Mass Transfer Questions and Answers – Correlations for Free Convection

This set of tough Mass Transfer Questions focuses on “Correlations For Free Convection”.

1. In respect to free convection over a vertical flat plate, the Nusselt number for laminar and turbulent flows varies respectively with Grashoff number G r as
a) G r and G r1/4
b) G r1/2 and G r1/3
c) G r1/4 and G r1/3
d) G r1/3 and G r1/4
View Answer

Answer: c
Explanation: It varies as G r1/4 and G r1/3.

2. Experimental results indicate that the local heat transfer coefficient h g for flow over a flat plate with an extremely rough surface is approximated by the relation
h x = a x -0.12
Where a is a constant coefficient and x is distance from the leading edge of the plate. What is the relation between this local heat transfer coefficient and the average heat transfer coefficient for a plate of length x?
a) 1.136 h x
b) 2.136 h x
c) 3.136 h x
d) 4.136 h x
View Answer

Answer: a
Explanation: h = 1.136 a x -0.12 = 1.136 h x.

3. For the free convection circumstances, the following dimensionless numbers apply
(i) Grashof number
(ii) Prandtl number
(iii) Nusselt number
Identify the correct answer
a) ii and iii
b) ii only
c) i and ii
d) i, ii and iii
View Answer

Answer: d
Explanation: All these numbers are useful in free convection.

4. Two horizontal steam mains with diameters 5 cm and 15 cm are to be laid in a boiler house that any mutual heat effect is precluded. The mains are at the same surface temperature of 500 degree Celsius whilst the ambient air is at 50 degree Celsius. Workout the ratios of the heat transfer coefficients
a) 0.136
b) 1.316
c) 2.136
d) 3.136
View Answer

Answer: b
Explanation: h 1/h 2 = (d 2/d 1) 0.25 = 1.136.

5. Consider the above problem, workout the heat losses from one meter length of the mains
a) 0.458
b) 0.448
c) 0.438
d) 0.428
View Answer

Answer: c
Explanation: Q 1/Q 2 = h 1 d 1 l/h 2 d 2 l = 0.438.

6. A spherical heater of 20 cm diameter and at 60 degree Celsius is immersed in a tank of water at 20 degree Celsius. Determine the value of convective heat transfer coefficient
a) 458.32 W/m2 K
b) 558.32 W/m2 K
c) 658.32 W/m2 K
d) 758.32 W/m2 K
View Answer

Answer: a
Explanation: h = Nusselt number (k/d) = 458.32 W/m2 K.

7. A steam pipe 50 mm diameter and 2.5 m long has been placed horizontally and exposed to still air at 25 degree Celsius. If the pipe wall temperature is 295 degree Celsius, determine the rate of heat loss. At the mean temperature difference of 160 degree Celsius, the thermo-physical properties of air are
k =3.64 * 10 -2 W/m K
v =30.09 * 10 -6 m2/s
P r =0.682
Β =1/160 + 273 = 2.31 * 10 -3 per K
a) 1156.5 W
b) 1146.5 W
c) 1136.5 W
d) 1126.5 W
View Answer

Answer: d
Explanation: Q = h A d t, where h = 14.60 k/D.

8. Consider the above problem, find out the Grashof number
a) 0.8447 * 10 5
b) 0.8447 * 10 6
c) 0.8447 * 10 7
d) 0.8447 * 10 8
View Answer

Answer: b
Explanation: Grashoff number = D 3 P 2 (β g d t)/µ 2.

9. Estimate the heat transfer from a 40 W incandescent bulb at 125 degree Celsius to 25 degree Celsius in quiescent air. Approximate the bulb as a 50 mm diameter sphere. The approximate correlation for the convection coefficient is
Nu = 0.60 (Gr Pr) 0.25
Where the different parameters are evaluated at the mean film temperature, and characteristics length is the diameter of the sphere
a) 4.71 W
b) 5.71 W
c) 6.71 W
d) 7.71 W
View Answer

Answer: d
Explanation: Q = h A d t = 7.71 W.

10. Consider the above problem, find what percent of power is lost by free convection?
a) 19.478%
b) 19.378%
c) 19.278%
d) 19.178%
View Answer

Answer: c
Explanation: Percentage of power lost by free convection = 7.71 * 100/40 = 19.278%.

Sanfoundry Global Education & Learning Series – Mass Transfer.

To practice tough questions on all areas of Mass Transfer, here is complete set of 1000+ Multiple Choice Questions and Answers.

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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