ht cengel chapter 3 - mazlan 2016
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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
mazlan@fkm.utm.my07-5534766
Steady Heat Conduction
AA
PP
TT
EE
RR
33DrDr Mazlan 2016Mazlan 2016
2
2
07-5534766
Towards an Entrepreneurial University
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
Thermal resistance Contact resistance Heat generation Fins
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
Thermal resistance Contact resistance Heat generation Fins
Temperature distribution
Thermal resistance Contact resistance Heat generation Fins
Temperature distribution
Thermal resistance Contact resistance Heat generation Fins
Boundary conditions
Thermal resistance Contact resistance Heat generation Fins
Thermal resistance
Thermal resistance Contact resistance Heat generation Fins
Thermal resistance
Electrical analogy
Electrical Thermal circuit
Electrical
circuit
Thermal resistance Contact resistance Heat generation Fins
Thermal resistance
Conductive
resistance
Convective
resistanceRadiative
resistanceresistance resistance resistance
Thermal resistance Contact resistance Heat generation Fins
Thermal Resistance for plane wall
Thermal resistance Contact resistance Heat generation Fins
Thermal resistance Contact resistance Heat generation Fins
Composite wall
Express the following geometry in terms of an equivalent thermal circuit
Thermal resistance Contact resistance Heat generation Fins
Composite wall
1
Thermal resistance Contact resistance Heat generation Fins
Composite wall
What is the heat transfer rate
for this system?for this system?
Thermal resistance Contact resistance Heat generation Fins
Composite wall
Thermal resistance Contact resistance Heat generation Fins
Parallel thermal resistance
Thermal resistance Contact resistance Heat generation Fins
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
Thermal resistance Contact resistance Heat generation Fins
Generalized thermal resistance
Thermal resistance Contact resistance Heat generation Fins
Cylindrical coordinates
Hollow cylinder with different thermal conditions at the
inner and outer surfaces
Thermal resistance Contact resistance Heat generation Fins
Cylindrical coordinates
Thermal resistance Contact resistance Heat generation Fins
Spherical coordinates
Thermal resistance Contact resistance Heat generation Fins
Problem 3-77
Thermal resistance Contact resistance Heat generation Fins
Contact resistance
Thermal resistance Contact resistance Heat generation Fins
Contact resistance
• Rough surface
produces air produces air
pockets
Thermal resistance Contact resistance Heat generation Fins
Thermal contact conductance
Thermal resistance Contact resistance Heat generation Fins
Thermal contact resistance
Thermal resistance Contact resistance Heat generation Fins
Thermal resistance Contact resistance Heat generation Fins
Problem 3-49
Thermal resistance Contact resistance Heat generation Fins
Problem 3-49
Thermal resistance Contact resistance Heat generation Fins
Problem 3-49
Thermal resistance Contact resistance Heat generation Fins
Problem 3-49
Thermal resistance Contact resistance Heat generation Fins
Conduction with heat generation
Thermal resistance Contact resistance Heat generation Fins
Conduction with heat generation
Thermal resistance Contact resistance Heat generation Fins
Plane wall
• One dimensional
• Steady state• Steady state
• Constant thermal
conductivity
• Uniform heat
generation
• Asymmetric surface • Asymmetric surface
condition
Thermal resistance Contact resistance Heat generation Fins
Plane wall
Thermal resistance Contact resistance Heat generation Fins
Thermal resistance Contact resistance Heat generation Fins
Thermal resistance Contact resistance Heat generation Fins
Thermal resistance Contact resistance Heat generation Fins
Symmetrical Distribution
Thermal resistance Contact resistance Heat generation Fins
Symmetrical Distribution
Thermal resistance Contact resistance Heat generation Fins
Adiabatic surface
Thermal resistance Contact resistance Heat generation Fins
Example
Thermal resistance Contact resistance Heat generation Fins
Example
Thermal resistance Contact resistance Heat generation Fins
Extended surface (fins)
Thermal resistance Contact resistance Heat generation Fins
Thermal resistance Contact resistance Heat generation Fins
Thermal resistance Contact resistance Heat generation Fins
Extended surface (fins)
• Combined effect of
conduction + conduction +
convection (and
radiation)
• Heat transfer direction
transverse to that of
conductionconduction
Thermal resistance Contact resistance Heat generation Fins
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
Thermal resistance Contact resistance Heat generation Fins
Steady heat conduction
Increase
heat heat
transfer
rate
Thermal resistance Contact resistance Heat generation Fins
Fin equation
Thermal resistance Contact resistance Heat generation Fins
Fin equation
Thermal resistance Contact resistance Heat generation Fins
Fin equation
Thermal resistance Contact resistance Heat generation Fins
Tip boundary conditions
Infinitely Negligible Infinitely
long fin
Negligible
heat loss
Specified Specified
temperatureConvection
Thermal resistance Contact resistance Heat generation Fins
Infinitely long fin
Thermal resistance Contact resistance Heat generation Fins
Infinitely long fin
Thermal resistance Contact resistance Heat generation Fins
Negligible heat loss
Thermal resistance Contact resistance Heat generation Fins
Negligible heat loss
Thermal resistance Contact resistance Heat generation Fins
Specified temperature
Thermal resistance Contact resistance Heat generation Fins
Specified temperature
Thermal resistance Contact resistance Heat generation Fins
Convective
Thermal resistance Contact resistance Heat generation Fins
Corrected fin length
Rectangular fin
Cylindrical fin
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