Introduction to Heat Transfer
*Energy transfer
Pre work :
+ how does passive cooling work in fanless laptop?
+ how does wireless charging work to charge the smartphone?
+ how does microwave work?
Home appliances use
heat transfer process
Fanless laptop
https://picklaptop.com/best-fanless-laptops/
March 8, 2021, 9.01 am
Cooling Fan laptop
https://surfacetip.com/is-surface-go-fanless/
March 8, 2021, 9.01 am
https://www.ifixit.com/Guide/HP+Notebook
+15-ba018ds+Fan+Replacement/115650
March 8, 2021, 9.01 am
Cooling Fan PC
Electronic devices use
heat transfer process
Automotive devices use
heat transfer process
Laboratory equipments
use heat transfer
process
Mechanism of Heat Transfer
*atomic/molecular point of view
• Dynamics of atomic structure
• Atom movement
• vibration
v=0
• rotation
electron
neutron
v=0
• translation
electron
proton
Electromagnetic wave
v0
• Temperature is a physical
quantity which describes
how quickly atoms are
moving inside a material.
• Temperature also
represents the
momentum of atoms.
Conduction (atoms do not have
translation movement, collides
between atoms due to vibration
or rotation = momentum transfer)
before collision
after collision
Convection (atoms have
translation movement, collides
between atoms due to
translation = momentum
transfer)
v
before collision
v’ v”after collision
Mechanism of Heat Transfer
*atomic/molecular point of view
Radiation (atoms that have dynamics
atomic structure produce electromagnetic
wave, the propagation of electromagnetic
wave that is called electromagnetic
radiation) influences the other atomic
structure.
Electric field
electron
neutron
Magnetic field
proton
Electromagnetic
radiation
Atom 1
proton
neutron
Atom 2
electron
Modelling of Heat Transfer (Conduction)
*microscopic point of view
𝑄
𝐴
Heat flow
𝑄
𝐴
Heat flow
T1 > T2
𝑄
𝐴
Heat flow
T
𝑘
𝑄
𝑑𝑇
= −𝑘
𝐴
𝑑𝑥
Fourier’s formula
Group 1
of atoms
Group 2
of atoms
atomic/molecular point
of view
x
x
microscopic point of view
Physical phenomena
Mathematical model
Application of Heat Transfer Study
*example of one dimensional steady
state heat conduction
The inside space of a refrigerator is
maintained to be constant at 15 oC,
where the surrounding temperature is
about 32 oC all the day. The wall of
refrigerator is isolated to prevent heat
transfer from surrounding except the
glass door. The thickness of glass door is 4
mm. If the thermal conductivity of glass is
about 0.96 W/(m.oC) and the area of glass
door is about 1.5 m2, determine the heat
that flows from surrounding into the
freezer to estimate the minimum power
of refrigerator.
Assumptions : the problem
becomes simple but still
physically realistic.
1. The outside surface
temperature of glass door is
homogeny and same with the
temperature of surrounding.
2. The inside surface
temperature of glass door is
homogeny and same with
inside temperature of
refrigerator.
3. The situation is steady state.
Heat flow
T1
Temperature
of surrounding
(T1) = 32 oC
Inside
temperature
of refrigerator
(T2) = 15 oC
T
Boundary condition:
x1 ⎯→ T1
x2 ⎯→ T2
T2
𝑄
𝑑𝑇
= −𝑘
𝐴
𝑑𝑥
𝑄
𝑑𝑥 = −𝑘𝑑𝑇
𝐴
𝑄
𝑘
x1
x
𝑇2
𝑄 𝑥2
න 𝑑𝑥 = −𝑘 න 𝑑𝑇
𝐴 𝑥1
𝑇1
x2
𝑄 = −𝑘𝐴
𝑄
𝑥 − 𝑥1 = −𝑘 𝑇2 − 𝑇1
𝐴 2
𝑇2 − 𝑇1
𝑥2 − 𝑥1
𝑊
15 − 32 𝑜 𝐶
2
𝑄 = −0.96 𝑜 1.5𝑚
𝑚. 𝐶
4. 10−3 𝑚
𝑄 = 2520𝑊
Important factors of Heat Transfer Study Applications
•
•
•
•
•
•
Steady state or Transient Heat Transfer
Multidimensional Heat Transfer
Multilayer Plane Walls
Heat Generation
Variable conductivity
Boundary and initial conditions
• Specified Temperature Boundary Condition
• Specified Heat Flux Boundary Condition
• Insulated Boundary
• Thermal Symmetry
• Convection Boundary Condition
• Radiation Boundary Condition
• Interface Boundary Conditions
• Generalized Boundary Conditions
• Solutions of Differential Equations
*Conduction
Modelling of Heat Transfer (Convection)
*microscopic point of view
Heat flow and
𝑄 Fluid flow
𝐴
Heat flow and
𝑄 Fluid flow
𝐴
T
T
>
v
ℎ
v
v
v
v
𝑄
= ℎ 𝑇 − 𝑇∞
𝐴
Newton’s law of cooling
Group 1
of atoms
Group 2
of atoms
atomic/molecular point
of view
microscopic point of view
Physical phenomena
Mathematical model
Important factors of Heat Transfer Study Applications
• Classification of fluid flows
• Laminar flow
• Turbulent flow
• Boundary layer
• Velocity boundary layer
• Thermal boundary layer
• External forced convection
• Internal forced convection
• Natural convection
*Convection