heat transfer co-efficient ce

8/13/2019 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/

heat transfer co-efficient ce

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