# Heat Transfer Questions and Answers – Nusselt Number

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This set of Heat Transfer Multiple Choice Questions & Answers (MCQs) focuses on “Nusselt Number”.

1. Which quantity signifies the ratio of temperature gradient at the surface to a reference temperature gradient?
a) Reynolds number
b) Nusselt number
c) Fourier number
d) Stanton number

Explanation: It is given by h l/k.

2. The determination of a value of Nusselt number or the convective film coefficient forms a basis for the computation of heat transfer by convection. Towards that end, following approaches have been suggested
(i) Non-dimensional analysis and experimental correlations
(ii) Hydrodynamic concept of velocity boundary layer
(iii) Reynolds similarity between the mechanism of fluid friction in the boundary layer and the transfer of heat by convection
Identify the correct one
a) i, ii and iii
b) i and ii
c) ii and iii
d) i and iii

Explanation: It should be dimensional analysis and experimental correlations.

3. Nusselt number is given by
a) h l/k
b) 2 h l/k
c) 3 h l/k
d) 4 h l/k

Explanation: The length parameter l specifies the geometry of solid body.
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4. The temperature profile at a particular location in a thermal boundary layer is prescribed by n expression of the form
t (y) = A- B y + C y 2
Where, A, B and Care constants. What is the value of heat transfer coefficient?
a) B k/ (t s – t infinity)
b) 2 B k/ (t s – t infinity)
c) 3 B k/ (t s – t infinity)
d) 4 B k/ (t s – t infinity)

Explanation: h = – k/ (t s – t infinity) [d t/d y] y = 0 and (d t/d y) y = 0 = – B.

5. The temperature profile at a particular location on a surface is prescribed by the identity
(t s – t) / (t s – t infinity) = sin (π y/0.015)
If thermal conductivity of air is stated to be 0.03 W/m K, determine the value of convective heat transfer coefficient
a) 6.48 W/m 2 K
b) 6.38 W/m 2 K
c) 6.28 W/m 2 K
d) 6.18 W/m 2 K

Explanation: h = – k/ (t s – t infinity) [d t/d y] y = 0. Therefore h = – k/ (t s – t infinity) [π (t s – t infinity)/0.015].

6. Air at 20 degree Celsius flows over a flat plate maintained at 75 degree Celsius. Measurements shows that temperature at a distance of 0.5 mm from the surface of plate is 50 degree Celsius. Presuming thermal conductivity of air is 0.0266 W/m K, estimate the value of local heat transfer coefficient
a) 23.18 W/m 2 K
b) 24.18 W/m 2 K
c) 25.18 W/m 2 K
d) 26.18 W/m 2 K

Explanation: h = – k/ (t s – t infinity) [d t/d y] y = 0 and d t/d y = – 50 * 10 3 degree Celsius/m.

7. Air at 20 degree Celsius flows over a flat surface maintained at 80 degree Celsius. Estimate the value of local heat transfer coefficient if the local heat flow at a point was measured as 1250 W/m2. Take thermal conductivity of air as 0.028 W/m K
a) 23.83 W/m 2 K
b) 22.83 W/m 2 K
c) 21.83 W/m 2 K
d) 20.83 W/m 2 K

Explanation: Q = h A (t s – t infinity).

8. Consider the above problem, calculate the temperature gradient at the surface
a) – 44636 degree Celsius/m
b) – 34636 degree Celsius/m
c) – 24636 degree Celsius/m
d) – 14636 degree Celsius/m

Explanation: (d t/d y) y = 0 = – h (t s – t infinity)/k.

9. At the interface of solid body, heat flows by conduction and is given by
a) A (t s – t infinity)
b) h A (t s – t infinity)
c) h (t s – t infinity)
d) h A

Explanation: Q = – k A (d t/d y) y = 0.

10. For a given value of Nusselt number, the convective surface coefficient h is directly proportional to
a) Length
b) Mass
c) Thermal conductivity
d) Density

Explanation: It is directly proportional to k and inversely proportional to significant length.

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