Classical thermodynamics, heat transfer Definition

Nusselt number

Builds on Dimensionless numbers in fluid mechanics Heat transfer coefficient Thermal conductivity Convective heat transfer

Definition

Let a surface exchange heat by convection with a fluid of thermal conductivity $k_f > 0$ . Let $h$ be the heat transfer coefficient and $L > 0$ a characteristic length of the flow. The Nusselt number is the dimensionless number $\mathrm{Nu} = \frac{h L}{k_f}.$

Remarks

The length is, for example, the diameter of a pipe or the length of a plate along the flow.

Example: in a fluid layer of thickness $L$ at rest between plates at temperatures $T_1$ and $T_2$ , the flux is $k_f (T_1 - T_2) / L$ . So $h = k_f / L$ and $\mathrm{Nu} = 1$ .

Non-example: the number $h L / k_s$ with the conductivity $k_s$ of the solid wall is not the Nusselt number. It is the Biot number, which compares resistances inside and at the surface of the solid.

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Convective heat transfer concept
Dimensionless numbers in fluid mechanics concept
Heat transfer coefficient concept
Thermal conductivity concept
Nusselt number definition

Tag	`2VRV90Z4GZ`
URI	`m2://2VRV90Z4GZ`
Kind	definition
Writing	W0 stub
MSC	80A19, 76R05, 76R10
Defines	Nusselt number
Uses	Dimensionless numbers in fluid mechanics, Heat transfer coefficient, Thermal conductivity, Convective heat transfer
Changed	3 November 2026

Nusselt number

Definition

Remarks

Reading path

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