# Food Engineering Questions and Answers – Unit Operations – Heat Transfer by Convection-2

This set of Food Engineering Interview Questions & Answers focuses 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

Explanation: 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

Explanation: 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

Explanation: Both the statements pertaining to Nusselt’s film theory are true.

4. Find heat transfer in an agitated jacketed vessel if NRei is 1.5 * 105 and NPr is 20.
a) 0.9
b) 1.2
c) 1.8
d) None of the mentioned

Explanation: NNu = 0.36 * (NRei) * 0.67 * (NPr) * 0.33 * фv, NNu = hd/k, which is the heat transferred.
Substituting values, we get 0.9.
фv 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

Explanation: Pectit number is NOT a real dimensionless number.

6. Statement 1: NPr= (Cpm)/kl
Statement 2: Stanton number is a product of NRe and NPr.
a) True, False
b) True, True
c) False, False
d) False, True

Explanation: NGz= (Cpm)/kl and NPr= (Cpμ)/k.
Pecklet number is a product of NRe and NPr.

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

Explanation: G is known as the mass flux and it is the product of density and velocity. Grashof number is associated with natural convection.

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

Explanation: фv is generally taken as 1 in food processing as generally very high wall temperatures aren’t reached and hence (µ/µw) * 0.14 is 1 as there isn’t much difference in µ and µw.

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

Explanation: Sieder-Tate and dittus-boelter equations imply only to turbulent flow conditions. Under laminar conditions, different equations apply both to Newtonian and non-Newtonian fluids.

10. Dittus-boelter equation is applicable for fluids with phase change.
a) True
b) False

Explanation: Dittus-boelter equation is applicable for fluids without phase change.

Sanfoundry Global Education & Learning Series – Food Engineering.

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