modes of heat transfer in pharmaceuticals

modes of heat transfer

1. Modes of Heat and Mass Transfer
P M V Subbarao
Associate Professor
Mechanical Engineering Department
IIT Delhi
Just as Intelligent as an Human Being .. ..

2. Modes of Heat & Mass Transfer
•Conduction or Diffusion
•Convection
•Radiation

3. Conduction by lattice vibration

5. Circumstances Leading to Heat Conduction

6. The Chip Carrier
•The chip is housed in a chip carrier or
substrate made of ceramic, plastic, or
glass in order to protect its delicate
circuitry from detrimental effects of the
environment.
•The basic part:
–The chip
–Bond
–Lead frame
–Bond Wires
–Lid
–Pins
•The design of the chip carrier is the first
level of thermal control of electronic
devices.

7. Heat Generation & Conduction through A Basic Chip
The circuitry of an electronic component through which electrons flow
and thus heat is generated is referred as the junction
Junctions are the sites of heat generation and thus the hottest spots in a
component.
Modern chips can with stand a Maximum Junctions temperature of
150
o
C.
Lower Junction temperatures are desirable for extended life and lower
Maintenance costs.
The basic thermal issue is : Removal Heat generated in 1 micron thick
chip made of silicon wafer.

8. Global heat transfer rate due to conduction :






dx
dT
kAq
x

Summary of Fourier’s Law of Conduction

9. Mathematical Description
•Temperature is a scalar quantity.
•Heat flux is defined with direction and Magnitude : A Vector.
•Mathematically it is possible to have:kqjqiqq
zyx
ˆ
''ˆ''ˆ'' '' 

Using the principles of vector calculus:Tkq  ''
 
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



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
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 k
z
T
j
y
T
i
x
T
kq
ˆˆˆ''


10. Further Physical Description
•Will kbe same in all directions?
•Why kcannot be different each direction?
•Why k cannot be a vector?kkjkikk
zyx
ˆˆˆ 

Will mathematics approve this ?
What is the most general acceptable behavior of k, approved by
both physics and mathematics?

11. Most General form of Fourier Law of Conduction






dx
dT
kq
x '' Tkq  ''
 Tkq  ''
 kkjkikk
zyx
ˆˆˆ 

We are at cross roads !!!!!

12. Physical-mathematical Feasible Model
•Taking both physics and mathematics into consideration,
the most feasible model for Fourier’s Law of conduction
is:Tkq .
~
~
''

Thermal conductivity of a general material is a tensor.

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13. Surprising Inventions !!!

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14. Fire Resistant Wood
•Among the assessment properties of wood composite of
structural members in building construction, fire
performance is important and getting more attention
nowadays.
•A new composite called molded carbon phenolic spheres
(CPS), a mixture of sugi wood charcoal powders and
phenol formaldehyde resin molded with a hot press is
developed by a research group in Japan.
•The heat due to a fire accident should be thrown out fast
outside the building.

15. Spread of Fire in A Room

16. Micro-structure of CPS

17. Thermal Performance

18. Another Physical Phenomena analogous to Heat
Conduction
Diffusive Mass Transfer

19. Physics of Mass Transfer
•During a mass transfer process individual chemical species
travels from high concentration region to low
concentration region.
•In this regard mass transfer is analogous to heat transfer.
•Conduction (Diffusion) Mass Transfer is governed by
Fick’s Law of diffusion.
•Fick's laws of diffusion were derived by Adolf Fick in the
year 1855.

20. Rate of
molecular
flow
=
Difference in concentration
Resistance: depends on ability of molecules to pass
through, k and membrane dimensions, Dxand A
Fick's law of diffusiondx
dC
DAJ
x
where Cis the concentration, J
xis the rate of mass diffusion,
and Dis the diffusion coefficient.

21. In two or more dimensionsCDAJ 

Equations based on Fick's law have been commonly used to model
transport processes in foods, neurons, biopolymers, pharmaceuticals,
porous soils, population dynamics, semiconductor doping process, etc.
A large amount of experimental research in polymer science and food
science has shown that a more general approach is required to describe
transport of components in materials undergoing glass transition.

22. 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.

23. 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

24. Forced Convection
•Forced convectionuses 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.