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tech_ed 55(d recl i f fm a i I. co nr

AIM: -l o clctermine the emissivity of gray surface

THEOI{\:

An1 hot body maintained by a constant ltear source' loses heat to surroundings by

conducri.rr, convection and radiation. If two boclies made of same geornetry are heated

under idcntical conditions, the heat loss by contlrrction and convection can be assumed

same fo| irorh the bodies, u4ien the difference in lctllperatures betw'een these two bodies

is not irigS l' such a case, r,viren one body is blrrcl< & tlie other bodf is gray from the

values o1' different surface ternpelatures of thc trvo bodies maitrtained by a constaut

po\ver sor.rrce emissivity can be calculated. The heat loss by radiation depends on:

a) Cltarrcterislic of the ntatelial

b) Georretry of the surface and

c) Ternperature of the surface

-l-[e lic:at loss by radiatiop u,hen one body is corlpletely enclosed by

givett l,'

Q: [ "

l;rrli'a body is losing

A.2 >> at.ea of body A1.

atmosphere equation ( I )

heat to the surrout-tding attrlosphere,

"flius if any body is loosing heat by

takes the form.

the other bodY is

then the area of atmosPhere

radiation to the surrounditrg

.\ r (T4' --ft)

_J^:E-]

Q:oAre(t'r -Taz)

Wherc.c, = Stefan Bolizman constant = 5-6697 x

Ar : Surface area in m2

e: Emissivity

Tr : surface temperature of the body in K and

T2 : Sul.tounding atmospheric temperature in K

Let us consider a black body & a gray body with identical geometry being heated

under identical conditions, assuming ccinduction & convection heat loss to rcrnain the

same. Let Q6 and Q* be the heat supplied to black & gray bodies respectively.

lf heat input to both the bodies are same

Qu: Qg

DE,SCRIPTION:

The experimental set up consists of trvo circulal brass plates of identical

dimensions. One of the plates is nrade black by applying a thick layer of lamp black

u'hile the other platc whose enrissivity, is to be nieasuled is a gray body. Heating coils are

provided at the bottom of the plates. The plates are mounted on asbestos cenrent sheet

and kept in an enilosure to providc undisturbed natLrral convection condition. Three

thertnocotiples are ,nounted ci-l each plate tc neasu)'e the average temperature. Oneal

thermocouple is in the chamber to rneasure the ambient temperature or chamber air

temperature. The heat input can be varied with the help of variac for both the plates that

can be measured using digital volt and ammeter.

Assuming, heat loss by condLrction and cf,nvectior"r from botl-r bodies to remain

sall1e.

llcat loss by radiatiou by the black bod' . lrcat loss bl,rzrcliation by the gray,bo,ll

o x ,4.6 x e5x (T{6 -T{u) : o x A" x e" x 1Ta* -Tao;

'

As geometry of two bodies are identical.A : Ar: 4.6 and sr, : 1 for black body.

Therefore, c=

Where,

Sutfix'b'

Suffix'g'

Suff.-rx'a'

stands for black bod1,.

stands for gray botlr.

stands for ambient.

.EE#

Specirlert material

Specirren Size

Voltrtreter

Amnretet

Dimnter-stat

Tenrpet'ature Indicator

Thelurocouple Used

I Ieatcr

D. P. D.T Selector Sr.r'itcl'r

Brass

+ i60 mnr. 6 mm thickness (gray & black body)

Digital type, 0-300v

Digital type,b-2 amps

0-240 V. 2 amPs

Digiral r1'pe, 0-300'C

7 nos.

Nichronte r.n'ire,

6A

PROCE,DURtr:

I . Su,itch on the electric tnains.

2 OPe rate lhe dinlnrcrslat \/cl'y sloivly artd qive Sallle pgwer inllut to both the heatcr,.

Sal,50Vbyusing/operatirrgcanrswitclrespror,ided.

3. \\ihen steady state is reached note dou,n the temperatures Tr to Tz by rotatirlg thc

ietltpctature selection sr't'itclt.

4 Also tiote dorvn the volt & anlmeter reading

5R.:llcattireexperitrrentfordiffbrentlreatinputs

OBSE,RVATION TABLtr:

st.No

Heater input Temperature ofblack surface "C

ffip"t?t*.., ofgray su'face "C

Chamber'

Temperatureoc

i.!ii.r€*tr;

SPECIMEN CALCULATIONS:

1. Temperature of the black body'

T6: (Tr + T, + T,) + 773 .15 K"3

2. Temperature of the graY bodY

T*:_(Totfrifil +213 15 K

3. Ambient ternperature

Tu: (Tr I 273.15) K

4 Heat input to the coils = V x I rvattj.

5. Emissivity of gray body,

rs: (Too -'fou)

(To* -t'.)

_ r.,:.i, 1

COMMENTS:'.

Do's: *'

1. Before switching ON

Zero Position.

unit, make sure

{

tl-rat the variac is ttl+

the

3. Operale tlr4 Increase the

2.operatethermocoupleselectorswitch(TSS)gently.

r-rnit minimuur tu'ice a rveek'

voltage ve]')'l sl on'lY'

Don'ts. I '

l.Dotrotgoabovel50voltspo\^/erinputtoheater.

2.Donot operate the equipment if liire voltage is less than

200 volts.