FINS MUST BE ARRANGED AT RIGHT ANGLES TO THE DIRECTION OF FLOW OF T...
4. Fins must be arranged at right angles to the direction of flow of the
working fluid.
Of
these
statements:
(a) 1 and 2 are correct
(b) 2 and 4 are correct
(c) 1 and 3 are correct
(d) 2 and 3 are correct.
Previous 20-Years IAS Questions Heat Transfer from a Bar Connected to the Two Heat Sources at Different, TemperaturesIAS-1.
A metallic rod of uniform diameter and length L connects two heat
sources each at 500°C. The atmospheric temperature is 30°C. The
temperature gradient
dT
dL
at the centre of the bar will be:
[IAS-2001]
500
(a)
500
/ 2
L
(b)
−
L
(c)
470
−
L
(d)
Zero
Page 33 of 97
Answers with Explanation (Objective) Previous 20-Years GATE AnswersGATE-1. Ans. (b)
Q
=
h p K A
θ
tan (
h ml
)
π
π
m
hp
P
rl
A
d
=
=
=
;
2
,
2
4
KA
Substituting we are getting
∴
Q =5 watt
GATE-2. Ans. False
Previous 20-Years IES AnswersIES-1. Ans. (b)
IES-2. Ans. (c) By the use of a fin, surface area is increased due to which heat flow rate
increases. Increase in surface area decreases the surface convection resistance,
whereas the conduction resistance increases. The decrease in convection
resistance must be greater than the increase in conduction resistance in order to
increase the rate of heat transfer from the surface. In practical applications of
fins the surface resistance must be the controlling factor (the addition of fins
might decrease the heat transfer rate under some situations).
IES-3. Ans. (b) Heat dissipated by fin surface
t
t
−
=
×
×
−
=
hP
=
1
2
0.0025 2
100 40
3.4 W
/
0.17 1
1 / 0.17 2
kA x kA
×
×
l
or Heat dissipated by fin surface =
(
)
h Pdx
∫
× −
t t
α
0
IES-4. Ans. (d)
IES-5. Ans. (b)
IES-6. Ans. (a)
dT
⎛
⎞
IES-7. Ans. (a) hA(T
at tip
– T
a
) = – KA
⎜
⎟
dx
=
⎝
⎠
= Negligibly small.
x l
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Therefore, the temperature gradient
⎝
⎠
at the tip will be negligibly small
i.e. zero.
IES-8. Ans. (a) The effectiveness of a fin can also be characterized as
=
=
=
−
=
q
q
T
T
R
R
/
f
f
b
t f
t h
ε
∞
(
)
(
)
(
)
,
,
,
,
f
q
hA T
T
T
T
R
R
−
−
C
b
b
t h
t f
∞
∞
It is a ratio of the thermal resistance due to convection to the thermal resistance
of a fin. In order to enhance heat transfer, the fin's resistance should be lower
than that of the resistance due only to convection.
IES-9. Ans. (b)
IES-10. Ans. (c)
IES-11. Ans. (b)
IES-12. Ans. (b) Effectiveness (ε
fin
)
(
)
Q
kP
hPkA t
t
=
=
=
−
with fin
cs
a
ε
(
0
0
)
Q
h A
h A t
t
fin
−
without fin
cs
cs
a
P
is
If the ratio
fin
A
↑
ε
↑
cs
IES-13. Ans. (a)
KP
KP
IES-14. Ans. (a)
0.75
20 1
∈=
hA
⇒
=
×
×
C
( )
θ
=
fin
0
q
hPKA
C
20 1
20 1 0.75 20
=
× ⋅
× ×
×
×
×
=
20 0.75 20 300
W
300
400
Q
W
∈ =
=
=
fin
75
Q
without fin
∈<
If
1; fins behave like insulator.
IES-15. Ans. (d)
IES-16. Ans. (c)
IES-17. Ans. (a)
IES-18. Ans. (c)
IES-19. Ans. (c) Addition of fin to the surface increases the heat transfer if
hA KP
/
<<1.
IES-20. Ans. (d)
Previous 20-Years IAS AnswersIAS-1. Ans. (d)
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