| 13| Annular Cavities

With annular cavities the fluid is considered as an additional conductor in the system so the Nusselt Number does not need to be calculated. Instead the thermal heat flux across the system can be calculated directly. This occurs between Concentric Cylinders and Concentric Spheres.

Concentric Cylinders

‘Keff’ is the effective thermal conductivity (the thermal conductivity of the moving fluid)

The thermal conductivity ‘K’ is the thermal conductivity of the of the fluid if there was no convection and therefore fluid motion. I.e. simply acting as a solid conductor.

The relationship between K and Keff is given by the following equation:

Note- If Keff is less than K, then it is assumed  Keff = K.

The length scaling equation for Ras is given by:

Concentric Spheres

For concentric sphere the equations used calculate thermal heat flux q’, and the relationship between Keff and K is exactly the same!

However, the length scaling equation for Ras is given by: