Given two parallel plates at different temperatures, the Nusselt number gives the ratio of actual heat transferred between the plates by a moving fluid to the heat transfer that would occur by conduction. It is defined by
where H heat transfer per unit area per unit time, d is the distance scale, is the temperature differential, k is the thermal conductivity, Ra is the Rayleigh number, and is the critical Rayleigh number.
Critical Rayleigh Number, Rayleigh Number
