This Slide explain the basic of Heat Transfer

1. INTRODUCTION TO HEAT TRANSFER
YOGESH KUMAR SHARMA
Assistant Professor,
GNA University, Phagwara, Punjab(India)
09/11/17

2. Lecture Strategy
Please
3- make your
mobile Silent
1- Smile
2- Respect the
lecture time09/11/17

3. 09/11/17

4. What is Heat?
This is the movement of thermal
energy from a substance at a
higher temperature to another at a
lower temperature.
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5. Heat vs. Temperature
Heat is the actual energy…
Temperature is the measure of average
kinetic energy of particles in a
substance.
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6. Thermodynamics tells us:
 How much heat is transferred (δQ)
 How much work is done (δW)
 Final state of the system
Heat transfer tells us:
 How (with what modes) δQ is transferred
 At what rate δQ is transferred
 Temperature distribution inside the body
Heat transfer Thermodynamicscomplementary
Thermodynamics Vs Heat Transfer
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7. Heat TransferHeat Transfer
Heat always moves from a warmer place to aHeat always moves from a warmer place to a
cooler place.cooler place.
Hot objects in a cooler room will cool to roomHot objects in a cooler room will cool to room
temperature.temperature.
Cold objects in a warmer room will heat up toCold objects in a warmer room will heat up to
room temperature.room temperature.
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8. Question?
Question: If a cup of hot coffee and a red popsicle
were left on the table in this room what would happen
to them? Why?
Ans: The cup of coffee will cool until it reaches room
temperature. The popsicle will melt and then the liquid
will warm to room temperature.
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9. Significance of Heat Transfer
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10. Heat Transfer Methods
Heat transfers in three ways:
Conduction
Convection
Radiation
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11. MODESOFHEATTRANSFER
WATER MAY BE TREATED AS HEAT AND PEOPLE MAY BE HEAT TRANSFER MEDIUM
1- Radiation
2-conduction
3-convection
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12. Conduction
Conduction heat transfer is the flowing of heat
energy from a high-temperature object to a
lower-temperature object within the medium/
different medium in direct physical contact
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13. When you heat a metal strip at one end, the heat
travels to the other end.
As you heat the metal, the particles vibrate, these
vibrations make the adjacent particles vibrate, and so on
and so on, the vibrations are passed along the metal and
so is the heat. We call this? Conduction
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14. The outer e______ of metal atoms
drift, and are free to move.
When the metal is
heated, this ‘sea of
electrons’ gain k_____
energy and transfer it
throughout the metal.
Insulators, such as w___ and p____, do not
have this ‘sea of electrons’ which is why they
do not conduct heat as well as metals.
lectrons
inetic
ood lastic
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15. Metal is a conductor, wood is an insulator. Metal
conducts the heat away from your hands. Wood
does not conduct the heat away from your hands as
well as the metal, so the wood feels warmer than
the metal.
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16. Convection
Convection takes place when heated molecules
move from one place to another, taking the heat
with them. Convection is common in both the
atmosphere, as well as in the oceans.
Convection is the primary way that heat moves
through gases and liquids.
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17. Convection
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18. What happens to the particles in a liquid or a
gas when you heat them?
The particles spread out and
become less dense.
This effects fluid movement.What is a fluid?A liquid or gas.
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19. Cooler, more d____, fluids
sink through w_____, less
dense fluids.
In effect, warmer liquids and
gases r___ up.
Cooler liquids and gases s___.
ense
armer
ise
ink
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20. Hot water
rises
Cooler
water sinks
Convection
current
Cools at the
surface
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21. Where is the
freezer
compartment
put in a fridge?
Freezer
compartment
It is put at the
top, because
cool air sinks,
so it cools the
food on the
way down.
It is warmer
at the
bottom, so
this warmer
air rises and
a convection
current is
set up.
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22. 09/11/17

23. CONVECTION
Convection: The mode of energy transfer between a solid surface and the
adjacent liquid or gas that is in motion, and it involves the combined effects
of conduction and fluid motion.
The faster the fluid motion, the greater the convection heat transfer.
Forced convection: If the fluid is
forced to flow over the surface by
external means such as a fan, pump,
or the wind.
Natural (or free) convection: If the
fluid motion is caused by buoyancy
forces that are induced by density
differences due to the variation of
temperature in the fluid.
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24. 09/11/17

25. How does heat energy get
from the Sun to the Earth?
There are no particles
between the Sun and the
Earth so it CANNOT
travel by conduction or
by convection.
?
RADIATION
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26. Unlike conduction and convection, heat transfer by radiation can occur
between two bodies, even when they are separated by a medium colder
than both of them.
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27. Radiation
Propagation of heat in the form of electromagnetic
waves.
Unlike conduction and convection, the transfer of
energy by radiation does not require the presence of
an intervening medium. In fact, energy transfer by
radiation is fastest (at the speed of light) in a vacuum.
This is exactly how the energy of the sun reaches the
earth.
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28. For conduction between two plane surfaces (eg heat
loss through the wall of a house) the rate of heat
transfer is
energy
transferred
through slab
Q
TH
TC
L
H CQ T T
k A
t L
∆ −
=
∆
Thermal conductivity k (W.m-1.
K-1
)
steady-state
Q QA
which is known as Fourier’s law of heat conduction
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29. H CQ T T
k A
t L
∆ −
=
∆
dQ dT
k A
dt dx
= −
steady-state
Thermal Conduction through a uniform slab
0 xL
TC
TH
temperature gradient
dT
dx
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30. Thermal conductivity, k property of
the material
kdiamond very high: perfect heat
sink, e.g. for high power laser
diodes
khuman low: core temp relatively
constant (37 o
C)
kair very low: good insulator
* home insulation
* woolen clothing
* windows double glazing
Metals – good conductors: electrons
transfer energy from hot to cold
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31. The Cost of Heat Loss through a Roof
The roof of an electrically heated home is 6 m long, 8 m
wide, and 0.25 m thick, and is made of a flat layer of
concrete whose thermal conductivity is k 0.8 W/m-°C.
The temperatures of the inner and the outer surfaces
of the roof one night are measured to be 25°C and
0°C, respectively, for a period of 10 hours.
Determine:
(a) the rate of heat loss through the roof that night and
(b) the cost of that heat loss to the home owner
if the cost of electricity is $0.2/kWh.
25 C
0 C
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32. (a) Heat transfer through the roof is by conduction and the
area of the roof is:
A= 6 m×8 m=48 m2
.
The steady rate of heat transfer through the roof is
Q·
=kA(T1-T2)/L= (0.8)(48 )(25-0)/0.25= 3840 W= 3.84 kW
(b) The amount of heat lost through the roof during a 10-hour
period and its cost:
Q = Q·
∆t =(3.84 kW)(10 h) = 38.4 kWh
Cost/day = (Amount of energy)(Unit cost of energy)
= (38.4 kWh)($0.2/kWh) =$7.68
Cost/month = (cost/day)×(30day/month)= $7.68×30=$230.4
Why is
the bill
so high?
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33. Newton’s law of cooling
h convection heat transfer
coefficient, W/m2
· °C
As the surface area through which
convection heat transfer takes place
Ts the surface temperature
T∞ the temperature of the fluid
sufficiently far from the surface.
The convection heat transfer coefficient
h is not a property of the fluid, but
depends on all the variables influencing
convection such as
- the surface geometry
- the nature of fluid motion
- the properties of the fluid
- the bulk fluid velocity
Convection
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34. All bodies at a temperature above absolute zero emit
thermal radiation.
The maximum rate of radiation that can be emitted
from a surface at an absolute temperature Ts is given
by the Stefan–Boltzmann law as
where A is the surface area and σ = 5.67 * 10-8
W/m2
·
K4
is the Stefan–Boltzmann constant. Neither it is
property nor depend upon the property of
substance.
Radiation
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35. Radiation travels in straight lines
True/False
Radiation requires particles to travel
True/False
Radiation travels at the speed of light
True/False
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36. Think about
* Why are fireplace pokers made of iron and not copper?
* Steel reinforcement bars add stability to concrete walls. Do they also enhance
the insulating value of concrete?
* Drapes hung close to a cold window
* Should you lower the blinds and draw the curtains on a hot day?
* When one steps from a shower on a cold morning, why does the tile floor seem
so much colder than the air?
* Place a wooden spoon and a metal spoon in the freezer. Which will
cool faster? After several hours, what would you feel like?
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37. 1. Which of the following is not a
method of heat transfer?
A. Radiation
B. Insulation
C. Conduction
D. Convection
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38. 2. In which of the following are
the particles closest together?
A. Solid
B. Liquid
C. Gas
D. Fluid
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39. 3. How does heat energy reach
the Earth from the Sun?
A. Radiation
B. Conduction
C. Convection
D. Insulation
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