1. For a straight rectangular fin of thickness δ and width b, choose the correct option
a) P/AC = 1/ δ
b) P/AC = 2/ δ
c) P/AC = 3/ δ
d) P/AC = 4/ δ
Explanation: As fin effectiveness in case of straight rectangular fin is (2k/h δ)1/2. So, P/A = 2(b + δ)/b δ.
2. “Effectiveness of fin is the ratio of the fin heat dissipation with fin to that of no fin”.
a) true
b) false
Explanation: Fins are used to enhance heat transfer rate and the use of fin on a surface can’t be recommended unless the enhancement in heat transfer justifies the extra cost and complexity associated with the fins.
3. Three fins of equal length and diameter but made of aluminum, brass and cast iron is heated to 200 degree Celsius at one end. If the fins dissipate heat to the surrounding air at 25 degree Celsius, the temperature at the free end will be least in
a) Brass fin
b) Cast iron fin
c) Aluminum fin
d) Each fin will have the same temperature
Explanation: Thermal conductivity of aluminum is higher than that of others.
4. Two long rods A and B of the same diameter have thermal conductivities k and 4k and have one of their end inserted into a furnace at 400 K. At 9.5 m away section from the furnace, the temperature of rod B is120 degree Celsius. So find out at what end from the furnace end, the same temperature would be reached in the rod A?
a) 0.25 m
b) 0.75 m
c) 0.15 m
d) 0.50 m
Explanation: k A /l A2 = k B /l B2, so it is 0.25 m.
5. The following factors need consideration for the optimum design of fins
(i) Cost
(ii) Space considerations
(iii) Weight considerations
Choose the correct option
a) i only
b) i and ii only
c) i, ii and iii
d) ii only
Explanation: The design will be considered optimum when the fins require minimum cost of manufacture are light in weight.
6. 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 with fin
a) 1234 .98 W
b) 1004.84 W
c) 6539.83 W
d) 3829.46 W
Explanation: Heat flow rate convicted from the base, Q b = h A b (t 0 – t INFINITY) and heat flow rate convicted from the fins, Q f = n k A C m (t 0– t a).
7. The fins would be effective for heat conduction if the ratio P k/h A C is
a) Greater than 5
b) Less than 5
c) Equal to 5
d) Varies between 2 to 9
Explanation: The ratio perimeter divided by area multiply by length must be greater than 5.
8. 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) iii and iv
b) i and iv
c) ii and iii
d) i and ii
Explanation: The statements made at serial number 3 and 4 are wrong. Fins are located on the side where the convective coefficient has a low value.
9. An increase in fin effectiveness is caused by a high value of
(i) Convective coefficient
(ii) Thermal conductivity
(iii) Circumference
(iv) Area
Identify the correct statement
a) i and iii
b) iii and iv
c) ii and iv
d) ii and iii
Explanation: Refer to the expression for fin effectiveness, an increase in fin effectiveness is caused by high value of circumference and thermal conductivity.
10. Choose the correct option regarding fin efficiency and fin effectiveness
a) 2 Fin effectiveness = A FIN/A B (Efficiency of fin)
b) 3 Fin effectiveness = A FIN/A B (Efficiency of fin)
c) Fin effectiveness = A FIN/A B (Efficiency of fin)
d) ½ Fin effectiveness = A FIN/A B (Efficiency of fin)
Explanation: On simplify the equations of fin efficiency and fin effectiveness we get the result.