I understand that there is such a metric as the Rayleigh number which governs convective cooling in a medium. Assume for example, a fist sized heatsink like that below in free air:-
Does the Rayleigh number mean that convection might not start between the fins? So in effect, the finned heatsink might as well be a large solid lump? And insofar as thermal resistance goes, the thermal conductivity would only depend on the gross overall dimensions (height x width x length) and not the actual finned area?