|  5| Mixed Convection

Often there will be a mixture of forced and free convection interactions and it is important to know if one of these is dominant or whether the both mechanisms must be considered. To do this a measure of forced - free convection forces is used.

Forced and free convection are comparable if:

Forced convection dominates if:

Heat transfer correlations for mixed convection are calculated by taking a form of weighted average of the Nusselt numbers for forced convection and for free (natural) convection.

FC represents Forced Convection

NC represents Free (Natural) Convection

+ is used for assisting and transverse flows (buoyancy occurring in the same direction)

- is used for opposing flows

n = 3 (approximately)

The use of +/- depends on the direction of the buoyancy forces. Below is a diagrammatic explanation of how to tell if a flow is assisting or opposing.

In both cases a air is moving upward along a wall and forming a boundary layer of flow. This flow will affect the surrounding quiescent air in different ways dependent on whether the flow is hotter or colder. If T1 >T2 then the quiescent air will be warmed and will circulate upwards (seen in the left half of the diagram). This is an assisting flow since the buoyancy in the quiescent fluid acts in the same direction as the boundary layer of flowing fluid.

In the reverse case where the air flowing is cooler than the surrounding quiescent air, the buoyancy will be opposing as the near the boundary layer is being cooled and is sinking.

To continue to Part 6, click the link below.

|6| Heat Transfer Correlations