Therefore, the Nusselt number represents heat transfer enhancement through a fluid layer due to convection relative to conduction across the same fluid layer. The preceding equation defines the Nusselt number. In the case of convection, the heat flux can be calculated using Newton’s law of cooling. In the case of conduction, the heat flux can be calculated using Fourier’s law of conduction. Heat transfer through the fluid layer will be by convection when the fluid involves some motion and conduction when the fluid layer is motionless. H is the convective heat transfer coefficient įor illustration, consider a fluid layer of thickness L and temperature difference ΔT. K f is the thermal conductivity of the fluid The Nusselt number is to the thermal boundary layer what the friction coefficient is to the velocity boundary layer. The conductive component is measured under the same conditions as the heat convection but with stagnant fluid. Nusselt number is equal to the dimensionless temperature gradient at the surface, and it provides a measure of the convection heat transfer occurring at the surface. The Nusselt number is a dimensionless number used to describe the ratio of the thermal energy convected to the fluid to the thermal energy conducted within the fluid.įor fully developed (hydrodynamically and thermally) turbulent flow in a smooth circular tube, the local Nusselt number may be obtained from the well-known Dittus-Boelter equation.
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