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
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
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
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
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
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
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
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
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
Explanation: Q = h A d t = 7.71 W.
10. Consider the above problem, find what percent of power is lost by free convection?
Explanation: Percentage of power lost by free convection = 7.71 * 100/40 = 19.278%.
Sanfoundry Global Education & Learning Series – Mass Transfer.
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