250+ TOP MCQs on Unit Operations – Heat Transfer by Convection-2 and Answers

Food Engineering Interview Questions & Answers on “Heat Transfer by Convection-2”.

1. According to Nusselt’s film theory, the film thickness decreases as we move from bottom to top. The heat transfer increases.
a) True
b) False
Answer: b
Clarification: According to Nusselt’s film theory, the film thickness increases as we move from bottom to top. The heat transfer decreases.

2. Statement 1: Unsteady state heat transfer equation can be used when there is negligible internal resistance.
Statement 2: Unsteady state heat transfer equation can be used for liquid food items which are well stirred, because, temperature gradient within won’t be there.
a) True, False
b) True, True
c) False, False
d) False, True
Answer: b
Clarification: Both the statements pertaining to the unsteady state heat transfer equation are true.

3. Statement 1: In nusselt’s film theory, velocity of the falling film is not affected by the velocity of the vapors.
Statement 2: LMTD can be used provided the overall heat transfer co-efficient is constant over a large range of temperatures.
a) True, False
b) True, True
c) False, False
d) False, True
Answer: b
Clarification: Both the statements pertaining to Nusselt’s film theory are true.

4. Find heat transfer in an agitated jacketed vessel if N_Rei is 1.5 * 105 and N_Pr is 20.
a) 0.9
b) 1.2
c) 1.8
d) None of the mentioned
Answer: a
Clarification: N_Nu = 0.36 * (N_Rei) * 0.67 * (N_Pr) * 0.33 * ф_v, N_Nu = hd/k, which is the heat transferred.
Substituting values, we get 0.9.
ф_v is neglected.

5. Which of the following is NOT a real dimensionless number?
a) Pectit number
b) Stanton number
c) Graetz number
d) Grashof number
Answer: a
Clarification: Pectit number is NOT a real dimensionless number.

6. Statement 1: N_Pr= (C_pm)/kl
Statement 2: Stanton number is a product of N_Re and N_Pr.
a) True, False
b) True, True
c) False, False
d) False, True
Answer: c
Clarification: N_Gz= (C_pm)/kl and N_Pr= (C_pμ)/k.
Pecklet number is a product of N_Re and N_Pr.

7. Statement 1: G is known as the mass flux and it is the product of density and velocity.
Statement 2: Grashof number is associated with forced convection.
a) True, False
b) True, True
c) False, False
d) False, True
Answer: a
Clarification: G is known as the mass flux and it is the product of density and velocity. Grashof number is associated with natural convection.

8. Ф_v is generally taken as 1 in food processing as generally very high wall temperatures aren’t reached and hence (µ/µ_w) * 0.14 is 1 as there isn’t much difference in µ and µ_w.
a) True
b) False
Answer: a
Clarification: ф_v is generally taken as 1 in food processing as generally very high wall temperatures aren’t reached and hence (µ/µ_w) * 0.14 is 1 as there isn’t much difference in µ and µ_w.

9. Statement 1: Sieder-Tate and Dittus-boelter equations imply only to laminar flow conditions.
Statement 2: Under laminar conditions, the same equation applies both to Newtonian and Non-Newtonian fluids.
a) True, False
b) True, True
c) False, False
d) False, True