Heat Transfer Basic Laws and their applications in simple language .pdf

basic laws of heat transfer

1. Basic Laws of Heat Transfer

2. Outline
Revise mechanisms of Heat Transfer
Check our understanding
Fourier’s law of heat conduction
Newton’s law of cooling
Stefan Boltzmann law
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3. Heat Transfer Mechanisms or Modes
1.Conduction
2.Convection
3.Radiation
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4. Conduction
The transferof energy fromthe
more energetic particles ofa
substance to the adjacent less
energetic ones as aresult of
interactionsbetween the
particles
It can take place in solids,
liquids as well as gases
In solids, it is due to the
combinationof vibrationsof
the molecules in a lattice and
the energy transport byfree
electrons.
In gases and liquids, conduction
is due to the collisionsand
diffusionof themolecules
during their random motion.
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5. Convection
Convection:The mode of energy transfer between a solid surface and the
adjacent liquid or gas that is in motion
It involves the combined effectsof conductionand fluid motion
Faster the fluid motion, greater the convection heat transfer
In the absence of any bulk fluid motion, heat transferbetween a solid surface
and the adjacent fluid is by pure conduction.
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6. Convection
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Natural (or free) convection:If the
fluid motion iscaused by buoyancy
forces that are induced by density
differences due to thevariation of
temperature in the fluid.
Forced convection:If the fluid is
forced to flow overthe surface by
external means such as a fan,
pump, or the wind.
Heat transfer processes that involve change of phaseof a fluid are also
considered to be convection because of the fluid motion induced during
theprocess, such as the rise of the vapor bubbles during boiling or the
fall ofthe liquid droplets duringcondensation.

7. Radiation
7
•Radiation:The energy emitted by matter in the form of electromagneticwaves
(or photons) as a result of the changes in the electronic configurations of the
atoms or molecules.
•Radiation does not require the presence of an intervening medium.
•Radiation is fastest mode of heat transfer
•In heat transfer studies we are interested in thermal radiation, which isthe form
of radiation emitted by bodies because of their temperature
•All bodies at a temperature above absolute zero emit thermal radiation.
•All solids, liquids, and gasesemit, absorb, or transmit radiation to varying
degrees.
•Radiation is a volumetric phenomenon, however, radiation isusually considered
to be a surface phenomenonfor solids.

8. Summery of basic modes
Conduction
-needs matter
-molecular phenomenon (diffusion process)
-without bulk motion of matter
-dominant in solids
Convection
-heat carried away by bulk motion of fluid
-needs fluid matter
Radiation
-does not needs matter
-transmission of energy by electromagnetic waves

9. Lets check our understanding

10. 3-convection
Barn
Water
1-Radiation
2-conduction

11. 11
1. The driving force for heat transfer is____________
2. Heat flows in the direction of __________ temperature.
6. Conduction in solids is due to combination of _________ & __________.
Lattice vibrations &Energy transport by Free electrons
7. __________ is the fastest mode of heat transfer.
8. Conduction in liquids & gases is due to _______&_______
Collision& Diffusion
3. Heat Transfer problems are classified as _________&_________
4. The measure of thermal energy is called as ________
5. The flow of thermal energy is called as ________
Fill in the blanks with appropriate words
Temperature difference
Decreasing
Rating Sizing
Temperature
Heat
Radiation

12. 12
Identify the method of heat transfer taking place in each of the following situations,
(conduction, convection, radiation).
1. Hot coffee is stirred with a spoon, the spoon gets hot due to
_______________.
2. A chair is placed several feet from a fire in a fireplace. The fireplace has a
glass screen. The side of the chair facing the fireplace gets warm because
of_______________.
3. A certain type of decorative lamp contains colored liquids. These liquids form
globs that break off and rise to the top of the liquid. The globs rise due to
_______________.
4. . Blood circulation in warm blooded animals is an example of __________.
5. Ironing a shirt is an example of_______________.
Conduction
Radiation
Convection
Convection
Conduction

13. 13
Identify the method of heat transfer taking place in each of the following situations,
(conduction, convection, radiation)
6. The movement of cool air inside refrigerator compartment is due to ________
convection.
7. A group of friends gather around a fire to stay worm __________.
8. You are in the top bunk of a bunk bed and you want to turn the air conditioner
on while your friend on the bottom bunk is fine is caused by _______.
9. A huge rock at the park gets so hot during the day that you can’t sit on it from
_______________.
10. You lay on that same rock at night so that you can keep warm by
_______________.
Radiation
Convection
Radiation
Conduction
Natural

14. Notations
Quantity Notation Unit
Theamount of Heat
transfer
Q or q Joules(J)
Rate of heat transfer
(Amount of heat transfer
per unit time)
or J/sec (W)
Heat Flux(Heat transfer
rate/unit area)
Q’’ or q’’ W/m
2

15. Fourier’s Law of Heat Conduction
The rate of heat conduction through a medium depends on
The geometry of the medium
Thickness of the medium
Material of the medium
Temperature difference across the medium
The rate of heat conduction through a plane layer is
Proportionalto the temperature difference across the
layer
Proportionalto the heat transfer area
Inversely proportional to the thickness of the layer.
Heat conduction
through a large plane
wall of thickness x
and area A.

16. Fourier’s Law of Heat Conduction
Where dT/dxis called as temperature gradient
(variation of temperature along the distance)
A is the area normal to the direction of heat flow
kis the constant of proportionality called as thermal
conductivity
kis ability of the material to conduct the heat
Negative sign ensures that the heat transfer is positive
quantity in positive x direction
Whenx → 0
Fourier’s law of
heat conduction
Statement: The rate of heat conduction through a
plane layer is directly proportional to the area (A)
normal to the direction of heat flow and the
temperature gradient (dT/dx) in the direction of
heat flow
In heat conduction
analysis, Arepresents
the area normal to the
direction of heat
transfer.

17. Newton’s law of Cooling
Statement:The rate of heat transfer by convection is proportional to surface
area and the temperature difference between the solid surface and the fluid
hconvection heat transfer
coefficient
A
sthe surface area through
which convection heat transfer
takes place
T
sthe surface temperature
T
the temperature of the
fluid sufficientlyfar from the
surface.
Mathematically
Where

18. Newton’s law of Cooling
•Unit of his
W/m
2
.
o
Cor W/m
2
.
o
K
•The convection heat transfer
coefficienthis not a fluid property
but a flow property
•Itis an experimentallydetermined
parameter
•Its valuedepends on all the
variables influencingconvection
such as
the surface geometry
the nature offluid motion
the properties of the fluid
the bulk fluid velocity

19. Stefan Boltzmann Law
•The rate of radiant energy emitted heat by a surface is proportional to the
surface area and forth power of its absolute temperature
•The maximum rate of radiation that can be emitted from a surface at an
absolute temperature T
sis given by
= 5.670 10
8
W/m
2
· K
4
Stefan–Boltzmann constant
Where
Blackbody:The idealized surface that
emits radiation at the maximum rate
Radiation emitted byreal surfaces
Blackbody radiation represents themaximum
amount of radiation thatcan be emitted from
a surfaceat a specified temperature.
Emissivity:Ameasure of how
closely a surfaceapproximates a
blackbody for which = 1of the
surface. 01.