Select one correct answer: [2 Marks] Consider heat transfer between two identica

Question

Select one correct answer:                                                                                   [2 Marks]

Consider heat transfer between two identical hot solid bodies and the air surrounding them. The first solid is being cooled by a fan while the second one is allowed to cool naturally. For which solid is the lumped system analysis more likely to be applicable?                                                                                              

a.

For the body cooled naturally

b. For the body cooled by a fan

c. Lumped system analysis will applicable to both

be

d. Lumped system analysis is not possible for the given condition

Select one correct answer:                                                                                   [2 Marks]

A system has a characteristic length of 0.1m. If the value of its thermal conductivity is half the value of the heat convection coefficient (?? = ?), then the lumped system analysis is not applicable on this system.

a. True

b. False

a.

For the body cooled naturally

b. For the body cooled by a fan

c. Lumped system analysis will applicable to both

be

d. Lumped system analysis is not possible for the given condition

Explanation / Answer

1) option a is correct. Because, for a body that is cooling naturally have less convection heat transfer coefficient than in forced convection.

Bi = hL/K

If Bi<0.1, then lumped analysis is possible.

For that h should be as minimum as possible.

So natural convection has less h value than forced convection.

2) Given L= 0.1m

K= h/2

Bi = hL/K = 2K×0.1/K

= 0.2

So, lumped analysis is not possible.

So, True

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