1. The document discusses various modes of heat transfer including conduction, convection, and radiation. 2. Conduction involves the transfer of heat through direct contact of materials. Convection uses fluid motion to transfer heat, and can be natural or forced. 3. Radiation transfers heat via electromagnetic waves and does not require a medium, allowing it to occur even in a vacuum. It occurs as objects emit and absorb thermal radiation.

1. Modes of Heat Transfer-1
P M V Subbarao
Professor
Mechanical Engineering Department
Various Possibilities of a Natural Happening
…..
Change in Class Room : LH 519

2. General Conduction Heat Transfer in a System
Conduction Heat Transfer from/to a System:
T
k
q 


'
'


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



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







 k
z
T
j
y
T
i
x
T
k
q ˆ
ˆ
ˆ
'
'

A
k
z
T
j
y
T
i
x
T
k
A
q
Q



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
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


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

 ˆ
ˆ
ˆ
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'
Heat flux due to conduction

3. Heat transfer through a plane slab
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







x
T
kA
Q

4. HEAT TRANSFE IN TURNING
• Cooling of a workpiece on a lathe is affected not only by
the usual factors such as exposure of its surfaces and their
velocities, but also by the presence of the chuck and the jet
the cutting fluid splashing on the workpiece while moving
with the tool.
• The cooling effect of the surface area under the impinging
coolant jet is not uniform and therefore the heat transfer
coefficient in this zone should be treated as a distribution.
• The initial conditions of cooling at the beginning of the
pass, when the jet flashes also on the side face of the
workpiece, poses additional challenge in determination of
heat transfer coefficients.

5. Heat Convection
• Convection uses the motion of fluids to transfer heat.
• In a typical convective heat transfer, a hot surface heats the
surrounding fluid, which is then carried away by fluid
movement such as wind.
• The warm fluid is replaced by cooler fluid, which can draw
more heat away from the surface.
• Since the heated fluid is constantly replaced by cooler
fluid, the rate of heat transfer is enhanced.

6. Natural Convection
• Natural convection (or free convection) refers to a case
where the fluid movement is created by the warm fluid
itself.
• The density of fluid decrease as it is heated; thus, hot fluids
are lighter than cool fluids.
• Warm fluid surrounding a hot object rises, and is replaced
by cooler fluid.
• The result is a circulation of air above the warm surface

7. Forced Convection
• Forced convection uses external means of producing fluid
movement.
• Forced convection is what makes a windy, winter day feel
much colder than a calm day with same temperature.
• The heat loss from your body is increased due to the
constant replenishment of cold air by the wind.
• Natural wind and fans are the two most common sources
of forced convection.

8. Newton's Law of Cooling
• The rate at which a hot body cools to the temperature of its
surroundings is given by an empirical formula first discovered by
Sir ISAAC NEWTON
• Newton's law of cooling states, "For a body cooling in a draft, the
rate of heat loss is proportional to the difference in temperatures
between the body and its surroundings."
• Since the temperature change is proportional to the heat transfer.
ambient
body
body
T
T
dt
dT


Energy transport due to two combined effects:
• diffusion: random molecular energy
• advection: bulk (macroscopic) fluid motion
The process of convection occurs between a moving fluid and
a surface at different temperatures.

9.  
ambient
surface T
T
q 

'
'
Convection Heat Transfer
ambient
surface T
T 
 
ambient
surface T
T
h
q 

'
'
ambient
surface T
T 
 
surface
ambient T
T
h
q 

'
'
where h is the local heat transfer coefficient.
Both the flux and transfer coefficient vary along the surface.

10.   s
ambient
surface dA
T
T
h
Q 


The local heat transfer rate is
Local and Average Heat Transfer Rate
  s
A
ambient
surface
A
s A
d
T
T
h
dA
q
s
s

 

"
The Total heat transfer rate is
 
ambient
surface
s
avg
A
s T
T
A
h
dA
q
s


 "
Where h is average heat transfer coefficient:

11. The total heat transfer rate Q is
avg
S
avg
A
s T
A
h
dA
q
Q
s


 
'
'
Where, havg is the average convection heat transfer coefficient for
the entire surface.
avg
A
s
s
avg
T
dA
q
A
h s



'
'
1
 
 



s
A
s
s
s
avg dA
T
T
A
T
1
where
 




 s
A
s
s
A
s
avg
dA
T
T
dA
q
h
'
'
Therefore

12. Thermal Radiation
• Radiation does not require a medium to pass through; thus, it is
the only form of thermal energy travel present in vacuum.
• It uses electromagnetic (photons) means of transportation, which
travels at the speed of light.
• Thermal radiation is emitted by any matter with temperature
above 0 degree Kelvin (-273 °C).
• Radiative heat transfer occurs when the emitted radiation strikes
another body and is absorbed.
• The electromagnetic spectrum classifies radiation according to
wavelengths of the radiation.
• Main types of radiation are (from short to long wavelengths):
gamma rays, x-rays, ultraviolet (UV), visible light, infrared (IR),
microwaves, and radio waves.
• Radiation with shorter wavelengths are more energetic and
contains more heat.