ht cengel chapter 3 - mazlan 2016

HEAT

TRANS

SKMM 3443

CCHH

AA

SFER

LECTURER: ASSOC PROF. DR. MAZLAN ABDUL WAHIDhttp://www.fkm.utm.my/~mazlan

HEAT TRANSFERHEAT TRANSFER

AA

PP

TT

EE

RR

33DrDr Mazlan 2016Mazlan 2016

TEXT: Heat and Mass TransferFundamental and Applications, 5th Edition by Cengel & GhajarMc Graw Hill

HEAT

TRANS Steady Heat Conduction

CCHH

AA

SFER

ASSOC. PROF. DR. MAZLAN ABDUL WAHIDC24-333

[email protected]

Steady Heat Conduction

AA

PP

TT

EE

RR

33DrDr Mazlan 2016Mazlan 2016

2

2

07-5534766

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Recap

• Generalized heat conduction equation may include:

– 3-D conduction– 3-D conduction

– Transient

– Heat generation

– Different coordinate system

• Conduction can be simpler:

– 1-D conduction– 1-D conduction

– Steady

– No heat generation

One dimensional steady conduction

Thermal Contact Thermal

resistance

Contact

resistance

Conduction Fins (Extended

Conduction

with heat

generation

Fins (Extended

surface)

Thermal resistance Contact resistance Heat generation Fins

One dimensional analysis

• Multi-dimensional conduction analysis is

complexcomplex

• Hard to determine temperature profile

• One-dimensional for the accurate engineering

analysis


The plane wall

• 1-D (x-axis)

• Hot and cold fluid on • Hot and cold fluid on different sides

• No heat generation

• Convection from hot fluid to wall

• Conduction through • Conduction through wall

• Convection from wall to cold fluid


Temperature distribution


Boundary conditions


Thermal resistance


Thermal resistance

Electrical analogy

Electrical Thermal circuit

Electrical

circuit


Thermal resistance

Conductive

resistance

Convective

resistanceRadiative

resistanceresistance resistance resistance


Thermal Resistance for plane wall


Composite wall

Express the following geometry in terms of an equivalent thermal circuit


Composite wall

1


Composite wall

What is the heat transfer rate

for this system?for this system?


Composite wall


Parallel thermal resistance


Generalized thermal resistance

Draw thermal resistance

network for this circuitnetwork for this circuit

Work out the heat transfer

rate as a function of

temperatures and thermal

resistance


Generalized thermal resistance


Cylindrical coordinates

Hollow cylinder with different thermal conditions at the

inner and outer surfaces


Cylindrical coordinates


Spherical coordinates


Problem 3-77


Contact resistance


Contact resistance

• Rough surface

produces air produces air

pockets


Thermal contact conductance


Thermal contact resistance


Problem 3-49


Conduction with heat generation


Plane wall

• One dimensional

• Steady state• Steady state

• Constant thermal

conductivity

• Uniform heat

generation

• Asymmetric surface • Asymmetric surface

condition


Plane wall


Symmetrical Distribution


Adiabatic surface


Example


Extended surface (fins)


Extended surface (fins)

• Combined effect of

conduction + conduction +

convection (and

radiation)

• Heat transfer direction

transverse to that of

conductionconduction


Types of fins

Uniform

cross section

Non-uniform

cross section

Annular

cross sectionPin fin

Uniform

cross section

Non-uniform

cross section

Annular

cross sectionPin fin


Steady heat conduction

Increase

heat heat

transfer

rate


Fin equation


Tip boundary conditions

Infinitely Negligible Infinitely

long fin

Negligible

heat loss

Specified Specified

temperatureConvection


Infinitely long fin


Negligible heat loss


Specified temperature


Convective


Corrected fin length

Rectangular fin

Cylindrical fin

ht cengel chapter 3 - mazlan 2016

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