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In thermal fluid dynamics, the Nusselt number (Nu, after Wilhelm Nusselt[1]: 336 ) is the ratio of total heat transfer to conductive heat transfer at a boundary in a fluid. Total heat transfer combines conduction and convection. Convection includes both advection (fluid motion) and diffusion (conduction). The conductive component is measured under the same conditions as the convective but for a hypothetically motionless fluid. It is a dimensionless number, closely related to the fluid's Rayleigh number.[1]: 466
A Nusselt number of order one represents heat transfer by pure conduction.[1]: 336 A value between one and 10 is characteristic of slug flow or laminar flow.[2] A larger Nusselt number corresponds to more active convection, with turbulent flow typically in the 100–1000 range.[2]
A similar non-dimensional property is the Biot number, which concerns thermal conductivity for a solid body rather than a fluid. The mass transfer analogue of the Nusselt number is the Sherwood number.
- ^ a b c Çengel, Yunus A. (2002). Heat and Mass Transfer (2nd ed.). McGraw-Hill.
- ^ a b "The Nusselt Number". Whiting School of Engineering. Retrieved 3 April 2019.