Chapter 1INTRODUCTION AND BASIC

CONCEPTS

Heat Transfer: A Practical ApproachSecond Edition

Yunus A. CengelMcGraw-Hill, 2002

Universitry of Technology Materials Engineering DepartmentMaE216: Heat Transfer and Fluid

ObjectivesUnderstand 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 heat conduction, Newton's law of cooling, and the Stefan–Boltzmann law of radiation

Identify the mechanisms of heat transfer that occur simultaneously in practice

Develop an awareness of the cost associated with heat losses

ERMODYNAMICS AND HEAT TRANSFERHeat: The form of energy that can be transferred from one system to another as a result of temperature difference.

Thermodynamics is concerned with the amount of heat transfer as a system undergoes a process from one equilibrium state to another.

Heat Transfer deals with the determination of the rates of 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:

pplication Areas of Heat Transfer

storical Background Kinetic theory: Treats molecules as tiny balls that are in motion and thus possess kinetic energy.

Heat: The energy associated with the random motion of atoms and molecules.Caloric theory: Heat is a fluidlikesubstance called the caloric that is a massless, colorless, odorless, andtasteless substance that can be poured from one body into another

It was only in the middle of the nineteenth century that we had a true physical understanding of the nature of heat.Careful experiments of the Englishman James P. Joule published in 1843 convinced the skeptics that

GINEERING HEAT TRANSFERansfer equipment such as heat exchangers, boilers, condensers, radiators,s, furnaces, refrigerators, and solar collectors are designed primarily on the f heat transfer analysis. at transfer problems encountered in practice can be considered in two : (1) rating and (2) sizing problems.ting problems deal with the determination of the heat transfer rate for ang system at a specified temperature difference. zing problems deal with the determination of the size of a system in order to r heat at a specified rate for a specified temperature difference.ineering device or process can be studied either experimentally (testing and measurements) or analytically (by analysis or calculations).perimental approach has the advantage that we deal with the actual physical , and the desired quantity is determined by measurement, within the limits of

mental error. However, this approach is expensive, timeconsuming, and often tical.alytical approach (including the numerical approach) has the advantage that it

odeling in Engineering

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 mass basis) of a system.The sum of all microscopic forms of energy is called the

f

AT AND OTHER FORMS OF ENERGY

Internal energy: May be viewed as the sum of the kinetic and potential energies of the molecules.

Sensible heat: The kinetic energy of the molecules.

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

Chemical (bond) energy: The internal energy associated with the atomic bonds in a molecule.

Nuclear energy: The internal energy associated with the bonds within the nucleus of the atom itself.

What is thermal energy?

What is the difference between thermal energy and heat?

ernal Energy and Enthalpy

n the analysis of systems hat involve fluid flow, we requently encounter the ombination of properties und Pv.

The combination is defined s enthalpy (h = u + Pv).

The term Pv represents the ow energy of the fluid (also alled the flow work).

ecific Heats of Gases, Liquids, and Solids

pecific heat: The energy required to aise the temperature of a unit mass of a ubstance by one degree.wo kinds of specific heats:

specific heat at constant volume cv

specific heat at constant pressure cp

he specific heats of a substance, in eneral, depend on two independent operties such as temperature and essure.

low pressures all real gases approach eal gas behavior, and therefore their

pecific heats depend on temperature nly.

ncompressible substance: A ubstance whose specific volume (or ensity) does not change with

emperature or pressure.

he constant-volume and constant-ressure specific heats are identical

or incompressible substances.

he specific heats of incompressible ubstances depend on temperaturenly.

rgy Transfery can be transferred to or from a given by two mechanisms:

heat transfer and work.ransfer rate: The amount of heat erred per unit time.lux: The rate of heat transfer per unit ormal to the direction of heat transfer.

when is constant:

r: The work per unit time.

E FIRST LAW OF THERMODYNAMICS

The energy balance for any system undergoing any process in the rate form

irst law of thermodynamics (conservation of energyiple) states that energy can neither be created nor destroyed g a process; it can only change forms.

The net change (increase or decrease) in the total energy of the system during a process is equal to the difference between the total energy entering and the total energy leaving the system during that process.

In heat transfer problems it is convenient to write a heat balance and to treat the conversion of nuclear, chemical, mechanical, and electrical energies into thermal energy as heat generation.

ergy 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 stationary systems since they don’t involve any changes in their velocity or elevation during a process.

rgy Balance for ady-Flow Systems

number of engineering devices such as eaters and car radiators involve mass flow out of a system, and are modeled as volumes.

ontrol volumes are analyzed under steady ng conditions.m steady means no change with time at a

ed location.ow rate: The amount of mass flowing a cross section of a flow device per unit

e flow rate: The volume of a fluid flowing a pipe or duct per unit time.

face Energy Balance

relation is valid for both steady and ent conditions, and the surface

gy balance does not involve heat ration since a surface does not a volume.

face contains no volume or mass, hus no energy. Thereore, a surfacee viewed as a fictitious system e energy content remains constant g a process.

AT TRANSFER MECHANISMSHeat as the form of energy that can be transferred from one ystem to another as a result of temperature difference.

A thermodynamic analysis is concerned with the amount of heat ransfer as a system undergoes a process from one equilibrium tate to another.

The science that deals with the determination of the rates of such nergy transfers is the heat transfer.

The transfer of energy as heat is always from the higher-emperature medium to the lower-temperature one, and heat ransfer stops when the two mediums reach the same temperature.

Heat can be transferred in three basic modes: conduction convection radiation

Heat conduction through a large plane

wall of thickness x and area A

ONDUCTIONduction: The transfer of energy from the more getic particles of a substance to the adjacent less getic ones as a result of interactions between the les.ses and liquids, conduction is due to the ions and diffusion of the molecules during their om motion. ids, it is due to the combination of vibrations of olecules in a lattice and the energy transport by

electrons. ate of heat conduction through a plane layer is

ortional to the temperature difference across the and the heat transfer area, but is inversely

ortional to the thickness of the layer.

n x → 0 Fourier’s law of heat conduction

mal conductivity, k: A measure of the ability of terial to conduct heat. perature gradient dT/dx: The slope of the erature curve on a T-x diagram.is conducted in the direction of decreasing erature, and the temperature gradient becomes tive when temperature decreases with asing x. The negative sign in the equation res that heat transfer in the positive x direction ositive quantity.

The rate of heat conduction th h lid i di tl

In heat conduction alysis, A represents

ermal nductivityrmal conductivity:rate of heat transfer ugh a unit thickness e material per unit per unit

perature difference.thermal conductivity material is a sure of the ability of

material to conduct

gh value for thermal uctivity indicates the material is a d heat conductor, a low value indicates the material is a

A simple experimental setup to determine the thermal conductivity of a material.

The range of thermal conductivity ofvarious materials at room temperature.

echanisms of heat

The thermal conductivities of gases such as air vary by a factor of 104 from those of pure metals such as copper. Pure crystals and metals have the highest thermal conductivities, and gases and insulating materials the lowest.

The variation of the thermalconductivity of various solids,liquids and gases

ermal Diffusivitypecific heat, J/kg · °C: Heat capacity er unit massHeat capacity, J/m3·°C: Heat capacity er unit volumehermal diffusivity, m2/s: Represents ow fast heat diffuses through a material

aterial that has a high thermal ductivity or a low heat capacity will ously have a large thermal diffusivity. larger the thermal diffusivity, the faster

propagation of heat into the medium.mall value of thermal diffusivity means heat is mostly absorbed by the

ONVECTIONvection: The mode of rgy transfer between a d surface and thecent liquid or gas that is otion, and it involves combined effects of duction and fluid motion. faster the fluid motion,

greater the convection t transfer.

e absence of any bulk motion, heat transfer

ween a solid surface and adjacent fluid is by pure duction.

Heat transfer from a hot surface to air by convection.

ced convection: If fluid is forced to flow r the surface by ernal means such as n, pump, or the wind.

ural (or free) nvection: If the fluid tion is caused by yancy forces that are

uced by density erences due to theation of temperature he fluid.

The cooling of a boiled egg by forced and natural convection.

at transfer processes that involve change of phase of a fluid are alsonsidered to be convection because of the fluid motion induced during e process, such as the rise of the vapor bubbles during boiling or the

Newton’s law of cooling

convection heat transfer coefficient, W/m2 · °C the surface area through which convection heat transfer takes placethe surface temperaturethe temperature of the fluid sufficiently far from the surface.

he convection heat transfer efficient h is not a property the fluid. s an experimentally

etermined parameter hose value depends on all e variables influencingnvection such as he surface geometry he nature of fluid motion he properties of the fluid

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

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

nlike conduction and convection, the transfer of heat by radiation does ot require the presence of an intervening medium.fact, heat transfer by radiation is fastest (at the speed of light) and it

uffers no attenuation in a vacuum. This is how the energy of the sunaches the earth.

heat transfer studies we are interested in thermal radiation, which ise form of radiation emitted by bodies because of their temperature.

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

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

owever radiation is usually considered to be a surface phenomenon

Stefan–Boltzmann law

5.670 108 W/m2 · K4 Stefan–Boltzmann constantkbody: The idealized surface that emits radiation at the maximum rate.

kbody radiation represents the maximum

ssivity : A measure of how closely rface approximates a blackbody for h = 1 of the surface. 0 1.

Radiation emitted by real surfaces

sorptivity : The fraction of the radiation energy incident on a rface that is absorbed by the surface. 0 1blackbody absorbs the entire radiation incident on it ( = 1).

rchhoff’s law: The emissivity and the absorptivity of a surface at iven temperature and wavelength are equal.

diation heat transfer:fference between the

of radiation emitted by the e and the radiation bed.etermination of the net f heat transfer by radiation en two surfaces is aicated matter since it ds onroperties of the surfacesorientation relative to

other nteraction of the medium en the surfaces withon

iation is usually ificant relative to duction or natural

When a surface is completely enclosed by a much larger (or black) surface at temperature Tsurr separated by a gas (such as air) that does not intervene with radiation, the net rate of radiation heat transfer between thesetwo surfaces is given by

Combined heat transfer coefficient hcombined

Includes the effects of both convection and radiation

When radiation and convection occur simultaneously between a surface and a gas:

ULTANEOUS HEAT ANSFER MECHANISMS

Although there are three mechanisms of heat transfer, a medium may involve

only two of them simultaneously.

transfer is only by conduction in opaque solids, y conduction and radiation in semitransparent s. d may involve conduction and radiation but not

ection. A solid may involve convection and/or tion on its surfaces exposed to a fluid or other ces.transfer is by conduction and possibly by tion in a still fluid (no bulk fluid motion) and by ection and radiation in a flowing fluid.

absence of radiation, heat transfer through a s either by conduction or convection, depending e presence of any bulk fluid motion.ection = Conduction + Fluid motiontransfer through a vacuum is by radiation.gases between two solid surfaces

ROBLEM-SOLVING TECHNIQUE

Step 1: Problem StatementStep 2: SchematicStep 3: Assumptions and ApproximationsStep 4: Physical LawsStep 5: PropertiesStep 6: CalculationsStep 7: Reasoning, Verification, and Discussion

S (Engineering Equation Solver)nounced as ease): EES is a program that es systems of linear or nonlinear braic or differential equations numerically. s a large library of built-in thermodynamic erty functions as well as mathematical ions. Unlike some software packages, does not solve engineering problems; it solves the equations supplied by the

ineering Software Packages

king that a person who can use the neering software packages without er training on fundamentals can ice engineering is like thinking that a

on who can use a wrench can work as mechanic.

Remark on Significant Digitsngineering calculations, the rmation given is not known to e than a certain number of ificant digits, usually three s.

sequently, the results ined cannot possibly be

urate to more significants.

orting results in more ificant digits implies greater uracy than exists, and it uld be avoided.

A result with more significant digits than that of given data falsely implies more accuracy.

SummaryThermodynamics and Heat Transfer Application areas of heat transfer

Historical background

Engineering Heat Transfer Modeling in engineering

Heat and Other Forms of Energy Specific heats of gases, liquids, and solids

Energy transfer

The First Law of Thermodynamics Energy balance for closed systems (Fixed Mass)

Energy balance for steady-flow systems

Surface energy balance

Heat Transfer Mechanisms

Conduction Fourier’s law of heat conduction

Thermal Conductivity

Thermal Diffusivity

Convection Newton’s law of cooling

Radiation Stefan–Boltzmann law

Simultaneous Heat Transfer Mechanisms

Problem Solving Technique Engineering software packages

Engineering Equation Solver (EES)

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