Heat and Mass Transfer Balamurgan Notes

8/3/2019 Heat and Mass Transfer Balamurgan Notes

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BBALAMUGUND

ANSENIORLECTURER PAN

IMALAR ENGGCOLLEGECHEN

NAI


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36. What is

meant

by Newtonianheating or

coolingprocess?

The process inwhich the


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internalresistance is

assumed asnegligible

in comparisonwith its surface

resistance is


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known asNewtonian

heatingor cooling

process.37. What is

meant by


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Lumped heat

analysis?

In a Newtonianheating or

cooling processthe temperature

throughoutthesolid is


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consideredto be uniform at

a given time.Such an

analysisis called Lumpe


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d heat capacityanalysis.

38. Whatis meant by

Semi-infinitesolids?

In a semiinfinite solid, at


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any instant oftime, there is

always a pointwherethe effect

of heating orcooling at one

of its


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boundaries isnot felt at all.

Atthis point thetemperature

remainsunchanged. In

semi infinite


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solids, the biotnumber value is

.39. What is

meantby infinite

solid?


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A solid whichextends itself

infinitely in alldirections of

space is knownasinfinite solid.

In infinite


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solids, the biotnumber value is

in between 0.1and100..1001.0


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40. Define Biot

number.

It is defined asthe ratio of

internalconductive

resistance tothe


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surface convective resistance.ceresisconvectiveSurfacecere

sisconductive Internal Bi

tantan

=k hL B

ci

=


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41. What is

the significance

ofBiot Number?

B

BALAMUGUND

ANSENIOR


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LECTURER PAN

IMALAR ENGG

COLLEGECHEN

NAI

Biot number is

used to findLumped heat

analysis, Semi


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infinite solidsand Infinite

solids42. Explain

the significanceof Fourier

number.


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It is defined asthe ratio of

characteristicbody

dimension totemperature wa


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ve penetrationdepth in time.timeindepthn penetratiowav

eeTemperatur ensionbody sticCharacterinumber Fourier

dim=

It signifies thedegree


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thermal

conductivity?

1. Moisture. 2.Density of

material. 3.Pressure. 4.

Temperature.5.


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Structure ofmaterial.

44. Explainthe significance

of thermaldiffusivity.

The physicalsignificance of


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thermaldiffusivity is

that it tells ushow

fast heat is propagated or it dif

fuses through a


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0.3m thick. If the

temperatures of

the inner and outer

surface aremaintained at 50C

and 30Crespectively.

Calculate the heatlossthrough one


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square meter area.

Find also the

temperature at an

interior pointof the wall 24cm

distance from theouter wall.

(Ans : 46.67w/m2


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, 34C)

2.A steam pipe

200mm OD

is covered with25mm thich layer

of insulationmaterial with an

average thermalconductivity of


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0.08W/mK. The

temperature of the

pipesurface is

400C and that ofthe outer surface

of insulation is50C. Find the loss

of heat from a


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length of 10m of

the pipe line.(Ans: 7.88kW)

B

BALAMUGUND

ANSENIOR


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LECTURER PAN

IMALAR ENGG

COLLEGECHEN

NAI3.Computethe heat loss per

square meter ofthe surface area

of furnace wall25cmthick. The


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inner and outer

surface

temperatures are

400C and 40Crespectively. The

variation of thethermal

conductivity inW/mK with


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temperature in C is

given by the

following

equation: k =0.002T - 10

-6T2

(Ans: 575 W/ m


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2

)4.The temperature

distribution in along cylindrical

tube at a certaininstant is given by

T= 800 + 1000r

5000r2


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. Find (1) The

rate of heat flow

at the inside

and outsidesurface per m

length. (2) Therate of heat

storage per mlength. (3)


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Therate of change

of temperature

with time at the

inner and outersurfaces. Take

the following data ID = 60cm, O

D = 1m, k = 58


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W/mK, = 0.00

4m2

/h.(Ans: 219kW,

730kW, -511kW,-72C/h)

5.An

aluminium spher


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e weighing 7kg a

nd initially

at a temperature

of 260Cis suddenly

immersed in afluid at 10C. If h =

50W/m2


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K, compute the

time requiredto

cool the sphere to

90C.3

/2707mkg

=


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, c = 9000 J/kgK,

k = 204 W/mC.

(Ans: 1580sec)

UNITIICONVECTIO

N HEAT

TRANSFER


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to the bulk motion

of thefluid

particles; through

the molecularconduction within

the fluid itself alsocontributes

tosome extent. If this motion is mai


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nly due to the den

sity variations ass

ociated with tempe

rature gradientswithin the fluid,

the mode of heattransfer is said to

be due to


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freeor

natural convectio

n

. On the otherhand if this fluid

motion isprincipally

produced bysomesuperimposed


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velocity field like

fan or blower, the

energy transport is

said to be due toforced convection

. B

BALAMUGUNDANSENIOR


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LECTURER PAN

IMALAR ENGG

COLLEGECHEN

NAIConvection

BoundaryLayers:Velocity B

oundary Layer


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: Consider the

flow of fluid over

a flat plate as

shown in thefigure. The fluid

approaches theplate in x direction

with uniformvelocity u


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. The fluid particlesin the fluid layer

adjacent to thesurface get zero

velocity. Thismotionlesslayer

acts to retart the

motion of particles


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in the adjoining

fluid layer as a

result of

friction betweenthe particles of

these twoadjoining fluid

layers at twodifferent


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velocities.

Thisfluid layer

then acts to retart

the motion ofparticles of next

fluid layer and soon, until

adistancey =


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from the surface

reaches, where

these effects

become negligibleand the

fluid velocity ureaches the free

stream velocity u.


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as a result of

frictional effects

between thefluid

layers, the localfluid velocity u

will vary from x=0, y = 0 to y =


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.The region of the

flow over the

surface bounded

by

in which theeffects of

viscous shearingforces caused by


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fluid viscosity are

observed, is called

velocity boundary

layer or hydrodynamic

boundary layer.The thickness of

boundary layer


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is

generallydefined

as a distance from

the surface atwhich local

velocity u = 0.99of free

streamvelocity u.


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The retardation of

fluid motion in

the boundary

layer is due to theshear stresses

acting in oppositedirection with

increasing thedistance y from


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the surface

shear stress

decreases, the

local velocity uincreases until

approaches u

. With increasing

thedistance from


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the leading edge,

the effect of

viscosity

penetrates furtherinto the free stream

and boundarylayer thickness

grows.


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Thermal boundar

y Layer:

If the fluid

flowing on asurface has a

different temperature than the

surface, thethermal boundary


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aconstant

temperature Ts.

The fluid particles

in adjacent layerto the plate get the

same temperaturethat of surface. T

he particles exchange heat energy


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with particles in a

djoining fluid

layers and so on.

As a result, thetemperature

gradients aredeveloped inthe

fluid layers and atemperature


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profile is

developed in the

fluid flow, which

ranges from Ts atthe surface to

fluid temperatureT


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sufficiently far

from the surface

in y direction.

Heat and Mass Transfer Balamurgan Notes

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