insulating powder

8/12/2019 Insulating Powder

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THERMAL CONDUCTIVITY OF INSULATING POWDER

OBJECTIVE:

To determine the thermal conductivity of insulating powder at various heatinputs.

THEORY

i) FORIER LAW OF HEAT CONDUCTION

This law states that rate of heat flow through a surface is directly

proportional to the area normal to the surface and the temperature gradient

across the surface.

q α A d T d X

q = -KA d T

d X

Negative sign indicates that the heat flows from higher temperature to the

lower temperature. K is called the thermal conductivity.

ii) THERMAL CONDUCTIVITY

This can be defined as the amount of heat that can flow per unit time across

a unit cross sectional area when the temperature gradient is unity. The units

of thermal conductivity are wm-K. !aterials having higher thermal

conductivity are called conductors while those having lower thermal

conductivity are called insulators. "#amples for good conductors include all

metals. $hile asbestos% magnesia% glass wool etc.% are some the e#amples for

insulators.

iii& The radial heat conduction for single hollow sphere transferringheat from inside to outside is given by

'Π K ri ro (Ti-To&

q = ro ) ri

*



Where

q r!"e #$ he!" "r!%&$er i% '!""&V(I

Ther*!+ ,#%-,"i/i"0 '1*23

ri r!-i& #$ i%%er &4here i% *e"er&

r# r!-i& #$ #"er &4here i% *e"er&

TiTe*4er!"re #$ "he i%%er &4here

T#Te*4er!"re #$ "he #"er &4here

DESCRIPTION OF APPARATUS

The apparatus consists of two concentric copper spheres. +eating coil is

provided in the inner sphere. The space between the inner and outer spheresare filled by the insulating powder whose thermal conductivity is to be

determined. The power supply to the heating coil is ad,usted by using variac.

hromel-Alumel thermocouples are used to record the temperatures.

Thermocouples * to are embedded on the surface of inner sphere and / to

*0 are embedded on the outer shell surface.

SPECIFICATIONS

*. 1adius of inner sphere = 23mm

0. 1adius of outer sphere = *33 mm4. 5oltmeter 3-4335 6 Ammeter 3-2amps

'. 5aric ) 0amps

2. Temperature 7ndicator )3-433 dg

PROCEDURE

*. onnect the unit to an A source 0'3 5 2amps and switch on the !

0. 8perate the 5ariac slowly to increase the heat input to the heater and

ad,ust the voltage to any desired voltage (do not e#ceed *235&4. !aintain the same heat input throughout the e#periment until the

temperatures reaches a steady state.

0



'. Note down the following readings provided in the 8bservation table.

2. 1epeat the e#periment for other heat inputs.

OBSERVATION TABLE

He!" I%4" I%%er Sr$!,e "e*4 C O"er Sr$!,e "e*4 C

S+.

N#.

V A W!""

&

T5 T6 T7 T8 T9 T T; T< T= T5> T55 T56

A/er!?e Ti T5 "# T

-? C

A/er!?e T# T; "# T56

-? C

q @ r#2ri8 r# @ ri Ti2T#) ' 1*

P+#" "he ?r!4h /& He!" I%4"

4

insulating powder

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