Nusselt Number

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Nusselt number, abbreviated as Nu, a dimensionless number, is heat transfer analysis to characterize the convective heat transfer coefficient.  It relates the rate of convective heat transfer to the rate of conductive heat transfer in a fluid flow over a solid surface.

The Nusselt number is often used to analyze and design heat exchangers, as it provides a measure of the efficiency of convective heat transfer.  It is also important in the analysis of natural and forced convection problems in fluid dynamics.  The value of the Nusselt number depends on various factors, such as the properties of the fluid (such as viscosity and thermal conductivity), the flow conditions (such as velocity and turbulence), and the geometry of the system.  Empirical correlations and experimental data are often used to estimate the Nusselt number for different types of flow and geometries.

Nusselt number formula

$$Nu \;=\; h \; l_c \;/\;k$$     (Nusselt Number)

$$h \;=\; Nu \; k \;/\;l_c$$

$$l_c \;=\; Nu \; k \;/\;h$$

$$k \;=\; h \; l_c \;/\;Nu$$

Symbol English Metric
$$Nu$$ = Nusselt number $$dimensionless$$ $$dimensionless$$
$$h$$ = heat transfer coefficient $$Btu\;/\;hr-ft^2-F$$  $$W\;/\;m^2-K$$
$$l_c$$ = characteristic length $$ft$$ $$m$$
$$k$$ = thermal conductivity $$Btu-ft\;/\;hr-ft^2-F$$ $$W\;/\;m-K$$