## HEAT AND MASS TRANSFER

INTRODUCTION TO
HEAT TRANSFER Transmission of energy from one area to the other as a
result of the temperature gradient is known as heat
transfer. APPLICATION Internal combustion engine, Refrigeration and
air conditioning, Design of cooling systems for electrical motors, generation
and transformer, Heating and cooling of fluid, Heat treatment of metals,
Dispersion of atmospheric pollutants. MODE OF HEAT TRANSFER is CONDUCTION,
CONVECTION and RADIATION.

CONDUCTION:- Conduction
is the mode of heat transfer which generally occurs in solid due to temp.
Difference associated with molecular lattice vibrational energy transfer and
also by free electron transfer. The reason behind all electrically good
conductor are also in general good conductor of heat is that the present of
abundant {planetary}free electron. Notable explain to the above statement
is Diamond whose 'K' value 2300 w/mk. The highest thermal
conductivity of diamond is due to its perfect crystalline molecular lattice
arrangement. Definition of 'K' is 'K' is the thermo physical property
of material which tells about the ability of the material to allow the heat
energy to get conduction through the material more rapidly.

Insulator
have very low thermal conductivity thereby they can present or reduce heat
transfer rate. Ex. Asbestos- K is a 0.2 w/mk, Refractory bricks –K is a 0.9
w/mk, Glasswool-K is a 0.075 w/mk, Polyurethane-K is a 0.02
w/mk HEAT CONDUCTION IN GASES Heat conduction occurs in gases by
molecular momentum transfer when high velocity high temp.
Molecules colloid with the low velocity low temp. molecules but in
general gases are very bad conductor of heat. Kair is a 0.026 w/mk at room
condition. NOTE- As the temp of the gases increases there thermal
conductivity is also increase because at higher temp. Greater molecular
activity may result in more no. of collision per unit time and hence more
momentum transfer rate.

Convection is the mode of heat
transfer which generally occurs between a solid surface and the surrounding
fluid due to temp. Difference associated with microscopic bulk motion of fluid
transporting thermal energy. In case of Forced convection heat transfer this motion of fluid is
provided by external agencies like fan or a blower or a pump. Whereas force convection heat
transfer the motion of fluid occurs naturally due to Buoyancy force arising in
out of density change of fluid because of its temperature Change.

Medium for its propagation and
hence occurs by electromagnetic wave propagation traveling with speed
of light. Solar radiation is short wavelength Radiation is the mode of heat
transfer which does not required any material emission. Low temperature
long wavelength emission, Radiation mode of heat transfer completely
predominates over conduction and convection particularly when the temperature
Difference is sufficiently large. NOTE:-In any practical situation all the three
mode of heat transfer may simultaneously exists.

Fourier law of conduction is the law states that the rate of Heat transfer by
conduction along a given direction is directly proportional to the temperature
gradient along that direction and is also directly proportional to the area of
the heat transfer lying perpendicular to the direction of heat
transfer. (-ve sign shows that heat always flows in the direction of
decreasing temperature that is to satisfy clauses statement of second law of
thermodynamics.

Newton’s
law of cooling ( for convection ) The law
states that the rate of Heat transfer by convection between a solid body and
the surrounding fluid is directly proportional to the temperature difference
between them and is also directly proportional to the area of contact or area
of explosion between them. H is a convective heat transfer coefficient or
film heat transfer coefficient in (w/ Sq.mk ) Unlike thermal conductivity
k h is not property of material but is depends upon some of the thermo physical
properties of fluid like.

In force convection heat
transfer, Where, V is a velocity of
fluid. D = characteristics dimension of body. (Dimension of body
that is used in the calculation of dimension less number.) In free convection heat transfer, Where, g
is acceleration due to gravity. B is an Isobaric volume expansion
coefficient of fluid. dT is a (Tw-T enfinity ) C is a
characteristics dimension of body. Practical ranges of h are Free
convection in gases and vapor h is a 3 to 25 w/ m Sq. K. Forced
convection in games:-h = 25 to 400 w/ m Sq. K Free convection in liquids
(water) h = 250 to 600 w/ m Sq. K. For study of convection (
viscosity, incompressible fluid flow through pipes, bouncy theory
) Forced convection in liquids:- h is 600 to 4000 w/ m Sq. K.
Condensation heat transfer:- vapor to liquid H is a 3000 to 25000 w/ m Sq. K.
Boiling heat transfer: - liquid to vapor h is a 5000 to 50000 w/ m Sq. K.

STEFAN AND BOLT MAN LAW :-(
RADIATION) The law states that the radiation energy emitted from the
surface of a black body per unit time per unit area is directly proportional to
the fourth power of the absolute temperature of the black body. Black body
Definition It is the body which absorbs all the thermal radiation energy
incident upon the body whereas and body absorbs certain amount of thermal
radiation, all this absorb energy will get completely transformation into
internal energy mode which will be used to increase its temperature. Black
body is perfect absorber, Diffusive in nature, Ideal emitter, Note: - A
thermally black body absorbing all the incident thermal radiation may not
appear black in color to the human eye. Ex. Ice and snow.

Conduction heat transfer:-Integration of Fourier law of
conduction is Conduction heat transfer through a slab one- dimensional
heat conduction, T=f(x) only, At x= O == T=T1 , x= b === T= T2
No heat generation in the slab and constant 'K' of material, qx is not
equal F(x) to satisfy steady state heat transfer conditioning That is qx = q
x+dx In any conduction through slab 'A' remains constant in the direction
of heat flow. Heat transfer rate for stab per unit area, Electrical
analogy of heat transfer Note: - More the thickness of slab and lesser
it's thermal conductivity, higher the thermal Resistance affect by the slab and
lesser will heat transfer rate. Conduction heat transfer through a
composite slabs Assumption is steady state, 1-D conduction heat transfer through
composite slab. Rate of conduction heat transfer through composite slab.

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