Heat Transfer Questions and Answers Part-3

1. The famous Fourier series is named after
a) Diller and Ryan
b) J.B. Joseph Fourier
c) Stefan- Boltzmann
d) Wein’s

Answer: b
Explanation: He gave this theory in 1824

2. Fourier law of heat conduction is best represented by
a) Q = -k A d t /d x
b) Q = k A d x/d t
c) Q = -k A
d) Q = k d t/d x

Answer: a
Explanation: According to Fourier law of heat conduction, Q = -k A d t /d x.

3. Here are some assumptions that are made for Fourier law. Identify the wrong one
a) No internal heat generation
b) Steady state heat conduction
c) Non- linear temperature profile
d) Isotropic and homogenous material

Answer: c
Explanation: It has constant temperature gradient and a linear temperature profile.

4. Consider the following statements:
The Fourier heat conduction equation
Q = -k A d t /d x
Presumes
i) Steady state conditions
ii) Constant value of thermal conductivity
iii) Uniform temperature at the wall surface
iv) One dimensional heat flow
Which of these statements are correct?
a) i, ii and iii
b) i, ii and iv
c) i, iii and iv
d) i, iii and iv

Answer: d
Explanation: Thermal conductivity is different for different materials.

5. Negative sign in Fourier heat conduction equation indicates
a) Heat always flow is in the direction of positive temperature gradient
b) Heat always flow in the direction of negative temperature gradient
c) No heat flow is there
d) Data is insufficient

Answer: b
Explanation: The ratio d t/d x represents the change in temperature per unit thickness i.e. the temperature gradient. So it represents heat flow in the direction of negative temperature gradient.

6. Transmission of heat i.e. molecular is smallest in case of
a) Gases
b) Liquids
c) Alloys
d) Solids

Answer: a
Explanation: In gases, atoms are arranged loosely, there is less molecular transmission of heat as compared to solids, liquids and alloys.

7. Which one is not the unit of thermal conductivity?
a) kcal/m hr k
b) KJ/m hr K
c) W/m s K
d) Cal/cm s K

Answer: c
Explanation: The unit kcal/m hr K could also be specified as J/m s K or W/m s K and this is actually done while quoting the numerical values of thermal conductivity.

8. “Thermal conductivity represents the amount of heat conducted across the unit area when a temperature difference of one kelvin”.
a) True
b) False

Answer: b
Explanation: It is across unit area and through unit distance.

9. Which of the following is the unit of thermal resistance?
a) degree/kcal
b) hour degree
c) s degree/kcal
d) degree/w

Answer: d
Explanation: Thermal resistance is expressed in the unit’s hr degree/kcal, degree/W and s degree/J.

10. Thermal conductivity is defined as the heat flow per unit time
a) When the temperature gradient is unity
b) Across the wall with no temperature
c) Through a unit thickness of the wall
d) Across unit area where the temperature gradient is unity

Answer: d
Explanation: Thermal conductivity of a material is because of migration of free electrons and lattice vibrational waves