Correct Answer - Option 4 : the ratio of internal conductive resistance to external convective resistance
Explanation:
Biot number:
It is defined as the ratio of internal conductive resistance offered by the body to the external convective resistance.
\(Bi=\frac{\frac{L}{kA}}{\frac{1}{hA}}=\frac{hL}{k}\)
Biot number properties:
- Biot number is used as a criterion for lumped heat analysis.
- Low biot number values signify that the body offers very little conductive resistance for any internal heat transfer within the body.
- During unsteady-state heat transfer if the biot number is less than 0.1 then the temperature is the function of time only, it is not the function of space.
- On the other hand, if the biot number is more than 0.1 then the temperature is the function of space and time.
Lumped heat analysis:
In this analysis it is assumed that the temperature of the body is uniform throughout its mass at any instant of time i.e., Internal temperature gradient are negligibly small at any instant of time.
i.e.,
T = f(time), T ≠ f(space)
where, T ⇒ Temperature.
