The lumped heat transfer analysis is suitable for small bodies with high thermal conductivity.
1. The lumped system analysis is valid when there is uniform temperature distribution throughout the body. To have uniform temperature distribution throughout the body the thermal resistance of the body to conduct heat must be negligible so that the heat is distributed evenly throughout the body.
2. To quantify this a non-dimensional number called Biot number is used which is a ratio of internal conductive resistance to convection resistance at the surface of the body. This number should be as small as possible.(close to zero preferred).
3. Small bodies with high thermal conductivity can readily transfer heat throughout the body and get to a uniform temperature immediately. Being small body the volume to surface area ratio is also small.