heat transfer co-efficient ce
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HEAT TRANSFER CO-EFFICIENT
Pritam Kambli
Amruta KarbelkarVikrant YelveKshama SawantVidhi Shah
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HOW IS HEAT TRANSFER ACCOMPLISHEDHeat can transfer between the surfaceof a solid conductor and thesurrounding medium whenever
temperature gradient exists.
There are three ways:
ConductionConvectionRadiation
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It is generally considered as the heat transfer phenomena for
the solids
Itfollows the Fourier Law of Heat Conduction:
Q = -kAT
Where,Q = transfer of heat per unit time
k = conductive heat transfer coefficient
A = heat transfer areaT = temperature difference between two systems
The thermal conductivity units in SI system is W/mK.
CONDUCTION
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Convective heat transfer occurs when the heat istransferred from a solid surface to a moving fluid owingto the temperature difference between the solid and thefluid.
It follows the Newtons Cooling Law of HeatConvection:
Q = hA(T T')Where,
Q = transfer of heat per unit timeh = convection heat transfer coefficientA = heat transfer areaT = temperature of fluidT'= temperature of solid
CONVECTION
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Natural ConvectionNatural convection occurs whenever heat flows
between a solid and fluid, or between fluid layers.
As a result of heat exchangeChange in density of effective fluid layers taken
place, which causes upward flow of heated fluid.
If this motion is associated with heat transfer mechanismonly, then it is called Natural Convection
Forced ConvectionIf this motion is associated bymechanical means such as pumps, gravityor fans, the movement of the fluid isenforced.
Convection is further classified as:
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.
RADIATION
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whereQ= heat flow in input or lost heat flow , J/s = W
h= heat transfer coefficient, W/(m2K)
A= heat transfer surface area, m2
= difference in temperature between the solid surface and
surrounding fluid area, K
HEAT TRANSFER CO-EFFICIENT?
The efficiency of a bodys ability to transfer heat by convection to thesurrounding environment is defined by the convective heat transferco-efficient.
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It can be estimated by dividing the thermal conductivity of theconvection fluid by a length scale.
The heat transfer coefficient is often calculated from the Nusseltnumber.
An understanding of convection boundary layers isnecessary to understanding convective heat transfer between asurface and a fluid flowing past it.
Methods of calculating heat Transfer Co-efficient
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Q hot Q cold
Th Ti,wall
To,wall
Tc
Region I : Hot Liquid-
Solid Convection
NEWTONS LAW OF
CCOLING
dqxhh .ThTiw .dA Region II : ConductionAcross Copper Wall
FOURIERS LAW
dqx
k.dT
dr
Region III: Solid
Cold Liquid
Convection
NEWTONS LAW OF
CCOLINGdqxhc .TowTc .dA
THERMAL
BOUNDARY LAYER
Energy moves from hotfluid to a surface byconvection, through thewall by conduction, andthen by convection fromthe surface to the coldfluid.
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Experiment
AIM:To establish wall heat transfer in packed bed and determinerelationship between nusselt number and reynolds number.
APPARATUS: Hot water with constant inlet temperature as heatingmedium, Platform Scale balance, stop watch, weighing container, Auxiliarypiping and pumping equipment.
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Experimental SetupA 5.0 cm copper tube is fitted with an external jacket. The packing is 3/8"
3/8" raschigrings, labeled inside the tube up to 45.7 cm length and rest on a perforatedplate,having 45% hole area.The packed bed exchanger is equipped with thermometers to measure the inleand outlet temperatures of cold water and hot water owing respectively on thetube side and jacket side.
DataInner diameter of packed tube (Di) = 50 mm.Outer diameter of packed tube (Do) = 60.6 mm.
Inner diameter of jacket (Di) jacket = 86 mm.Length of packed test section = 457.2mm.Raschig Ring Dimensions: Length = 13.6 mm,Inner diameter = 7.6 mm,Outer diameter = 13.6mm.
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PROCEDURE
1. Admit the cold water through the packed tube and set its flow rate to thedesired value asindicated by the calibrated Rota meter.
2. Admit hot water through the jacket at 60C and at constant flow rate of about
10 kg/min.
3. When the steady state is reached, record the inlet and outlet temperatures ofcold water andhot water.
4. Determine the exact flow rates of cold water and hot fluid by weighing theliquids collectedin a known interval of time.
5. Repeat the above procedure for several low rates. Flow rate of cold water inthe packed bed
should be in the range of 2 to 7 kg/min for constant operating conditions of
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Method of Calculation
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PRO LEMA steel pipe of diameter 10cm ismaintained at 170oC and exposed to air at30oC. Length of the pipe is 2m and is kepthorizontally. Find the heat lost by pipe perhour. Following are the properties at mean
temperature.Thermal conductivity = 0.0322W/m/KKinematic Viscosity = 23.3X10-6
Prandlt number = 0.69
For horizontal cylinders:Nu =hL/k = 0.53(Gr.Pr)1/4{104
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Formulae UsedGr = BgmL3/2
Pr = Cp./k
B = 1/T
Q = hAt
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REFERENCE
www.wikipedia.com
Lab manual of chemical engineering, exp no.19, heat transfer inPacked bed.
Unit operation of chemical engineering, mccabe and smith, 5thedition,Pg.403 to pg.465.
Heat transfer co-efficient experiment in fin surface, pdf.
http://www.wikipedia.com/http://www.wikipedia.com/