# Nusselt Number

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## Nusselt number

[′nu̇s·əlt ‚nəm·bər] (physics)

A dimensionless number used in the study of mass transfer, equal to the mass-transfer coefficient times the thickness of a layer through which mass transfer is taking place divided by the moleculor diffusivity. Symbolized

*Nu*_{m };*N*_{Nu m }. Also known as Sherwood number (*N*_{Sh }). (thermodynamics)

A dimensionless number used in the study of forced convection which gives a measure of the ratio of the total heat transfer to conductive heat transfer, and is equal to the heat-transfer coefficient times a characteristic length divided by the thermal conductivity. Symbolized

*N*_{Nu }.## Nusselt Number

a dimensionless parameter that characterizes the intensity of convective heat exchange between the surface of a body and a flow of gas (or liquid). It is named after the German physicist W. Nusselt (1882–1957). The Nusselt number Nu = α*l/λ*, where α = *Q/(S* · Δ*T*) is the heat-exchange coefficient, *Q* is the heat transfer across the surface of the body per unit time, Δ*T* > O is the difference of temperature between the surface of the body and the gas (or liquid) measured outside the boundary layer, S is the area of the surface, *l* is a characteristic dimension, and λ is the coefficient of thermal conductivity of the gas.