Emissivity Measurement Apparatus
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Transcript of Emissivity Measurement Apparatus
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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.