts can be presented

along with the physical mechanismsinvolved and practical applications.

Issues such as prevention of thermal burn,fire hazard, and thermal failure in systemsare topics that can relate the concepts ofPtD with the basic science of heat and masstransfer.

The process of solving heat and masstransfer problems, along with theapplication of PtD concepts, involves

incorporating prescribed PtD criteria,the prevention of burn injury, fire hazard, or

system failure, to the solutions.

To successfully arrive at a solution thatsatisfies prescribed PtD criteria requires theunderstanding of how the physicalmechanisms of heat and mass transferinterrelate with the concepts of PtD.

'"*LE#+S"L(I&G TECH&I)E

tep : *roblem tatement

tep +: chematic

tep : %ssumptions and %pproximations

tep -: *hysical as

tep /: *roperties tep 0: 1alculations

tep 2: 3easoning, 4erification, and 5iscussion

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EES (Engineering Equation Solver)(Pronounced as ease): EES is a program that

solves systems of linear or nonlinearalgebraic or differential equations numerically.

It has a large library of built-in thermodynamic

property functions as well as mathematicalfunctions. Unlike some software packages,

EES does not solve engineering problems; itonly solves the equations supplied by the

user.

Engineering Software Packages

Thinking that a person who can use theengineering software packages withoutproper training on fundamentals can

practice engineering is like thinking that a

person who can use a wrench can work asa car mechanic.

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A Remark on Significant Digits

In engineering calculations, the

information given is not known tomore than a certain number of

significant digits, usually three

digits.

Consequently, the resultsobtained cannot possibly beaccurate to more significant

digits.

Reporting results in more

significant digits implies greater

accuracy than exists, and itshould be avoided.

A result with more significant

digits than that of given datafalsely implies more accuracy.

Su--ary

Thermodynamics and 'eat Transfer

%pplication areas of heat transfer

'istorical background

Engineering 'eat Transfer

Modeling in engineering

'eat and 6ther 7orms of Energy pecific heats of gases, li"uids, and solids

Energy transfer

The 7irst a of T hermodynamics

Energy balance for closed systems (7ixed Mass)

Energy balance for steady&flo systems

urface energy balance

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'eat Transfer Mechanisms

1onduction

7ourier8s la of heat conduction

Thermal 1onductivity

Thermal 5iffusivity

1onvection

9eton8s la of cooling

3adiation

tefan;olt

Documents

Heat Chap01 Lecture Note1