1. The handle of a saucepan, 30 cm long and 2 cm in diameter, is subjected to 100 degree Celsius temperature during a certain cooking operation. The average unit surface conductance over the handle surface is 7.35 W/m2 degree in the kitchen air at 24 degree Celsius. The cook is likely to grasp the last 10 cm of the handle and hence the temperature in this region should not exceed 38 degree Celsius. What should be the thermal conductivity of the handle material to accomplish it? The handle may be treated as a fin insulated at the tip
a) 18.36 W/m degree
b) 17.36 W/m degree
c) 16.36 W/m degree
d) 15.36 W/m degree
Explanation: α x/α 0 = t x –ta/t 0 –t a= cos m (l – x)/cos ml. Now, for a circular handle of diameter d, P/A = 4/d.
2. Let us assume a square section fin split longitudinally and used as two fins. This will result in
a) Increase or decrease in heat transfer depending on the material of fin
b) Heat flow remains constant
c) Decrease in heat transfer
d) Increase in heat transfer
Explanation: Heat transfer will definitely increases because it split into two fins i.e. more surface area.
3. Mark the false statement regarding effectiveness of fin
a) A high value of film coefficient adversely affects the fin effectiveness
b) Fin effectiveness is improved if the fin is made from a material of low conductivity
c) Fin effectiveness is improved by having thin fins
d) It can also be improved by having closely spaced fins
Explanation: It should be of high thermal conductivity.
4. Which fin yields the maximum heat flow per unit weight?
a) Straight triangular fin
b) Curved triangular fin
c) Parabolic fin
d) Hyperbolic fin
Explanation: In straight triangular fin, there is maximum heat flow.
5. Heat dissipation by every segment of the fin is
a) Sometimes same
b) Same
c) Not same
d) Sometimes same or sometimes not same
Explanation: It is always different as fins are not uniform with respect to cross-sectional area.
6. “If a fin of a constant cross section is used, there would be wastage of material”. Chose the correct option
a) true
b) false
Explanation: Cross section must vary to utilize the material.
7. Which one is true regarding parabolic fin?
a) It dissipates the minimum amount of heat at a minimum material cost
b) It dissipates the minimum amount of heat at a maximum material cost
c) It dissipates the maximum amount of heat at a maximum material cost
d) It dissipates the maximum amount of heat at a minimum material cost
Explanation: In this case, a parabolic fin is of great practical importance.
8. For parabolic fin, the curve follows which law?
a) y = C/x2
b) y = C x4
c) y = C x2
d) y = C x1/2
Explanation: Equation of parabola is y = 4 x2 or x = 4 y2.
9. The correction length for cylindrical fin is
a) L C = L + d/4
b) L C = 2 L + d/4
c) L C = 3 L + d/4
d) L C = 4 L + d/4
Explanation: Area = π d2/4. Where, d is the diameter.
10. Provision of fins on a given heat transfer surface will be more effective if there is
a) Fewer but thick fins
b) Large number of thick fins
c) Fewer but thin fins
d) Large number of thin fins
Explanation: Increase in ratio of perimeter P to be cross sectional area A C brings about improvement in the effectiveness of fins.