11. Heat Transfer
• Is a heat energy being transferred between systems
by either conduction, convection, or radiation.
•. THE RATE OF HEAT TRANSFER IS DEFINED
BY:
H= RATE OF HEAT TRANSFER
Q= HEAT
T= TIME IN SECOND
12. CONVECTION
• Is a transfer of heat or flow
of heat through currents
within a fluid (liquid or gas).
13. Convection Heating Depends
On Several Factors:
1. The temperature of the body
2. The temperature
3. Velocity of the fluid surroundings, and other
factors, including even the detailed geometry of
the body's surface.
14. Heat transfer in atmosphere result in
upward movement of hot air and subsequent
replacement by cold air from surrounding places.
This convectional movement in atmosphere result
in the high and low pressure zones and on a
larger scale causes the formation of clouds and a
weather pattern.
15. EXAMPLE:
•1. In geology the slow
movement of molten matter
inside the earth crust is also
an good example of
convection current.
16. EXAMPLE:
•2. Apart from this there are
examples of heating of
water in kettle where the
hot water rises to the top
and cold layer comes in
that place.
17. EXAMPLE:
•3. Radiator in a car engine
helps in heat from engine
to diffuse into which rises
to the top and cold water
from the top layer takes the
place of the same.
18. EXAMPLE:
•4. The heat energy which
drives the hot air balloon to
move upward also follows
the same theory of
convection where the hot air
rises to the top and cold from
surrounding comes into
continue the cycle.
19. EXAMPLE:
•5. The heat energy of
microwave unit follows the
same convectional
movement and cooks that
food faster than other
convectional method.
20. EXAMPLE:
•6. The oceanic current follows
the same convectional
movement where the hot water
from equator flows toward the
north and south pole resulting
in the cold water from these
regions moving towards the
equator only to continue the
same cycle.
21. The formula of convection
is:
H=hA∆T
•H=rate of rate transfer
•H=convection coefficient (W/m2-co)(
W/m2-K)
•A=area of the surface in m2
22. The rate of convective heating is found to be
approximately proportional to the product of the
body’s area and the temperature difference
between the body and the surrounding fluids.
24. EXAMPLE PROBLEM:
•Calculate the rate of heat loss by convection for
the situation described, exposed skin of area
1.5m2 at 32°C and surrounding air at 20°C: (a)
in still air, for which h=7.0 W/m2-c°; (B) in air
moving at a speed of 2.0m/s, for which
h=20 W/m2-c°.
26. To Get The Heat Loss :
Q=Ht
Q=Ht
=130W(3600s)
=4.7x105J (1kcal/4186J)
=110 kcal
27. EXAMPLE PROBLEM:
•Calculate the rate of heat loss by convection for
the situation described, exposed skin of area
1.5m2 at 32°C and surrounding air at 20°C: (a)
in still air, for which h=7.0 W/m2-c°; (B) in air
moving at a speed of 2.0m/s, for which
h=20 W/m2-c°.
Paste the formula on the board.
Student, can you please derive the formula of heat transfer the value of heat. Show it on the whiteboard.
Who can give another example of convection?
Volunteer, who wants to derived the formula of convection to get the value of:
Who wants to answer in letter be problem? Show it on the whiteboard
When the skin is warm, expanded capillaries enhance the flow of blood and heat near the skin.
When the skin is cold, constrition of capillaries reduces the flow of blood and heat.
