lect - 7 examples transient conduction (only questions).pptx

8/17/2019 Lect - 7 Examples Transient Conduction (Only Questions).pptx

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Examples

Dr. Senthilmurugan S. Department of Chemical Engineering IIT Guwahati - Part 7

Transient conduction


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Transient Conduction

The temperature of a gas stream is to be measured by a thermocouple hose

!unction can be approximated as a 1"mm"diameter sphere# as shon in the

figure$ The properties of the !unction are k= %5 &/m'(# )*+5,, -g/m%#and c p *

%2, ./-g'(# and the conection heat transfer coefficient beteen the !unction

and the gas is h=21, &/m2'($ 0etermine ho long it ill ta-e for the

thermocouple to read percent of the initial temperature difference$

Example %"1 3unus# 4fshin


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5/12/16 | Slide %


An ordinary egg can be approximated as a 5-cm-diameter sphere (See

Figure). The egg is initially at a uniform temperature of 5° and is

dropped into boiling !ater at "5°. Ta#ing the con$ection heat

transfer coe%cient to be h=&' *m'+, determine ho! long it !ill

ta#e for the center of the egg to reach °.

Example %"2 3unus# 4fshin


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5/12/16 | Slide 7


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5/12/16 | Slide 5


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5/12/16 | Slide 6

Steel 8all Cooling in 4ir

A steel ball /c 0 .12 #3*#g.° # 0 45 *m+° 5. cm in diameter

and initially at a uniform temperature of 15° is suddenly placed in a

controlled en$ironment in !hich the temperature is maintained at

&6. The con$ection heat-transfer coef7cient is & *m'.°.

alculate the time re8uired for the ball to attain a temperature of

&5°.

Example %"% 9olman

T:* 75, ;C

T,* 75, ;C


7/155/12/16 | Slide =


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5/12/16 +

A large slab of aluminum at a uniform temperature of '° suddenly

has its surface temperature lo!ered to °. hat is the total heat

remo$ed from the slab per unit surface area !hen the temperature at a

depth 1. cm has dropped to &'°9

Ti * 2,,

;C

T, * =, ;C

x* 7 cm

Tx# > *12, ;C

9eat ?emoal from semi"infinite solid

Example %"7 9olman


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5/12/16


10/155/12/16 | Slide 1,

Sudden Exposure of Semi"@nAnite Slab to Conection

The slab of aluminium is suddenly exposed to a con$ection-surface

en$ironment of ° !ith a heat-transfer coef7cient of 5'5 *m '.°.

alculate the time re8uired for the temperature to reach &'° at the

depth of 1. cm for this circumstance.

Example %"5 9olman

Ti * 2,,

;C

T, * =, ;C

x* 7 cm

Tx# > *12, ;C


11/155/12/16 | Slide 11

4luminum Blate Suddenly Exposed to Conection

A large plate of aluminum 5. cm thic# and initially at '° is

suddenly exposed to the con$ection en$ironment of :xample 4-5.

alculate the temperature at a depth of &.'5 cm from one of the faces

& min after the plate has been exposed to the en$ironment. ;o!

much energy has been remo$ed per unit area from the plate in this

time9

Example %"6 9olman

Ti * 2,,

;C

T, * =, ;C

x* 5 cm

x* 1$25 cm > * 1 min


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5/12/16 | Slide 15


A long '-cm-diameter cylindrical shaft made of stainless steel 41

comes out of an o$en at a uniform temperature of 2° (See Fig belo!).

The shaft is then allo!ed to cool slo!ly in an en$ironment chamber at

'° !ith an a$erage heat transfer coe%cient of h 0 = *m'+,.

>etermine the temperature at the center of the shaft 15 min after the

start of the cooling process. Also determine the heat transfer per unit

length of the shaft during this time period.

Example %"+ 3unus# 4fshin

lect - 7 examples transient conduction (only questions).pptx

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