| 6| Vertical Plates
In order to more fully understand free convection, the next step is to consider examples of free convection. This should highlight the similarities between free and forced convection and demonstrate that problems can be tackled using similar approaches.
Free convection boundary layer development on a hot plate. The hot plate heats the air near it causing it to rise and generating both a thermal and velocity boundary layer.
Maximum velocity occurs in the boundary layer and is zero at the wall at edge of the boundary layer. The velocity at the edge of the boundary layer is zero since the free stream is quiescent.
The Rayleigh number is used to describe the transition from laminar to turbulent flow in free convection. This occurs at approximately
Rax,c = 109. The critical Rayleigh number can be calculated from the critical Grashof number and using the same expression as has been used previously.
The Nusselt number at a point is found at using the above functions. This correlation holds when the flow is both laminar and turbulent.
The average Nusselt number is calculated by finding the average heat transfer coefficient across the length of the plate and back-calculating Nusselt number from this.
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