CHAPTER 1111 HEAT TRANSFER - Faculty of Mechanical ...mazlan/?download=Heat+Transfer+Chapter+1... · CHAPTER 1111 Heat transfer is thermal energy in transit ... Rate of net energy

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CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER CHAPTER

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HEAT TRANSFER

SME 4463SME 4463

INSTRUCTOR: ASSOC PROF DR. MAZLAN ABDUL WAHID

http://www.fkm.utm.my/~mazlan

TEXT: Introduction to Heat Transfer

by Incropera, DeWitt, Bergman, Lavine

FACULTY OF MECHANICAL ENGINEERINGUNIVERSITI TEKNOLOGI MALAYSIASKUDAI, JOHOR, MALAYSIA HEAT TRANSFER INTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTIONINTRODUCTION

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by Incropera, DeWitt, Bergman, Lavine5th Edition, John Wiley and Sons


11111111TEXT: Inroduction to Heat Transfer

by Incropera, DeWitt, Bergman, Lavine John Wiley and Sons


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Thermodynamics is about:

Interaction of energy with system and surroundings.

Thermodynamics

systemsurroundings

boundaryWQ


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Energy can move in and out of a system in two forms Work (W) and Heat (Q)


11111111ObjectivesWhen you finish studying this chapter, you should be able to:• Understand how thermodynamics and heat transfer are related to

each other,• Distinguish thermal energy from other forms of energy, and heat • 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


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• Identify the mechanisms of heat transfer that occur simultaneously in practice,

• Develop an awareness of the cost associated with heat losses, and

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11111111Thermodynamics and Heat Transfer

• The science of thermodynamics deals with the amount of heat transfer as a system undergoes a process from one equilibrium state to another, and process from one equilibrium state to another, and makes no reference to how long the process will take.

• The science of heat transfer dealswith the determination of the rates of energy that can be transferred from one system to another as a


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from one system to another as a result of temperature difference.


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• Thermodynamics deals with equilibrium states and changes from one equilibrium state to another. Heat

Thermodynamics and Heat Transfer

changes from one equilibrium state to another. Heat transfer, on the other hand, deals with systems that lack thermal equilibrium, and thus it is a nonequilibriumphenomenon.

• Therefore, the study of heat transfer cannot be based on the principles of thermodynamics alone.

• However, the laws of thermodynamics lay the


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• However, the laws of thermodynamics lay the framework for the science of heat transfer.

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11111111Heat Transfer

• The basic requirement for heat transfer is the presence of a temperature difference.

• The second law requires that heat • The second law requires that heat be transferred in the direction of decreasing temperature.

• The temperature difference is the driving force for heat transfer.• The rate of heat transferin a certain direction depends on the

magnitude of the temperature gradientin that direction.


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• The rate of heat transferin a certain direction depends on the magnitude of the temperature gradientin that direction.

• The larger the temperature gradient, the higher the rate of heat transfer.


11111111But thermodynamics …..

……deals with the end states of the processor

when the system reach equilibrium at the end states

It provides no information concerning

• The nature of the interactions• The time rate at which it occurs

Heat transfer describes the nonequilibrium phenomena of the


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Heat transfer describes the nonequilibrium phenomena of the transfer of energy due to temperature difference.

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11111111EXAMPLE

Consider a can of drinks which you want to cool down – you would put it in a refrigerator.

20oCSurrounding Air

We know from experience that if we leave it in the fridge –ultimately – it will reach equilibrium with its surroundings

AT WHAT RATE ? HOW?

Surrounding Air

T = 4oC


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AT WHAT RATE ? HOW?

Thermodynamics can not answer that.


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When there is temperature difference between two medium, energy will transfer from a hotter medium to a colder medium in a form of HEAT.

The nature of heat transfer

So heat transfer will occur when there is an energy difference (temperature difference) in a medium or between mediums


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Heat transfer is thermal energy in transit due to

temperature difference

The topic of Heat Transfer is about…

All of Heat Transfer study is about answering the

understanding, determining and predicting flows of heat

temperature difference


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All of Heat Transfer study is about answering the question:

What is the heat flow rate from A to B?


11111111What is temperature ?

• Thermal energy: atomic/molecular/electronic kinetic energyenergy

• Measure to determine how hot/cold a material is (intensity of thermal energy)

• Criterion to determine the direction of thermal-energy transport

From a microscopic view, temperature represents atomic ormolecularkineticenergy(translation/ vibration/ rotation)


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molecularkineticenergy(translation/ vibration/ rotation)

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11111111Where is heat transfer falls at?There are three principle laws upon which Engineering studies are derived

•Conservation of Momentum (Fluid Mechanics, •Conservation of Momentum (Fluid Mechanics, Mass Transfer)

•Conservation of Energy (Thermodynamics, Heat Transfer)

•Conservation of Mass (Continuity, Mass Transfer)


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In this course we are primarily interested in the

Conservation of Energy in Heat Transfer


11111111Application Areas of Heat Transfer


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11111111Importance of heat transfer in engineering

Power

• High turbine inlet temperatures desired for efficiency.

• Heat transfer from gas or steam to turbine blades (convection, radiation) – blades may fail.

• Predict/control temperature of blades. Cooling strategies – internal cool air passages,

cool air bleed through perforated blade surface.

Power


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cool air bleed through perforated blade surface.


11111111Turbine blade cooling


DR. MAZLANFaculty of Mechanical EngineeringUniversiti Teknologi Malaysia81310 Skudai, Johor, Malaysia

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11111111Biomedical

• Thermal cancer treatments – electromagnetic radiation (laser, radio), ultrasonic waves, etc used to heat tumor.

• Necessary to predict tumor temperature and understand heat transfer to surrounding

tissue (conduction, convection).

• Sometimes whole body temperature needs to be raised, lowered, maintained – water

ultrasonic waves, etc used to heat tumor.


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maintained – water

and air blanket devices (convection and conduction), IR lamps (radiation).


11111111Building

• Heat is transferred through walls (conduction) to outside air (convection), through(convection), through

windows (radiation, convection, conduction), open doors/windows (convection)…

• Heat loss (or gain) determines heating (air-conditioning) requirements.


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11111111Heat exchangers

• devices designed specifically to promote heat transfer between two fluids

• car radiators, boilers, condensers, chip cooling, equipment cooling …

and so on…and so on…


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11111111Fuel cells


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11111111Energy Transfer

• Energy can be transferred to or from a given mass by two mechanisms: – heat transfer, and – work.

• The amount of heat transferred during a process is denoted by Q. • The amount of heat transferred during a process is denoted by Q. • The amount of heat transferred per unit time is called heat

transfer rate, and is denoted by Q.• The total amount of heat transfer Q during a time interval ∆t can

be determined from

⋅

0

(J)t

Q Qdt∆

= ∫ &


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• The rate of heat transfer per unit area normal to the direction of heat transfer is called heat flux, and the average heat flux is expressed as

0

2 (W/m )Q

qA

=&

&

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11111111The First Law of Thermodynamics

• The first law of thermodynamics states that energy can neither be created nor destroyed during a process; it can only change forms.

Total energy Total energy Change in the

• The energy balance for any system undergoing any process can be expressed as (in the rate form)

Total energyentering the

system

Total energyleaving the

system

Change in thetotal energy of

the system- =

(W)E E dE dt− =& &


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(W)in out systemE E dE dt− =& &

Rate of net energy transfer

by heat, work, and mass

Rate of change in internal kinetic, potential,

etc., energies


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Notation used in this course

Notation

Q - energy transfer in the form of heat (same as in thermo), J

q - heat transfer rate, W

q’ - heat transfer rate, per unit length, W/m

q’’ - heat flux, heat transfer rate per unit area, W/m2

.


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q - rate of energy generation per unit volume, W/m3.

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11111111Methods of Heat Transfer

Objectives are to:

• describe the three methods of heat transfer

• give practical/environmental examples of each


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11111111Modes of Heat Transfer

Heat transfers across the system’s boundary in three modes

• Conduction

• Convection


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• Radiation

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11111111Conduction

• Conduction is the transfer of energy from the more energetic particles of a substance to the adjacent less energetic ones as a result of interactions between the particles.particles.

• Conduction can take place in solids, liquids, or gases– In gases and liquids conduction is due to

the collisions and diffusion of the molecules during their random motion.


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– In solids conduction is due to the combination of vibrations of the molecules in a lattice and the energy transport by free electrons.


11111111Conduction


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11111111Conduction

-Usually in solid(s) , maybe liquids -Rarely gases (negligible to convection)

• Straightforward transmission of heat within a stationary medium

• Solid, liquid, or gas (usually most important in solids)

• Mechanisms are on molecular/atomic level: molecular vibrations,


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• Mechanisms are on molecular/atomic level: molecular vibrations, motion of free electrons

• Can often come up with exact mathematical solutions

Need a temperature gradient


11111111Conduction


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11111111Conduction is simply:Transfer of energy from more energetic to lessenergetic particles of a substance due to interactionsbetweenparticles

Conduction

Fourier’s Law

betweenparticles

From empirical observations (experiments)


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L

TkAqcond

∆−=

Conduction

• q: heat transfer rate• A: cross-sectional area• L: length

LkAqcond −=


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• L: length• k: thermal conductivity• ∆T: temperature difference across conductor

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11111111Thermal Conductivity

• The thermal conductivity of a material is a measure of the ability of the material to conduct measure of the ability of the material to conduct heat.

• High value for thermal conductivity

good heat conductor

• Low value


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• Low value

poor heat conductor or insulator.


11111111Thermal Conductivities of Materials

• The thermal conductivities of gasesconductivities of gasessuch as air vary by a factor of 104 from those of pure metals such as copper.

• Pure crystals and metalshave the highest


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thermal conductivities, and gases and insulating materials the lowest.

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11111111Thermal Conductivities and Temperature

• The thermal conductivities of materials vary with temperature.

• The temperature dependence of thermal conductivity causes considerable complexity in


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conduction analysis.

• A material is normally assumed to be isotropic.


11111111Thermal diffusivity

2Heat conducted (m s)

Heat stored p

k

cα

ρ= =

• The thermal diffusivity represents how fast heat diffuses through a material.

• Appears in the transient heat conduction analysis.

• A material that has a high thermal conductivityor a low heat

Heat stored pcρ


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• A material that has a high thermal conductivityor a low heat capacity will have a large thermal diffusivity.

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

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11111111Example

A plain carbon steel rod of diameter 15 mm and length 420 mm has its ends held at steady temperatures of 20°C and 90°C, respectively. What is

Conduction

ends held at steady temperatures of 20°C and 90°C, respectively. What is the heat flow rate?

k = conductivity = 60.5 W/m.K for plain carbon steel

(Fourier’s Law).

q = -kAdT/dx = kA(T - T )/L


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q = -kAdT/dx = kA(T1- T2)/L

= 60.5 × π (0.015)2/4 × (90 – 20) / 0.42

= 1.78 W


11111111Under steady state conditions where temperature distribution is linear, the temperature gradient may be expressed as

and the heat flux is then

or


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Note: heat flux is rate per unit area. The heat rate by conduction, qx (W), through a plane wall of area A is qx = qx " • A

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11111111Example

The heat flow rate through a wood board of 2 cm thick for a temperature difference of 25 oC between the two surfaces is 150 W/m 2. Calculate

Conduction

between the two surfaces is 150 W/m 2. Calculate the thermal conductivity of the wood.

T

50oC

25oC


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25oC

Convection

• Convection is the mode of energy transfer between a

CONVECTION = Conduction + Advection (fluid motion)

• Convection is the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion.

• Convection is commonly classified into three sub-modes:– Forced convection,– Forced convection,

– Natural (or free) convection,

– Change of phase (liquid/vapor,

solid/liquid, etc.)

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ConvectionT ∞

T s

The convection heat transfer mode is comprised two mechanisms:

1. Energy transfer due to random molecular motion (diffusion)

2. Energy transfer due to bulk (or macroscopic) motion of thefluid(called advection)

•If both transport of energy is present, the termCONVECTION is generallyused.•If transport of energy due only to bulk motion of the fluid, the termADVECTION is used.

• h is the convection heat transfer coefficient in W/m2°C.

Convection

W/m °C.

• h depends on variables such as the

surface geometry, the nature of fluid

motion, the properties of the fluid,

and the bulk fluid velocity.

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Convection is what happens when the motion of a heat conducting

fluid increases the rate of

heat transfer.

Convection

heat transfer.

In other words, the convective

air currents increase the rate

of heat transfer by improving of heat transfer by improving

the conduction at the surface.

•Convection heat transfer normally takes place in a moving

liquid or gas

Convection

• Conduction still takes place

• Usually interested in cooling or heating of a solid object by a fluid stream – e.g. pipes in a boiler, cooling fin on an engine…

• Exact mathematical analysis usually impossible – usually rely on empirical correlations

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11111111Convection


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Convection


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11111111Convection at Home


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We are interested mainly in cases where there is heat transfer between a fluid in motion and a bounding surface.

a. Velocity boundary layer

Convection

a. Velocity boundary layer

b. Thermal boundary layer

There are two types of convection:

Forced convection - flow caused by external means


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Forced convection - flow caused by external means

Free convection - caused by buoyancy forces


11111111Newton’s Law of Cooling:

Convection

• q” is the convective heat flux (W/m 2), and is proportional to the

difference between surface and fluid temps.

• h (W/m2 K) is convective heat transfer coefficient -depends on


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depends on

conditions in boundary layer, surface geometry, nature of fluid motion, and fluid thermo and transport properties.

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11111111Convection


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11111111RADIATION

Radiation is energy emitted by matter that

is at a finite temperature.

•The emission is due to changes in

electron configurations of constituent

atoms or molecules.

•Transported by electromagnetic radiation.


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• Does not require a material medium,

occurs most efficiently in vacuum.

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11111111Radiation

• Radiation is the energy emitted by matter in the form of electromagnetic waves (or photons) as a result of the changes in the electronic configurations of the atoms or molecules.molecules.

• Heat transfer by radiation does not require the presence of an intervening medium.

• In heat transfer studies we are interested in thermal radiation (radiation emitted by bodies because of their temperature).


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temperature).• Radiation is a volumetric phenomenon. However, radiation

is usually considered to be a surface phenomenon for solids that are opaque to thermal radiation.


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11111111Ideal Radiator

The maximum rate of radiation that can be emitted from a surface at a thermodynamic temperature Ts (in K) is given by the Stefan–Boltzmann law as

Ideal radiator

or Blackbody

where,

Ts is the absolute temp (K) of the surface

σ is the Stefan Boltzmann constant (5.67 x 10-8 W/m2K4)

The idealized surface that emits radiation at this maximum rate is called a blackbody .


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blackbody .

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Heat flux emitted by a real object (less than that of a blackbody)

0 ≤ ≤ 1

: emissivity, a radiative property of surface, how efficient radiation emission is compared to blackbody

Determination of the net rate at which radiation is


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Determination of the net rate at which radiation is exchanged between surfaces is complicated

Most often, we only need to know the net exchange between a small surface and the surroundings.


11111111Small Surface and Large Surroundings

The net rate of radiation heat exchange between a small surface and

a large surroundings per a unit area of the small surface

• ε: emissivity

T su r

T s

A

A su r

( )44SURS TTAq −= εσ


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• ε: emissivity

Maximum ε = 1.00, black charcoal surface,

Minimum ε = 0.01, shiny gold surface

• σ: Stefan-Boltzmann constant, 5.67 x 10-8 W/m2K4

0 ≤ ≤ 1

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Above eqn. can also be written in the following form,

( )44SURS TTAq −= εσ

Above eqn. can also be written in the following form,

q = h rA(Ts – Tsur )

Where hr is the radiation heat transfer coefficient

h = εσεσεσεσ(T + T ) (T 2 + T 2)


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hr= εσεσεσεσ(Ts + Tsur ) (Ts2 + Tsur

2)

where we have linearized the equation shown earlier.


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Greenhouse Greenhouse Greenhouse Greenhouse EffectEffect


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11111111


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11111111Conservation of Energy

Control Volume

Surroundings, S

-Accumulation (Storage) stE&

• Energy conservation on a rate basis:

Control Volume (CV)

Boundary, B (Control Surface, CS)

(Storage)

-GenerationAdditionthrough inlet

Lossthrough outlet

stst

outgin Edt

dEEEE &&&& ==−+

inE& outE&

gE&stE&

Units W=J/s


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basis: dt

� Inflow and outflow are surface phenomena� Generation and accumulation are volumetric phenomena

Units W=J/s

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Conservation of energy


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11111111Ein

= dEst/dt =


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st=

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11111111Surface Energy Balance

For a control surface:

qrad”

0

0

""" =−−

=−

radconvcond

outin

qqq

or

EE &&T1

T2

qcond ”qrad”

qconv ”


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T

x

T2

∞T

control surface


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Surface energy balance


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11111111The surface energy balance


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11111111Analysis of h.t problem – Mesti buat seperti ini !!!


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11111111Important!


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CHAPTER 1111 HEAT TRANSFER - Faculty of Mechanical ...mazlan/?download=Heat+Transfer+Chapter+1... · CHAPTER 1111 Heat transfer is thermal energy in transit ... Rate of net energy