Fluid mechanics/heat transfer - meaning of Nusselt number

[RobosaurusRex]

The Nusselt number represents the ratio of heat transfer by convection relative to conduction.

At the onset of convection does this correspond to a Nusselt number of 1?

 

[dRic2]

Not sure what "At onset of convection" means, but, as you pointed out, Nusselt number represents the ration of convection and conduction; in fact, by multiplying for $$ΔT*S/ΔT*S$$ you get $$Nu = Q_{convection}/Q_{conduction} $$ So this means simply that conduction is the "limit" of convection: When the fluid slows down its velocity the heat transfer by convection decreases until the fluid stops ( $v = 0$ ) and the heat transfer becomes by pure conduction; in that case the 2 terms of the fraction are equal and thus $Nu = 1$.