Nusselt number

The Nusselt number (symbol: after Wilhelm Nusselt ) is a dimensionless measure of the similarity theory of heat transfer, which measures the enhancement of heat transfer from a surface when you compare the actual conditions when only thermal conduction would occur through a stationary layer. Thus, it sets the intensity of convective heat transfer at a solid surface in relation to an imaginary fluid is at rest, when pure heat conduction effect.

Normally, using the Nusselt number to describe the transfer of heat to fluids flowing. The Nusselt number can, however, also be interpreted as a dimensionless gradient of the temperature at the surface. It is formally formed equal to the Biot number. Thermal conductivity and characteristic length here do not relate to the fixed body, but to the fluid.

  • α = coefficient of heat transfer to the flowing fluid ( SI unit: W / ( m2 K) ),
  • L = Characteristic length ( SI unit: m ) (For example, the length of an overflowed area in the flow direction or the diameter of a tube through which flow. )
  • ? L = thermal conductivity of the fluid ( SI unit: W / ( m K) ).

The units of the coefficients of the right term must be used that is unitless (eg SI units only ).

The similarity theory is that the heat transfer between two geometrically similar structures are equal if their Nusselt numbers are the same, regardless of the real extent have the right transport. This applies to both free and forced convection. The equation of the Nusselt number is used to determine the heat transfer coefficient α of certain fluids in certain geometries.

Considering the above-mentioned heat transfer between a solid and a liquid medium, the average Nusselt number as

Are formed. Here, an average for the Nusselt number, but on the heat flux.

  • = Heat flow, SI unit W, by the area
  • = Heat flux density, SI unit W / m2
  • = Temperature of the wall, and the fluid at a large distance from the wall, SI unit K
  • = Characteristic length, SI - unit m. This may be the length of a laminar boundary layer flow over plate, for example.

For both sides suffused with laminar boundary layer plate of length applies:

  • Code ( Fluid Mechanics )
  • Code ( thermodynamics)
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