Heat Transfer Test

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Heat Transfer Test - 4

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Question 1
4 / -1

For conduction heat transfer, the heat energy propagation will be minimal for __________

A
Copper

B
Air

C
Water

D
Lead

Solution

It’s because air has the lowest heat conductivity of all the possibilities.
Question 2
4 / -1

A 25 mm diameter egg roll (k = 1 W/m degree) is roasted with the help of microwave heating. For good quality roasting, it is desired that temperature at the center of roll is maintained at 100 degree Celsius when the surrounding temperature is 25 degree Celsius. What should be the heating capacity in W/m ³of the microwave if the heat transfer coefficient on the surface of egg roll is 20 W/m² degree?

A
613.31 k W/m³

B
93.31 k W/m³

C
6713.31 k W/m³

D
213.31 k W/m³

Solution

t _max= t _a+ q _gR/2h + q _gR²/4k.
Question 3
4 / -1

The rate of heat transfer for a plane wall of homogenous material with constant thermal conductivity is given by which of the following equation?

A
Q = 2k/δ x

B
Q = 2kAδx

C
Q = kA (t₁-t₂)/δ

D
Q = 2kAx/ δ

Solution

The value of the temperature gradient can be substituted into the general equation to compute heat flow. The heat flow is independent of x in some way.
Question 4
4 / -1

The appropriate rate equation for convective heat transfer between a surface and adjacent fluid is prescribed by which law?

A
Newton’s law of cooling

B
Kirchhoff’s law

C
Newton’s first law

D
Wein’s displacement law

Solution

When a solid surface is cooled by a fluid, the rate equation used to describe the mechanism of convection is called Newton’s law of cooling.
Question 5
4 / -1

A wire of radius 3 mm and 1.25 m length is to be maintained at 60 degree Celsius by insulating it by a material of thermal conductivity 0.175 W/m K. The temperature of surrounding is 20 degree Celsius with heat transfer coefficient 8.5 W/ m² K. Find percentage increase in heat loss due to insulation?

A
124.23 %

B
100.00 %

C
12.55 %

D
134.46 %

Solution

Q = 8.5 (2 π 0.003 1.25) (60 – 20) = 8.01 W. % increase = (18.78 – 8.01/8.01) (100) = 134.46 %.
Question 6
4 / -1

Why fins are provided on a heat transfer surface?

A
Pressure drop of the fluid should be minimized

B
Increase turbulence in flow for enhancing heat transfer

C
Surface area is maximum to promote the rate of heat transfer

D
Increase temperature gradient so as to enhance heat transfer

Solution

Fins are added to the surface of a heat exchanger to improve heat transmission by increasing the surface area exposed to the environment.
Question 7
4 / -1

A heating unit is made in the form of a vertical tube of 50 mm outside diameter and 1.2 m height. The tube is fitted with 20 steel fins of rectangular section with height 40 mm and thickness 2.5 mm. The temperature at the base of fin is 75 degree Celsius, the surrounding air temperature is 20 degree Celsius and the heat transfer coefficient between the fin as well as the tube surface and the surrounding air is 9.5 W/m² K. If thermal conductivity of the fin material is 55 W/m K, find the amount of heat transferred from the tube without fin

A
118.44 W

B
98.44 W

C
8.44 W

D
908.44 W

Solution

Q = h A d t = h (π d ₀H) (t ₀– t _INFINITY).
Question 8
4 / -1

What is the rate of heat transfer from the fin in case of fin insulated at the tip?

A
(h P k)^1/2 (t₀– t_a) tan h ml

B
(h P A)^1/2 (t₀– t_a) tan h ml

C
(h P k A)^1/2 (t₀– t_a) tan h ml

D
(h k A)^1/2 (t₀– t_a) tan h ml

Solution

It should contain all the terms i.e. h, A, P, k.
Question 9
4 / -1

Consider the following statements pertaining to heat transfer through fins

(i) They must be arranged at right angles to the direction of flow of working fluid

(ii) The temperature along the fin is variable and accordingly heat transfer rate varies along the fin elements

(iii) Fins are equally effective irrespective whether they are on the hot side or cold side of the fluid

(iv) Fins are made of materials that have thermal conductivity higher than that of wall

Identify the correct statements

A
i and ii

B
iii and iv

C
i and iv

D
ii and iii

Solution

The statements made in serial number 3 and 4 are incorrect. Fins are seen on the side of the body when the convective coefficient is low.
Question 10
4 / -1

In which of the following cases provision of fins on a given heat transfer surface will be more effective?

A
Fewer but thin fins

B
Large number of thin fins

C
Large number of thick fins

D
Fewer but thick fins

Solution

Increase in ratio of perimeter P to be cross-sectional area A _C brings about improvement in the effectiveness of fins.

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Heat Transfer Test - 4

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Heat Transfer Test - 4

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SHARING IS CARING

Question 1

Copper

Air

Water

Lead

Solution

Question 2

613.31 k W/m3

93.31 k W/m3

6713.31 k W/m3

213.31 k W/m3

Solution

Question 3

Q = 2k/δ x

Q = 2kAδx

Q = kA (t1-t2)/δ

Q = 2kAx/ δ

Solution

Question 4

Newton’s law of cooling

Kirchhoff’s law

Newton’s first law

Wein’s displacement law

Solution

Question 5

124.23 %

100.00 %

12.55 %

134.46 %

Solution

Question 6

Pressure drop of the fluid should be minimized

Increase turbulence in flow for enhancing heat transfer

Surface area is maximum to promote the rate of heat transfer

Increase temperature gradient so as to enhance heat transfer

Solution

Question 7

118.44 W

98.44 W

8.44 W

908.44 W

Solution

Question 8

(h P k)1/2 (t 0 – t a) tan h ml

(h P A)1/2 (t 0 – t a) tan h ml

(h P k A)1/2 (t 0 – t a) tan h ml

(h k A)1/2 (t 0 – t a) tan h ml

Solution

Question 9

i and ii

iii and iv

i and iv

ii and iii

Solution

Question 10

Fewer but thin fins

Large number of thin fins

Large number of thick fins

Fewer but thick fins

Solution

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613.31 k W/m³

93.31 k W/m³

6713.31 k W/m³

213.31 k W/m³

Q = kA (t₁-t₂)/δ

(h P k)^1/2 (t₀– t_a) tan h ml

(h P A)^1/2 (t₀– t_a) tan h ml

(h P k A)^1/2 (t₀– t_a) tan h ml

(h k A)^1/2 (t₀– t_a) tan h ml