Heat transfer Heat transfer BASICS OF HEAT TRANSFER.ppt

Heat transfer

1. Assist. Prof. Dr. Rasim BEHÇETAssist. Prof. Dr. Rasim BEHÇET

2. THERMODYNAMICS AND HEAT TRANSFER
Heat, which is the form of energy that can be
transferred from one system to another as a result of
temperature difference. The science that deals with the
determination of the rates of such energy transfers is
heat transfer.
Thermodynamics deals with equilibrium states and
changes from one equilibrium state to another. Heat
transfer, on the other hand, deals with systems that
lack thermal equilibrium, and thus it is a
nonequilibrium phenomenon. Therefore, the study of
heat transfer cannot be based on the principles of
thermodynamics alone.

3. APPLİCATİON AREAS OF HEAT TRANSFER
3

4. HEAT AND OTHER FORMS OF ENERGY
Energy can exist in numerous forms such as thermal,
mechanical, kinetic, potential, electrical, magnetic,
chemical, and nuclear, and their sum constitutes the
total energy E (or e on a unit mass basis) of a system.
The sum of all microscopic forms of energy is called
the internal energy of a system, and is denoted by U
(or u on a unit mass basis).
Internal energy may be viewed as the sum of the
kinetic and potential energies of the molecules. The
portion of the internal energy of a system associated
with the kinetic energy of the molecules is called
sensible energy or sensible heat.

5. The average velocity and the degree of activity of the
molecules are proportional to the temperature. Thus,
at higher temperatures the molecules will possess
higher kinetic energy, and as a result, the system will
have a higher internal energy.
The internal energy associated with the phase of a
system is called latent energy or latent heat.
The internal energy associated with the atomic bonds
in a molecule is called chemical (or bond) energy,
whereas the internal energy associated with the
bonds within the nucleus of the atom itself is called
nu- clear energy.

6. THE FİRST LAW OF THERMODYNAMİCS
The first law of thermodynamics is essentially an
expression of the conservation of energy principle.
Energy can cross the boundaries of a closed system
in the form of heat or work.
If the energy transfer across the boundaries of a
closed system is due to a temperature difference, it is
heat; otherwise, it is work.

7. The energy balance for any system undergoing
any process can be expressed as:

8. •Taking heat transfer to the system and work done
by the system to be positive quantities, the energy
balance for a closed system can also be expressed
as:
•where:

9. •Various forms of work are expressed as follows:

›Electrical work: (kJ)
›Boundary work: (kJ)
›Gravitational work (=DPE): (kJ)
›Acceleration work (=DKE): (kJ)
›Shaft work: (kJ)
›Spring work: (kJ)

10. •For ideal gases u, h, C
v
, and C
p
are functions of
temperature alone. The u and h of ideal gases can be
expressed as:
3-32

11. HEAT TRANSFER MECHANISMS
We defined heat as the form of energy that can be
transferred from one system to another as a result of
temperature difference.
There are three types of heat transfer:
•CONDUCTIONCONDUCTION
•CONVECTIONCONVECTION
•RADIATIONRADIATION

12. Conduction
Conduction is transfer through
direct contact.
On a molecular level, hotter
molecules are vibrating faster
than cooler ones.
When they come in contact, the
faster moving molecules “bump
into” the slower moving
molecules and heat is transferred!
This is how heat is transferred to
your finger if you touch a hot
stove!

13. 13
Fourier's law of heat conduction is

Q Ak
dT
dx
cond t


Q
cond
dT
dx
here:
= heat flow per unit time (W)
k
t
= thermal conductivity (W/mK)
A= area normal to heat flow (m
2
)
= temperature gradient in the direction of
heat flow (C/m)
Integrating Fourier's law:
cond t
T
Q k A
x




Since T
2
>T
1
, the heat flows from right to left in the above figure.

14. Convection
When fluids are heated, currents are
created.
This is because the individual molecules
that come in contact with a hot surface
expand, become less dense, and rise.
(this is how hot air balloons work!)
When this happens, other molecules
circulate down and take their place, and
a cycle is established.
An example of this can be observed in
the air currents that are created in a
room with a radiator against one wall.
The air in contact with the radiator
rises, moves across the ceiling to the far
wall, sinks, and then comes back to the
radiator across the floor.

15. 15
The rate of heat transfer by convection is determined
from Newton's law of cooling, expressed as
Q
conv

16. 16

( )Q hATT
conv s f
 here

Q
conv
= heat transfer rate (W)
A= heat transfer area (m
2
)
h = convective heat transfer coefficient (W/m
2
K)
T
s
= surface temperature (K)
T
f
= bulk fluid temperature away from the surface (K)
•The convective heat transfer coefficient is an experimentally
determined parameter that depends upon the surface
geometry, the nature of the fluid motion, the properties of the
fluid, and the bulk fluid velocity. Ranges of the convective
heat transfer coefficient are given below. h W/m
2
K
free convection of gases 2-25
free convection of liquids 50-
100
forced convection of gases 25-
250
forced convection of liquids 50-20,000
convection in boiling and condensation 2500-
100,000

17. Radiation
Radiated heat energy travels as
an electromagnetic wave.
Sometimes these waves are in
the visible part of the spectrum,
like when something is “red hot.”
You can see how hot it is, but
you can also feel it from a
distance, as your skin absorbs
the energy.

18. 18
Radiative Heat Transfer
Radiative heat transfer is energy in transition from the surface
of one body to the surface of another due to electromagnetic
radiation. The radiative energy transferred is proportional to
the difference in the fourth power of the absolute temperatures
of the bodies exchanging energy.

19. 19
= heat transfer per unit time (W)
A= surface area for heat transfer (m
2
)
σ= Stefan-Boltzmann constant, 5.67x10
-8
W/m
2
K
4
and
0.1713x10
-8
BTU/h ft
2
R
4
= emissivity
T
s
= absolute temperature of surface (K)
T
surr= absolute temperature of surroundings (K)
here
Q
rad

 
44
surrsrad
TTAQ 


20. Conduction, Convection and Radiation

21. THANK YOU