# 2 1. (a) Give three examples for each of natural (free) convection and forced convection which occur in industrial processes. (b) The Grashof number and the Reynolds number appear in most correlations...

2
1. (a) Give three examples for each of natural (free) convection and forced convection which occur in industrial processes.
(b) The Grashof number and the Reynolds number appear in most correlations of experimental data for convective heat transfer. Explain, in a maximum of 150 words, the mechanisms of natural and forced convection with particular reference to the above non-dimensional groups.
(c) An appropriate correlation for heat transfer by natural convection from a horizontal pipe to the atmosphere is
where
and
Nu = 0.5 3Gr0.25 pr0.25
ocp2d3 (Ts - Tf )g Gr = ju2
c au Pr = P k
Show that the above correlation can be simplified to
025 h w 1.34 Ts Tf W m-2 K
when air has the values listed below a = 3.077 x 10-3K-1, p =1.086 kg m-3, cp = XXXXXXXXXXkJ kg-1K-1 k = 2.816 x 10-5 kW m-11(-1, p. = XXXXXXXXXXkg m-1- s-1

## Solution

David answered on Dec 20 2021
Examples of free convection:
i. Melting of metals in the furnace.
ii. Venting hot flue gases or smoke from a boiler, stove, furnace or fireplace to the outside atmosphere
iii. Convection through the pipes of hot fluid to the su
oundings.
Examples of forced convection:
i. Heat transfer through condenser is forced convection
ii. Metal industries provide forced convection for even cooling of melted metal.
iii. Venting of hot gases or smoke from the working spaces is also forced convection.
c) Solution:
Where,
We know,
Now, putting the given data for and numbers, we get:
Hence,
d) Solution: