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HEAT TRANSFERHEAT TRANSFER

Prof. Dr. Melek Tter

Istanbul Technical UniversityDepartment of Chemical Engineering


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TextbookTextbook

HeatHeat and Mass Transferand Mass Transfer::A Practical ApproachA Practical Approach

byby

Yunus A.Yunus A. engelengel

McGrawMcGraw--Hill, NYHill, NY, 2007, 2007


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Basic Concepts of

Thermodynamics and

Heat Transfer

CHAPTER 1CHAPTER 1


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What is going on?What is going on?

What do you feel when you enter awarm classroom?

What happens to a cold canned drinkor a hot cup of coffee left in a room?

What do you feel when you jump into

cold sea (from a hot beach)?

Why do put on more clothing on acold day?

Heat transfer


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Heat Transfer :

It is the science that deals with the determination of

the rates of such energy transfers.

What is Heat Transfer ?What is Heat Transfer ?

Heat :

It is the form of energy that can be transferred from

one system to another as a result of temperature

difference.


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Thermodynamics and Heat TransferThermodynamics and Heat Transfer

What is the difference?

They are called thermal sciences

Thermodynamics :

It concerns with the amount of heat transfer(between equilibrium states).

Time independent

Equilibrium phenomenon


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Heat Transfer :

Time dependent processes

Non-equilibrium phenomenon

Driving force: temperature difference Thermodynamic principles alone are notenough for heat transfer analysis

It deals with the rate of heat transfer.


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TThehe DDifferenceifference bbetweenetween

HHeateat TTransferransfer &&TThermodynamicshermodynamics

How long will it take to cool the coffee in a thermos

bottle from 90oC to 70oC?

Which one, heat transfer or thermodynamics, can

answer this question?

Heat transfer can calculate the time needed


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Some application areas of heat transfer


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Thermal Mechanical Kinetic Potential Electrical Magnetic

Chemical Nuclear

Total Energy of aTotal Energy of a SSystemystem (E) =(E) = all energiesall energies

Heat and Other Forms of EnergyHeat and Other Forms of Energy

Forms of Energy :


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Internal energy may be viewed as the sum of theInternal energy may be viewed as the sum of the

kinetic and potential energies of the molecules.kinetic and potential energies of the molecules.

The kinetic energy of the molecules is calledThe kinetic energy of the molecules is called sensiblesensibleheatheat..

The internal energy associated with the phase of aThe internal energy associated with the phase of asystem is calledsystem is called latent heatlatent heat..

The internal energy associated with theThe internal energy associated with the atomic bondsatomic bondsin a molecule is calledin a molecule is called chemicalchemical (or(or bondbond)) energyenergy..

The internal energy associated with theThe internal energy associated with the bonds withinbonds withinthe nucleusthe nucleus of the atom itself is calledof the atom itself is called nuclearnuclearenergyenergy..


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In the analysis of systemsIn the analysis of systemsthat involve fluid flow, wethat involve fluid flow, wefrequently encounter thefrequently encounter thecombination of propertiescombination of properties uu

andand PvPv..

The combination is definedThe combination is definedasas enthalpyenthalpy ((hh==uu++PvPv).).

The termThe term PPvv represents therepresents theflow energyflow energy of the fluid (alsoof the fluid (also

called the flow work).called the flow work).

Internal Energy and EnthalpyInternal Energy and Enthalpy


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Specific Heats of Gases, Liquids and SolidsSpecific Heats of Gases, Liquids and Solids

Specific heat:

Energy required to raise the temperature of a unit mass

of substance by one degree (denoted by C).

In thermodynamics, there are two kinds of specificsheats:

1. Specific heat at constant volume (Cv)

2. Specific heat at constant pressure(Cp)

Units: kJ/kg.C or kJ/kg.K

For ideal gases: Cp

= Cv

+ R


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dTCdhanddTCdu PV

(J/kg)TChandTCu ave,Pave,V (J)TmCHandTmCU ave,Pave,V

C)(kJ/kgheatspecificaveragetheC

(kg)systemtheofmassm

o

ave

Changes in the internal energy (u) and enthalpy (h) of

ideal gases :

Differential :

Finite :


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Heat transfer:

Driving force is temperature difference (T ).Q T Q, T or Q, T

EnergyEnergy TransferTransfer

Energy can be transferred to or from a given mass bytwo mechanisms:

Heat (Q) Work (W)

Heat transfer rate:

The amount of heat transferred per unit time (J/s)


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The total amount of heat transferred during time

interval dt is: t0 dtQQ )J((J/s)transferheatofratetheQ

tQQ =

)J(

Heat Flux (q): The rate of

heat transfer per unit area

per unit time

A

Qq )sm/J( 2

If is constant:

Q

when Q is uniform overthe area A.

A = area normal tothe direction of HT


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TheThe first law of thermodynamicsfirst law of thermodynamics states that energy canstates that energy can

neither be created nor destroyed during a process; it can onlyneither be created nor destroyed during a process; it can only

change forms.change forms.

TheThe energy balanceenergy balance for any system undergoing any processfor any system undergoing any process

can be expressed as (in the rate form)can be expressed as (in the rate form)

The First Law of ThermodynamicsThe First Law of Thermodynamics


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In heat transfer problems it is convenientIn heat transfer problems it is convenientto write ato write a heat balanceheat balance and to treat theand to treat the

conversion of nuclear, chemical,conversion of nuclear, chemical,

mechanical, and electrical energies intomechanical, and electrical energies intothermal energy asthermal energy as heat generation.heat generation.

TheThe energy balanceenergy balance in that case can bein that case can be

expressed asexpressed as


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Energy BalanceEnergy Balance

Closed systemsClosed systems

Stationary closedStationary closed

system, no work:system, no work:

SteadySteady--Flow SystemsFlow Systems

When kinetic and potentialWhen kinetic and potential

energies are negligible,energies are negligible,and there is no workand there is no workinteractioninteraction


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Heat can be transferred in three basicHeat can be transferred in three basic

modes:modes: conductionconduction,,

convectionconvection,,

radiationradiation.. All modes of heatAll modes of heat

transfer require thetransfer require the

existence of a temperature difference.existence of a temperature difference.

All modes are from the highAll modes are from the high--temperature mediumtemperature medium

to a lowerto a lower--temperature one.temperature one.

Heat Transfer MechanismsHeat Transfer Mechanisms


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CONDUCTION

Conduction: the transfer of heatfrom one part of a material toanother part of the same material, or

from one material to another inphysical contact with it, without anyappreciable displacement of themolecules.

The rate of heat conduction through amedium depends on:

the geometry of the medium the thickness the material of the medium the temperature difference across

the medium.


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The rate of heat conduction through aplane layer is proportional to thetemperature difference across the layerand the heat transfer area, but is inverselyproportional to the thickness of the layer.

or

where the constant of proportionality kis the thermal conductivity of thematerial, which is a measure of the

ability of a material to conduct heat


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In the limiting case of reduces to

which is called Fouriers law of heat conductionafter J. Fourier, who expressed it first in his heattransfer text in 1822.

Here, dT/dxis the temperature gradient, whichis the slope of the temperature curve on a T-xdiagram at locationx.

The negative sign in the above equation ensures that heat transferin the positivex direction is a positive quantity.


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Thermal Conductivity

Thermal Conductivity: a measure of a solid material to conduct heat(the rate of heat transfer through a unit thickness of the material per unitarea per unit temperature difference.


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Thermal Diffusivity

Heat Capacity of a Material: the product Cp which is frequentlyencountered in heat transfer analysis.

Thermal Diffusivity: another material property that appears in

the transient heat conduction analysis, representing how fastheat diffuses through a material.

k represents how well a material conducts heat, and the heatcapacity Cp represents how much energy a material stores per unit

volume.

The thermal diffusivity of a material can be viewed as the ratio of theheat conducted through the material to the heat stored per unit

volume.


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CONVECTION

Convection: the mode of energy transfer between a solid surface and

the adjacent, moving liquid or gas (involving the combined effects ofconduction and fluid motion).

Forced Convection: where the fluid is forced to flow over the surfaceby external means such as a fan, pump, or the wind.

Natural (or Free) Convection: where the fluid motion is caused by

buoyancy forces that are induced by density differences due to thevariation of temperature in the fluid.


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The rate of convection heat transfer is observed to beproportional to the temperature difference, and is conveniently

expressed by Newtons law of cooling as

h : Convection heattransfer coefficient inW/m2C

As : Surface area throughwhich convection heattransfer takes placeTs : Surface temperature

T : Temperature of thefluid sufficiently far fromthe surface


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RADIATION

Radiation: the energy emitted by matter in the form ofelectromagnetic waves (orphotons) as a result of the changes in theelectronic configurations of the atoms or molecules.

Thermal Radiation: the form of radiation emitted by bodies because oftheir temperature.

Radiation is a volumetric phenomenon, and all solids, liquids, and

gases emit, absorb, or transmit radiation to varying degrees.

Radiation is usually considered to be a surface phenomenon for solidsthat are opaque to thermal radiation such as metals, wood, and rockssince the radiation emitted by the interior regions of such material

can never reach the surface, and the radiation incident on such bodiesis usually absorbed within a few microns from the surface.


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The maximum rate of radiation that can be emitted

from a surface at an absolute temperature Ts (in K orR) is given by the StefanBoltzmann law as

: The StefanBoltzmann constant(5.66910-8W/m2K4)

Blackbody: the idealized surface that emits radiation atthis maximum rate.

Blackbody Radiation:the radiation emitted bya blackbody.


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The radiation emitted by all real surfaces is less than the radiationemitted by a blackbody at the same temperature, and is expressed as

where is the emissivity of the

surface.Emissivity: a measure of how closely asurface approximates a blackbody forwhich . Its value is in the range

.

Absorptivity (): the fraction of theradiation energy incident on a surface

that is absorbed by the surface.

A blackbody absorbs the entireradiation incident on it and is a perfectabsorber (= 1) as it is a perfect

emitter.


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Kirchhoffs Law of Radiation:the emissivity and the

absorptivity of a surface at agiven temperature andwavelength are equal.

where is the rate at whichradiation is incident on the surface

and a is the absorptivity of the surface.

The net rate of radiation heat

transfer between two surfaces:


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Combined Heat Transfer Coefficient (hcombined): including the

effects of both convection and radiation.

The total heat transfer rate to or from a surface by convectionand radiation:

Radiation is usually significant relative to conduction or naturalconvection, but negligible relative to forced convection. Thusradiation in forced convection applications is usuallydisregarded, especially when the surfaces involved have lowemissivities and low to moderate temperatures.

SIMULTANEOUS HEAT TRANSFER


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SIMULTANEOUS HEAT TRANSFERMECHANISMS

Heat transfer is only by conduction in opaquesolids, but by conduction and radiation insemitransparent solids.

In the absence of radiation, heat transferthrough a fluid is either by conduction orconvection, depending on the presence of any bulkfluid motion.

Convection can be viewed as combinedconduction and fluid motion, and conduction in afluid can be viewed as a special case of convectionin the absence of any fluid motion.

Heat transfer through a vacuum is by radiationonly since conduction or convection requires thepresence of a material medium.

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