Biot number signifies the ratio of ____
A. convective resistance in the fluid to conductive resistance in the solid
B. conductive resistance in the solid to convective resistance in the fluid
C. inertia force to viscous force in the fluid
D. buoyancy force to viscous force in the fluid

Biot number is used to decide if the lumped system analysis can be applied to the heat flow problem where conduction and convection mode of heat transfer is going to take place.

Lumped system analysis: In this approximation the conductive resistance of solid material is assumed to be negligible compared to convective resistance in the fluid.

In other words the solid body is assumed to have large thermal conductivity that is it readily spreads the heat with least resistance.

Therefore Biot number is defined as ratio of conductive resistance of the solid and the convective resistance of fluid.

Bi = Rcond of solid / Rconv of fluid

The lumped system approach is used when the Biot number is very very small.

(Bi << 0.1)

Biot Number Bi = h*Lc/Ksolid

where,

h = heat transfer coefficient (W/m^2)

Lc = Characteristic length(m) = Volume/surface area = V/As

Ksolid = thermal conductivity of solid. (W/mK)