Friday, 16 March 2018

HEAT AND MASS TRANSFER ke important kuch point


                 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/ ) 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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