ERT 216
INTRODUCTION TO HEAT TRANSFER
MISS HANNA ILYANI ZULHAIMI
S
WHAT IS HEAT TRANSFER?
Heat transfer is thermal energy transfer that is induced by a
temperature difference (or gradient)
Mode of heat transfer
• Conduction : Occurs when a temperature gradient exists through a
solid or a stationary fluid (liquid or gas). (Surface to surface)
• Convection : Occurs within a moving fluid, or between a solid
surface and a moving fluid, when they are at different temperatures.
(Surface to air)
• Thermal radiation: Heat transfer between two surfaces (that are not
in contact), often in the absence of an intervening medium. (Direct
exchange across space)
HEAT TRANSFER VS
THERMODYNAMICS
Example:
Cooling of hot steel bar placed in a pail water
THERMODYNAMICà predict the final equilibrium
temperature of the steel bar-water combination.
HEAT TRANSFER à predict the temperature of both the
bar and the water as a function of time also the
temperature of the bar will be after a certain length of
time.
How is Heat Transferred? There are THREE ways heat can move.
Ø  Conduction
Ø  Convection
Ø  Radiation
CONDUCTION
S  Transfer of energy from the more energetic to less energetic particles of
a substance by collisions between atoms and/or molecules.
S  Conduction is the transfer of heat from a hotter area to a cooler area .
S  Heat is transferred from one par/cle of ma2er to another in an object without the movement of the object. S  Conduc3on = CONTACT Conduction
Fourier’s Law à
•  Temperature different: T1 and T2
[K]
•  Thermal conductivity: k [W m-1
K-1]
•  Heat transfer area: A [m2]
•  Thickness of the material: x [m]
•  Heat transfer rate by conduction: qc
•  Heat flux: q/A
Since T2>T1, the heat flows from right to
left
Fourier’s Law
•  Heat transfer by conduction from a high
temperature region to the low temperature
region
•  The driving force of this heat transfer is the
temperature gradient
Have you ever… S  Touched a metal spoon si=ng in a pan of boiling water only to be surprised by HOW hot it is? Think back to what you know about metals and
nonmetals. What conducts heat better, metal or
nonmetal? Why?
THERMAL CONDUCTIVITY
•  Experiment measurement made to determine the value
of k (different material, different k)
•  The k value is a physical property of each solid, liquid
and gas material
•  The numerical value of k indicates à how fast heat will
flow in a given material.
•  If molecules move fasters (gas), the faster they will
transport energy.
•  So, value of k depends on the molecules structure (gas,
liquid or solid)
•  k is strongly temp-dependent
•  Unit W m-1 K-1
K of various
materials at
0 °C à
Calculate the heat loss per m2 of surface area for an insulating wall
composed of 25.4 mm thick fiber insulating board, where inside
Temperature is 352.7L and outside temperature is 297.1 K.
From Appendix A.3, thermal conductivity for fiber insulating board
is 0.048 W/m.K. The thickness is 25.4 mm. Thus:
THERMAL CONDUCTIVITY
1)  Gas
•  If temp. higher, the molecules have higher
velocities
•  Molecules are in continuous random motion and
will colliding with one another and exchanging the
energy and momentum
•  The random motion of gas molecules happen
whether/ not a temp. gradient exist in gas
•  If a molecules move from higher temp. region to
low temp. region, it transport kinetic energy to
lower temp. part of system (through collisions
molecules)
•  The faster molecules moveà faster they will
transport energy
THERMAL CONDUCTIVITY
2) Liquid
•  The molecules are move closely space
•  Exchange energy in collision process
3) Solid
•  Thermal energy conducted in solid by 2 modes:
lattice vibration and transport by free electron
CONVECTION
•  Transfer of heat between an object and fluid (Surface to fluid).
•  Convection = VENTS (through air and liquid particles)
•  Transfer of heat by bulk transport and mixing of macroscopic elements
of warmer portions with cooler portion in a gas or liquid.
# Transfer is affected by
i) the speed with which the air is moving
ii) the ability of the object to conduct heat
Convection
• 
• 
• 
• 
• 
Temperature different between wall and fluid: Tw and Tf [K]
Convective heat transfer coefficient: hcv [W m-2 K-1]
Heat transfer area: A [m2]
Heat transfer rate by convection: qcv
Heat flux: q/A
Also referred to as the “Newton rate equation”
or “Newton’s law of cooling”
Appproximate
value of h à
Convection
Two types:
(i)  Forced convection:
# Fluid is forced to flow
past a solid surface by a
mechanical means
(ii) Natural (free)
convection:
# Heat circulation due to
fluid density difference
resulting from the
temperature variation
throughout a region of
the fluid
Convection
•  The temperature of the plate is Tw and the temperature of the
fluid is T∞.
•  The velocity, u reduce to 0 at the plate as a result of viscous
action. Because of that, heat must be transferred only by
conduction at that point (wall).
•  convection heat transfer, hcv will have a dependence on the
viscosity of the fluid in addition to its dependence on the thermal
properties of the fluid (thermal conductivity, specific heat, density).
Radiation
•  Transfer of energy across the space by means of
electromagnetic waves- in the same way as the
electromagnetic light waves transmitting light (Direct
exchange across space)
•  Radia/on does NOT require ma2er to transfer thermal energy •  Radia/on = Radiates (heat escaping the sun) Examples of RADIATION Fire Heat Lamps
Sun Radiation
•  Electromagnetic radiation which is propagated
because of temp. different called à thermal
radiation
•  Ideal thermal radiation/ black body radiation,
will emit energy at a rate proportional to the fourth
power of the absolute temp. of the body and
directly proportional to it’s surface area.
•  Stefan-Boltzmann Law; q emitted = σ A T4
•  Black-body = Black surface (a piece of metal
recovered with carbon black)
Radiation
q emitted = σ A T4
σ = 5.669 x 10-8 W/m2.K4
•  Stefan-Boltzmann Lawà thermal radiation, it
govern only radiation emitted by black-body.
•  The net radiant exchange between 2 surface
proportional to the different in absolute
temperature to the fourth power
q emitted = σ A (T14 – T24)
Radiation
•  Emissivity, ϵ à relates the radiation on the ‘gray surface’
•  Not all the radiation leaving one surface will reach the
other surface since electromagnetic radiation travels in
straight line & some will be lost to the surroundings.
•  Because of that, 2 new factors will be considered ;
Fϵ: Emissivity function and
FG: Geometric “view factor’ function
Radiation
Radiation in an enclosure
Simple radiation problem is encounter when a
heat transfer surface at temp., T1 completely
enclosed by a much large surface maintain at
temp. T2. So that, the net radiant exchange: